FEATURE
Intrinsic Remediation Under the Microscope Concerns about the use of "natural attenuation" have led to a major review of the method's effectiveness. REBECCA
mid widespread concern that the growing use of intrinsic remediation, or "natural attenuation," is outpacing the development of adequate guidelines for its use, the National Research Council (NRC) launched a two-year study of the cleanup approach. Intrinsic remediation, which relies on natural subsurface processes rather than engineered processes to eliminate contaminants in groundwater or soil, recently spawned several protocols from industry and government agencies. However, "major differences" in the specifications of these protocols for crucial activities such as site characterization and monitoring have, in part, prompted the NRC evaluation, said study director Jackie MacDonald. There are more than a half-dozen protocols for using intrinsic remediation to clean up contaminated groundwater, and more are in development. "There are widespread concerns, both inside and outside the NRC committee, about how intrinsic remediation is being used," said Committee Chair Bruce Rittmann of Northwestern University, Evanston, 111. The NRC Intrinsic Remediation Committee includes experts in intrinsic remediation, such as Stanford University's Perry McCarty, EPA's John Wilson, and DuPont's David Ellis. Specialists in hydrology, microbiology, environmental engineering, and environmental health are also on the committee. The widespread interest in intrinsic remediation is reflected by the study's sponsors: EPA, U.S. Army and Navy, Nuclear Regulatory Commission, Chemical Manufacturers Association (CMA), American Petroleum Institute (API), and Oxygenated Fuels Association. Rittmann said that the concerns stem from several observations. Research scientists are alarmed by some of the simplifications used in the field application of intrinsic remediation, such as the use of predictive mathematical models failing to treat bacte-
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ria as living organisms and monitoring strategies failing to measure the appropriate geochemical indicators. These concerns are amplified by significant differences among emerging protocols in terms of the evidence required to determine whether intrinsic remediation is appropriate and effective. "There is a public perception that this is a donothing approach," said David Mentall, explaining CMA's sponsorship. "We believe that the NRC study is necessary to give some validation of intrinsic remediation as a real way to do site cleanup." Many organizations have developed protocols for using natural attenuation in the cleanup of contaminated sites. These documents are often used by responsible parties or regulators to plan, implement, justify, or evaluate using natural attenuation at a contaminated site. A cooperative effort between EPA and the U.S. Air Force led to the publication of a detailed protocol providing guidance about using intrinsic remediation to clean up petroleum fuel spills (i). "The protocol was very successful with the responsible party community," said Wilson, an EPA microbiologist involved with the guidance. "In fact, they started to use the fuels protocol to describe chlorinated solvents." More protocols being developed The Air Force and EPA are currently working on a protocol for the intrinsic remediation of chlorinated solvents. API is developing a guidance manual about field methods for measuring natural processes that occur in intrinsic remediation, and Mobil Oil Corp. and Chevron are among the companies developing protocols for using intrinsic bioremediation at their own sites. The American Society for Testing and Materials (ASTM), which has developed guidance for risk-based corrective action at underground fuel tank sites, is also working on a protocol for intrinsic re0013-936X/98/0932-180A$15.00/0 © 1998 American Chemical Society
A broad study of natural attentuation processes supported by the Bioremediation Consortium of the Remediation Technologies Development Forum is under way at Dover Air Force Base in Delaware. A sampling crew collects readings of dissolved oxygen from a monitoring well at a contamination site. (Courtesy Robert Baird, DuPont)
mediation. EPA produced its first major document about intrinsic remediation in December 1997 when the Office of Solid Waste and Emergency Response issued a directive clarifying its policy on the use of natural attenuation. EPA's Office of Research and Development is working on technical guidance (2). Although intrinsic remediation does not require construction of engineering systems, it does require initial studies of site hydrology, geochemistry, and microbiology as well as subsequent monitoring. "People need to realize that there is no donothing approach. Intrinsic remediation relies on natural processes, but these have to be monitored. Investment shifts from engineering intervention to monitoring, before and during," stressed Rittmann. The use of natural attenuation as a remediation strategy has increased exponentially over the past few years (3). Several different events have