Monitored Natural Attenuation: Demonstration and review of the ... · The evaluation made clear...

NICOLE

Monitored Natural Attenuation:Demonstration and review of the applicability of MNA at 8 field sitesPart 1: Main report

NICOLE secretariatMarjan Euserc/o TNOLaan van Westenenk 501P.O. Box 3427300 AH ApeldoornThe NetherlandsP +31 55 549 39 27www.nicole.org

Hans Slenders (TNO)Alette Langenhoff (TNO)

Hendrik Ballerstedt (TNO)Jeroen Ter Meer (TNO)

Anja Sinke (BP)

Date: September 2005

All rights reserved.Na part of this publication may be reproduced and/orpublished by print, photoprint, microfilm or any other means without the previous written consentof TNO and Nicole.

Nicole-report Demonstration project MNA

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Preface

This report has been prepared for a consortium of Nicole members; the Network of Industrially Contaminated Land. The consortium included: BP (UK), ChevronTex-aco (The Netherlands), DOW Benelux BV (The Netherlands), ExxonMobil (Europe, Africa Middle East), Ford Werke AG (Germany), Port of Rotterdam (The Netherlands), Shell (UK), Solvay (Belgium), ENI group (Italy) and Total (France). The report describes the current approach to investigate the possibilities to apply Monitored Natural Attenuation (MNA) as a sound remedial alternative at contami-nated sites. MNA may be a stand alone option or a part of a strategy involving other activities, for instance source treatment. The report refers to and makes use of previously published literature and guidelines, which are considered as key docu-ments and to which the reader is referred for extended explanation. Eight sites have been investigated as demonstration sites in 6 different European countries during the last four years. These sites were selected on the basis that they were either in the early stages of MNA evaluation or that an evaluation was about to start. The outcome with respect to the efficacy of MNA was not known before-hand. The condensed site reports have been reviewed by 12 independent reviewers. Apart from a prescribed questionnaire, the reviewers were free to make whatever comments they wished. Comments were inevitably influenced by their legislative background, personal opinion and sphere of interest.

Acknowledgements: The authors acknowledge Nicole and the consortium for their support, scientific input and comments on the report. We appreciate the thorough evaluation and comments on the demonstration sites by the reviewers: – Dick Corpel, Province Gelderland, the Netherlands; – Andreas Dahmke, University Kiel, Germany; – Daneel Geysen, OVAM, Belgium; – Alessandro Gargini, University Ferarra, Italy; – Richard Lookman, VITO, Belgium; – Jean-Remi Mossman, BRGM, France; – Michel Nomine, INERIS, France; – John Parsons, University Amsterdam, The Netherlands; – Francesca Quercia, EA, Italy; – Jonathan Smith, EA, England and Wales; – Steve Thornton, University Sheffield, United Kingdom; – Gerd Wolff, Umweltschutz Stuttgart, Germany.


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Executive summary (English, French, Italian, Dutch, German)

Back in 2000 NICOLE set up a data sharing program for industrial sites to in-vestigate the possibilities for MNA. Over the last 3-4 years, 8 industrial partners and their consultants carried out investigations at their sites as part of the Nicole MNA demonstration project. The 8 site reports were put into a summarized for-mat and reviewed by 12 independent reviewers from universities and authorities. Although the reviewers had a lot of comments, mainly caused by their need for information, the general outcome was that MNA is applicable and effective at many sites. The proposed methodology with the common 3 lines of evidence was embraced as being very useful. The difference in opinions arising from the coun-try backgrounds of the reviewers was striking. This report demonstrates the ap-plicability of MNA at industrial sites. Application and acceptance of NA and MNA The 8 cases that have been investigated are considered representative for European industry. The majority of the cases were complex, with either layered hydrology, or multiple contaminants or phase present. The outcomes of the MNA site assess-ments were not known beforehand, but at all these sites the site owners (in consul-tation with the authorities) were looking to fit MNA into a risk based site approach, were MNA can be a stand alone option or a cost-effective addition to active source measures. Each of the reviewers reviewed two sites, each site was reviewed 3 times. During the review process, the majority of reviewers recognized the occurrence of NA (degradation processes) at the sites, but sometimes felt uncertain (25%) and in-dicated a need for more information than offered in the summarized reports. The original site reports were summarized and put into a standard format to enable an efficient reviewing process. It was not realistic to expect reviewers to review all the underlying reports and data. The application of MNA as a management option is not always accepted by the re-viewers. In some cases the plume is growing (site A, C, E) despite degradation. Of-ten the flux out of the source zone into the plume is significantly larger than the degradation and other NA processes, and MNA can only be applied in combination with active measures. In other cases the national legislative framework in the coun-tries of some reviewers is not (yet) adapted to a risk based approach, and the accep-tance of MNA is difficult (site G). Methodology and review process For this project NICOLE chose independent reviewers, with a background in Uni-versity or authorities. This led to critical reviews, because both groups wanted to check/challenge the conclusions on a data level, which was not fully included in the summarized reports. The reviewers were positive about the summarized re-ports, and although wanting more data, 75% were very happy with the systematic


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approach to the lines of evidence. This proved to be very efficient and commonly accepted. These 3 lines consist of: 1. a documented loss of contaminants or receding plume; 2. documented NA conditions; 3. documented microbiological activity (presence of relevant bacteria, column

tests, microcosms). The approach following the lines of evidence seems to be commonly accepted amongst authorities and scientists, and was clear for people with different country backgrounds, and enabled comparison. The influence of their legislative back-ground was noticeable; risk-based or not. Points of attention regarding information and presentation The reviewers indicated they needed more extensive information to be able to judge the applicability of MNA. This concerned spatial data, soil profile, vertical distribution of contaminants and groundwater chemistry, and a solid conceptual model of basic processes and plume behaviour. Sometimes the expressed data need was more applicable to conditions for a scientific research project than for an eve-ryday remediation project where 100% proof and certainty is not a realistic de-mand. The evaluation made clear that with summarized information the applicability of MNA can be estimated already. The extent of the site specific information needed should therefore depend upon the obviousness that MNA is occurring and the risk involved for receptors. A structured argumentation with a conceptual model and a comprehensive data set is always necessary to get MNA approved, and to remove remaining doubt with authorities and site owners. Recommendations Experiences with soil remediation worldwide have made abundantly clear that our society is not able to achieve a complete removal of contaminants in the subsurface within time frames of a few decades. Technically a 100% removal in most cases is impossible. Financially the costs of full removal are not in line with the benefits and risks involved. The Nicole demonstration project underlined that in this per-spective, MNA is a cost-effective option that can be used to obtain satisfactory re-sults. MNA is often not a stand alone option, but part of a risk based site manage-ment plan MNA should be incorporated into the various soil and groundwater policies in European member states and DG-environment of the EU-commission, as one of the tools to manage historical soil and groundwater pollution, and to support the objec-tives of the EU Water Framework and Groundwater Directives and Soil Thematic Strategy.


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En 2000, le réseau NICOLE a démarré un programme collaboratif d’étude de l’Atténuation Naturelle (AN) sur des sites industriels. Au cours des 3-4 dernières années, 8 partenaires industriels et leurs consultants ont conduit sur leurs sites des investigations dans le cadre du projet NICOLE de démonstration de l’atténuation naturelle. Les synthèses des études sur chacun des 8 sites ont été présentées sous un format commun et soumis à la revue critique de 12 lecteurs du monde académique ou représentants des autorités. Bien que les relecteurs aient faits de nombreux commentaires, découlant surtout du besoin de disposer de plus de données, il en résulte que l’AN est applicable et efficace sur beaucoup de sites. La méthodologie classique proposée des trois faisceaux de preuve est adaptée et considérée comme très utile. Les différences entre contextes nationaux des relecteurs transparaissent dans leur avis. Ce rapport démontre l’applicabilité de l’AN sur des sites industriels. Mise en œuvre et acceptation de l’AN et l’AN contrôlée Les 8 cas étudiés sont considérés comme représentatifs de l’industrie européenne. La majorités des cas sont complexes, de par une hydrogéologie stratifiée ou de par la présence de contaminants multiples et de phases séparées. Les résultats de l’évaluation de l’AN n’étaient pas connus d’avance. Les sites (en accord avec leurs autorités) envisagent l’AN dans une approche de gestion des risques où l’AN seule peut être une option suffisante ou un complément économiquement viable à des mesures actives de gestion des sources. Pour la procédure de relecture, chaque évaluateur a reçu deux sites, chaque site a été évalué trois fois. La majorité des relecteurs reconnaissent l’occurrence de l’AN (processus de dégradation), mais parfois ils doutent (25%) et expriment le besoin de disposer de plus d’information que ce qui était disponible dans les synthèses. Les rapports des études des sites ont été résumé et mis dans un format de présenta-tion qui permette un processus de relecture efficace. L’application de l’AN comme option de gestion n’est pas toujours acceptée par les relecteurs. Dans certains cas, le panache s’étend (site A, C, E) malgré une dégrada-tion. En cause un le flux sortant de la zone source supérieur à la dégradation et aux autres processus d’atténuation, l’AN n’est alors envisageable qu’en combinaison avec des mesures actives. Dans d’autres cas, le contexte législatif du pays du relec-teur n’est pas (encore) adapté à l’approche de gestion du risque et l’AN controlée est difficilement acceptée (site G). Méthodologie et processus de relecture Pour ce projet, NICOLE a choisi des relecteurs indépendants issus des autorités ou du monde académiques. Il en résulte des avis critiques, chaque groupe souhaitant vérifier/contrôler les conclusions sur base des données brutes, ce qui n’était pas in-clus dans les rapports des sites. Les relecteurs sont très positifs quant aux rapports bien que souhaitant plus de données de détail, 75% sont satisfaits avec l’approche


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par « faisceaux de preuve ». Cette approche s’est révélée efficace et généralement bien acceptée. Ces trois faisceaux consistent en : 1. la démonstration d’une perte de masse de contaminant ou un panache en ré-

gression 2. la démonstration de l’existence de conditions favorables à l’AN 3. des preuves complémentaires d’une activité microbiologique (présence de bac-

térie spécifiques, test en colonne, microcosmes) L’approche par les faisceaux de preuve apparaît généralement bien acceptée parmi les autorités et les scientifiques, elle est suffisamment claire pour des personnes dont le contexte national diffère, elle permet des comparaisons entre cas. L’influence du contexte législatif des relecteurs, gestion du risque ou pas, est nota-ble dans leurs réponses. Remarque particulière concernant l’information et la présentation Les relecteurs jugent avoir besoin de données plus détaillées pour pouvoir se pro-noncer sur l’applicabilité de l’AN. Ceci concerne les données spatiales, les profils de sols, la distribution verticale des contaminants, la géochimie de la nappe souter-raine ainsi qu’un modèle conceptuel des processus de base et du comportement du panache. Parfois, les besoins exprimés sont ceux qui le seraient pour un projet de recherche plutôt que pour un projet de remédiation pour lequel une preuve à 100% et la certitude n’est pas une exigence réaliste. L’évaluation a mis en évidence qu’une information résumée permet déjà d’estimer la faisabilité de l’AN contrôlée. Sur un site, des données nécessaires à l’acceptation de l’AN devrait dépendre de l’évidence des phénomènes d’atténuation et des risques impliqués pour les cibles. Une argumentation structu-rée basée sur un modèle conceptuel et un ensemble de données complet sont tou-jours nécessaires pour obtenir l’acceptation de l’AN et pour effacer les doutes l’esprit des autorités et des gestionnaires de sites. Recommandation L’expérience en remédiation de sols de par le monde a clairement démontré que notre société ne peut pas arriver à éliminer les contaminants du sol dans des délais de quelques décades. Techniquement, l’élimination de 100% des contaminants est dans la plupart des cas impossible. Financièrement les coûts de l’élimination com-plète ne sont pas en rapport avec les bénéfices et les risques impliqués. Ce projet de démonstration de NICOLE a souligné dans cette perspective que l’AN est une op-tion économiquement viable qui peut être utilisée pour obtenir des résultats satis-faisant. Souvent, l’AN n’est pas une option suffisante à elle seule, mais est un volet d’un plan de gestion des risques d’un site contaminé L’AN controlée devrait être considérée dans les diverses politiques relatives aux sols et eaux souterraines des pays membres de l’UE, de la DG environnement de la Commission européenne comme un des outils pour gérer les pollutions historiques de sols et de nappes souterraines, ceci en accord avec la directive cadre sur l’eau, la directive eau souterraine et la stratégie thématique pour la protection des sols.


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A partire dall’anno 2000, NICOLE ho predisposto un programma di valutazione dei dati relativi ai siti industriali per riscontrare quali fossero le possibilità di applicare la MNA (monitored natural attenuation). Durante gli ultimi 3-4 anni, 8 realtà industriali con le società di ingegneria ambientale loro consulenti, hanno portato avanti indagini scientifiche nei siti di loro pertinenza facendo parte del progetto dimostrativo di Nicole sulla MNA. I report relativi agli 8 siti sono stati redatti in una forma riassuntiva e revisionati da 12 reviewers indipendenti appartenenti ad Università e ad Enti Pubblici. Nonostante i reviewers abbiano presentato molti commenti, la maggior parte dei quali relativi ad una più vasta richiesta di informazioni, la risposta generalizzata indica come la MNA sia applicabile ed effettivamente efficace in molti siti. La metodologia proposta con una base condivisa delle 3 Linee di Evidenza è stata giudicata molto funzionale. Va sottolineata una differenza del retroterra culturale specifico dei reviewers. Questo report fornisce la dimostrazione dell’applicabilità della MNA nei siti industriali. Applicazione e accettabilità della NA e della MNA Gli 8 casi che sono stati studiati sono considerati rappresentativi della realtà industriale europea. La maggioranza dei casi si presenta complessa, con caratteristiche idrologiche con stratificazioni, contaminanti multipli o con presenza di fasi separate. I risultati dell’applicazione della MNA nei siti non erano disponibili in anticipo, ma per tutti questi siti le Società proprietarie (in accordo con le prescrizioni delle Autorità) hanno proposto la MNA all’interno di un approccio basato sulla valutazione del rischio, dove la MNA poteva essere sia una scelta singola oppure una opzione di casto sostenibile accoppiata a misure attive di intervento. Durante l’esame dei revisori, la più parte ha riscontrato l’effettiva presenza di Attenuazione Naturale (processi di degradazione), ma taluni hanno ritenuto esserci una incertezza (25%) e ravvisano la necessità di maggiori informazioni di quanto venga esposto nei report riassuntivi. I report originali dei siti erano stati ridotti e posti in un formato agevole per permetterne una revisione più efficiente. L’applicazione della MNA come opzione di gestione della bonifica non è sempre ben accetta da parte dei revisori. In alcuni casi la plume si sta espandendo (nei siti A, C, E) nonostante la degradazione. Spesso il flusso di ricarica dalla zona sorgente verso la plume è superiore alla quantità di degradazione e agli altri processi di NA, e la MNA può essere unicamente applicata in combinazione con altre misure attive di intervento. In altri casi l’impostazione sulla propria legislazione nazionale da parte dei revisori non è (per ora) adattata ad un approccio di valutazione basato sull’analisi di rischio, e l’accettabilità della MNA risulta difficoltosa (sito G).


