Monitored Natural Attenuation (MNA)

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Monitored Natural Attenuation (MNA) is an important, common groundwater remediation technology used for treating some dissolved groundwater contaminants. MNA relies on natural attenuation processes to achieve site-specific remediation objectives within a reasonable time frame compared to more active approaches. While MNA has primarily focused on managing plumes with low residual contamination, there is an growing movement to also apply it to source zones via natural source zone depletion (NSZD). Long-term monitoring is required to determine if the concentrations of target contaminants are behaving as predicted.

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CONTRIBUTOR(S): Dr. John Wilson

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A number of natural processes can attenuate the concentrations of contaminants in groundwater including biological degradation, abiotic degradation, sorption, dispersion into groundwater adjacent to the contaminant plume, and volatilization to soil gas above the groundwater. As the concentration declines, it may reach a point where it is no longer considered hazardous. If the natural processes that attenuate the concentrations of a particular hazardous chemical can meet the cleanup goals for a site, the processes can provide the basis for a cleanup technology. The United States Environmental Protection Agency (U.S. EPA), defines Monitored Natural Attenuation (MNA) as “the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods”[1].

The concentration at which a contaminant is no longer hazardous is defined by U.S. EPA and state regulations. The U.S. EPA regulates the maximum concentration of contaminants that are allowed in water that is supplied as drinking water. These U.S. EPA regulations are referred to as the Maximum Contaminant Level (MCL)[2]. Often, the MCL is selected as the cleanup goal for MNA. However, other goals[3] are occasionally selected.

To accept MNA as remedial technology on the same basis as engineered remedial technologies, it is necessary to characterize the distribution of contamination at a site, characterize the flow of groundwater, understand the processes that contribute to natural attenuation and use this information to build a conceptual model of the site. Sometimes the site conceptual model is used to organize an analytical model of the transport and fate[4] of the contaminants in groundwater. The forecasts of the transport and fate model are compared to the cleanup goals for the site to determine if natural attenuation is an appropriate remedy. If natural attenuation is selected as a remedy, the site is monitored over time to ensure that the attenuation of the contaminant proceeds as anticipated. The entire package of site characterization[5], a site conceptual model, and monitoring[6] are necessary components of MNA as a formal remedy for any site selected.

The U.S. EPA considers three lines of evidence[1] before MNA can be accepted as the remedy for a site:

  • Historical groundwater 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.
  • Hydrogeologic and geochemical data that can be used to demonstrate indirectly the type(s) of natural attenuation processes active at the site, and the rate at which such processes will reduce contaminant concentrations to required levels.
  • Data from field or microcosm studies (conducted in or with actual contaminated site media) which directly demonstrate the occurrence of a particular natural attenuation process at the site and its ability to degrade the contaminants of concern (typically used to demonstrate biological degradation processes only).

At most sites, U.S. EPA requires the first two lines of evidence. The third line of evidence is reserved for contaminants that are not well understood.

MNA is often used as a remedy, or part of a remedy, where contaminants have been demonstrated to be degrading or sequestered in groundwater. A number of technical protocols have been developed to guide the application of MNA for particular contaminants, including fuel hydrocarbons[7], chlorinated solvents[8], methyl tert-butyl ether (MTBE[9]), inorganics , metals , radionuclides[10], and explosives[11][12]. These protocols were developed from 1999 to 2010, in the same time period when U.S. EPA developed its policy guidance. Since that time, there have been significant advances[13] in our understanding of the processes that degrade contaminants in groundwater.

Abiotic Process

Abiotic processes[14] can contribute to natural attenuation of certain contaminants such as chlorinated solvents. For example, chlorinated alkenes can react with naturally occurring magnetite or other iron minerals in aquifer materials[15]. The rate constants are generally slow, but abiotic degradation can be important if the travel time of the contamination to the point of compliance is long.

