Engineering and Expeditionary Warfare Center

Monitored Natural Attenuation

Schematic of Monitored Natural Attenuation for Groundwater Plume
Schematic of Monitored Natural Attenuation for Groundwater Plume



Monitored natural attenuation (MNA) is an in situ remediation technology where naturally occurring physical, chemical, and biological processes sufficiently attenuate contaminant concentrations in soil and groundwater to protect human health and the environment and achieve remedial goals within a reasonable timeframe. Biodegradation is often the predominant MNA mechanism to reduce the mass of the contaminants of concern (COCs) at a site. It should be noted that the impacts of chemical degradation pathways (or abiotic degradation) are becoming better understood and are recognized to contribute more significantly to MNA than previously thought.

  Abstract      Names      Description      Applicability     Cost     Duration     Limitation     Publications     Related Sites 


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Other Technology Names

  • Natural Attenuation
  • Intrinsic Remediation



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For natural attenuation to be considered a viable remedial strategy, the natural subsurface processes such as dilution, volatilization, degradation (biotic or abiotic), and adsorption must reduce contaminant concentrations to acceptable levels within a reasonable timeframe. To assess whether MNA is a viable remedy at a site, data from the site should indicate the following items per the Groundwater Management Handbook (NAVFAC, 2008):

  • Stable to decreasing concentration trends are observed in the plume,
  • Analytical results indicate that decreasing trends are statistically significant at a 95% confidence level,
  • Estimated dissolved mass of contaminants onsite, offsite, and within the entire plume continue to remain stable to decreasing, and
  • Groundwater chemistry data support geochemical conditions that are suitable for biodegradation and indicate whether active biodegradation has or is occurring.

Similarly, EPA (1998) outlined three lines of evidence that are required to provide sufficient affirmation that the contaminant at a site can be remediated via natural attenuation processes:

  1. Examine historical contaminant trends over time to determine if contaminant loss is occurring. This line of evidence corresponds to the first three bulleted points listed above which are explicit ways of documenting MNA trends at a site.
  2. Use hydrogeological and geochemical data to determine if the aquifer environment is amenable to natural attenuation for the COCs. This second line of evidence corresponds to the last bullet above which is to determine whether an aquifer’s redox condition is amenable to MNA currently as well as into the future.
  3. If the first two lines of evidence are inconclusive, then site-specific laboratory studies are recommended to determine if the contaminant can be remediated naturally. If this approach is selected, then a microcosm study is typically undertaken to assess the biodegradation potential.

For reference material, a variety of guidance documents have been produced by the Navy, EPA, and the Interstate Technology and Regulatory Council (ITRC). These documents cover a wide range of contaminants (e.g., petroleum hydrocarbon constituents and chlorinated solvents) and the natural processes that can reduce contaminant concentrations. The typical degradation pathways for petroleum hydrocarbon constituents include aerobic and anaerobic biodegradation, abiotic degradation, and pyrolysis. Specifics on the fate and transport of methyl-tert butyl ether (MTBE) have been addressed as well. For chlorinated solvents, biotic reductive dechlorination is the focus of numerous publications. However, pathways such as abiotic degradation, anaerobic and aerobic oxidation, and aerobic cometabolism should not be overlooked.

The MNA figure (top of page) provides a basic conceptual site model for a site including possible locations for monitoring wells. These monitoring wells are vital for establishing that MNA is an option at a site as well as documenting the progression of MNA if selected as a remedial strategy. Although only monitoring of COCs occurs at a MNA site, MNA should not be perceived as the same as "no action.” MNA can be described as a passive remedial approach that ensures protectiveness and uses the natural conditions (such as microorganisms) as a means for reducing contaminant concentrations. Moreover, this passive remedial approach should also be recognized as a green and sustainable remediation process in that additional energy inputs are not warranted or used. Based on the above discussion, advantages of MNA can be summarized to include:

  • Less waste generation,
  • Less intrusive,
  • Specific to locations at a site,
  • Works well in conjunction with other remediation technologies,
  • Typically overall lower cost.