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Metodologia e procedure di revisione applicate Per il presente progetto, NICOLE ha scelto dei revisori indipendenti, che provenissero dal corpo docente universitario o da Enti pubblici. Ciò ha portato a revisioni critiche, visto che ciascun gruppo desiderava verificare/controllare le conclusioni di particolari livelli di dettaglio che non erano completamente esplicitati nei report sintetici. I revisori hanno espresso giudizi positivi sui report sintetici e, nonostante desiderassero conoscere più dati, per il 75% erano molto soddisfatti dell’approccio sistematico che utilizza le Linee di Evidenza. Le seguenti prove vengono considerate molto efficienti e comunemente accettate. Le 3 linee sono così definite: 1. una documentata diminuzione dei contaminanti o una plume in contrazione; 2. condizioni favorevoli per l’esistenza della NA; 3. una evidenza microbiologica di supporto (presenza di batteri significativi, test

in colonna, microcosmi). L’approccio che prevede le Linee di Evidenza sembra essere comunemente accettato dalle autorità e dai tecnici, inoltre è chiaro anche per persone con impostazioni diverse per provenienza e nazionalità e favorisce così la comprensione dei dati. Molto marcata si è presentata l’impostazione relativa alle varie legislazioni, che fosse basata o meno sull’analisi di rischio. Punti salienti riguardanti le informazioni e la presentazione Per poter meglio valutare l’applicabilità della MNA, i revisori richiedevano informazioni più estese concernenti i dati spaziali, i profili del suolo, la distribuzione verticale dei contaminanti ed il comportamento chimico dell’acquifero, oltre ad un modello concettuale solido sui processi di base e sul comportamento della plume. Talvolta il bisogno di una esplicitazione di dati era applicabile ad una condizione di un progetto di ricerca scientifico, piuttosto che ad un normale progetto di bonifica, per il quale non è una domanda realistica ottenere il 100% di certezza e di conferme del funzionamento. Le valutazioni hanno chiarito che l’applicabilità della MNA può essere valutata anche con le informazioni dei report sintetici. La richiesta di specifiche informazioni riguardo ad un sito dovrebbe dunque dipendere da quanto sia persuasiva la sua valutazione e quale sia il rischio definito per i recettori. Una argomentazione strutturata con un modello concettuale ed un volume di dati appropriato, è sempre necessaria perchè la MNA sia approvata, oltre a rimuovere ogni dubbio residuo nelle Autorità e nelle Proprietà del sito. Raccomandazioni Ovunque nel mondo le esperienze di bonifica hanno chiarito bene che la nostra società non è in grado di raggiungere la rimozione completa dei contaminanti nel sottosuolo in archi temporali di poche decenni.


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Una rimozione del 100% nella maggior parte dei casi è tecnicamente impossibile. Dal punto di vista economico, i costi di una bonifica completa non sono accettabili considerando i relativi benefici e rischi. Il progetto dimostrativo di NICOLE sottolinea questo fatto ed in questa prospettiva, la MNA è una opzione con un buon rapporto costo-efficacia che può essere applicata per ottenere risultati soddisfacenti. La MNA spesso non è un’opzione singola di bonifica, ma è parte di un piano di gestione ambientale basato sull’analisi di rischio. La MNA dovrebbe essere considerata nelle politiche di gestione ambientale per suoli ed acque sotterranee definite dagli stati della Comunità Europea e dal DG ambientale della Commissione Europea, come uno degli strumenti per gestire casi storici di contaminazioni di suoli e falde, rimanendo coerente con il Piano coordinato UE per le acque, con le direttive sulle acque sotterranee e con le strategie in tema di suoli.


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In 2000 is door NICOLE een gemeenschappelijk data programma opgezet om de mogelijkheden te onderzoeken van de toepassing van Monitored Natural Attenu-ation (MNA; gemonitoorde natuurlijke afname) op industriële locaties. Als on-derdeel van het NICOLE demonstratie project hebben 8 industriële partners en hun adviseurs in de afgelopen 3-4 jaar onderzoeken uitgevoerd op hun locaties. De 8 locatierapporten zijn afzonderlijk samengevat volgens een uniform format en vervolgens beoordeeld door 12 onafhankelijke reviewers van bevoegde geza-gen of van universiteiten. Hoewel de reviewers veel opmerkingen hebben ge-plaatst, vooral met betrekking tot de sterke behoefte aan meer informatie, is de algemene conclusie gerechtvaardigd dat MNA toepasbaar en effectief is op veel locaties. De voorgestelde methodologie met 3 veel gebruikte bewijslijnen (lines of evidence) werd door de reviewers erg nuttig gevonden. Het verschil in mening, als gevolg van de nationale beleidskaders van de reviewers was wel opvallend. Dit rapport demonstreert de toepasbaarheid van MNA op industriële locaties. Toepasbaarheid en acceptatie van NA en MNA De 8 onderzochte locaties worden representatief geacht voor de Europese industrie. De meerderheid van de locaties was complex, met een heterogene, gelaagde bo-demopbouw, of met meerdere verontreinigingen en de aanwezigheid van puur pro-duct. De resultaten van de MNA site assessments op de locaties waren vooraf niet bekend, maar op alle locaties wordt MNA ingepast in de saneringsaanpak. Daarbij kan MNA de enige techniek zijn, of een kosteneffectieve combinatie met actieve saneringsmaatregelen. De reviewers hebben elk 2 locaties beoordeeld, zodat elke locatie 3x is gereviewd. In vrijwel alle gevallen is het optreden van NA onderkend. In sommige gevallen was er echter een behoefte naar meer informatie om de resterende onzekerheid (25%) weg te nemen. De oorspronkelijke locatierapporten waren om redenen van efficiency en tijdsbeslag samengevat in een uniform, vlot lezend format. Niet in alle gevallen wordt MNA door de reviewers geaccepteerd als management optie. In sommige gevallen is ondanks NA sprake van een groeiende omvang van de verontreiniging (site A, C, E). Vaak is de nalevering uit de bronzone groter dan de afbraak en andere NA processen, zodat MNA alleen kan worden toegepast in combinatie met actieve saneringsmaatregelen. In andere gevallen heeft het nationa-le of regionale beleidskader van de reviewers (nog) geen omslag gemaakt naar een risico gebaseerde of functiegerichte aanpak, en is de acceptatie van MNA moeilijk (site G). Methodologie en review proces Voor dit demonstratie project zijn door NICOLE onafhankelijke reviewers geko-zen, met een achtergrond in de Universitaire wereld of het bevoegde gezag. Deze keuze heeft geleid tot kritische reviews, want beide groepen hebben behoefte om


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de conclusies tot op een gedetailleerd niveau te controleren, een detail niveau dat doorgaans niet aanwezig was in de samengevatte rapporten. Alhoewel de reviewers een data behoefte hebben geuit, waren ze wel positief over de beknopte rapporta-ges. 75% was erg gelukkig met de gestructureerde aanpak met de “lines of eviden-ce”. Dit bleek een efficiënte en algemeen geaccepteerde aanpak. De 3 bewijslijnen: 4. Gedocumenteerde afname van verontreinigingen of vervagende pluim; 5. Gunstige omstandigheden voor NA 6. Ondersteunend microbiologisch bewijs (aanwezigheid bacteriën, kolomtesten,

batch experimenten)

De aanpak volgens de “lines of evidence” wordt algemeen geaccepteerd door re-viewers van overheid en wetenschap, bleek inzichtelijk ongeacht het land van her-komst, en was een prima gemeenschappelijk beoordelingskader. De invloed van nationaal beleid was wel merkbaar; functiegericht (risico) of niet. Aandachtspunten betreffende informatie en presentatie De reviewers gaven aan de uitgebreidere informatie nodig te hebben voor een in-schatting van de toepasbaarheid van MNA. Dit betrof: ruimtelijke data, bodempro-fiel, verticale verdeling van de verontreiniging en grondwaterchemie, en een solide conceptueel model van basale processen en pluimgedrag. In sommige gevallen kwam de geuite informatie behoefte bijna op het niveau van een wetenschappelijk onderzoeksproject, en niet op het niveau van een alledaags saneringsproject, waar-bij 100% zekerheid geen realistische eis is. Het review proces heeft duidelijk gemaakt, dat ook met samengevatte informatie de toepasbaarheid van MNA kan worden ingeschat. Een (verdergaande) locatiespeci-fieke informatie behoefte kan vervolgens worden afgestemd op de overtuigings-kracht van deze schatting, en het risico voor receptoren. Om toestemming te krij-gen voor de toepassing van MNA, en om de laatste twijfel weg te nemen bij over-heden en probleembezitters, is altijd een gestructureerde argumentatie met concep-tueel model en samenhangende data set nodig. Aanbevelingen Ervaringen met bodemsanering wereldwijd hebben overduidelijk aangetoond dat een volledige verwijdering van verontreinigingen uit de ondergrond niet haalbaar is in een tijdspanne van enkele tientallen jaren. In de meeste gevallen is 100% ver-wijdering technisch niet mogelijk. Financieel zijn de kosten van volledig verwijde-ring niet in verhouding met de baten en de betreffende risico’s. Dit NICOLE de-monstratie project onderstreept dat MNA in dergelijke gevallen een kosteneffectie-ve optie is, die kan worden gebruikt om bevredigende resultaten te bereiken. MNA is vaak geen zelfstandig gebruikte techniek, maar onderdeel van een combinatie van maatregelen. Het verdient aanbeveling om MNA op te nemen in de diverse richtlijnen voor grond en grondwater in de Europese deelstaten en DG-Milieu van de Europese commisie als een van de instrumenten voor de aanpak van de historische belasting


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van de ondergrond, en in lijn met de EU Kader en dochter Richtlijn Water en Grondwater, and de EU Bodemstrategie.


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Im Jahr 2000 wurde durch NICOLE ein gemeinsames Datenaustausch-Programm ins Leben gerufen um die Möglichkeit der Anwendung von Monitored Natural Attenuation (MNA; beobachteter bzw. überwachter natürlicher Abbau) zu untersuchen. Als Teil des NICOLE Demonstrations-Projekts haben 8 (industrielle) Partner und deren Berater in den vergangenen 3-4 Jahren auf ihrem Gelände Untersuchungen durchgeführt. Die 8 Berichte wurden in ein einheitliches Format gebracht und danach durch 12 unabhängige Gutachter (von Universitäten bzw. Behörden) beurteilt. Obwohl die Gutachter zahlreiche Rückfragen hatten – insbesondere bestand der Bedarf nach mehr Informationen – ist die allgemeine Schlussfolgerung, dass MNA bei vielen Standorten anwendbar und effektiv ist. Die vorgelegte Methode mit den üblichen 3 Beweislinien (lines of evidence) wurde durch die Gutachter sehr positiv beurteilt. Der gesetzliche Hintergrund des Herkunftlandes der Gutachter beeinflusste auffallend die Meinung der Gutachter. Dieser Bericht behandelt die Anwendbarkeit von MNA auf Industriegeländen. Anwendbarkeit und Akzeptanz von NA und MNA Die 8 untersuchten Standorte wurden für die europäische Industrie als repräsentativ gewertet. Die Mehrheit der Standorte war komplex, mit heterogenem, geschichtetem Bodenaufbau, oder mit mehrfachen Verunreinigungen bzw. der Anwesenheit von freier Phase. Die Ergebnisse der MNA site assessments auf den Standorten waren vorher nicht bekannt, aber auf allen Standorten wird MNA im Sanierungsverfahren mit verwendet. Dabei ist es möglich, dass MNA die einzige zur Anwendung kommende Technik ist, oder dass eine kosteneffektive Kombination mit aktiven Sanierungsmethoden verwendet wird. Die Gutachter haben jeweils 2 Standorte beurteilt. Somit wurde jeder Standort dreimal beurteilt. Die Mehrheit der Gutachter erkannte in nahezu allen Fällen ist das Auftreten von NA (natürlicher Abbau) an. In einigen Fällen bestand jedoch Bedarf nach mehr Informationen, um eventuelle Unwägbarkeiten entkräften zu können (25%). Die ursprünglichen Berichte per Standort wurden aus Gründen der Effizienz in ein standardisiertes Format zusammengefasst, um eine effiziente Beurteilung zu gewährleisten. Es war unrealistisch zu erwarten, dass die Gutachter alle zugrundeliegende Berichte und Daten in ihre Beurteilung einbeziehen würden. MNA wird durch die Gutachter nicht in allen Fällen als reine Sanierungsmöglichkeit akzeptiert. In einigen Fällen ist trotz NA eine Ausbreitung der Verunreinigung zu bemerken (Standorte A, C, E). Dann ist meist ist der Strom aus der Kontaminationsquelle grösser als der Abbau und andere NA-Prozesse, somit kann MNA nur Anwendung finden in Kombination mit aktiven Sanierungstechniken. In anderen Fällen ist der nationale bzw. regionale rechtliche Hintergrund der Herkunftsländern der Gutachter (noch) nicht ausgerichtet auf


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risikobasierte Vorgehensweisen und somit die Akzeptanz von MNA schwierig (Standort G). Methode und Beurteilungsprozess Für dieses Demonstrationsprojekt sind durch NICOLE unabhängige Gutachter von Universitäten bzw. Behörden ausgewählt worden. Diese Wahl führte zu kritischen Beurteilungen, da jede der beiden Gruppen die Schlussfolgerungen auf Detail-Niveau kontrollieren wollte. Die gewünschte Datendichte war nicht vollständig in den zusammengefassten Berichten verfügbar. Die Gutachter beurteilten die standardisierten Berichte durchweg positiv. Trotz dem Wunsch nach höherer Informationsdichte äusserten sich 75% der Gutachter sehr zufrieden mit der systematischen Vorgehensweise der „lines of evidence“. Diese zeigte die Effizienz und allgemeine Akteptanz der Methode. Die drei Beweislinien beinhalten: 1. nachgewiesene Abnahme von Kontaminationen oder Konzentrationsabnahme

in der (Kontaminations-)Fahne; 2. nachgewiesene günstige Bedingungen für NA; 3. unterstützender mikrobiologischer Beweis (Anwesenheid von Bakterien,

