Natural Attenuation of Groundwater

What is Natural Attenuation?

"Natural attenuation" refers to the ability of a ground water system to rid itself of contamination resulting from a spill or improper disposal of wastes. It is emerging as a viable -- and in some cases, the preferred -- remedy for contaminated ground water.

The basic concept of natural attenuation is not new. We depend on it, for example, in municipal wastewater permits that allow treatment plants to discharge a certain amount of wastes without unduly depleting oxygen in surface water. Another example is the use of drainfields in septic systems.

Bacteria that naturally inhabit many ground water environments are able to break down chemicals once thought to be virtually non-biodegradable. For example, components of gasoline such as benzene, toluene, xylene, and ethylbenzene are now known to biodegrade in ground water to carbon dioxide and water. Other contaminants, including chlorinated solvents (e.g., dry-cleaning solvents) can also biodegrade under particular conditions. In some cases, natural biodegradation may break down contaminants in ground water faster than they can be removed by engineered systems.

Natural attenuation is sometimes a preferred remedy because it does not transfer pollutants from one location to another. Rather they are broken down in place, converted usually to non-toxic end products. However, natural attenuation is not always a completely effective remedy by itself. In cases where the contamination is spreading more quickly than it can break down, where drinking-water wells are threatened by contamination, or when toxic breakdown products occur, engineered systems are needed so that exposure to contaminants is eliminated.

Background

In 1976, the National Organics Monitoring Study presented the first evidence that ground water in the United States was contaminated with chlorinated solvents. By 1980, numerous other local studies showed that ground water contamination was widespread. In 1981, the Council on Environmental Quality published the Contamination of Ground Water by Organic Chemicals and reported "serious contamination of drinking water wells in 34 states."

In response, enormous efforts to remediate contaminated ground water were made at RCRA and Superfund sites, employing systems designed to physically remove contaminated ground water from the aquifer, followed by treatment to destroy or remove the contaminants, with the goal of restoring the ground water systems to cleanup standards. In many cases, these efforts were unsuccessful. In 1994, a National Research Council Committee on Ground Water Cleanup Alternatives evaluated 77 sites where "pump and treat" systems were used and reported that cleanup goals had been achieved at only eight of them. Other studies confirmed that pump-and-treat remediation systems have serious performance limitations. The primary reason for their failure was the presence of non-aqueous phase liquids (NAPL) at sites with contaminated ground water. NAPL is a long-term source of contamination to aquifers and cannot be removed from the subsurface by pumping ground water.

However, laboratory research has revealed that contaminants previously regarded as non-biodegradable, such as trichloroethylene, benzene or PCB are, in fact, biodegradable under conditions that commonly exist in the ground water environment. Although the rates of biological degradation are often low, contaminants can be destroyed within the soil and ground water, giving the environment some degree of "assimilitive capacity" for pesticides, chlorinated solvents, and petroleum-related chemicals.

This is supported by two studies of over 500 BTEX plumes in California and Texas that showed 75% of the plumes analyzed stabilize within about 250 feet of the source of contamination. Approximately 80% of the contaminant plumes appeared to be either stable or shrinking while 8% appeared to be expanding. A similar national study of 88 sites with chlorinated solvents suggested the median length of chlorinated solvent plumes is approximately 1,200 feet.

Evidence that natural attenuation can function at least as a partial remedy notwithstanding, the public may perceive natural attenuation as a "do nothing" response to ground water contamination. This may be due to the understandable desire to "clean up" the affected ground water as soon as possible. In addition, the public may perceive that a natural attenuation approach to ground water contamination seems to allow responsible parties to simply "walk away" from contamination problems they caused. Finally, where natural attenuation is functioning as a remedy, nothing appears to be "happening" at the site. In response to these concerns, the Committee on Intrinsic Bioremediation convened by the National Research Council in 2000 concluded that the public should be included very early in any discussions of possible natural attenuation remedies. Early involvement on the part of the public can prevent misconceptions that may degenerate into a deep mistrust of the regulatory staff and responsible party or parties addressing the site.

Although natural attenuation is not a highly "visible" approach to dealing with ground water contamination, the up-front work is more demanding. For chlorinated solvent sites, the extensive sampling and analysis involved in evaluating the feasibility of natural attenuation and demonstrating that it can work is very expensive. Chlorinated solvents biologically degrade under particular environmental conditions, and demonstrating that these conditions are present at a site is important to evaluating natural attenuation as a potential remedy. Trends in contaminant concentrations require multiple sampling events over time from monitoring wells at the site, including background wells to establish ambient ground water conditions. Evaluation of risk often involves the use of fate and transport models that may require more data than is expected for sites with engineered remedies. Measurements of degradation rate constants for the contaminant versus its migration rate in ground water are needed prior to such modeling efforts. Thus, when natural attenuation is explored as a remedy, work that normally would be invested in an engineered remedy is instead focused on site evaluation, analysis of contaminant fate, and long-term ground water monitoring to verify that predictions of efficacy are accurate.

Strategies to remediate ground water are still evolving. However, remediation approaches now often reflect the understanding that contaminants in ground water systems can naturally attenuate in timeframes that are competitive with other approaches to ground water remediation. On a national level, EPA's Monitored Natural Attenuation Policy (1) indicates that natural attenuation is a valid alternative to engineered remediation efforts for ground-water contamination, provided that the risk to receptors is eliminated and the time to ground water cleanup is reasonable. In addition, EPA has published detailed technical guidance on evaluating natural attenuation remedies (2, 3). Other resources are available from the American Society for Testing and Materials (ASTM) (4), and the Interstate Technology and Regulatory Cooperation Work Group (ITRC) (5).

