Peer Review Of EPA Natural Attenuation Research Program
 
October 11, 2000
 
Rory C. Ryan- Orlando

The unenhanced use of natural processes for cleanup of site contamination is called “natural attenuation,” and it is commonly considered a remedy at waste sites. A peer review of the U.S. EPA’s Office of Research and Development Natural Attenuation Research Program is underway. The peer review subcommittee is reviewing the EPA’s research program for monitored natural attenuation of contaminants in groundwater, soils, and sediments by evaluating the research in terms of its scientific quality as well as the effectiveness and utility for exercising regulatory judgment concerning the use of monitored natural attenuation as a site specific remedy. The final report of the subcommittee is expected soon. The peer advisors so far have made interesting, preliminary findings.

First, the existing EPA Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater provides only a framework for regulatory decision making which, “. . . should not and cannot be credibly employed as a generic protocol.” The subcommittee also determined that the EPA should develop a similar framework for natural attenuation for inorganics and in sediments. The subcommittee felt the emphasis of these frameworks should be mathematical models to extrapolate the results from the lab to the field as well as incorporating uncertainty into the predictions.

Concerning the Protocol, the subcommittee found that there were major uncertainties remaining as to the use of natural attenuation as a remedy at a chlorinated solvent site. Specifically, they found that (i) biodegradation rates for chlorinated solvents can be estimated reliably only if site-specific field data are used and (ii) the role of DNAPLs and the source strength in limiting the attenuation of chlorinated solvent plumes may be significant, but is not yet very well understood. The subcommittee recommended the EPA broaden the focus of its research on natural attenuation of chlorinated solvents in order to have the scientific fundamentals in place and a more complete gathering of relevant field measurements.

Concerning natural attenuation of fuel hydrocarbons, the subcommittee recognized that this is an accepted remediation approach to certain underground storage tank sites in the United States. However, it did recognize that too little is known about the fate and transport of oxygenates, such as MTBE, to evaluate whether monitored natural attenuation is an acceptable remedy. The subcommittee recommended that the regulatory agency develop a better understanding of MTBE and other oxygenates, which may be replacing MTBE in the near future, because these constituents tend to behave differently than the traditional BTEX.

Concerning monitored natural attenuation of metals in soils and groundwater, the subcommittee determined that existing protocols, such as for chlorinated solvents, are not directly applicable to inorganics and that there is an urgent need to develop a framework addressing natural attenuation for inorganics. Among the preliminary recommendations of the subcommittee include that DEP develop a flexible framework rather than a single protocol that’s applicable for metals.

As part of their charge, the subcommittee reviewed the present state of monitored natural attenuation at EPA, and found that contaminated sediments had received much less attention than contaminated soil and groundwater and that the existing protocols developed and tested for contaminated soil and groundwater cannot be applied to contaminated sediments without further work. The subcommittee recommended the EPA collaborate with other agencies responsible for managing contaminated sediments to develop the necessary research programs and a technical protocol for contaminated sediments.

For additional reading, see the Committee on Intrinsic Remediation of the National Research Council’s recent book, Natural Attenuation for Groundwater Remediation. The subcommittee made use of the National Research Council’s Book during its peer review. Also, the subcommittee’s report will be posted on the Science Advisory Board’s Web site (www.epa.gov.sab).