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A series of carbon-14 (14C) radiotracer-based microcosm experiments was conducted to assess the mechanisms and products of degradation of cis-dichloroethene (cis-DCE) and vinyl chloride (VC) in streambed sediments at the U.S. Department of Energy, Kansas City Plant in Kansas City, Missouri. The focus of the investigation was the potential for biotic and abiotic cis-DCE and VC degradation in surficial and underlying hyporheic sediment from the Blue River and its tributaries, Indian Creek and Boone Creek. Substantial degradation of [1,2-14C] cis-DCE and [1,2-14C] VC to 14C-carbon dioxide (CO2) was observed in all viable surficial sediment microcosms prepared under oxic conditions. No significant accumulation of reductive dechlorination products was observed under these oxic incubation conditions. The results indicate that microbial mineralization processes involving direct oxidation or co-metabolic oxidation are the primary mechanisms of cis-DCE and VC biodegradation in oxic stream sediment at the Kansas City Plant. Substantial mineralization of [1,2-14C] VC also was observed in all viable surficial sediment microcosms incubated in the absence of detectable oxygen (dissolved oxygen concentrations less than 25 micrograms per liter). In general, the accumulation of mineralization products (CO2 and 14C-methane [CH4]) predominated with only trace-level detection of the reductive dechlorination product, 14C-ethene. In contrast, microbial degradation of [1,2-14C] cis-DCE by reductive dechlorination or mineralization was not significant in the absence of detectable oxygen. The potential for [1,2-14C] VC biodegradation also was significant in sediments from the deeper hyporheic zones under oxic conditions and in the absence of detectable oxygen. In this study, microbial degradation of [1,2-14C] cis-DCE was not significant in hyporheic sediment treatments under either oxygen condition. Taken together, the results indicate that microbial mineralization processes in streambed sediments at the Kansas City Plant can be an important component of cis-DCE and VC degradation under oxic conditions and of VC degradation even in the absence of detectable oxygen. These results demonstrate that an evaluation of the efficiency of in situ cis-DCE and VC biodegradation in streambed sediments, based solely on observed accumulations of reduced daughter products, may underestimate substantially the total extent of contaminant biodegradation and, thus, the potential importance of the hyporheic zone and streambed sediments as barriers to the discharge of contaminated groundwater. Introduction Mixed chlorinated solvent plumes derived from multiple historical releases characterize the shallow groundwater system at the U.S. Department of Energy (DOE), Kansas City Plant (KCP; figs. 1 and 2). The primary contaminants in these plumes are trichloroethene (TCE) and the reductive degradation products, 1,2-dichloroethene (1,2-DCE) and vinyl chloride (VC). The general stratigraphy of the contaminated shallow aquifer is characterized by surface fill underlain by an extensive silty-clay layer and then coarse sand and gravel underlain by bedrock. The sand and gravel layer is expected to provide a zone of preferential transport for contaminants, particularly those with a density greater than water. In the northeastern part of the study area (fig. 2), shallow groundwater flow and contaminant plume migration are generally east and northeast toward the Blue River and its tributary, Boone Creek (U.S. Department of Energy, 2006). The ongoing operation of several pumping wells in this area of the site appears to intercept effectively the primary contaminant plume and to flatten or reverse the potentiometric gradient between the pumping wells and the putative surface-water receptors, thereby preventing further contaminant transport toward the Blue River and Boone Creek (U.S. Department of Energy, 2006). Similarly, in the southern part of the study area, groundwater flow and possible contaminant migration toward Indian Creek is contained hydraulically by building footing tile drains and pumping interceptor wells (U.S. Department of Energy, 2006). Fundamental to the adoption of the current hydraulic containment strategy was the conclusion that natural attenuation in the shallow groundwater system prior to the operation Potential for Microbial Degradation of cis-Dichloroethene and Vinyl Chloride in Streambed Sediment at the U.S. Department of Energy, Kansas City Plant, Missouri, 2008 By Paul M. Bradley 2 Microbial Degradation of cis-Dichloroethene and Vinyl Chloride in Streambed Sediment, Kansas City Plant, MO of contaminant-interception wells was not sufficient to prevent contaminant migration to the Blue River. In the absence of a demonstrated natural buffer that could effectively prevent contaminant discharge to the Blue River and Boone Creek, the groundwater-interception strategy was implemented. The apparently successful implementation of the current pumpand-treat strategy at the KCP has created an opportunity to investigate the feasibility of alternative remedial strategies that do not involve the long-term operation and maintenance costs associated with pump-and-treat remediation. The results of recent U.S. Geological Survey (USGS) investigations (Bradley and Chapelle, 1997, 1998a, 1998b, 1999a, 1999b, 2000a, 2000b, 2002; Bradley, Chapelle, and Lovley, 1998; Bradley, Chapelle, and Wilson, 1998; Bradley, Landmeyer, and Dinicola, 1998; Bradley, 2003) have demonstrated that the potential for chloroethene biodegradation is pronounced particularly in wetlands and streambedsediment environments. Spatial and temporal variations in the hydrologic and geochemical characteristics of the sediments in these environments can lead to highly dynamic microbial communities with a wide range of metabolic capabilities and the demonstrated potential for rapid reductive and oxidative chloroethene biodegradation. Thus, because the reduced daughter products, DCE and VC, are the predominant chloroethene contaminants in the downgradient portions of the chloroethene plumes at the KCP, and because the extent and efficiency of DCE and VC biodegradation in streambed sediments has been reported to be much greater than that observed in adjacent aquifer systems (Bradley and Chapelle, 1997, 1998a, 1998b, 1999a, 1999b, 2000a, 2000b, 2002; Bradley, Chapelle, and Figure 1. Location of the U.S. Department of Energy, Kansas City Plant in Kansas City, Missouri, near the Blue River. DOE Kansas City Plant 71 435 470 M is so u r i K a n sa s B lue R i v er I ndia n Creek yke Banch Boone reek Base from U.S. Department of Agriculture, National Agriculture Imagery Program, 1-meter resolution orthoimagery, 2007; U.S. Department of Transportation, Bureau of Transportation Statistics 1:100,000-scale road network, 2003; and U.S. Geological Survey 1:100,000-scale national hydrography dataset, 2005 Universal Transverse Mercator projection, zone 15, NAD 83 datum 0 5,000 Feet 0 1,000 Meters 94°32' 94°34' 94°36'

scientific investigations report

Potential for microbial degradation of cis-dichloroethene and vinyl chloride in streambed sediment at the U.S. Department of Energy, Kansas City Plant, Missouri, 2008