Säulentests, Batch-Experimente). Die Vorgehensweise mittels der „lines of evidence“ wird allgemein anerkannt durch die Gutachter aus Behörden und Wissenschaft. Dies zeigte sich ungeachtet des Herkunftlandes und lieferte eine gut funktionierende Beurteilungsbasis. Der Einfluss nationaler Politik und Rechtssprechung war allerdings signifikant, ob der gesetzliche Hintergrund einen mehr oder weniger risiko-basierten Ansatz verfolgt. Anmerkungen bezüglich Information und Präsentation Die Gutachter gaben an, umfassendere Informationen für eine Einschätzung der Anwendung von MNA zu benötigen bezüglich: räumliche Daten, Bodenprofile, vertikale Ausbreitung von Kontaminationen und Grundwasser-Chemie, und ein solides konzeptionelles Modell der Basis-Prozesse und dem Verhalten der (Kontaminations-)Fahne. In einigen Fällen ähnelten die Informationswünsche eher einem wissenschaftlichen Forschungsprojekt und nicht einem anwendungsbezogenem Sanierungsprojekt, bei dem 100%ige Sicherheit keine realistische Forderung darstellt. Der Bewertungsprozess hat deutlich gezeigt, dass auch mit zusammengefassten Informationen die Anwendbarkeit von MNA eingeschätzt werden kann. Ein (weitergehender) standortspezifischer Informationsbedarf kann danach abgestimmt werden in Abhängigkeit der bisherigen Einschätzung und dem Risiko für Rezeptoren. Eine strukturierte Argumentation mit konzeptionellem Modell und umfangreichen Datenerhebungen wird immer nötig sein um Zweifel bei Behörden und Standortverantwortlichen auszuräumen und MNA zu ermöglichen. Empfehlungen Erfahrungen mit Bodensanierungen haben weltweit gezeigt, dass die vollständige Beseitigung von Verunreinigungen aus Boden und Grundwasser in einer


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Zeitspanne von einigen Jahrzehnten nicht realistisch ist. In den meisten Fällen ist eine 100%ige Entfernung technisch nicht möglich. Die Kosten für eine vollständige Beseitigung sind unverhältnismässig gegenüber dem Nutzen und den potentiellen Restrisiken. Dieses NICOLE Demonstrations-Projekt macht deutlich, dass MNA in solchen Fällen eine kosteneffektive Möglichkeit ist, die befriedigende Ergebnisse erzielt. MNA ist häufig keine eigenständig angewandte Technik, sondern ein Teil aus einer Kombination mit anderen Massnahmen. Es wird vorgeschlagen, um MNA in diverse Richtlinien für Boden und Grundwasser in den europäischen Teilstaaten und in der DG-Umwelt der Europäischen Kommission aufzunehmen als ein Instrument für die Sanierung bei historisch belastetem Boden und Grundwasser, um die EU Tochterrichtlinie Wasser und Grundwasser und der EU Bodenstrategie mit der Sanierungsvariante MNA zu unterstützen.


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Table of Contents

Part 1: Main report Preface Executive Summary (English, French, Italian, Dutch and German)

1. Introduction..............................................................................................23 1.1 Why a NICOLE project on MNA?...........................................23 1.2 Background MNA ....................................................................23 1.3 NICOLE Field demonstration of MNA....................................24

1.3.1 Demonstration sites...................................................24 1.3.2 A common procedure for the evaluation of

MNA .........................................................................25 1.3.3 Review process..........................................................25

1.4 Lay-out of the report.................................................................25

2. Technical procedures for assessment of NA............................................27 2.1 Technical protocols...................................................................27 2.2 Lines of evidence......................................................................28 2.3 First line of evidence and plume life cycle evaluation .............29

2.3.1 Direct proof for decreasing contaminants .................29 2.3.2 Concentrations in individual wells ............................29 2.3.3 Plume behaviour........................................................30

2.4 Second line of evidence............................................................31 2.4.1 Understanding plume conditions and processes........31 2.4.2 Redox conditions and availability redox species ......31 2.4.3 Metabolites................................................................32 2.4.4 Screening and fate & transport models. ....................33 2.4.5 Stable isotope fractionation.......................................33

2.5 Third line of evidence...............................................................35 2.5.1 Conventional microbial techniques...........................35 2.5.2 Molecular techniques ................................................36

3. A procedure for evaluating NA................................................................39 3.1 Nicole Evaluation of NA: Tiered approach ..............................39 3.2 Acquisition of field data (step 1) ..............................................40

3.2.1 Hydro geological data ...............................................40 3.2.2 Chemical and biological parameters .........................41

3.3 Site description, conceptual model and risk assessment...........43

4. Getting MNA approved and decided .......................................................45 4.1 Legislation and policies on MNA in Europe ............................45


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4.2 Contracts and documents..........................................................46 4.3 Choice and justification of remedial objective .........................47 4.4 Monitoring strategy ..................................................................48 4.5 Performance criteria and contingency plan ..............................49

5. Demonstration of feasibility MNA at 8 sites ...........................................51 5.1 Choice of new sites with unknown outcome ............................51 5.2 Complexity of the sites .............................................................52 5.3 Lines of evidence for MNA at the sites ....................................53

6. Independent review ..................................................................................55 6.1 Reviewers .................................................................................55 6.2 Set-up of the review and questionnaire ....................................55 6.3 Evaluation of site reviews.........................................................56

6.3.1 Method and considerations........................................56 6.3.2 Summary reviews site A ...........................................57 6.3.3 Summary reviews site B............................................58 6.3.4 Summary reviews site C............................................59 6.3.5 Summary reviews site E............................................60 6.3.6 Summary reviews site F ............................................62 6.3.7 Summary reviews site G ...........................................63 6.3.8 Summary reviews site H ...........................................64 6.3.9 Summary reviews site J.............................................65

6.4 Overall evaluation for MNA.....................................................66

7. Conclusions, discussion and recommendations .......................................69 7.1 Conclusions ..............................................................................69 7.2 Best practice for MNA investigation ........................................70 7.3 Recommendations ....................................................................70

8. Web sites, research projects and literature ...............................................71 8.1 Websites....................................................................................71 8.2 Research projects and programs ...............................................71 8.3 Literature ..................................................................................72

Part 2: Appendices (reported separately on CD and available on request at the NICOLE secretary)

A. Quick scan score systems B. Third line of evidence; Molecular techniques C. Electron acceptor – Electron donor model D. Questionnaire E. Individual site reports sites A-B-C-E-F-G-H-J


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1. Introduction

1.1 Why a NICOLE project on MNA?

Over the past several years, natural attenuation (NA) has received increasing atten-tion as it became more and more clear that naturally occurring processes are able to remediate contaminated groundwater. There are large differences between the dif-ferent European countries with respect to the acceptance and implementation of Monitored NA (MNA) as a remedial option. In the Netherlands and the UK, deci-sion tools and guidelines have been developed that help site owners and consultants to evaluate NA (NOBIS 1998, EA E&W 2000). In both these countries MNA is be-ing applied regularly and the national legislation allows the implementation of MNA, provided that NA is sufficiently protective for the environment. Sometimes additional source control measures are required. In some of the other countries the authorities are more reserved and the legislation is formulated in such a way that MNA is not easily accepted as remedial option. To overcome this hurdle, in 1997 the Nicole workgroup MNA was formed. This working group started off with the following general objectives: 1. Promote the acceptance of natural attenuation as part of a cost-effective and

environmentally sound solution for contaminated sites. 2. Provide a technical basis for risk-based application of natural attenuation.

After the publication of a review of existing guidelines and protocols (Sinke and Le Hecho, 1999) and the publication of an information booklet in comic form (Nicole 1999), it became obvious that to achieve the aforementioned objectives, the actual evaluation of real sites as demonstration was essential. It was decided to start with new sites, without knowing the outcome.

The additional objectives are: 3. Underpin the technical basis of MNA with the results of field sites, and pooling

of experiences; 4. Provide best practice procedure for the evaluation of site specific applicability

of NA.

1.2 Background MNA

The term Natural Attenuation (NA) refers to “naturally occurring processes in soil and groundwater environments that act without human intervention to reduce the mass, toxicity, mobility, volume and/or concentration of contaminants in those me-dia” (US-EPA, 1997).

MNA is the monitoring and evaluation of these processes. An often used definition of monitored natural attenuation formulated by The Environment Agency of Eng-land and Wales (EA E&W, 2000) is:


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“Monitoring of groundwater to confirm whether natural attenuation processes are acting at a sufficient rate to ensure that the wider environment is unaffected and that remedial objectives will be achieved within a reasonable timescale; this will typically be less than one generation or 30 years”

The most important destruction mechanism during natural attenuation is microbial mediated degradation. Non-destructive attenuation mechanisms include sorption, dispersion, dilution and volatilisation. During the former processes the contami-nants are broken down ultimately into harmless components such as water, chlo-ride, and carbon dioxide. In the latter processes the contaminants remain but the concentrations in the groundwater and the associated risks decrease. Inorganic con-taminants like heavy metals and arsenic can be attenuated by chemical and physical processes but cannot be degraded.

Although the occurrence of natural attenuation processes has been demonstrated at many sites, the actual use of these processes as remedial strategy at contaminated sites is a rather new phenomenon in Europe. Despite the fact that many of the proc-esses underlying NA have been investigated and described thoroughly, many coun-tries started up national research projects (NL: NOBIS-SKB, UK: NNAGS, SIREN, D: KORA) to tackle specific (scientific) problems and to get the approach accepted amongst national stakeholders such as authorities, consultants and service providers. Also the European Commission granted several research projects to in-vestigate innovative techniques to underpin NA and to disseminate knowledge and thereby enhance acceptance (EU 5th Framework project CORONA).

1.3 NICOLE Field demonstration of MNA

1.3.1 Demonstration sites

In 2000 NICOLE supported a data-sharing demonstration program that was set up for industrial sites located in different European countries to investigate the possi-bilities for MNA. The industrial partners with their local consultants carried out their part of the MNA project at their sites. In several cases the MNA work at the site was integrated in a larger R&D project and co-financed by a national or EC project. This of course enabled the local project leader to go deeper into specific scientific details. During the project the industrial members reported regularly on the results and field data measured at their sites and exchanged ideas and experiences on the most appropriate ways to determine the possibilities for MNA. Obviously, the obstacles to investigate and demonstrate MNA are not merely “scientific” but also financial, organisational, and practical. Also the fact in itself, that MNA is not yet a com-monly accepted remedial technique, posed difficulties in some countries. This re-


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port will not focus on these types of hurdles but on the systematic evaluation of MNA on the 8 demonstration sites.

1.3.2 A common procedure for the evaluation of MNA

Due to the large differences in legal framework in Europe, the Nicole members de-cided to use a standardized procedure to evaluate the possibilities and efficacy for NA at their sites: Nicole Evaluation of Natural Attenuation (Sinke, 2000). This common procedure is based on existing guidelines and protocols and tested at the demonstration sites in several European countries. Obviously this best practise has no formal status and does not pretend to prescribe a certain approach. It is merely intended to help out the Nicole members how to handle, what to measure and how to evaluate the results with respect to the possibilities of MNA at their specific sites. This procedure is described in chapter 3 and optimized further during the NICOLE demonstration project.

1.3.3 Review process

The Nicole MNA workgroup decided to actively seek the participation of other stakeholders. In the final phase of the demonstration, representatives of academia and authorities have been invited to give their opinion on the MNA evaluation that has been carried out at the demonstration sites. They have been asked to give their opinion on the quality of the work, the reliability of the results and the interpreta-tion. Also they were asked whether the provided information is sufficient to start off with the implementation of MNA at the specific site.

1.4 Lay-out of the report

This report describes in the introductory chapters the state of the art with respect to guidelines and protocols evaluating the possibilities for MNA at industrial sites (chapter 2) and the data requirements to describe the field situation adequately (chapter 3). Chapter 4 focuses on the preconditions for actual implementation of MNA such as the contingency plan, the monitoring strategy and the duration and remedial objectives. In chapter 5 an overview is given of the demonstration sites, including the general features of the sites. The reports of the demonstration sites themselves are summarised in the appendix E. In chapter 6 the set-up of the review, the background of the reviewers and the outcome of the independent review proc-ess is presented. Finally, in chapter 7 conclusions and recommendations are pre-sented.


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2. Technical procedures for assessment of NA

The implementation of MNA as a remedial measure requires that natural attenua-tion acts efficiently to achieve site-specific remedial targets within a reasonable time frame. To evaluate the potential applicability of MNA it is necessary to make use of a reliable technical protocol to confirm and quantify the efficacy. A series of protocols and guidelines have been published to evaluate MNA as a remedial ap-proach at contaminated industrial sites. There is much consensus amongst pub-lished guidelines and protocols on the general approach, the data requirements and the evaluation process for determining the feasibility of MNA (Rittmann 2000, Sinke, 2000, Small and White 1998).

2.1 Technical protocols

With the increased interest in applying MNA as part of a remedial strategy, an in-creased number of guidelines, protocols and recommendations have been devel-oped or are currently being developed. Much information can be found on the web-sites listed in the literature list. In the USA the first initiatives, listed in Table 2.1, were taken at the end of the nineties to develop technical protocols to judge sites with petroleum hydrocarbons, chlorinated hydrocarbons or recalcitrant compounds (ASTM 1997, RTDF 1997, US-EPA 1997, Wiedemeier et al. 1994). These protocols are updated and adapted regularly to keep up with scientific innovations and extending expertise in the field of MNA. An overview of protocols is given by Rittmann et al. 2000. Also in Europe (NOBIS 1998, EA E&W 2000) guidelines have been formulated.

The protocols and guidelines all have as the most important criteria to allow moni-tored natural attenuation as a remedial strategy: MNA has to be protective to human and environment, MNA has to be effective within a reasonable time frame.

Table 2.1 Overview of initial protocols and guidelines.

