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

The biogeochemical effects of a large surface water on a chloroethene contaminated anaerobic groundwater at the groundwater / surface water interface (GSI) were evaluated using spatially discretized multilevel arrays, and statistically interpreted in light of natural bioattenuation mechanisms. Groundwater samples, collected during a five-month evaluation period with increasing storm activity, were evaluated to determine the effect of lake activity 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. cis-1,2-Dichloroethene (DCE) remained unchanged or increased at the same locations indicating that the decrease in methane and chloroethene was not due to dilution effects from lake water infiltration. Moreover, negative correlation of chloroethene and methane data with oxygen suggest that chloroethene is co-oxidized by methane-oxidizing bacteria in the shallow zone of the plume. Laboratory microcosm experiments using methane-oxidizing enrichment derived from GSI solids confirmed that chloroethene and ethene were co-oxidized, while cis-DCE was not. Based on the co-oxidation rates obtained from these model systems, methanotrophs contributed approximately 1% to the degradation of the chloroethene flux observed at the GSI. (C) 1999 Elsevier Science Ltd. All rights reserved.