led to this rise. Over the past 15 years, although the flaws in conventional pump-and-treat technology were becoming apparent, scientists were also developing a quantitative understanding of the fate of chemical contaminants in the subsurface. The realization that natural processes can and do degrade contaminants was part of this new understanding. Furthermore, its most significant and practical manifestation is the now-widespread acceptance that petroleum hydrocarbons, particularly the BTEX compounds (benzene, toluene, ethylene, and xylene), biodegrade in almost all groundwater systems (4). As a result, most states now routinely accept intrinsic remediation as a remedy for such contamination, said microbiologist Francis Chapelle of the U.S. Geological Survey in Columbia, S.C. The importance of intrinsic remediation for underground storage tanks is clearly demonstrated in figures from 1995. Intrinsic remediation was being used to clean up contaminated groundwater at just under half (48%) of the 19,200 underground storage tank sites surveyed and at 28% of the 103,000 sites where soil cleanup was under way (5). Beyond petroleum hydrocarbons There are signs that intrinsic remediation is poised to make the jump to other types of cleanups. Intrin-
sic remediation may, through sorption or oxidationreduction reactions, reduce the groundwater concentrations of inorganic contaminants such as metals and radionuclides. However, the real growth in its use is with chlorinated solvents. The method can have impressive results. Ellis cited one case in which there was a 99% decline in source-area contamination in five years. Although intrinsic remediation does not figure as a treatment technique at many Superfund sites—it has been selected as a cleanup method at 73 groundwater contaminated sites but is the sole treatment option at only 6—its use has been much studied at a Superfund site in St. Joseph, Mich. Studies by Wilson and co-workers at EPA's R. S. Kerr Laboratory in Ada, Okla., have demonstrated that microorganisms are effectively removing trichloroethylene (TCE) and other chlorinated solvents from groundwater as it travels about half a mile from the site to where it enters Lake Michigan. At the site, TCE concentrations >200,000 micrograms per liter (pg/L) were measured, but at the shore of Lake Michigan, the concentrations were about 1000 times less. About 100 meters (m) offshore in the lake, the concentrations were below EPA's allowable levels. Wilson and coworkers estimate that the plume took about 20 years to move from the source of the contamination to Lake Michigan (6, 7). State regulators report that cleanup proposals involving intrinsic remediation for sites contaminated by chlorinated solvents are on the rise. However, said Ellis, they are concerned that many of these proposals fail to present sufficient evidence, for example, the appropriate hydrological and geochemical data, to evaluate whether intrinsic remediation is an appropriate technique. Ellis, whose biotechnology group at DuPont in Wilmington, Del., has made several discoveries about the biodegradation of chlorinated solvents, gets feedback from regulators when he teaches a course on biodegradation of chlorinated solvents for the Interstate Technology and Regulatory Cooperation Work Group, a coalition of state and federal environmental regulatory agencies (8). "There is a bandwagon effect," he said. "I fear that if intrinsic remediation is used at sites where it APRIL 1, 1998/ENVIRONMENTAL SCIENCE & TECHNOLOGY / N E W S " 1 8 1 A
is not appropriate, there could be unfortunate consequences." Bruce Bauman, a spokesman for API, agrees that interest in intrinsic remediation is high. "Intrinsic remediation is hot. This is one of the most important topics in remediation today," he said. Chlorinated solvents, widely used for degreasing equipment and clothing, represent the most prevalent organic groundwater contaminants in the United States and are major contaminant problems in Canada and Europe (3). In fact, TCE is present at more than half of all Superfund sites. Pump-and-treat, the conventional technology to control the movement of TCE in contaminated groundwater, requires costly disturbance of cleanup sites and fails to completely remove pollutants. Wilson, one of the most experienced scientists studying intrinsic remediation of chlorinated solvents, estimates that at about two-thirds of TCE-contaminated sites, bacterial breakdown is occurring at an environmentally significant rate. However, these processes are likely to be protective of human health and the environment at only one-fifth, he said. "Intrinsic remediation requires time and space, so sites need to be far enough away from a receptor that could be harmed by the contamination." Applying natural attenuation to chlorinated solvent sites is a more complicated task than applying it to hydrocarbons, in part because the biological processes are more complicated, explained Ellis. Petroleum hydrocarbons act as electron donors, an energy source for bacteria. However, the most important biological process for degrading chlorinated solvents is completely different.