Protocol/guideline Contaminant type

AFCEE 1995 Petroleum hydrocarbons *Wiedemeier et al. 1996 (1998) Chlorinated solvents ASTM 1997 Petroleum hydrocarbons RTDF 1997 Chlorinated solvents *OSWER 1997 (1999) General *NOBIS 1998 (2000) Chlorinated solvents and BTEX EA E&W 2000 General

* From these protocols new, adapted, editions are available.


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The technical protocols give recommendations for site-specific data collection and analysis to support remediation (see also chapter 3). The protocols generally de-scribe a stepwise framework to demonstrate the feasibility of MNA at a given site. The protocols include tables with parameters that may be used and methods to de-termine them. The parameters to be collected include field data on (geo)hydrology, spatial distribution of contaminants and presence of receptors. Naturally, there are some differences between the different protocols and guidelines, but these are pri-marily in tone and emphasis, reflecting the different perspectives and responsibili-ties of the entities that developed them.

2.2 Lines of evidence

All protocols state that to support remediation by MNA, the site owner must con-vincingly demonstrate that attenuation, or degradation1 of site contaminants is oc-curring at rates sufficient to be protective of human health and the environment. So the sole occurrence of NA is not satisfactory, the rates of attenuation have to be high enough to be protective. The data and type of information that can be used to underpin the demonstration of NA can be classified in three “lines of evidence”. Where in most protocols and guidelines the first two lines of evidence are indis-pensable, the third line is usually considered as additional. This third line is brought up as additional evidence, for instance in case of complex situations and compounds, or to narrow the bandwidth of predictions, or to reduce uncertainty. Sometimes it may even provide conclusive evidence for the occurrence of NA (e.g. presence of specific bacterial enzymes for degradation of contaminants). 1. First line: documented loss of contaminants or direct proof of decreasing con-

taminant concentrations Means: historical contaminant data for the plume or single wells, application of screening models, Historical groundwater data and/or soil chemistry data that demonstrate a clear and meaningful trend of decreasing contaminant mass and/or concentration over time at appropriate monitoring or sampling points. In case of a groundwater plume the decreasing concentrations should not be solely the result of plume migration. In that case simple transport or screening models can be useful. In case of inorganic chemicals the primary attenuation mechanism should be iden-tified.

2. Second line: documented NA conditions means: geochemical indicators (e.g. redox), biochemical indicators (electron balance) presence of metabolites and stable isotope shift. (Bio-)geochemical data can be used to demonstrate that favourable conditions for NA processes are present. In addition, the presence of metabolites directly proves the degradation of the primary contaminant. This does indicate that natu-

1 In the Netherlands and Germany degradation must be the key mechanism responsible for

NA.


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ral attenuation processes are active at the site and the rates at which such proc-esses can reduce contaminant concentrations to required levels can be esti-mated. Stable isotope data are not obligatory, but can come in very useful, be-cause in certain cases they can prove that the occurrence of metabolites is caused by degradation and not transport from upstream.

3. Third line: documented microbial activity means: additional laboratory or field data such as RNA-DNA analysis, enzyme data, bacterial counts, microcosm studies Data from field or microcosm studies which directly demonstrate the occurrence of particular natural attenuation processes at the site and its ability to degrade the contaminants of concern.

2.3 First line of evidence and plume life cycle evaluation

2.3.1 Direct proof for decreasing contaminants

The first line of evidence can be demonstrated at different levels of detail and per-suasion. One can use data from individual wells that have been followed for a pro-longed period of time or, more convincing, demonstrate the historical behaviour and development of the source and plume zone. The first line of evidence can al-ready prove that NA is sufficiently protective. But proving that actual degradation of the contaminants takes place may be necessary, because the mere dilution or volatilisation of the compounds is, at least in some countries in Europe, not consid-ered as an acceptable sole basis for MNA.

Obviously, the impact of the first line of evidence increases significantly when data trends are consistent with time and have been followed for a series of years. A good impression of the overall first order natural attenuation rate constants can be obtained by plots of concentration versus time or by plots of concentration versus distance along the plume axis (Newell et al 2002). The first can be used as indica-tive for the remediation timeframe and the latter for plume length.

2.3.2 Concentrations in individual wells

The concentration data of individual wells can be used to underpin the occurrence of natural attenuation. Without pretending to have characterised the full dimensions of the plume in all directions, this type of evidence gives information on several single locations and/or depths. The crucial factor in this is the positioning of these individual wells with respect to the source zone and the groundwater direction and velocity. A test that can be used to identify concentration trends is the Mann-Kendall statistical test (Gilbert 1987).


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2.3.3 Plume behaviour

In a series of well-described and investigated cases, trends can be observed in the development of a plume and plumes can be grouped in different categories (figure 2.1): – expanding: residual source present. Mass flux of contaminants exceeds the as-

similative capacity of the aquifer, – stable: insignificant changes in both plume dimensions and mass, the level of

attenuation processes equals the amount of leaching, – fading: residual source (nearly) exhausted. Significantly reduction in plume

mass as inferred from either plume dimensions or concentrations, – fading and-or detaching: average plume concentration very low and changing

slowly over time. Plume may detach from the original source zone.

A stabilised or receding plume is direct and irrefutable evidence of natural attenua-tion. Several studies addressed the changes in plume length and concentration for dissolved petroleum hydrocarbon plumes and plumes of chlorinated solvents (Mace et al. 1997, Newell et al.1990, Rice et al. 1995). As a general trend it can be seen that BTEX plumes are shorter than plumes of chlorinated ethenes or other chlorinated solvents (Newell and Connor 1998). Most of the studies on plume be-haviour were carried out at US sites where aerobic background conditions prevail. Generally aerobic conditions are favourable for the degradation of petroleum hy-drocarbons including BTEX. In the US the majority of the petroleum hydrocarbon plumes appear to be either stable or fading and only as little as 5-10 % is still ex-panding (Newell and Connor 1998).

detaching & fadingSource

Expanding

Source

Stable

Source declining

Fading

Former source

Fading

detaching & fadingSource

Expanding

Source

Stable

Source declining

Fading

Former source

Fading

Source

Expanding

Source

Stable

Source declining

Fading

Former source

Fading

Figure 2.1 The life cycle of a contaminant plume that is naturally attenuated with the four succeeding stages of expanding, stable, fading and vanishing.

Documented data on the historical plume behaviour are rare. A consistent historical picture of the plume behaviour, even with several information gaps, is important


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evidence It rarely happens that the plume dimensions have been followed with time for all relevant aspects such as depth distribution, size, volume and mass. Besides, the interpretation of the plume behaviour and the reliability of the often scattered historical data, are not in all cases straightforward as different analytical methods have been used through time and wells have not been positioned optimally. Also at sites with multiple source regions and mixed contaminants, the historical develop-ment of the plumes is seldom described with confidence. Collecting conclusive data, preferably at an early stage, is therefore important.

2.4 Second line of evidence

2.4.1 Understanding plume conditions and processes

The second line of evidence involves the determination of redox- or biochemical and geochemical conditions. This also involves the occurrence of metabolites, sup-ported by modelling and perhaps stable isotope data. The main questions for this line of evidence are: 1. Are the prevailing redox-conditions optimal for the microbial degradation of

the contaminants present? 2. Are the available amounts of electron donor or acceptor sufficient for the com-

plete transformation of the amount of contaminant? 3. Can the presence of metabolites be explained by degradation or transport (from

the source zone)?

2.4.2 Redox conditions and availability redox species

The different redox-conditions are often referred to as to which Terminal Electron Acceptor Process is dominant (oxygen, nitrate-reducing, iron-reducing, sulphate-reducing or methanogenic).The change in these conditions can already demonstrate that the observed decrease in concentrations (first line) can be attributed to intrinsic processes such as degradation or chemical destruction.

Necessary redox conditions Depending on the type of contaminant the type of dominating redox condition is crucial: − Chlorinated solvents

Dechlorination of the higher chlorinated solvents such as PCE and TCE can only be achieved under strongly reducing (anaerobic) conditions. In a homog-onous situation complete dechlorination will only occur if sulphate is com-pletely reduced. The chlorinated solvents can be used directly as primary sub-strate (electron acceptor). Prerequisite for this process is the supply and the quality of dissolved organic carbon (electron donor). If there is an imbalance


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between the amount of chlorinated compounds and the supply of organic car-bon, the natural attenuation process may come to an end before all chlorinated solvents have been degraded.

− (petroleum) hydrocarbons In general, the microbial degradation of these compounds can take place under oxidative, nitrate reducing, and sometimes iron- or sulphate reducing condi-tions. The oxidative capacity determines the extent and rate of the degradation of these compounds. The supply of electron acceptors must be sufficient to de-grade the entire load of contaminants. In cases where the oxidative capacity is sufficient to balance the amounts of hydrocarbon present, natural attenuation is likely to take place and the plume will be stable or fade. In cases where the oxidative capacity is insufficient and the supply of electron acceptor is limit-ing, the plume will certainly expand further.

Electron acceptor – electron donor balance The balance between the amount of electron donors (ED) and the available electron acceptors (EA) determines continuation of the degradation processes. If e.g. the contaminant is PCE (normally an electron acceptor) than the amount of electron donor (hydrogen, organic matter etc.) available or becoming available, must theo-retically be sufficient to lead to complete transformation. This calculation can be done in simple tables with total amounts of EA-ED equivalents or with the use of a model. However other species compete in the consumption of electron donors, and the solid state species (e.g. Fe/Mn) are often overlooked. In active bioremediations, often a surplus of 20-times the calculated amount of electron donor or acceptor is needed to accomplish degradation of the contaminant. In appendix C a box model is presented that can be used to calculated steady state plume lengths (CORONA, 2005).

2.4.3 Metabolites

Depending on the type of contaminant and the situation, the presence of metabo-lites may be used as indication for the occurrence of NA. For instance chlorinated solvents such as trichloroethene (TCE) can be dechlorinated stepwise under an-aerobic conditions and the presence of daughter-products such as cis-1,2-dichloroethene (cis-1,2-DCE), vinyl chloride (VC) and ethene, indicate that NA is occurring. However, cisDCE and VC are even more toxic than PCE or TCE. The final product ethene must be found as indication that NA possibly is sufficiently protective. For BTEX compounds elevated levels of carbon dioxide can be a proof of degradation. Additional modelling or stable isotope data often are needed to state that the me-tabolites are a result of degradation in the plume, and not in the source zone. Me-tabolites formed in the source zone, can be transported to the plume zone, without degradation taking place in the plume.


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2.4.4 Screening and fate & transport models.

By the use of solute transport models the groundwater transport and, after correc-tion for retardation, the contaminant transport can be calculated (e.g. Bioscreen, Biochlor, Aziz et al.). These models are typically used to quantify groundwater flow, sorption, dilution and biodegradation. The comparison of for instance the ex-pected plume length with the actual plume length can be used as evidence that other processes, besides transport and dilution, have influenced the plume behav-iour. The model is used to predict the plume behaviour, to identify mass loss of contaminants in relation to groundwater flow, and additional data to be collected at specific locations. The evidence grows stronger if a comparison can be made with a conservative tracer (e.g. chlorine) or with a relatively recalcitrant compound (e.g. MTBE to total petroleum hydrocarbons). For more complex situations a full analytical or numerical model can be used such as (combinations of) MODFLOW, RT3D.

Often fate and transport models are used with either analytical or numerical solu-tions to predict the effectiveness of MNA into the future. Preferably site-specific data and time series are used in the model. The model can be used to identify the key-processes by a sensitivity analysis of input parameters.

2.4.5 Stable isotope fractiona-tion

Recent research has shown that com-pound specific stable isotope analysis can be usefull to demonstrate degrada-tion at contaminated sites. Carbon, hydrogen, and chlorine isotope ratios are used to assess degradation. The theory behind this is that organic con-taminants have a characteristic isotope ratio (e.g. 13C/12C-ratio or δ13C) that changes during biodegradation. In mi-crobial degradation processes, light isotopes are preferentially used and the heavier isotopes are enriched in the residual substrate fractionation (figure 2.2). This process is called fractionation. Non-biological

Figure 2.2 Changes in isotope ratio

during biodegradation.


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processes like adsorption, dilution and volatilisation have no significant influence on the fractionation. However not all compounds show fractionation under differ-ent conditions. This means that an absence of fractionation does not mean that there is no degradation. A stable isotope shift however does prove that degradation took place.

The isotope ratio of a compound is given with the δ-value (in ‰). The δ-value is calculated from the isotope ratio from a sample relative to the isotope composition of an internationally recognized standard. A typical δ-value of petroleum related compounds is around -28 ‰.

An increase in isotope ratio along the central flow path of a groundwater plume can be evidence for biological degradation. The interaction of transport differences of different compounds along the flow field and the stable isotope shift however can complex to interpret. In figure 2.3 an example is given of a cisDCE/VC plume. It shows a very significant shift of cisDCE from -21‰ to +3 ‰. VC is first being produced out of cisDCE (so first a decrease from -28‰ to -34‰) and is then being degraded and shows a shift to -20‰. In this case the flow field and transport mechanisms were well known, and the stable isotope numbers could be interpreted.

Fractionation along flowpath

-40,00

-30,00

-20,00

-10,00

0,00

10,00

0 200 400 600 800

distance from source (m)

d13C

[‰]

delta 13C VCdelta 13C cisDCE

Figure 2.3 Fractioning along flow path (source Slenders et al. CORONA).

An increase in ratio in time in a specific monitoring well can be used similarly. For a compound like benzene, the carbon ratio and/or hydrogen ratio can be used (figure 2.4). The carbon fractionation of benzene under anaerobic conditions is known to be maximum 2 ‰, and only significant after more than 60% degradation. The maximum hydrogen fractionation is much higher, and occurs already at less than 30% degradation. The disadvantage of hydrogen isotope analyses is however a higher detection limit.


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-45

-43

-41

-39

-37

-35

0 10 20 30 40 50time (d)

delta

H (p

rom

ille

Figure 2.4 Fractionation of benzene in a monitoring well in time (source Langenhoff).

At this moment knowledge and experience is obtained with compound specific sta-ble isotope analyses of mono-aromatic compounds (e.g. BTEX), chlorinated ethe-nes (PCE, TCE DCE, and VC) and MtBE.

