Laboratory evaluation of temporal trends in biogeochemical conditions at agroundwater-surface water interface

The biogeochemical effects of a large surface water on a chloroethene conta minated anaerobic groundwater at the groundwater / surface water interface (GSI) were evaluated using spatially discretized multilevel arrays, and sta tistically interpreted in light of natural bioattenuation mechanisms. Groun dwater samples, collected during a five-month evaluation period with increa sing storm activity, were evaluated to determine the effect of lake activit y on the oxidation capacity and contaminant distribution within the plume. Our analyses indicate that concentrations of methane and chloroethene (VC) decreased as the groundwater became increasingly oxidized along the GSI in shallow sample points impacted by infiltration of oxygenated lake water. ci s-1,2-Dichloroethene (DCE) remained unchanged or increased at the same loca tions indicating that the decrease in methane and chloroethene was not due to dilution effects from lake water infiltration. Moreover, negative correl ation of chloroethene and methane data with oxygen suggest that chloroethen e is co-oxidized by methane-oxidizing bacteria in the shallow zone of the p lume. Laboratory microcosm experiments using methane-oxidizing enrichment d erived from GSI solids confirmed that chloroethene and ethene were co-oxidi zed, while cis-DCE was not. Based on the co-oxidation rates obtained from t hese model systems, methanotrophs contributed approximately 1% to the degra dation of the chloroethene flux observed at the GSI. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.