Research active on several key issues Under anaerobic conditions, microorganisms use chlorinated solvents as electron acceptors: They breathe the solvents just as aerobic organisms use oxygen. This reductive dechlorination process results in the breakdown of the solvents, one chlorine atom at a time. One of the practical impacts of this difference is that, in addition to favorable subsurface chemical conditions, there must also be a sufficient food source. These are just some of the complicating factors in this area where research is very active. "We still don't know such basic facts as what bacteria are involved, how do they do what they do, and how much solvent can be destroyed at a site," said Ellis. Organisms that carry out direct respiratory dechlorination of chlorinated solvents were first discovered in 1993, and the biochemistry of the reactions is still an active research topic (3). In addition, mathematical fate-and-transport models are generally not adequate to capture all of the possible processes involved in intrinsic remediation. Despite gaps in the scientific understanding of biodegradation, most bioremediation scientists accept that there are three lines of evidence necessary to establish that a site is suitable for intrinsic remediation. There must be documented loss of the contaminant at the site, geochemical data indicating that subsurface conditions favor biodegradation, and lab-
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oratory studies demonstrating that bacteria indigenous to the site are capable of degrading the contaminants. Some of the existing protocols, however, do not require all of these lines of evidence, which were first proposed in a 1993 NRC report, according to MacDonald (9). For example, ASTM is developing a formal standard, known as the Guide for Remediation by Natural Attenuation at Petroleum Release Sites, that enumerates procedures for using intrinsic remediation to manage underground spills of petroleum products, mainly gasoline. But this standard requires only one of the three types of evidence: documented loss of contaminants from the site. Under the ASTM standard, the other two types of evidence are optional. One of the tasks for the NRC committee will be to closely examine the existing protocols, explained Rittmann. In addition, the committee plans to assess current knowledge about the subsurface processes that play critical roles in intrinsic remediation. Using a combination of case studies and surveys, the committee plans to delineate what can and cannot be achieved through intrinsic remediation. It will also assess the risks associated with leaving contamination in place and identify the measurements, observations, and monitoring needed when intrinsic remediation is chosen as a remedy.
References (1) Weidemeyer, T. H. et al. Technical Protocol for Implementing the Intrinsic Remediation with Long-Term Monitoring Option for Natural Attenuation ofDissolved-Phase Fuel Contamination in Groundwater, U.S. Air Force Center for Environmental Excellence, Brooks Air Force Base: San Antonio, TX, 1995. (2) Use ofMonitored Natural Attentuation at Superfund, RCRA Corrective Action and Underground Storage Tank Sites; U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response Directive 9200.4-17 (http:// www.epa.gov/swerustl/directiv/d9200417.htm). (3) Proceedings of the Symposium on Natural Attenuation of Chlorinated Organics in Ground Water, EPA/540/R-97/ 504; U.S. Environmental Protection Agency, Office of Research and Development, U.S. Government Printing Office: Washington, DC, 1997. (4) Hinchee, R. E.; Kittel, J. A.; Reisinger, H. J. Applied Bioremediation of Petroleum Hydrocarbons; Batelle Press: Columbus, OH, 1995. (5) National Research Council. Innovations in Ground Water and Soil Cleanup: From Concept to Commercialization; National Academy Press: Washington, DC, 1997. (6) Wilson, J. X; Weaver, J. W.; Kampbell, D. H. Intrinsic Bioremediation of TCE in Groundwater at an NPL Site in St. loseph, Michigan. In U.S. EPA Symposium on Intrinsic Bioremediation of Ground Water, EPA/540/R-94/515; U.S. Environmental Protection Agency, U.S Government Printing Office: Washington, DC, 1994. (7) A Citizen's Guide to Natural Attenuation; EPA 542-F-96015; U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, U.S. Government Printing Office: Washington, DC, 1996. (8) Lecture and Workshop in Natural Attentuation of Chlorinated Solvents in Ground Water, Pennsylvania Department of Environmental Protection, Interstate Technology and Regulatory Cooperation Working Group (http:// www.biotreat.state.pa.us). (9) National Research Council. In Situ Bioremediation: When Does It Work?; National Academy Press: Washington, DC, 1993. Rebecca Rentier is a contributing editor o/ES&T.