As with all remedial actions, MNA is considered on a case-by-case basis. At sites where polychlorinated biphenyls (PCBs) are strongly sorbed to deep subsurface soils and are not migrating or where dense, nonaqueous-phase liquid (DNAPL) removal is technically impracticable, MNA is a viable remediation strategy. However, MNA is used seldom as the sole remediation strategy. Rather, active remedial strategies are employed often at select locations at a site (e.g., source zone) while MNA is selected to address the remaining dissolved phase plume. Active remedial strategies include technologies such as thermal treatment, in situ chemical oxidation, and in situ enhanced bioremediation (where the site is either biostimulated or bioaugmented). If MNA is to follow active treatment, it is important to recognize the effects the various active treatments can have on the natural attenuation capacity of the subsurface such as microorganism population and redox conditions.

During the consideration of MNA, modeling and evaluation of contaminant degradation rates and pathways should be undertaken. The primary objective of site modeling is to demonstrate that natural processes of contaminant degradation will reduce contaminant concentrations below regulatory standards before potential exposure pathways are completed. In addition, sampling and analysis must be conducted throughout the process to confirm that degradation is proceeding at rates consistent with meeting cleanup objectives. The Navy’s Natural Attenuation Software (NAS) can be used to assess the natural attenuation capacity of an aquifer and estimate the time of remediation depending on the amount of source reduction performed. The tool is meant to assist RPMs in decision making on the extent of source zone treatment required in conjunction with MNA using site-specific remediation objectives. This is a useful tool for the evaluation and design of treatment trains to optimize the overall remedy. This model has been validated with data from several Navy sites including NAWC Lakehurst, New Jersey, NAB Little Creek, Virginia, NAS Pensacola, Florida, and Naval Undersea Warfare Center Keyport, Washington. This software can be downloaded at Groundwater software models that can be used for MNA evaluations as well as fate and transport evaluations are located together on the EPA Web page “Center for Subsurface Modeling Support “ at



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MNA can be used to treat sites contaminated with both organics and inorganics. Organic compounds that may qualify for MNA include halogenated and nonhalogenated volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) and PCBs.  se of MNA at inorganic sites is possible if contaminants are sorbed to the soil or there is a change in redox state. Changes in the redox state of metals can cause them to become more mobile or even immobilize them. Therefore, the driving factors for redox conditions at the site must be understood. For this reason, use of MNA is limited at sites contaminated with inorganics.

For MNA to be applicable, there must be limited risk to human health and the environment and the site must be so governed that monitoring of contaminants in the groundwater can be done regularly. The land use in the area must also be controlled to prevent exposure to potential receptors.

In its technical protocol, EPA has outlined the site conditions at which MNA may be feasible for sites contaminated with chlorinated solvents. These include low redox/methanogenic conditions, presence of non-toxic daughter products at the site, chloride concentrations more than 2´ background and total organic carbon >20 mg/L.



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Cost ranges depend on the size of the site and the number of monitoring wells within the monitoring network. As a site progresses with MNA, the monitoring costs can be reduced through the optimization of the number of monitoring wells, sampling frequency, and number of analytes.

These costs assume that the site has been adequately characterized and a comprehensive conceptual site model exists.  Indirect costs, such as project management, design and engineering, vendor selection, administrative support, permit preparation and fees, regulatory interaction, site characterization, treatability testing, performance bond, and contingencies, are not included in the estimated cost.

Major cost items can be divided into three categories including pretreatment costs, fixed costs, and variable costs. These cost categories along with some of the more common factors that impact individual cost components are provided in the table below.