Tools for Assessing Monitored Natural Attenuation

  1. Statistical Tools to Evaluate Trends. Computer programs such as MAROS[16][17] and the Mann-Kendall Toolkit[18] can be used to help confirm trends in groundwater data used as a line of evidence for MNA.
  2. Molecular Biological Tools (MBTs). MBTs are used to identify and characterize the bacteria that carry out critical steps in the biodegradation of the contaminants in groundwater. In the case of chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE), a key bacterium is Dehalococcoides mccartyi[19]. In anaerobic groundwater, chlorinated alkenes can undergo a sequential reductive dehalogenation from PCE, to TCE, to dichloroethene (DCE) and then to vinyl chloride (VC) and finally to ethane. Anaerobic microbial communities that contain Dehalococcoides can degrade PCE and TCE all the way to harmless end products. The abundance of Dehalococcoides cells in groundwater can be determined by an assay based on the polymerase chain reaction[20]. Other assays can determine the abundance of reductase genes[19] that code for enzymes that can carry out specific steps in the dechlorination pathway. Similar assays are available to determine the abundance of Dehalobacter, Dehalogenimonas, and Desulfitobacterium strains that degrade chlorinated alkanes, and MBT assays are available for several of their reductase genes. A great variety of bacteria degrade petroleum hydrocarbons. Bacteria that degrade hydrocarbons using oxygen initiate degradation with an oxygenase enzyme, and qPCR assays are available for a variety of oxygenase enzymes[21]. The bacteria that degrade hydrocarbons under anaerobic conditions are particularly important for natural attenuation, and there are qPCR assays for the enzymes that initiate degradation under anaerobic conditions[22]. See an entire article on MBTs here: Molecular Biological Tools - MBTs
  3. Compound Specific Isotope Analysis (CSIA). CSIA can unequivocally demonstrate that a compound has degraded in groundwater. It is difficult to document the degradation of a compound in groundwater if the only information available is an apparent attenuation in concentrations along a flow path in the plume. There is always a possibility that a downgradient well is askew of the true flow path, and the attenuation is caused by dilution and not degradation. CSIA determines the ratio of stable isotopes in a compound. As a compound degrades, molecules with lighter isotopes degrade faster. As degradation progresses, the material that has not degraded becomes enriched in the heavier stable isotope. At many sites, degradation of the compound can be recognized and documented from a change in the ratio of isotopes[23]. At some sites, it is possible to use CSIA and reactive transport modeling[24] to evaluate the plausibility of alternate degradation pathways, and to estimate the extent of degradation. See an entire article on CSIA here: Compound Specific Isotope Analysis (CSIA)
  4. Computer Models. Groundwater fate and transport computer models are often used to evaluate how attenuation processes can control the migration of a plume. Public domain software is available that can incorporate terms for advective flow of groundwater, dispersion (and more recently diffusion) of contaminations in groundwater, and biotic or abiotic reactions. Examples of commonly used models include analytical models REMChlor[25] and REMFuel[25] , and the numerical models MODFLOW/RT3D[26], MODFLOW/MT3DMS, and the Natural Attenuation Software (NAS[27]).
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Figure 1a. Evolution of a plume when the plume and source do not attenuate.
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Figure 1b. Evolution of a plume when the source and concentrations in groundwater both attenuate.
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Figure 1c. Evolution of a plume when the source attenuates faster than the plume.

Source Area Considerations

In most plumes, the time frame that is required for natural attenuation to reach a cleanup goal across the entire plume is not controlled by the rate of attenuation in the groundwater. In many plumes, a source of contamination, such as residual oily phase material (non-aqueous phase liquid [NAPL]), contaminated soils, and matrix diffusion sources, provides a continuous supply of new contamination to the groundwater.

As a result, the lifecycle of the source[28] largely controls the lifecycle of contamination in groundwater. As a consequence, at many sites, some attempt is made to actively remediate the source of contamination. In almost every instance, active remediation is successful in reducing the concentration of the contamination, but fails to reduce the concentration to the cleanup goal. The final remedy is a pragmatic combination of active source remediation and MNA. Transport and fate models[29] can be used to evaluate the benefits from source remediation on the size and lifecycle of the plume of contaminated ground water. The models can estimate the reduction in concentration at the source that is necessary to pull a plume back behind a point of compliance and the time that is required for the plume to recede behind the point of compliance.