2.5 Third line of evidence

This line includes additional evidence from field, laboratory or mesocosm systems, directly aimed at microbial activity. Usually when the first two lines of evidence are strong enough, the third line is not required. However, additional evidence might help to strengthen the other observations, and to prove that decreasing con-centrations are a result of microbial degradation. Several techniques are currently being developed that might be used to demonstrate that degradation occurs or to prove the presence of the “right” bacteria in the aquifer capable of degrading the site contaminants. In general there are two ways to verify biotic degradation: con-ventional microbial techniques and molecular techniques.

2.5.1 Conventional microbial techniques.

These are the commonly used standard techniques such as bacterial plate counts, most probable number counts, batch experiments and column experiments. The ad-vantage is that only the active bacteria are counted, whilst the disadvantage is that only a small fraction of the soil bacteria can be detected. The obtained data give a rough indication for the third line of evidence. Counts: An indication of the presence of culturable (pollutant-)metabolising micro-organisms can be obtained by plate counts (colony forming units, cfu’s). For these plate counts either selective media can be used on which only specific bacteria can grow with for instance phenols as carbon source, or non-selective media giving a rough indication of total culturable bacteria. Plate counts can be carried out under aerobic or anaerobic conditions. For most probable number counts (MPN), multiple samples and dilutions in fluid media are used which are after incubation either scored positive or negative for


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growth. Also this type of measurement can be selective or non-selective and redox specific. A typical example is the MPN counting of sulphate reducing bacteria. Batch and column experiments: classical laboratory assays showing that micro-organisms in field samples are able to metabolise the contaminants under site con-ditions (temperature; geochemistry; substrates). Laboratory techniques used are mainly microcosm studies (batches) and soil / groundwater column systems, partly combined with in situ mesocosm systems. Advantages of these classical methods are the clear distinction between biotic / abiotic effects, the analysis of complex “ecosystems” under in situ conditions, and the option of combination with other techniques. Also the possible bottle-neck situations (pH, co-contaminants) or nutri-ent limitation (N,P) can be examined. The main disadvantages are the duration of the biodegradation tests, the physiological adaptation of the micro-organisms to laboratory conditions, thus the lack of comparability with field conditions.

2.5.2 Molecular techniques

Recent progress in molecular biology has made it possible to detect microbes by the analysis of their universal intracellular components (e.g. DNA, RNA, proteins, lipids) and provides insight into the regulation and expression of degradative genes via mRNA analysis. The use of these recently developed methods (Appendix B) poses the possibility of direct analyses of biological processes in field derived sam-ples. Here are some relevant techniques: – Determination of specific pollutant transforming micro-organisms by epifluo-

rescence microscopy, flow cytometry or fluorescence activated cell sorting. – Direct extraction of nucleic acid components of the microbes from environ-

mental samples. – PCR products of rRNA approaches are used to analyse the composition of

complex bacterial communities using 16S rDNA fingerprinting techniques (e.g. DGGE, RFLP) or clone libraries for gene sequence analyses. With these tools general changes in the composition of bacterial communities directly or indirectly influenced by biodegradation activities are examined.

For the verification of biodegradation, the detection of specific bacterial species or groups and their spatial dispersion over the plume can be useful (e.g. chlorinated solvents dechlorinating Dehalococcoides ethenogenes; naphthalene degrading Pseudomonas putida G7; sulphate reducing bacteria; iron-(III)-reducing Geobacte-riaceae). Combining the power of PCR with molecular probing and fluorescence analysis in “real-time” enables direct quantification of the pollutant-degrading bac-teria.

An even more elegant tool is the organism independent detection of genes respon-sible for enzymes, specifically necessary for e.g. aromatic ring cleaving Catechol-dioxygenases in BTEX. In figure 2.5 the effects of air-sparging to enhance BTEX-degradation have been monitored by the quantitative detection of genes coding for


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catechol-2,3-dioxygenase. During the airsparging there is a significant increase in specific genes pointing at growth.

Catechol-2,3-dioxygenase Detection

100

1000

10000

100000

1000000

10000000

100000000

0 50 100 150

Time (d)

Gen

e co

pies

/ m

l gr

ound

wat

er

IWEWMW-3MW-4MW-5

Figure 2.5 Increase of gene copies Catechol-2,3-dioxygenase in groundwater

samples during air sparging of BTEX-contaminated soil. With IW influent water and EW effluent water.


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3. A procedure for evaluating NA

3.1 Nicole Evaluation of NA: Tiered approach

All protocols and guidelines use a tiered procedure to investigate the possibilities for MNA at a site. The aim is to make as early as possible and with as little ex-pense as necessary, a good estimation on the probability that MNA is an appropri-ate strategy to achieve site specific remediation objectives within a reasonable time frame. The number of steps, the amount of detail and the sequence of steps, differ between the different protocols but are essentially comparable (Table 3.1). Typi-cally, the first steps are focussed on obtaining information for the first line of evi-dence while at a later stage the guidelines focus on the screening and modelling re-quired for the second line of evidence. The third line of evidence, referring to ex-periments under field conditions or in the laboratory to demonstrate biological abilities to degrade the contaminants of concern, is not specifically included in these protocols. 1. The first step is the collection of basic information such as historical, technical

and field data. The field data comprise contaminant data, biochemical and geo-chemical data. The structured evaluation of these data and the graphic repre-sentation allows the identification of data gaps.

2. A conceptual model has to be formulated to describe the origin and behaviour of the contaminant in the environment, including a hypothesis on the natural at-tenuation processes and the source-pathway-receptor approach. A qualitative procedure can be applied to assess the potential for NA. Some of the protocols (Appendix A: Wiedemeier et al. 1998, Sinke et al, 2000) use a type of quick scan giving points for field parameters but these “score lists” only serve as indication for the occurrence of NA and can never be used as such to “demonstrate” the efficacy of NA. In the case of chlorinated solvents the presence of daughter products are a strong indication for the occurrence of degradation processes. Also the redox conditions are included here as, for ex-ample, the degradation of petroleum hydrocarbons is favoured under aerobic conditions. Special attention has to be paid to the evaluation of the sustainabil-ity of the processes (the continuation of NA processes in the future), which is important for the long-term NA capacity.

3. A third step, that is not included in all protocols, includes different types of ex-periments under field conditions or in the laboratory measurements to demon-strate biological activity and the abilities of the soil biota to degrade the con-taminants of concern.

4. The next step is to simulate NA processes using a transport and fate model to predict the long-term behaviour of the plume. In some cases, simple models can be used such as BIOSCREEN but in other cases a more complex modelling approach using MODFLOW and RT3D is more suitable.


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5. The comparison of field data and the model output allows conclusions on the efficacy and the sustainability of NA and enables the risk assessment of current and future plume conditions.

6. Finally, provided that the authorities also approve, MNA can be implemented as remedial strategy. Additional agreements have to be made about a contin-gency plan and a long term monitoring strategy. The future plume development has to be followed, to validate or refine the modelling results.

Table 3.1 Overview of evaluating steps (after Sinke 2001).

Step Line of evidence

Activity

1 1st

2nd

Collect information on available site data; history and hydrogeology − concentrations of contaminants − redox conditions, macro chemistry, concentrations of metabolites

2 2nd Formulate conceptual model including hypothesis to explain NA Assess potential for NA Identify additional data requirements and collect data

3* 3rd Field and laboratory experiments to underpin microbial activity 4 2nd Use transport & fate model to confirm NA hypothesis and predict fu-

ture plume behaviour 5 2nd Test hypothesis and carry out risk evaluation 6 Getting MNA approved, and integration of Natural Attenuation in long-

term management strategy & verification of MNA

*not obligatory in all protocols, depending on evidence in 1st and 2nd line and type of contaminant.

Steps 1, 2, and 5 are explained in the next paragraphs. Steps 3 and 4 are already discussed in the previous chapter. Step 6: How to get MNA approved and agreed with the authorities is detailed in chapter 4.

3.2 Acquisition of field data (step 1)

The type of data to be collected to characterise a site and to determine the possibili-ties for NA can be divided into two categories: hydro geological (determines transport of the contaminants), chemical and biological (determines fate of the contaminant).

3.2.1 Hydro geological data

The properties of an aquifer that have the greatest impact on contaminant transport include hydraulic conductivity, hydraulic gradient, and porosity. In table 3.2 the parameters are listed that have to be determined to characterise the hydrogeology of the site and the presence of potential receptors. These data are used as a basis for the site-specific conceptual model.


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Table 3.2 Parameters advised to be measured to characterise the hydrogeology and receptors of the site.

Chlorinated compounds

Petroleum hydrocarbons

Characterisation of aquifer

Regional hydrogeology Yes Yes Definition of site lithology and stratigraphy Yes Yes Porosity Yes Yes Water level elevations Yes Yes Direction of ground water flow Yes Yes Travel rate of ground water Yes Yes Range of seasonal water level fluctuations Yes Yes Depth of pollution Yes Yes Extent of unsaturated soil impact Yes Yes Potential for vertical migration Yes Yes Push-pull tests or pumping tests Optional Optional

Risk assessment and characterisation of receptor

Current use Yes Yes Survey of nearby water supply wells Yes Yes Location of potential receptors Yes Yes Identification of high conductivity pathways If possible If possible Locations of groundwater recharge and discharge areas

Yes Yes

3.2.2 Chemical and biological parameters

Measurements of the concentration and spatial distribution of the major contami-nants and, in case of chlorinated solvents, the daughter-products are important to characterise the extent of the pollution (table 3.3). Also the distribution of contami-nant in the aquifer has to be determined, and whether it is present as pure product (LNAPL or DNAPL), or whether it is sorbed or dissolved in the groundwater. Fi-nally, the conditions in the aquifer as indicated by the chemical and biological pa-rameters determine which reactions may proceed: sorption, degradation, or chemi-cal transformation. The amount of organic carbon determines the sorption, and consequently, the retardation of contaminants. A factor that is often overlooked in many protocols is the presence of species that are available in the solid phase. Or-ganic matter, iron- and manganese complexes do play a significant role in redox processes, but can be difficult to interpret. The redox conditions determine the type of degradation process that might proceed in the aquifer as well as the rate at which degradation takes place. Aerobic condi-tions favour the degradation of petroleum hydrocarbons and lesser chlorinated sol-vents such as DCE and VC but inhibit the degradation of the higher chlorinated solvents such as PCE and TCE. For the aerobic degradation of petroleum hydro-carbons, the balance between the amount of organics (electron donor) present and the flux of oxidizing equivalents (electron acceptor i.e. oxygen, nitrate etc) deter-mines the efficacy of the degradation and thereby the plume behaviour.


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Table 3.3 Recommended parameters to characterise the dimensions of the contamina-tion and the biological and chemical conditions in an aquifer for chlorinated solvents and petroleum hydrocarbons.

Chlorinated compounds

Petroleum hydrocarbons

Characterisation of contaminant

PCE Yes No* TCE Yes No* DCE Yes No* VC Yes No* Ethene, ethane Yes No* Chloroethane Yes No* 1,1,1, trichloroethene Yes No* 1,1 dichloroethene Yes No* Aromatic hydrocarbons (BTEX) No* Yes Trimethylbenzene No* No* Methyltertiairybutylether (MTBE) No* Yes PAH’s No* No* Other contaminants (metals, arsenic) No* No* Carbon dioxide Optional Optional Ketones Optional Optional Volatile hydrocarbons Yes Yes

Characterisation of energy source

Dissolved organic carbon Yes No Total organic carbon # Yes# Yes# Quality natural organic matter Optional No Volatile fatty acids Optional No Propane/propene Optional Optional

Characterisation of redox

Oxygen Yes Yes Nitrate Yes Yes Manganese Optional Optional Iron(II) Yes Yes Sulphate Yes Yes Sulphide Optional Optional Methane Yes Yes Hydrogen Optional Optional Oxidation reduction potential Optional Optional Biologically available iron Optional Optional

Characterisation of geochemistry

Iron(III) species Yes Yes Manganese species Organic matter (%)

Optional Yes

Optional Yes

* Contaminants marked with *: only if historic data indicate that these contaminants might have been used

# is important with respect to calculation of retardation factor for transport o optional parameters might add to the lines of evidence in some cases


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Under strongly reducing conditions (e.g., sulphate reducing or methanogenic) the reductive dechlorination of the higher chlorinated solvents (PCE, TCE) is favoured. Under these reduced conditions, organic carbon is used as electron donor for the dechlorination of chlorinated solvents that are thereby used as electron acceptor. The electron donor source, present as dissolved organic carbon, total organic car-bon or as co-pollutant (BTEX), relative to the amount of electron acceptor (chlo-rinated solvent), determines the sustainability of the degradation process. Also the quality of the electron donor is important but can not yet be determined with stan-dardised methods. The amount of volatile fatty acids may give information on the “quality” of the energy source.

Several other parameters can be determined that help to describe the conditions present in the aquifer. Generally these parameters are not measured in all samples or are only measured in a later tier to get additional, more detailed information. – levels of phosphate and ammonia give an indication on the nutritional state of

the bacteria, – pH: when the pH is too low (< 5) or too high (> 9) the bacterial activity might

be hampered, – heavy metals: the presence of large amounts of heavy metals might inhibit the

bacterial activity, – temperature influences directly the rate of chemical, physical and biological

processes, – alkalinity: an increased alkalinity compared to background indicates the pro-

duction of carbon dioxide, – conductivity and chloride concentration: an increase in these values compared

to background indicates the release chlorine.

3.3 Site description, conceptual model and risk assessment

From site data to conceptual model Site investigation data should first be used to develop a site-specific conceptual model with source-pathway-receptor linkages. It should represent the origin of the contaminant, preferably both in time and location, and the hypothesis on how the contaminants behaved after liberation into the environment. Crucial parameters such as type of contaminant (DNAPL, LNAPL), mass, groundwater transport and type of natural attenuation processes have to be included. There is some confusion to whether MNA is a remedial measure only for plumes or also for source zones. Actually this is very site specific and depends on the balance between the mass flux out of the source zone, the rate of NA and the location of the plume. If, despite the presence of a source zone, the plume is stable, there is no sci-entific reasoning against MNA. The NA processes can keep up with the supply of contaminants from the source zone and the plume will remain in position. How-ever, the presence of a source zone may increase the duration of the MNA signifi-cantly.


