Biogeochemistry and transformation potential of chloroethene-contaminated sediments at the groundwater-surface water interface

A groundwater-surface water interface (GSI) was characterized for spatial distributions of contaminants and geochemical conditions to evaluate the in-situ bioremediation potential of this environmental system. The philosophical approach to the research emphasized that the field site must be characterized and carefully evaluated prior to initiating laboratory work. In this way relevant experiments could be performed to specifically evaluate the mechanisms for natural attenuation of the contaminant plume at the GSI. Based on three transects at the GSI, the following trends were observed.; cis-1,2-Dichloroethene was predominant in the presence of elevated ferrous iron concentrations and depressed aquifer solids-bound iron concentrations. Chloroethene and ethene were predominant contaminants in the presence of depressed sulfate and elevated aqueous sulfide and methane concentrations. Shallow monitoring points were generally hypoxic to aerobic and exhibited values of specific conductance reflective of near-shore lake water, indicating reoxygenation of the contaminant plume due to wave infiltration. An offshore transect did not yield significant contaminant concentrations but showed evidence of sulfate-reduction suggesting the contaminant plume discharged into the lake between the shoreline and 100 meters from shore.; A temporal study at the GSI was undertaken using spatially discretized arrays. Temporal effects of lake activity on the biogeochemistry of the plume indicate that concentrations of methane and chloroethene decreased as the groundwater became increasingly oxidized along the GSI in shallow sample points impacted by infiltration of oxygenated lake water. The lack of change in cis-1,2-dichloroethene indicates this decrease was not due to dilution. A negative correlation of chloroethene and methane data with oxygen suggest that chloroethene is co-oxidized by methane-oxidizing bacteria at shallow depths. In contrast with oxidative processes in the shallow zone, reductive dechlorination of contaminants remained the predominant (bio)-transformation process in the deep zones of the GSI. Laboratory microcosm experiments using methane-oxidizing enrichment cultures derived from GSI solids confirmed that chloroethene and ethene were co-oxidized, while cis-1,2-dichloroethene was not. However, flux calculations indicated that methanotrophic activity was a minor constituent of the oxidative flux observed at the GSI transect.; This study contributes to the understanding of temporal and spatial processes affecting contaminant fate and transport at near-shore GSIs.