Environ. Sci. Technol. 2001, 35, 4038-4045
Assessment of Natural Attenuation of Chlorinated Aliphatics and BTEX in Subarctic Groundwater S H A R O N A . R I C H M O N D , * ,†,‡ J O N E . L I N D S T R O M , †,§ A N D JOAN F. BRADDOCK† Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99775, and U.S. Geological Survey, 1150 University Avenue, Fairbanks, Alaska 99709, and Shannon & Wilson, Inc., 2055 Hill Road, Fairbanks, Alaska 99709
We examined biogeochemistry and microbiology associated with natural attenuation of trichloroethene (TCE), trichloroethane (TCA), and benzene in a subarctic aquifer. Identification of a predominant terminal electron-accepting process (TEAP) and characterization of typical natural attenuation footprints was difficult. Hydrogen and ferrous iron concentrations suggested that iron reduction was the predominant TEAP; calculated in situ Gibbs free energies for iron reduction were energetically feasible at all wells although a source of ferric iron has not been conclusively determined. The presence of dissolved sulfide and favorable free energies for sulfate reduction provided support of concurrent iron and sulfate reduction. Methanogenesis from H2/CO2 was generally not energetically favorable. The presence of TCE and TCA degradation intermediates suggested that biological reductive dechlorination occurred, although proportions of intermediates relative to parent compounds remained stable. By September 2000, contaminant concentrations were within regulatory standards at most sampling points. However, low rates of microbial activity and incomplete degradation imply that intrinsic bioremediation did not likely represent an important contribution to contaminant removal at this site, where dilution appeared to be the primary attenuation mechanism.
Introduction Monitored natural attenuation has become an increasingly attractive treatment strategy for remediation of contaminated groundwater and soils because it is generally less expensive and, in some cases, more practical than engineered cleanup solutions. As of 1997, this approach had been implemented at tens of thousands of contaminated sites in the temperate United States (1), either as the only treatment strategy or as a supplement to engineered solutions. It is currently being assessed for its applicability in cold climates. As of 2001, the Alaska Department of Environmental Conservation (ADEC) had documented 867 contaminated groundwater sites within the state; of those, 717 require remediation (W. Petrik, ADEC, personal communication, 2001). This study examined biogeochemistry and microbiology associated with natural attenuation of chlorinated aliphatics † ‡ §
University of Alaska. U.S. Geological Survey. Shannon & Wilson, Inc.
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at a subarctic site near Fairbanks, AK. The site is characterized by permanently cold groundwater co-contaminated with benzene and other aromatics. At the beginning of this study, concentrations of benzene and trichloroethene (TCE), although low, were well above drinking water standards, and implementation of some treatment strategy was necessary. The extraordinarily large volume of water requiring treatment (an estimated 1 million m3) coupled with low contaminant concentrations rendered any engineered solution prohibitively expensive; therefore, a natural attenuation feasibility study was undertaken. Currently accepted protocols for assessing the feasibility of intrinsic bioremediation have been developed using data primarily collected from highly contaminated sites in temperate climates; their applicability to cold or very dilute systems is unclear. This paper addresses some of the difficulties encountered when assessing the feasibility of natural attenuation of chlorinated aliphatics in a dilute subarctic aquifer. The combined effects of aquifer geochemistry, low contaminant concentrations, low levels of microbial activity, and very cold temperatures made it unlikely that intrinsic bioremediation was a significant mechanism of contaminant removal at this site.
Materials and Methods Site Description. Six Mile Village, near Fairbanks, AK, is located about 3 km north of the Tanana River within the Tanana-Chena flood plain (Figure 1). Alluvial sediments are primarily sand and gravel from glacial outwash from the Alaska Range (2). In the study area, depth to bedrock has not been conclusively determined, although in a nearby area a sediment thickness of ca. 180 m was reported (2). Discontinuous permafrost is present throughout the region (3), and permafrost lenses were encountered during drilling at this site. However, where permafrost is absent, the aquifer is considered unconfined. The alluvial plain is flat, and the regional hydraulic gradient is 1.23 mm/m, although this varies locally and seasonally (4). Depth to groundwater is between