Regulatory Considerations

If a site is regulated under the Resource Conservation and Recovery Act (RCRA)[30], the usual goal is for the contaminants to attenuate to acceptable concentrations before groundwater can migrate off-site and impact receptors. Under this MNA approach, the groundwater must reach a cleanup goal before it reaches a point of compliance. For this implementation, a quantitative framework (BioPIC)[31] is now available that integrates new discoveries on degradation processes into the U.S. EPA’s approach to evaluate MNA.

When a site is regulated under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or Superfund)[32], there is often an additional requirement that all the contamination must reach the cleanup goal by a specified date. The performance of a remedy at a Superfund site is reviewed on a five-year cycle. A framework[33] is available to review long-term monitoring data to determine whether the attenuation within the review cycle is adequate to meet the cleanup goal by the specified date.

In the USA, the individual states have provided regulations to supplement the U.S. EPA guidance. Examples include general guidance on MNA provided by California[34], Minnesota[35], New Jersey[36] , Ohio[37], and Texas[38]. In addition, California[39], Minnesota[40], Washington State[41], and Wisconsin[42] provide guidance on petroleum releases. Minnesota[43] and Wisconsin[44] provide guidance on chlorinated solvents.

Additional Information

Additional information on MNA is available on web pages that are maintained by the United State Environmental Protection Agency[45], the United States Geological Survey[46], Department of Energy, and the Interstate Technology Regulatory Council[47]. In addition, ESTCP has published “Frequently Asked Questions Regarding MNA in Groundwater” which provides a recent summary overview of key approaches, technologies, and best practices for applying MNA[13].