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From conceptual model to risk assessment The conceptual model should identify and describe the behaviour of source and the main processes in the pathway. From that point the conceptual model converts into a risk assessment, when it comes down to the threats to possible receptors. An ISO description of an exposure assessment comprises: – source identification and characterization; – identification of exposure routes (pathways); – identification of relevant receptors; – actual exposure assessment (possibly). If receptors are not, or not directly, exposed to a contaminant, the exposure assess-ment needs to consider the various ways by which exposure might occur in the fu-ture and their significance. This includes: extractions, surface water, evaporation etc. The contaminant might also undergo biological or chemical transformations, affecting its toxicity, availability and mobility. The risk depends on both the con-centration of the contaminant and the pathway/exposure route (skin, ingestion etc.). For the risk assessment in MNA studies the main focus is on pathways and the chance that the contaminant actually reaches a receptor.


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4. Getting MNA approved and decided

4.1 Legislation and policies on MNA in Europe

In order to implement MNA at a site, it has to be approved by the authority as a remedial strategy. Also there have to be clear-cut agreements on the monitoring strategy, the performance criteria, the contingency plan, the remedial objectives and the timeframe of the remediation. By now most countries in Europe have for-mulated a policy or a position on MNA. But very often there are regional differ-ences in the interpretation of the authorities; it may depend on the country or “county” or “provincie” or “region” or “Bundesland”. The major trend however is that MNA is an accepted option for cost-effective management of contaminated groundwater that cannot be ignored. There is awareness that a rapid and complete cleanup of mobile contaminants in groundwater in many cases is not feasible, both technically and financially. European Legislation: In the EU Groundwater directive, Annex IV, indent 1.4 (version submitted to the EU parliament) it is stated that:

“Specific trend assessment shall be carried out for relevant pollutants in bodies of groundwater that are affected by point sources of pollution, including historical point sources, in order to verify that plumes from contaminated sites do not expand over a defined area and deteriorate the chemical status of the groundwater body.”

The application of MNA in Europe can therefore be accepted. Groundwater plumes are even allowed to remain and expand within a defined area, unless the chemical status of the groundwater body is deteriorated according the definitions of the EU directive. The plumes however are not allowed to pose any risk for possible recep-tors such as surface water or public water supply wells.

Policy in the countries of the demonstration sites:

UK, Belgium and the Netherlands There are similarities in the regulatory approach to MNA in these countries. MNA may be considered as a risk management option if it is sufficiently protective of human and ecological receptors. The efficacy of NA has to be demonstrated clearly beforehand, with an assessment of the current and future conditions for NA (sus-tainability). The remedial goals have to be achieved within 30 years, leading to a stationary or receding plume, without a significant increase. If at any time NA does not meet the criteria a fall-back contingency plan has to be activated. Furthermore MNA is considered as one of the remedial options in a decision process, which is obligatory. In this process benefits and burdens can be weighed to demonstrate cost-efficiency (see 4.3). The EA for England and Wales have produced detailed


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guidance on the steps to go through to demonstrate that MNA is a technically ro-bust risk management strategy for a site. Differences do occur in the fact that in Netherlands and Belgium additional criteria can be put on residual concentrations and that a problem owner is asked to remove as much contaminant load as is cost-effectively feasible. In the Netherlands spe-cific legal possibilities have been created that allow MNA to be used, even with a slightly expanding plume, if it can be ascertained that within a reasonable time frame (30 years), the contaminant concentrations are stabilized to such an extent that no further care or control is needed.

Germany: Currently only a sparse body of legislation exists in the Federal German Republic which takes into account natural attenuation processes in soil conservation admini-stration. A position paper from the Commission on Contaminated Sites (ALA) of the German National/Federal States Committee on Soil Protection (LABO) was published recently, which proposes classifying MNA as self-monitoring measures in terms of Article 15 of the German Federal Soil Protection Act. A possible classi-fication of MNA as remediation measure (decontamination measures and securing containment measures, Article 2 (7)) or protection and restriction measure (Article 2(8)) was refused by the commission. As a general rule MNA is more and more accepted as a supplement or replacement of remediation measures, but the determining process regarding organic pollutants should be microbial decomposition and must not be dilution. The number of ap-proved projects is still limited, but twenty four sites are currently under investiga-tion within the BMBF-funding priority KORA - Retention and Degradation Proc-esses Reducing Contaminants in Groundwater and Soil. France and Italy: In France and Italy the acceptability of MNA as such is still in its infancy. In both countries only a few projects focussed on MNA have been proposed and approved. These projects were mainly prepared in compliance with international protocols and guidelines. However, the French approach for managing contaminated sites leaves open how far natural processes may or must be taken into account when determining rehabili-tation values. If natural processes are referred to, enough evidence of their occur-rence must be provided.

4.2 Contracts and documents

In some countries the formal approval is a contract that has to be signed by both parties (Germany), while in others the authorities approve a proposed remedial plan.


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In most countries one should provide: 1. Site description including the conceptual model, Risk assessment and Fate and

transport model, 2. Choice and justification of remedial objective, 3. Monitoring strategy, 4. Performance criteria, 5. Contingency plan.

Normally, at this stage, item 1 is already available.

4.3 Choice and justification of remedial objective

As in many decision processes, choosing an objective is choosing between alterna-tives. The final remedial objective is the outcome of a weighing process between the burdens and the benefits of remedial actions. This is a more specific expression of cost-effectiveness. If the costs for a full clean-up are too high in relation to its economical and societal benefits, the balance may switch over in favour of a pro-longed MNA process that is often also safer and environmentally friendly. In all cases it should be ascertained that the site-specific remediation is protective of hu-man health and the environment. Recently a Guidance was published in the Nether-lands on how to make decisions dealing with mobile contaminants Slenders et al. 2004 (ROSA).

This guidance is a description of the balancing process leading to the choice of a remedial alternative, and contains: – An inventory of stakes and commitment of all

parties concerned in the process; – A clear and unequivocal definition and mutual

choice of relevant benefits and burdens; – An open balancing of realistic remedial alterna-

tives; – A stepwise reduction of the number of remedial

alternatives, leading to the choice of the prefer-ential alternative.

Table 4.1 Decision aspects.

Burdens Benefits

1 kg1 kg1 kg

Cost Remediation time and aftercare Risk of failure Emissions and exhaustion of resources Other

Risk reduction Restoration of functional use Plume behaviour Amount contaminant removed Other (e.g. decrease of liability)


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4.4 Monitoring strategy

A fate and transport model often forms the basis for the monitoring strategy as it indicates the plume behaviour both in time and space, the expected trends in con-centration of parent and daughter products, and time-scale for remedial targets to be achieved. A model can provide insight in the optimised well position and the sampling frequency. For the requirements that should be set for the monitoring strategy there can be a difference between: I. plume/process monitoring and II. early warning monitoring to protect possible receptors. The monitoring program should specify the location, frequency, and type of samples and measurements to evaluate remedy performance as well as the possible threat to receptors. In addition an adequate monitoring program should: I. plume oriented: – identify any potentially toxic transformation products resulting from biodeg-

radation, – determine if a plume is expanding (either down-gradient, laterally or verti-

cally), – detect new releases of contaminants to the environment that could have an

impact on the effectiveness of the natural attenuation remedy, – detect changes in environmental conditions (e.g., hydrogeologic, geochemical,

microbiological, or other changes) that may reduce the efficacy of any of the natural attenuation processes.

– II. Early warning: – ensure adequate warning of potential impact to down-gradient receptors, – demonstrate the efficacy of institutional controls that were put in place to pro-

tect potential receptors.

For ideal cigar like plumes a monitoring network could look like the one drafted in figure 4.1, where R stands for Reference, S for Source, P for Plume, and M for the Early Warning Monitoring. L: lateral wells Figure 4.1 Monitoring net-work for ideal cigar shape plume


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4.5 Performance criteria and contingency plan

Performance criteria The evaluation on the efficacy of the MNA remedial action should be based on pre-defined performance criteria. These criteria should be realistic and process based. There is a noticeable growing trend towards a dual objective containing the follow-ing two performance indices: 1. a strict criterion that checks the actual plume development versus the agreed

development. E.g. If a plume keeps on expanding, this can be a direct reason to activate the contingency plan;

2. a target or expected trend or level of residual concentrations. This is not a strict criterion, but a switch to adjust the monitoring and aftercare programme or to check the plume prediction/modelling.

This split in performance criteria makes it possible to set clear fall-back criteria, but at the same time avoid the dependency on variations in concentration levels. Often these levels are put as an aim, not a realistic target. Variations in concentra-tion levels can be the result of natural processes such as (temporal) non-equilibrium situations due to for instance groundwater fluctuations, heavy rainfall or tempera-ture shifts, but also changing conditions. Also man-induced variations occur due to sampling procedures or minor analytical variability.

Contingency planning The implementation documents for MNA should include a contingency plan in case the MNA does not meet the performance criteria or is even failing. Important aspects that should be included in the contingency plan are: – Clear definition of the activation procedure for the contingency measures.

Preferably, the decision criteria should be based on process parameters and not purely on concentration data (see above), but the national legislation must offer this liberty. The contingency plan should be activated for example when several wells or sequential measurements indicate a deviation from the as-sumed trend. Obviously, concentrations in one well exceeding the expecta-tions cannot be the basis for activation of the contingency plan.

– Type of contingency measures. It should be indicated at least which contin-gency measures could be used. The contingency measures often refer to “con-ventional” techniques such as pump and treat or hydrological containment to prevent the plume from migration and/or further expansion. In some cases also an additional active remediation of the source zone could be considered.

– The changed use of the site that might result in an increase of the risk should automatically start a process of re-evaluating the possibilities of MNA. This concerns both the altered use (e.g. housing instead of industry) and changes in hydro geological conditions (e.g. increase or decreased groundwater extrac-tion regime, long lasting changes in groundwater level, changes in groundwa-ter fluctuations).


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5. Demonstration of feasibility MNA at 8 sites

5.1 Choice of new sites with unknown outcome

Eight sites were involved in the demonstration, and took part in the review process. These sites were all new with respect to a MNA evaluation and the outcome of a MNA evaluation was not known at the beginning of the project. The demonstration sites represent a variety of geological conditions including fractured bedrock and unconsolidated deposits (table 5.1). The types of contamination encountered in-clude hydrocarbons, chlorinated hydrocarbons, phenols and mixed contamination. The chosen dataset therefore was not biased. The full reports of the sites are at-tached in appendix E.

Table 5.1 Overview of site characteristics.

Site A B C E F G H J

Total area in ha ~100 330 20 200 1 150 4 Research area (ha) 19 330 20 200 1 8 4 1 Number of source zones unknown 6 1 1 1 1 1 1 Current use Industry Industry Industry Industry Sports-

field Industry Industry Industry

Planned future use Industry Industry Industry Industry Sports-field

Industry Industry Industry

Number of wells (MNA area / total area)

39 / >100 39 / 400 17 100 18 24/>200 2 29

Estimated year of origin 1946 - 1960

1965 1950-1995

Mid 50s – early 80s

1950-1960

From 1957

1998 1996

Monitored since 1999 1999 1995 1997 1996 1999 Risk assessment model None SUS RISC DOE-

ORO Guidance

ASTM RBCA

Yes RBCA

Nearby receptor1 SW, GW SW SW GW SW, GW SW, GW SW, GW Type of soil Fluvial,

anthropo-genic

Sand overlain by clay

antrhopo-genic, silt clay over bedrock

Alluvial, sand & gravel

sand, clay, sandstone

Alluvial, sand

Alluvial, sand, silt

Sand

Velocity of GW (m/yr) 6-30 2-22 Main contaminants2 CHC, mo

BTEX, styrene

CHC, TPH, BTEX styrene

TPH, phenol

CHC BTEX BTEX, MO

BTEX BTEX, TPH, PAH

Estimated plume extension (mxm)

Unknown unknown 270x260 3000x 1000

50x50 200x50 unknown 250x150

Depth of pollution 30 m 5-25 m unknown 3-12m 1m 5-20m 9-12 m

1. SW Surface water, GW Ground Water, H Housing 2. TPH total petroleum hydrocarbons, MO mineral oil, CHC chlorinated hydrocarbons, PAH polycyclic aromatic

hydrocarbons, DCM dichloromethane, CB chlorobenzene

Probably at some sites MNA will be a good and reliable approach as a sole meas-ure, while at others MNA will be used in addition to active methods such as pump


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and treat. Finally, at some sites MNA may not be applicable at all due to unfavour-able conditions. At some moment, for all these sites the decision had to be taken as whether or not to go forward with MNA. The demonstration sites have been inves-tigated according to a common strategy defined to be consistent with the procedure described in chapter 3.

5.2 Complexity of the sites

The sites differed with respect to the complexity of hydrology, the variety of con-taminants, the magnitude of the source zone, and the total contaminated area. The sites have been categorised based on these criteria (table 5.2). The composition of the contaminant and the complexity of the (geo)hydrological system varied from a single category of contaminants in a homogeneous aquifer (sites C, G and H) to intermingled multiple source and plume zones in a complex hydrological setting (sites A and B). For the former ones a well-defined source zone and accompanying plume has been evaluated for MNA. For the latter ones it is not practicable to evaluate all source and plume zones separately. For these com-plex sites an approach has been chosen where the impact of MNA has been evalu-ated at a site scale with respect to the protection of the off-site receptors such as surface water and deep groundwater.

Table 5.2 Increasing complexity of sites with respect to hydrology and contaminant composition and mass of contaminants.

Site Complexity1 Mass2 Site category3

Hydrology Contaminant

A *** *** *** III B *** *** *** III C ** * * I E ** ** ** II F ** ** ** II G * * ** I H * * * I J * * *** II

1. Complexity for hydrology scales from * to ***; with: * a single homogeneous aquifer; **layered aquifer; ***aquifer with multiple layers, fingering or gradients Complexity for contaminants scales from * to ***, with: * a well-defined source zone with a single class of compounds (phenol, BTEX) ; ** multiple source zones or multiple contaminants; *** multiple source zones and multiple contaminants.

2. Mass scales from * to *** with: * presence of dissolved contaminants; ** presence of dissolved contaminants and residual NAPL in pores; *** presence of NAPL (either DNAPL or LNAPL).

3. Category I: 3 times* or 4 *; category II: 5* or 6* and category III: 7* to 9 *


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5.3 Lines of evidence for MNA at the sites

In table 5.3 a summary is given of the lines of evidence for each site.

Table 5.3 Summary on the lines of evidence with the positive evidence for MNA and, in italics, the “negative” evidence against MNA.

