Groundwater-surface water interaction and its role on TCE groundwater plume attenuation

A field investigation of a TCE plume in a surficial sand aquifer shows that groundwater-surface water interactions strongly influence apparent plume attenuation. At the site, a former industrial facility in Connecticut, depth-discrete monitoring along three cross-sections (transects) perpendicular to groundwater flow shows a persistent VOC plume extending 700 m from the DNAPL source zone to a mid-size river. Maximum TCE concentrations along a transect 280 m from the source were in the 1000s of microg/L with minimal degradation products. Beyond this, the land surface drops abruptly to a lower terrace where a shallow pond and small streams occur. Two transects along the lower terrace, one midway between the facility and river just downgradient of the pond and one along the edge of the river, give the appearance that the plume has strongly attenuated. At the river, maximum TCE concentrations in the 10s of microg/L and similar levels of its degradation product cis-DCE show direct plume discharge from groundwater to the river is negligible. Although degradation plays a role in the strong plume attenuation, the major attenuation factor is partial groundwater plume discharge to surface water (i.e. the pond and small streams), where some mass loss occurs via water-air exchange. Groundwater and stream mass discharge estimates show that more than half of the plume mass discharge crossing the first transect, before surface water interactions occur, reaches the river directly via streamflow, although river concentrations were below detection due to dilution. This study shows that groundwater and surface water concentration measurements together provide greater confidence in identifying and quantifying natural attenuation processes at this site, rather than groundwater measurements alone.