Typical Cost Components

Cost Dependencies




Site preparation (utility locating, debris removal, well installation, fencing)

Size of site; soil type; average well depth; number of wells; groundwater monitoring and ancillary well drill methods; relative complexity of well installation and periphery requirements (e.g., piping and manhole covers); location of site (urban or rural); existing structures; fence perimeter; required signage; personal protective equipment (PPE) level; equipment decontamination

Mobilization/demobilization (transportation to site, mobile office, storage, fabrication, assembly, setup, dismantle, well abandonment)

Site location and facility requirements

Monitoring wells

Monitoring requirements and number of monitoring wells; lithology; depth to contaminated interval

Labor (for transportation and on-site setup)

Location and size of site; facility requirements; equipment complexity; number and depths of monitoring wells; PPE level

OPERATING COST (Variable Cost)


Utility rates; site size; equipment size

Waste disposal

Total number of wells; sampling requirements and frequency

Labor (for monitoring between applications and after final application)

Number of applications; number of monitoring wells; monitoring requirements; PPE level





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MNA is only a viable option when cleanup goals can be achieved within a reasonable timeframe. Reasonable timeframes are very subjective and depend on the contaminant, site-specific conditions, and state laws. However, a reasonable timeframe is sometimes defined as the time required for contaminant levels to reach acceptable levels as long as risks to human health and the environment are considered.




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The following factors can limit the applicability and effectiveness of the process:

  • Extensive amounts of data must be collected and analyzed to determine plume behavior.
  • MNA typically is not applicable when the plume is expanding.
  • MNA should be used only where there are no impacts to potential receptors.
  • Contaminants may migrate before they are degraded.
  • The site may have to be fenced and may not be available for re-use until contaminant levels are reduced.
  • If source material exists, it may have to be removed.
  • Some inorganics (e.g., metals) can be immobilized by natural mechanisms but are not degraded.
  • Halogenated organics may be relatively resistant to natural degradation processes.
  • Intermediate degradation products (e.g., vinyl chloride) may be more mobile or more toxic than the original contaminant.
  • Public acceptance may be difficult.
  • Redox environments in the aquifer may change over time which can stop (biotic/organic) or reverse (abiotic/inorganic) MNA processes.
  • Long-term monitoring at the site is required.



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Topics Title and Description Pages Size (KB)



Remedial Action

Removal Action

DON Policy for Optimizing Remedial and Removal Actions at all Department of Navy (DON) Environmental Restoration Program Sites, NAVFAC 5090-Ser 12005/EV3-KB (2 April 2012)

This updated Policy clarifies when Optimization Reviews are necessary and provides effective remedial strategies to meet remedial action objectives. It also mandates the following actions be performed on DON sites: 1) Green and Sustainable Remediation (GSR) be incorporated into the optimization process, 2) Perform an Remedial Alternatives Analysis (RAA) to ensure sites have been effectively optimized, and 3) Ensure use of SiteWise™ tool in all GSR actions. Finally, in order to facilitate the documentation of effective monitoring efforts during the optimization process, this Policy also recommends the use of the Management and Monitoring Approach (MMA) for DON sites.

12 1013




DON Guidance for Planning and Optimizing Monitoring Strategies, NAVFAC ESC UG-2081-ENV Rev1, November 2010

This guidance document provides Navy RPMs comprehensive information for optimizing monitoring programs at remediation sites. Part I of the document discusses key concepts such as conceptual site model, data quality objectives, monitoring program goals, and regulatory framework. This part also discusses approaches to optimize monitoring locations, monitoring frequency, list of analytes, data analysis, and reporting. Part II contains specific information on optimizing monitoring strategies for various media and site types including groundwater, sediments, groundwater discharge to surface water, ecological resources, vadose zone, landfills, and land use controls. This guidance document has been updated to include the main elements in designing and optimizing monitoring programs for vapor intrusion sites.