Site A B C E F G H J First line of evidence

Increasing concentra-tions

Fluctuating or decreas-ing concen-trations

Increasing plume con-centrations

Decreasing concentra-tions

Fluctuating/ stable con-centrations

Decreasing concentra-tion trends since 1996

Decreasing plume size since 2002

Decreasing concentra-tion trends since 1999

Plume behaviour Multiple/ expanding

Stable/ un-certain

Expanding Stable Stable Stable plume

stable stable

Second line of evidence

Redox favourable for NA of CHC

Redox buffer zones

Redox outside plume fa-vourable

Redox not suitable

Redox indi-cates an-aerobic degradation

Redox favourable

Redox zonation favourable for NA

Redox zonation favourable for NA

Fate and transport model

Modflow Modflow - - Bioscreen ModFlow RT3D

- ModFlow RT3D

C12/C13 indicative proof.

Stable iso-topes

Electron balance negative

US-EPA screening model

Compari-son species transport

C12/C13 indicative

Electron balance

Third line of evidence

Increased number D. ethano-genes

- CFU’s indicative

- Micro-cosms benzene

Batch data for all redox conditions

Aerobic mi-cro cosms

MPN and CFU’s in-dicative

In table 5.4 the conclusion of the site owner with respect to the management option at the site is presented. At three of the investigated sites is MNA chosen as a stand alone management option, at all other sites more active remedial actions are (or will be) taken and MNA is considered to be implemented as aftercare. In general it can be concluded that the more complex the site, the smaller the chance that MNA will be used as preferred stand-alone option.

Table 5.4 Category of the sites and management option.

Site Category Management option A III Regional approach with MNA B III Site approach with MNA buffer zone C I Active source treatment + MNA E II Source containment + restriction of use of

the groundwater + study of enhanced NA as contingency plan

F II MNA G I MNA H I MNA J II Active source treatment + MNA plume


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Table 5.5 Overview of the monitoring strategy and the contingency plan for the sites where MNA is applied as stand-alone solution.

Site Monitoring strategy Number of wells Contingency plan F Once a year 18 H Once a year 12 Air sparging/pumping G 2004-2005: once a year

2005-2011 once every 2nd year 12 10

Interception


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6. Independent review

6.1 Reviewers

The aim of this demonstration project is to demonstrate the applicability of MNA at new sites. By having the site reports reviewed by independent reviewers from different countries that have no relationship with the cases, the acceptance of the methodology and its application in real industrial context was checked. Representa-tives of the academia and authorities have been invited to give their opinion on the work / NA evaluation that has been carried out at the sites (table 6.1). The average experience on contaminated land issues of these reviewers is 12.5 years. They have been asked to give their opinion on the reliability of the results, the interpretation and the assessment of MNA. It proved very soon that the reviewers felt a need for more info that could be offered in a summarized report. They wanted to be able to check the origin of interpretations and conclusions. Obviously, this had conse-quences for their opinion (see 6.3). Summarized reports were provided to ensure consistency of data presentation and to facilitate the review process in a reasonable length of time. It was not realistic to expect reviewers to review all the underlying reports and data.

Table 6.1 Alphabetical overview of reviewers with affiliation.

Name Organisation Country Category 1) 1 Corpel, D. Province Gelderland NL A 2 Dahmke, A. Un. Kiel D U 3 Gargini, A. Un. Ferrara I U 4 Geysen, D. OVAM B A 5 Lookman, R. VITO B U 6 Mossmann, J. BRGM F U 7 Nomine, F INERIS F A 8 Parsons, J. Un. Amsterdam NL U 9 Quercia, F. EA Italy I A 10 Smith, J. EA England-Wales UK A 11 Thornton, S. Un. Sheffield UK U 12 Wolff, G. Umweltschutz Stuttgart G A

1) A: Authorities; U: Scientific (research institutes and universities)

6.2 Set-up of the review and questionnaire

Each reviewer received two site reports to evaluate. In most cases they received a report of a site in their own country and a report from elsewhere in Europe. The re-viewers received only the summary report of the sites as given in Appendix E and not the underlying full reports. These reports were written according to a pre-scribed format to enable efficient reviewing.


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As additional information they received the Nicole report on existing guidelines and protocols (Sinke and Lehecho, 1999). All sites were reported anonymously not referring to local (geological) conditions or rivers and not mentioning the consult-ant involved. The reviewers were free in their approach to evaluate the site reports and possibil-ity for MNA; we did not prescribe any protocol. However, to structure the review all reviewers have been requested to evaluate the anonymous sites on the basis of a series of questions in a standardized format. The questionnaire is given as appendix D. At each question ample space was provided to give additional comments and suggestions.

6.3 Evaluation of site reviews

6.3.1 Method and considerations

Consequence of summarized site information The lay-out of the site descriptions was prescribed and contained only summarized information. The demonstration project, and thus the reviews, is aimed at the ques-tion if MNA is a feasible and solidly proven option. The questions about the qual-ity of the site information provided are mainly meant to evaluate whether the sum-marized site info on site history and data representation has an influence on the re-viewers opinion regarding the acceptance of MNA and the lines of evidence, which are described more elaborately. The returned questionnaires made very clear that the provided level of detail in the reports did not meet the data needs of the review-ers, and in some cases this did affect the confidence in the remedial strategy of-fered. The fact that only condensed information was provided and not the underly-ing data did influence the level of confidence of the reviewers in the role of MNA at the sites. Lines of evidence NA adequate Regarding the principle aspect of the reviews, following the lines of evidence, in general the provided information was considered sufficient, convincing and con-clusive. Assembling results and way of presentation In the next chapters the reviews are briefly evaluated per site. Per category the an-swers are wrapped up, and the overruling impression is stated. The following cate-gories are used: 1. Description of site and contaminants (quality of); 2. Conceptual mode, risk assessment and flow model; 3. Lines of Evidence; 4. Agreement on implementation of MNA; 5. Other comments.


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On top of these categories the following questions are illustrated with simple pie charts: 1. Are the lines of evidence soundly presented (Y,?,N)? 2. Is NA occurring or sufficiently proven (Y,?,N)? 3. Is MNA sufficiently protective (Y,?,N)? The underlying scores of these pie charts are derived from the answers, and since only 3 reviews per site are available, only 3 points per question can be divided be-tween the answers Yes, ? and No. The returned questionnaires had to be interpreted for the awarding of points. The first line of evidence had extra weight in the inter-pretation of the score for the lines of evidence. Question 2 and 3 had to be ex-tracted from the extra comments of the reviewers.

6.3.2 Summary reviews site A

Site resume: Contaminants: CHC, BTEX, mineral oil, styrene Complex contaminant situation and hydrogeology 1st line: expanding and increasing concentrations 2nd line: redox favourable for CHC, not for benzene 3rd line: presence of specific CHC degraders chosen management option: MNA approach in regional context Site and contaminant description The reviewers would like to have had more detailed info about the site history and separate groundwater well data. There clearly is a need to be able to check e.g. in-vestigation strategy and redox interpretation. The description of geology, hydro-geology and of metabolites present is considered to be adequate to good. Conceptual model, risk assessment and flow model The site conceptual model in combination with transport modelling is found ade-quate, but the risk assessment is lacking. Lines of Evidence Due to the large amount of data a statistical representation is offered to demon-strate the trends in contaminant concentrations (L1). This does offer a clear picture of contaminant evolution, but some reviewers want to assess the underlying data. The sketched redox situation and the influence on NA are understood and makes NA probable, but needs underpinning (L2). The extra evidence with stable isotopes and molecular techniques are considered very valuable and convincing that degra-dation takes place (L3). Especially DNA testing provides useful supporting evi-dence of the capacity to biodegrade chlorinated hydrocarbons.


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Lines of Evidence sound

Yes

?

No

Is NA occurring?

Yes

?

No

Is MNA protective?

Yes?

No

Agree on implementation of MNA? For chlorinated solvents degradation proof is given and MNA is a realistic option, but the data are not convincing that NA is sufficiently protective for all contami-nants. Microbial degradation of benzene remains questionable at this site and makes MNA as an overall approach uncertain. Other comments? It is a nice report, but very little real data are presented (3x). Context actual approach versus reviews The site is located in a harbour area, surrounded by other contaminated sites. A re-gional strategy or integral management system, combining the groundwater prob-lems at a regional scale is going to be implemented. A stand alone approach is not possible or cost-effective. The basis for this strategy lies on MNA

6.3.3 Summary reviews site B

Site resume: Contaminants: CHC, BTEX, TPH, styrene Complex contaminant situation and hydrogeology 1st line: fluctuating or decreasing concentrations 2nd line: redox buffer zones present 3rd line: - chosen management option: MNA buffer zone concept Site and contaminant description The information about site and contaminants is considered to be good! Only one reviewer would like to have seen more information about separate well data (posi-tion, concentrations and redox species). Conceptual model, risk assessment and flow model The conceptual model is adequate and coherent, but the word buffer zone is found confusing. There are already contaminants present in this zone, and there is no evaluation about this zone being big enough. A RA is not really detailed in this case, and the flow and transport model is considered adequate.


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Lines of Evidence The documented loss of contaminants is adequately described, but the statistics and time series are considered limited (L1). The information on redox is adequate (L2). The data on stable isotopes of carbon and hydrogen offer additional underpinning but are not a common tool and sometimes difficult to interpret (L3).


Yes

?

No

Is NA occurring?

Yes

?

No

Is MNA protective?

Yes

?

No

Agree on implementation of MNA? Yes, but ongoing monitoring to verify the conceptual model is a necessity. This has to be accompanied with clear decision criteria and a robust contingency plan. Other comments? It is an interesting case study and conceptual model. Fairly convincing, and with additional data the remaining uncertainty would disappear. Context actual approach versus reviews The actual approach is approved by the reviewers, but some monitoring results are needed to found the decision.

6.3.4 Summary reviews site C

Site resume: Contaminants: TPH, phenols Simple contaminant situation and average complexity of hydrogeology 1st line: expanding and increasing concentrations 2nd line: redox favourable for degradation 3rd line: indicative bacteria counts chosen management option: active source treatment and MNA outer plume Site and contaminant description The description offered is considered reasonable to good, but all reviewers ex-pressed a need for more spatial data and information on soil stratigraphy, contami-nants and redox species. The vertical distribution is missing, and the data on redox are insufficient, but considered not necessary at the moment since the plume seems to be expanding fast anyhow.


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Conceptual model, risk assessment and flow model A flow model is missing, but the available data are put into a conceptual model and risk assessment in a correct way. Again the obvious expansion of the plume does make clear with little additional info that NA is no option. Lines of Evidence The first line of evidence is presented in a satisfying manner, although clearly there is no loss of contaminants in the plume (L1). The description of redox is adequate but a comparison of redox contours and contaminant plumes would help to found the conceptual model (L2). The additional bacteria counts mainly seem to demon-strate that there is an inhibitory effect at high concentrations and are regarded illus-trative.


Yes

?

No

Is NA occurring?

Yes

?

No

Is MNA protective?

Yes?

No

Agree on implementation of MNA? Clearly no, although degradation does occur, the load of contaminants does largely exceed the amount of available electron acceptors and an active remedial option is needed. Other comments? The obvious increase of concentrations and plume dimensions quickly leads to the conclusion that NA is not protective at all. One might conclude that additional in-formation on conceptual model, redox species etc. is not needed, but a solid under-standing of migration pathways and plume development is needed to plan future management options (monitoring and contingency plan). Context actual approach versus reviews The assessment of MNA and the reviewers are conclusive. The choice of this site may be considered unfortunate given the obvious shortcomings of NA. It does sup-port however the reliability of the MNA assessment and protocol.

6.3.5 Summary reviews site E

Site resume: Contaminants: CHC Average complexity contaminant situation and hydrogeology


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1st line: Stationary plume and decreasing concentrations 2nd line: redox not favourable 3rd line: - chosen management option: source containment and MNA plume Site and contaminant description The general description of site and site geology and hydrogeology was sufficient. The reviewers would like to have had more information about the vertical distribu-tion of the contaminants, the redox species and organic matter. Only trends of chlo-rinated solvents and degradation products are not sufficient in their view. Conceptual model, risk assessment and flow model The conceptual model and risk assessment are adequate to good. The risks are well determined. A flow model was built, but not discussed in detail. Lines of Evidence The document loss of contaminants is addressed adequately (L1) and accounts for the positive, but the presentation does not make clear if this is due to natural at-tenuation or source management. This is mainly due to the limited description of the geochemical indicators (L2). There is no additional microbial evidence given (L3).


Yes

?

No

Is NA occurring?

Yes

?

No

Is MNA protective?

Yes

?

No

Agree on implementation of MNA? All reviewers agree with the conclusions of the report that MNA alone will not solve this problem, and that active (source-) measures remain needed. Other comments? NA in the plume is mainly caused by dispersion and dilution, but the plume does seem to be stable when the source containment is in place. There is a lack of elec-tron donor in the plume. Enhanced NA could be an extra option or contingency measure. Context actual approach versus reviews The source obviously has to be contained, and redox conditions in the plume do not really point at NA. This lack of confidence in NA leads to a negative attitude to-wards MNA, although concentrations in the plume are clearly declining.


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6.3.6 Summary reviews site F

Site resume: Contaminants: BTEX Average complexity contaminant situation and hydrogeology 1st line: fluctuating concentrations and stationary plume 2nd line: redox favourable for aerobic and anaerobic degradation in different parts of the plume 3rd line: BTEX degrade rapidly in aerobic laboratory microcosms and TEX (but not benzene) degrade under anaerobic conditions chosen management option: MNA Site and contaminant description The opinions of the reviewers greatly differed, from good site info to reasonable and poor. The additional comments of the latter pointed at a need for very detailed information, more suitable for a research project. All reviewers however, expressed a need for more spatial data on soil profile and more information on source compo-sition and distribution. Conceptual model, risk assessment and flow model A flow model is not presented, but the impression of the conceptual model and risk assessment is adequate/good. However, one of the reviewers translates the poor source description into an uncertainty of the site conceptual model. Lines of Evidence Historical data need more explanation, but the data on contaminant reduction and time series are adequate (L1). The geochemical indicators are considered good, but could use additional explanation of spatial and temporal variations (L2). The mi-crocosm experiments are considered illustrative (L3).


Yes

?

No

Is NA occurring?

Yes

?

No

Is MNA protective?

Yes?

No

Agree on implementation of MNA? Yes, under the precondition that processes are better understood and solid monitor-ing and contingency plans are made.


