Temporal and Spatial Trends in Biogeochemical Conditions at a Groundwater−Surface Water Interface:  Implications for Natural Bioattenuation

The biogeochemical effects of a large surface water on a contaminated anaerobic groundwater at the groundwater−surface water interface (GSI) were evaluated using spatially discretized multilevel arrays and interpreted in light of natural bioattenuation mechanisms. Groundwater samples, collected during a 5-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 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 remained unchanged or slightly increased at the same locations, indicating that the decrease in methane and chloroethene was not due to dilution effects. 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. Contrary to oxidative processes in the shallow zone, reductive dechlorination of contaminants remained the predominant bio-transformation process in the deep zones of the GSI with chloroethene and ethene being the major contributors to total contaminant concentration. This study is the first to evaluate the effects of seasonal changes on a chlorinated ethene-contaminated plume at the GSI in spatial and temporal detail.