Two documents describe Minnesota's policy on natural attenuation for ground water: Assessment of Natural Attenuation at Petroleum Release Sites and Natural Attenuation of Chlorinated Solvents in Ground Water. These documents contain concise technical guidance on current methods for the evaluation of natural attenuation, including sampling techniques, sample analysis, and modeling at cleanup sites in Minnesota.

References:

1. U. S. Environmental Protection Agency. 1999. Final OSWER Monitored Natural Attenuation Policy (OSWER Directive 9200.4-17P). United States Environmental Protection Agency, Office of Solid Waste and Emergency Response.
   
   
2. U. S. Environmental Protection Agency. 1998. Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water. United States Environmental Protection Agency, National Risk Management Research Laboratory EPA/600/R-98/128. September, 1998.
   
   
3. U. S. Environmental Protection Agency. 1998. Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. Prepared for the Air Force Center for Environmental Excellence, Technology Transfer Division, San Antonio, Texas.
   
   
4. American Society for Testing and Materials. 1998. Standard Guide for Remediation of Ground Water by Natural Attenuation at Petroleum Release Sites. ASTM E-1943-98. pp. 875-916.
   
   
5. Interstate Technology and Regulatory Cooperation Work Group, Industrial Members of the Remediation Technologies Development Forum. Natural Attenuation of Chlorinated Solvents in Groundwater: Principles and Practices. May, 1999. 66 pages.

Publications

Fact Sheets

• Monitored Natural Attenuation of Chlorinated Solvents - U.S. Environmental Protection Agency Remedial Technology Fact Sheet (Kerr site).
  
  
• Monitored Natural Attenuation of Petroleum Hydrocarbons U.S. Environmental Protection Agency Remedial Technology Fact Sheet (Kerr site).

Reports

• Guidelines for Natural Attenuation of Chlorinated Solvents in Ground Water - Report prepared by the MPCA
  Site Remediation Section, 1999. This document provides guidance on the selection of natural attenuation as a remedy for chlorinated solvents in ground water at sites regulated under the Minnesota Environmental Response and Liability Act (MERLA), including the Minnesota Superfund and Voluntary Investigation and Cleanup (VIC) Programs. It is part of the Risk-Based Site Evaluation Manual that outlines the process for making decisions at sites on the basis of risk to human health and the environment.

  The MPCA Guidelines for Natural Attenuation of Chlorinated Solvents in Ground Water is consistent with the approach recommended by EPA: a phased process of site evaluation that begins with a screening of pertinent site data followed by detailed measurements of ground water flow, biodegradation rate, and sorption. These measurements are then combined in an evaluation of risk that quantifies the contribution of natural attenuation to ground water remediation. The Guidelines contains a summary of sampling requirements and analytical methods used in a natural attenuation investigation, as well as a workplan "checklist" that summarizes MPCA requirements for screening, verification, and implementation stages of a natural attenuation remedy. It also lists numerous references and provides links to other sources of information on natural attenuation.

• Assessment of Natural Attenuation at Petroleum Release Sites - a Fact Sheet prepared by the Leaking Underground Storage Program of the Minnesota Pollution Control Agency, April 2000.

Case Studies

• Evaluation of Natural Attenuation of Chlorinated Solvents in Ground Water at the Twin Cities Army Ammunition Plant – Site A. MPCA, June 2000. The goal of this study was to assess how natural attenuation has influenced the development of the ground water plume at Site A and its role in the eventual restoration of the aquifer.
• Evaluation of the Protocol for Natural Attenuation of Chlorinated Solvents: Case Study at the Twin Cities Army Ammunition Plant

Related Links

Natural Attentuation Modeling

• Modeling Subsurface Transport of Petroleum Hydrocarbons. U.S. EPA's Online Assessment Tool.
  
  
• BIOSCREEN Computer Tool. U.S. EPA Center for Subsurface Modeling Support. BIOSCREEN is a screening model that simulates remediation through natural attenuation of dissolved hydrocarbons at petroleum fuel release sites.
  
  
• BioChlor Computer Tool. U.S. EPA Center for Subsurface Modeling Support. BIOCHLOR is a screening model that simulates remediation by natural attenuation of dissolved solvents at chlorinated solvent release sites. BIOCHLOR can be used to simulate solute transport without decay and solute transport with biodegredation modeled as a sequential first-order process within one or two different reaction zones.
  
  
• Metropolitan Ground Water Model - the Metro Model. Minnesota Pollution Control Agency. A regional groundwater flow model encompassing the Twin Cities, seven-county Metropolitan area to be used as a groundwater management tool. The Metro Model provides regional flow conditions that can be quickly and easily modified to provide information on local sites.

Other Links

• Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural Attenuation of Fuel Contamination Dissolved in Groundwater. Wiedemeier, et al. 1998. Prepared for the Air Force Center for Environmental Excellence, Technology Transfer Division, San Antonio, Texas.
  
  
• National Research Council Committee on Intrinsic Bioremediation Report.
  
  
• Natural Attenuation of Chlorinated Solvents in Groundwater: Principles and Practices. Interstate Technology and Regulatory Cooperation Work Group, Industrial Members of the Remediation Technologies Development Forum.
  
  
• Evaluating Ground Water and Surface Water Interaction at Remediation Project Sites

More Information

• For more information about natural attenuation, contact Mark Ferrey at 651-757-2355, mark.ferrey@state.mn.us.
• For information regarding sampling supplies and procedures, please contact Paul Estuesta at 651-757-2165, paul.estuesta@state.mn.us