  1. ^ 1.0 1.1 1.2 U.S. Environmental Protection Agency, 1999. Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites. Report.pdf
  2. ^ U.S. Environmental Protection Agency (USEPA), 2016. Table of Regulated Drinking Water Contaminants.Table of Regulated Drinking Water
  3. ^ Deeb, R., Hawley, E., Kell, L. and O'Laskey, R., 2011. Assessing alternative endpoints for groundwater remediation at contaminated sites. ER-200832
  4. ^ VirginiaTech, United States Geological Survey (USGS), and Naval Facilites Engineering Command (NAVFAC). 2016. Natural Attenuation Software (NAS). Software
  5. ^ Pivetz, B.E., Abshire, D., Brandon, W., Mangion,S., Roberts, B., Stuart, B., Vanderpool, L., Wilson, B., Acree, S.D., 2012. Framework for Site Characterization for Monitored Natural Attenuation of Volatile Organic Compounds in Ground Water. EPA 600-R-12-712, 89 pgs. Report pdf
  6. ^ Pope, D.F., Acree, S.D., Levine, H., Mangion, S., Van Ee, J., Hurt, K., Wilson, B. and Burden, D.S., 2004. Performance monitoring of MNA remedies for VOCs in ground water. US Environmental Protection Agency, National Risk Management Research Laboratory. Report pdf
  7. ^ Wiedemeier, T.H., Wilson, J.T., Kampbell, D.H., Miller, R.N., Hansen, J.E., 1999. Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. Volume I. Report pdf
  8. ^ Wiedemeier, T.H., Swanson, M.A., Moutoux, D.E., Gordon, E.K., Wilson, J.T., Wilson, B.H., Kampbell, D.H., Haas, P.E., Hansen, J.E., Chapelle, F.H., 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. EPA-600-R-98-128. Report pdf
  9. ^ Wilson, J.T., Kaiser, P.M., Adair, C., 2005. Monitored Natural Attenuation of MTBE as a Risk Management Option at Leaking Underground Storage Tank Sites EPA/600/R-04/1790. Report pdf
  10. ^ Truex, M., Brady, P., Newell, C.J., Rysz, M., Denham, M., Vangelas, K. 2011. The Scenarios Approach to Attenuation-Based Remedies for Inorganic and Radionuclide Contaminants. Savannah-River National Laboratory U.S. Department of Energy. Report pdf
  11. ^ Pennington, J.C., Zakikhani, M., Harrelson, D., 1999. Monitored Natural Attenuation of Explosives in Groundwater. ESTCP Completion Report ER-199518. ER-199518
  12. ^ Borden, R.C., Knox, S.L., Lieberman, M.T., Ogles, D., 2014. Perchlorate natural attenuation in a riparian zone. Journal of Environmental Science and Health, Part A, Toxic/Hazardous Substances and Environmental Engineering, 49(10), 1100-1109. doi: 10.1080/10934529.2014.897145
  13. ^ 13.0 13.1 Adamson, D., Newell, C., 2014. Frequently Asked Questions about Monitored Natural Attenuation in the 21st Century. ER-201211. Environmental Security and Technology Certification Program, Arlington, Virginia. ER-201211
  14. ^ Darlington, R., Rectanus, H. 2015. Biogeochemical Transformation Handbook. TR-NAVFAC EXWC-EV-1601, 41 pgs. Report pdf
  15. ^ He, Y., Su, C., Wilson, J., Wilkin, R., Adair, C., Lee, T., Bradley, P., Ferrey, M., 2009. Identification and characterization methods for reactive minerals responsible for natural attenuation of chlorinated organic compounds in ground water. US Environmental Protection Agency. Report pdf
  16. ^ Aziz, J.J., Ling, M., Rifai, H.S., Newell, C.J., Gonzales, J.R., 2003. MAROS: A decision support system for optimizing monitoring plans. Ground Water, 41(3), 355-367. doi: 10.1111/j.1745-6584.2003.tb02605.x
  17. ^ Aziz, J.J., Newell, C.J., Rifai, H.S., Ling, M., Gonzales, J.R., 2000. Monitoring and Remediation Optimization System (MAROS): Software User’s Guide. Report pdf
  18. ^ Connor, J., Farhat, S. K., Vanderford, M. V., Newell, C. J., 2012. GSI Mann-Kendall Toolkit. Mann Kendall Toolkit
  19. ^ 19.0 19.1 Löffler, F.E., Ritalahti, K.M., Zinder, S.H., 2013. Dehalococcoides and reductive dechlorination of chlorinated solvents. Bioaugmentation for groundwater remediation, ed. H.F. Stroo, A. Leeson, C.H. Ward, Springer, New York, NY. pgs. 39-88. ISBN: 978-1-4614-4114-4. doi: 10.1007/978-1-4614-4115-1
  20. ^ Lebron, C.A., Petrovskis, E., Loffler, F., Henn, K., 2011. Application of Nucleic Acid-Based Tools for Monitoring Monitored Natural Attenuation (MNA), Biostimulation and Bioaugmentation at Chlorinated Solvent Sites (No. NFESC-CR-11-028-ENV). ER-200518. Naval Facilities Engineering Command Port Hueneme CA Engineering Service Center. ER-200518