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Other comments? The overall impression of the reviewers is quite good, but would be better if the de-scription of the vertical distribution of the contaminants and soil profile was de-tailed, and more effort was put into the understanding of MNA processes. Context actual approach versus reviews The site is managed with MNA, which is in coherence with the reviews. Especially because more data are available than presented in the summarized site report.

6.3.7 Summary reviews site G

Site resume: Contaminants: BTEX, mineral oil Simple contaminant situation and hydrogeology 1st line: decreasing concentrations and stable plume 2nd line: redox favourable for aerobic degradation 3rd line: positive batch experiments for all redox conditions chosen management option: MNA Site and contaminant description All reviewers expressed a need for more spatial data and information on soil strati-graphy, contaminants and redox species. They experienced difficulties in visualiz-ing the vertical distribution, mainly because the screen depths were not indicated. Conceptual model, risk assessment and flow model As a result of the difficulties in visualizing the plume, it was felt that the concep-tual model needed explaining. Lines of Evidence Generally the lines of evidence were considered convincing, especially the clear decreasing trend of concentrations (L1). The redox situation was also positively contributing, but not founded with factual data but with just an interpretation. A need for the underlying data was expressed to be able to check this (L2).The mi-crocosm studies were considered to be a valuable extra (L3).


Yes

?

No

Is NA occurring?

Yes

?

No

Is MNA protective?

Yes

?

No


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Agree on implementation of MNA? The reviewers do not agree. The opinions vary from: “NA clearly demonstrated” to “Implementation not recommended”. In this perspective, the country and legisla-tive background of the reviewers seems to play a role. Other comments? The key-aspects are mentioned, but need solid foundation. A 3D visualization could have been helpful. Context actual approach versus reviews Obvious decreasing concentrations and the plume being within the catchment area of a groundwater extraction for process water (and not reaching this extraction as a result of NA processes), make MNA a solid approach. This has lead to a contract between problem owner and authority. The availability of more data in reality, and the explanation of the site owner have removed the remaining uncertainty for the authority.

6.3.8 Summary reviews site H

Site resume: Contaminants: BTEX Simple contaminant situation and hydrogeology 1st line: decreasing concentrations and stable plume 2nd line: redox zonation favourable for NA 3rd line: presence of specific: micro cosms chosen management option: MNA Site and contaminant description The overall impression of the data is reasonable, but the data on the contaminant position is insufficient. All reviewers expressed a necessity for more spatial data and information on contaminants. Limited access makes it difficult to delineate the plume correctly, but the uncertainties in plume demarcation and characterization are thought to be big. Conceptual model, risk assessment and flow model The conceptual model is reasonable, but would benefit with additional data. A risk assessment and flow model are not presented or are described too briefly. It is also noted that the used expression “usage non-sensible” (insensitive use) is in fact not elaborated for risk assessments, and thus not used correctly. Lines of Evidence The limited amount of available data leads the reviewers to limited confidence in the first line of evidence, the documented loss of contaminants (L1). The redox data are clearer (L2). The additional microbial tests seem to be convincing, but


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need controls or explanation for the conditions applied during the experiments (L3).


Yes

?

No

Is NA occurring?

Yes

?

No

Is MNA protective?

Yes

?

No

Agree on implementation of MNA? The occurrence of NA under the conditions present is probable. The application of MNA is within reach if sufficient underpinning is given. Other comments? The site seems to be suitable for MNA, but the site research and the lines of evi-dence need to be developed in more detail. A source investigation and implementa-tion in conceptual model should make clear whether source remediation is cost-effective. Context actual approach versus reviews MNA is being used and approved by the reviewers (with side remarks).

6.3.9 Summary reviews site J

Site resume: Contaminants: BTEX, TPH, PAH Average contaminant situation and hydrogeology 1st line: decreasing concentrations and stable plume 2nd line: redox zonation favourable NA 3rd line: bacteria counts chosen management option: MNA plume and source treatment Site and contaminant description The site description is reasonable. The description could do with more information about chemicals used, the soil profile and geology. The vertical distribution of the contaminants needs detailing. Conceptual model, risk assessment and flow model These are all reasonable to adequate, but only if the current use and environment is considered. If additional receptors are to be included, additional detailing is needed. Information of soil contamination is missing, and this does influence the risk assessment and conceptual model.


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Lines of Evidence The documented loss of contaminants is clear (L1). The geochemical indicators and the bacterial counts furthermore support the conceptual model and explain the loss of contaminants (L2, L3).


Yes

?

No

Is NA occurring?

Yes

?

No

Is MNA protective?

Yes?

No

Agree on implementation of MNA? It is expected that MNA is applicable, but additional characterization of sources, land use and receptors and a contingency plan are needed. Other comments? No other comments Context actual approach versus reviews MNA is being used and most probably would be approved 100% by the reviewers if more factual data were presented.

6.4 Overall evaluation for MNA

Reasonable description, but data need All in all, the sites were considered to be reasonably described, but the amount of data provided was felt insufficient by nearly all reviewers. All reviewers had a background in University or Authorities. Academics want detailed information to fully understand and verify the soil system. Sometimes the information need of the academic reviewer seems to be more scientifically driven than practically needed. And in their position of controlling authority, the Authorities want to check and comprehend the provided information on a detailed level as well. It does stress however that an approach with MNA can only be approved if there are sufficient data to create confidence. However it is not possible to turn every site into a re-search site. The degree of proof should be based on the level of risk a site repre-sents. Proving MNA In all reports the lines of evidence are followed, and this was highly regarded by the reviewers. The overall score for the soundness of the lines of evidence was 75%. This is a high score if it is realised that they reviewers wanted more data to underpin their view on the conceptual model. It mainly indicates that the reviewers


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agree with the chosen methodology. The high score does not mean that the ques-tion “is MNA sufficiently protective?” can be answered. 38% wants more informa-tion to be able to answer this question.


75%

8%

17%Yes?

No

Is NA occurring?

71%

25%

4%

Yes

?

No

Is MNA protective?

33%

38%

29% Yes

?

No

There is hardly any doubt that NA occurs at all sites. There is a full 71% “yes” and 25% “uncertain or not clear”, the latter mainly being caused by the demand for ex-tra information.

The score for the protectiveness of MNA could have been higher than the actual 33% if the contingency plans, risk assessments and monitoring plan would have been more elaborate.

There is no relationship between the soundness of the lines of evidence and the protectiveness of MNA. In some cases where the lines of evidence are good with-out question, MNA is understandably rejected (Site C and to a lesser extent E), and in another case the lines of evidence were questionable, but the acceptance of MNA was in fact without doubt. Differences in reviews due to nationality? There were large differences between the sites and between the reviewers. Some reviewers deviated structurally from the average giving low scores, while others scored consequently above average. It was not really possible to track this back to their background in University or Authority, although the academic reviewers needed more information. There was however a noticeable relationship between country policy/legislation and the scores of the reviewers. In countries where risk-based approaches are accepted (e.g. UK) the reviewers are more positive than in countries where NA and risk based management is not yet or rarely approved (e.g. Germany). Complexity of sites and MNA acceptance In table 6.2 it is illustrated that the complexity of the site has no influence on the acceptance of MNA. Site C and E are rejected and they are in site category I and II (simple to average). It is a prerequisite that the site is adequately characterized and described, which obviously is more difficult with complex sites.


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Table 6.2 Complexity of sites and acceptance of MNA.

Site Complexity1 Mass2 Site MNA Acceptance4

Hydrology Contami-nant

category3 Yes More data needed

No

A *** *** *** III 0% 67% 33% B *** *** *** III 67% 33% 0% C ** * * I 0% 0% 100% E ** ** ** II 0% 33% 67% F ** *** ** II 67% 33% 0% G * * ** I 33% 33% 33% H * * * I 67% 33% 0% J * * *** II 33% 67% 0%

1,2,3 See 5.2 4See 6.2-6.9


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7. Conclusions, discussion and recommendations

7.1 Conclusions

Application and acceptance of NA and MNA – If the notes of the reviewers are taken into account, than in the majority of

cases (6 out of 8) MNA is considered as a solid stand alone option or part of a combination of techniques.

The 8 cases that have been investigated are considered representative for European industry. The majority of the cases were complex, with either layered hydrology, or multiple contaminants or phases present. The outcomes of the MNA site assess-ments were not known beforehand. But it is expected that at all these sites the site owners (in consultation with the authorities) will fit MNA into a risk based site ap-proach, were MNA can be a stand alone option or a cost-effective addition to ac-tive source remediation measures. During the review process, the majority of the reviewers recognized the occurrence of NA (degradation processes), but sometimes felt uncertain (25%) and would like to have had more information than offered in the summarized reports. The applica-tion of MNA as a management option is not always accepted by the reviewers. In some cases the plume is growing (site A, C, E) despite degradation. Often the flux out of the source into the plume is significantly larger than the degradation and other NA processes, and MNA can only be applied in combination with active measures. In other cases the national legislative background of the reviewers is not (yet) adapted to a risk based approach, and the acceptance of MNA is then difficult (site G). Methodology and review process – The lines of evidence and the protocol were regarded as useful and convincing

tools. – The country legislative background greatly influences the opinion of the re-

viewers. The reviewers were positive about the summarized reports, and although wanting more data, 75% was very happy with the systematic approach with lines of evi-dence. It was clear for people with different country backgrounds, and enabled comparison. The approach following the lines of evidence seems to be commonly accepted amongst authorities and scientists. For this project NICOLE chose independent reviewers, with a background in Uni-versity or authorities. This led to critical reviews, because both groups wanted to verify the conclusions at a data level, which was not fully included in the summa-


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rized reports. As mentioned before, the influence of their legislative background was noticeable (e.g. risk-based or not). The data the reviewers indicated they needed to be able to judge the applicability of MNA were: spatial data, soil profile, vertical distribution of contaminants and groundwater chemistry, and a solid conceptual model of basic processes and plume behaviour. Sometimes the expressed data need was more applicable for a scientific research project than for an everyday remediation project where 100% certainty is unrealistic.

7.2 Best practice for MNA investigation

The use of a common procedure (chapter 3) or the 3 lines of evidence proved to be very efficient and commonly accepted. These 3 lines consist of: 1. a documented loss of contaminants or fading plume; 2. favourable NA conditions; 3. supporting microbiological evidence (presence of relevant bacteria, column

tests, microcosms). The evaluation made clear that with summarized info the applicability of MNA can already be estimated. The site specific need for information should therefore de-pend upon the certainty of this estimate and the risk involved for receptors. A structured argumentation with a conceptual model and a comprehensive data set is always necessary to get MNA approved, and to remove remaining doubt with the authorities, experts and site owners.

7.3 Recommendations

Experiences with soil remediation worldwide have made abundantly clear that our society is not able to achieve a complete removal of contaminants in the subsurface within time frames of a few decades. Technically a 100% removal in most cases is impossible. Financially the costs of full removal are not in line with the benefits and risks involved. The Nicole demonstration project underlined that in this per-spective, MNA is a cost-effective option that can be used to obtain satisfactory re-sults. MNA is often not a stand alone option, but part of a risk based site manage-ment plan MNA should be incorporated in the various soil and groundwater policies in Euro-pean member states and DG-environment of the EU-commission, as one of the tools to manage historical soil and groundwater pollution, and to support the EU Water Framework and Groundwater Directives and Soil Thematic Strategy.


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8. Web sites, research projects and literature

8.1 Websites EPA: http://www.epa.gov (general EPA site)

http://www.epa.gov/ada/download/issue/540S02500.pdf http://www.epa.gov/swerust1/oswermna/index.htm http://www.epa.gov/swerust1/oswermna/mna_epas.htm http://www.deq.state.ok.us/lpdnew/FactSheets/MonitoredNaturalAttenuation.htm http://www.hqafcee.brooks.af.mil/products/techtrans/monitorednaturalattenuation/ evidence.asp http://www.epa.gov/swerust1/rbdm/issues.htm http://www.epa.gov/epaoswer/hazwaste/ca/resource/guidance/gw/gwhandbk/gwhb041404.pdf (Handbook of Groundwater Protection and Cleanup Policies for RCRA Corrective Action - April 2004 - Updated Version)

SKB/VROM http://www.bodembreed.nl/upload/documents/SV-

614%20eindrapport%20ROSA.pdf (ROSA guidance on mobile contaminants

OVAM: http://www.ovam.be/jahia/Jahia/pid/639 (Flemish NA guidance) Nicole: www.Nicole.org EA E&W: http://www.environment-agency.gov.uk

8.2 Research projects and programs

United Kingdom:

Contaminated Land Application In Real Environments (public/private partnership) www.claire.co.uk The Engineering and Physical Sciences Research Council (UK Government's lead-ing funding agency) www.epsrc.ac.uk and the linked www.subrim.org.uk

Germany:

KORA (Bundesministerium für Bildung und Forschung) www.KORA.de


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The Netherlands:

SKB Foundation for Knowledge Transfer&Development Soil www.skbodem.nl

8.3 Literature

ASTM. (1997). Standard guide for remediation of ground water by natural attenua-tion at petroleum release sites. ASTM.

Aziz, C.E. et al. BIOCHLOR, Natural Attenuation Decision Support Model, USEPA NRMRL 1999

BMBF Programm der Bundesregierung '' Forschung fur die Umwelt" 01-02-2000 Forschungs- und Entwicklungsvorhaben der Altlastsanierung zum Thema "Kontrollierter Naturlicher Rückhalt und Abbau von Schadstoffen bei der Sanierung kontaminierten Böden und Grundwässer"

Carey, M.A. et al (2000) Guidance on the assessment and monitoring of natural attenuation of contaminants in groundwater. Environment Agency R&D publication 95. UK

Gilbert, R.O. (1987) Statistical methods for Environmental Pollution Monitoring. Van Nostrand Reinhold Company, New York.

Langenhof, A (2003) Overdraagbaarheid van benzeenafbraak TNO-report 2003/208

Mace, R. E. et al. (1997). Extent, mass, and duration of hydrocarbon plumes from leaking petroleum storage tank sites in Texas.: University of Texas, Texas Natural Resource conservation commission and US-EPA.

Newell, C. J. and Connor, J. A. (1998). Characteristics of dissolved petroleum hydrocarbon plumes, results from four studies.: API Soil/ groundwater technical task force.

Newell, C. J. et al (1990). A hydrogeologic database for groundwater modelling. Ground Water 28, 703-714.

Newell, C.J. et al (2002) Calculations and use of first-order rate constants for monitored natural attenuation studies. EPA Ground Water Issue

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