Transport and Degradation of a Trichloroethylene Plume Within a Stream Hyporheic Zone

Abstract

In predicting natural attenuation rates for contaminant plumes, it is vital to determine flow patterns, suitability of chemical and microbial conditions, and seasonality. Savannah River Site’s CMP Pits operated from 1971 until 1979; receiving chemicals, metals, and pesticides. Now a Superfund site, monitoring-wells indicated perchloroethylene (PCE) and trichloroethylene (TCE) had seeped beneath the vadose zone. It was unknown how the plume was entering Pen Branch valley below and whether natural attenuation was degrading the contaminant load. Our study focused on plume transport and exchange within the critical hyporheic zone beneath Pen Branch and helped to ground-truth model the plume borders. We also determined reductive dechlorination of PCE and TCE into dichloroethylene and vinyl chloride. Over forty sampling holes were augered into the hyporheic zone and adjacent floodplain along with 12 stream stations. Chemical conditions linked to natural attenuation (e.g. H₂S, Fe⁺², and NH₃) were monitored to identify reductive dechlorination suitability along with temperature, pH, redox, and dissolved oxygen. Plume flow displayed complex entry patterns, but natural attenuation was documented by higher levels of cis-dichloroethylene (cis-DCE) (61.5 μg/l) compared to PCE or TCE. High means of hyporheic PCE (26.5 μg/l) and TCE (6.7 μg/l) compared to overlying stream water PCE (0.5 μg/l) and TCE (0.2 μg/l) raise new transport pathway questions.