  21. ^ Baldwin, B.R., Nakatsu, C.H., Nies, L., 2008. Enumeration of aromatic oxygenase genes to evaluate monitored natural attenuation at gasoline-contaminated sites. Water Research, 42(3), 723-731. doi:10.1016/j.watres.2007.07.052
  22. ^ da Silva, M.L.B., Corseuil, H.X., 2012. Groundwater microbial analysis to assess enhanced BTEX biodegradation by nitrate injection at a gasohol-contaminated site. International Biodeterioration & Biodegradation, 67, 21-27. doi:10.1016/j.ibiod.2011.11.005
  23. ^ Hunkeler, D., Meckenstock, R.U., Sherwood Lollar, B., Schmidt, T.C., Wilson, J.T., 2008. A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-08/148, 2008. Report pdf
  24. ^ Kuder, T., Philp, P., van Breukelen, B., Thouement, H., Vanderford, M., Newell, C. 2014. Integrated Stable Isotope-Reactive Transport Model Approach for Assessment of Chlorinated Solvent Degradation. ER-201029
  25. ^ 25.0 25.1 Falta, R.W., Stacy, M.B., Ahsanuzzaman, A.N.M., Wang, M., Earle, R., 2007. REMChlor remediation evaluation model for chlorinated solvents user’s manual Version 1.0. Cent. for subsurface model. support, US Environ. Prot. Agency, Ada, Okla.User's Manual v1.0
  26. ^ 2005. MODFLOW and Related Programs Modflow
  27. ^ Widdowson, M.A., Mendez III, E., Chapelle, F.H., Casey, C.C., 2005. Natural Attenuation Software (NAS) User’s Manual Version 2. Report pdf
  28. ^ Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward, C. H. Springer New York. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3
  29. ^ Widdowson, M., Chapelle, F., Casey, C., Kram, M., 2008. Estimating Cleanup Times Associated With Combining Source-Area Remediation With Monitored Natural Attenuation. ER-200436 ER-200436
  30. ^ US EPA RCRA Laws & Regulations
  31. ^ Lebron, C. A., Wiedemeier, T. H., Wilson, J.T., Löffler, F.E., Hinchee, R.E., Singletary, M.A., 2015. Development and Validation of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches at Chlorinated Solvent Sites. ER-201129. ER-201129
  32. ^ US EPA CERCLA Act
  33. ^ Wilson, J.T., 2011. An Approach for Evaluating the Progress of Natural Attenuation in Groundwater. EPA 600-R-11-204. Report pdf
  34. ^ California Regional Water Quality Control Board, 2014. Workshop - Monitored Natural Attenuation. Barstow, California, September 10 & 11. Report pdf
  35. ^ Minnesota Pollution Control Agency. Natural Attenuation of Groundwater. Natural Attenuation of Groundwater
  36. ^ New Jersey Department of Environmental Protection - Site Remediation Program. 2012. Monitored Natural Attenuation Technical Guidance. Report pdf
  37. ^ Ohio Environmental Protection Agency - Division of Environmental Response and Revitalization, 2001. Remedial Response Program Fact Sheet. Remediation Using Monitored Natural Attenuation.Report pdf
  38. ^ Texas Commission on Environmental Quality - Remediation Division, 2010. Monitored Natural Attenuation Demonstrations under TRRP. RG-366/TRRP-33. Report pdf
  39. ^ California State Water Resources Control Board. 2012. Low-threat Underground Storage Tank Case Closure Policy. Report pdf
  40. ^ Minnesota Pollution Control Agency, 2005. Assessment of Natural Biogradation at Petroleum Release Sites. Guidance Document 4-03. Report pdf
  41. ^ Washington State Department of Ecology, 2005. Guidance on Remediation of Petroleum-Contaminated Ground Water by Natural Attenuation. Publication Number 05-09-091 (Version 1.0). Report pdf
  42. ^ Wisconsin Department of Natural Resources, 2014. Guidance on Natural Attenuation For Petroleum Releases. Remediation and Redevelopment Program. RR-614. Report pdf
  43. ^ Minnesota Pollution Control Agency Site Remediation Section. 2006. Guidelines Natural Attenuation of Chlorinated Solvents in Ground Water. Report pdf
  44. ^ Wisconsin Department of Natural Resources, 2014. Understanding Chlorinated Hydrocarbon Behavior in Groundwater: Guidance on the Investigation, Assessment and Limitations of Monitored Natural Attenuation. RR-699. Report pdf
  45. ^ U.S. Environmental Protection Agency, 2016. Natural Attenuation Overview. Technology Innovation and Field Services Division. Natural Attenuation Overview
  46. ^ Natural Attenuation Definitions. 2015. United States Geological Survey.
  47. ^ ITRC, 2008. Enhanced attenuation of chlorinated organics (EACO): A decision framework for site transition. Report pdf

See Also