250 3911


Optimizing Remedy


DON Guidance for Optimizing Remedy Evaluation, Selections, and Design, NAVFAC ESC UG-2087-ENV (9 Mar 2010)

This document provides a general overview and explanation of key optimization concepts as they pertain to the Feasibility Study (FS), Record of Decision (ROD), and Remedial Design (RD) clean-up phases. This document is not intended to provide guidance on determining site-specific risk-based clean-up goals, performing risk assessments, conducting site assessments and background investigations, or other site-specific contaminant characterization activities for which United States Environmental Protection Agency (EPA) and Navy guidance already exists. Rather, this document complements these important components of the site remediation process by providing recommendations for optimizing remedy selection and design.

84 3193



Remedial Action Operation

NAVFAC Guidance for Optimizing Remedial Action Operation, UG-NAVFAC EXWC-EV-1301 (1 October 2012)

Discussion of the six general steps for optimizing Remedial Action Operations (RAO) as well as technology-specific optimization considerations. Optimization of RA-O programs is an important process which helps to ensure maximum remedial effectiveness, minimum negative environmental and societal impacts, and improved cost efficiency of a remedy. Optimization is an ongoing responsibility of Navy/Marine Corps RPMs and their contractors who operate, maintain, and monitor remediation systems. The goal of optimization is to achieve RC and ultimately site closeout in the shortest amount of time and with the least possible remedy footprint and expenditure.

94 2554
Groundwater Risk Management

Groundwater Risk Management Handbook, NAVFAC (25 January 2008)

This hanbook provides an overview of groundwater risk management strategies that can be used to support optimization concepts addressed in the Guidance for Optimizing Remedy Evaluation, Selection, and Design (NAVFAC 2004, updated 2010).

30 1253
DNAPL Management

DNAPL Management Overview Handbook, NAVFAC (April 2007)

The Dense Non-aqueous Phase Liquid (DNAPL) Management Overview provides an introduction on how to manage DNAPL contamination at a site.

17 993

DNAPL Source Zone


Assessing the Feasibility of DNAPL Source Zone Remediation: Review of Case Studies NAVFAC ESC CR-04-002-ENV (May, 2004)

This report discusses developing guidelines for selecting DNAPL source remediation guidelines by reviewing various DNAPL treatment technologies that have been field-deployed.

293 5823

Monitored Naturatl Attenuation

Petroleum Hydrocarbons

Chlorinated Solvents

Technical Guidelines for Evaluating Monitored Natural Attenuation of Petroleum Hydrocarbons and Chlorintated Solvents in Groundwater at Naval and Marine Corps Facilities, Department of the Navy (September, 1998)

Guidance for Navy RPMs on how to assess natural attenuation as a remedial strategy for groundwater systems contaminated with petroleum hydrocarbons and/or chlorinated solvents.

92 2732

Natural Attenuation


API Technical Protocol for Evaluating the Natural Attenuation of MTBE (2007)

This report discusses the natural attenuation mechanisms of MTBE.

186 2952

Natural Attenuation


An Approach for Evaluating the Progress of Natural Attenuation in Groundwater, EPA 600-R-11-204 (December 2011)

Monitoring over time insures that the future behavior of the plume is consistent with past behavior and that the risk of exposure to the contaminants is managed. The trend of contaminant concentrations over time in a particular well can be used to forecast the future concentrations in that well and predict when concentrations will attain a selected concentration level. The purpose of this document is to present a simple, statistically based approach for evaluating the progress of natural attenuation from the data collected during site characterization and long-term monitoring. The intended audience is technical professionals that actually perform the data analyses (i.e., hydrogeologists, engineers) as well as project managers who review those analyses and/or make decisions based on those analyses.

84 1152



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Related Sites

Topics Title and Description Pages Size (KB)

Monitored Natural Attenuation


EPA – CLU-IN Technology Focus Web Site on MNA Guidance

Natural attenuation relies on natural processes to clean up or attenuate pollution in soil and groundwater. Natural attenuation occurs at most polluted sites. However, the right conditions must exist underground to clean sites properly. If not, cleanup will not be quick enough or complete enough. Scientists monitor or test these conditions to make sure natural attenuation is working. This is called monitored natural attenuation or MNA.

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