IDENTIFICATION OF NATURAL ATTENUATION OF TRICHLOROETHENE AND TECHNETIUM ALONG LITTLE BAYOU CREEK, KENTUCKY, BY TRACER TESTS

Little Bayou Creek, a first-order tributary to the Ohio River in McCracken County, Kentucky, receives inflow from an aquifer contaminated by past activities at the Paducah Gaseous Diffusion Plant, a US Department of Energy facility and Superfund site. Contaminants include trichloroethene (TCE) and technetium-99 (⁹⁹Tc). We hypothesized that TCE and ⁹⁹Tc are naturally attenuated by dilution, sorption, and (in the case of TCE) volatilization and that these processes vary seasonally. We monitored contaminant concentrations and gaged stream flow (by the cross-section method) seasonally from January to October 2002. In addition, to account for hyporheic-zone flow, seasonal tracer tests were run along a 290-m reach of the creek. We added slugs of Br^- (as NaBr) as a conservative tracer, rhodamine WT as a visual tracer and a tracer of sorption to stream sediments, and NO₃^- (as NaNO₃) as a redox sensitive tracer to the stream. Propane gas was bubbled continuously into the stream as a non-conservative tracer to mimic TCE volatilization. Comparison of concentration breakthrough curves showed that rhodamine WT behaves conservatively, so it was used in place of Br^- beginning in August. Travel times for the rhodamine center of mass along the lower 241 m of the reach were 76 min in January, 31 min in June, 72 min in August, and 83 min in October. Similarly, flow rates at the downstream end of the reach (in m³/sec) increased from 0.032 in January to 0.059 in June before decreasing to 0.036 in August and 0.032 in October. TCE and ⁹⁹Tc concentrations also peaked in June (at 46 ppb for TCE and 74.8 pCi/L for ⁹⁹Tc at the downstream end of the reach). These results are consistent with prior studies showing that ground-water discharge and stream flow are minimal in winter and peak in spring. Breakthrough curves for NO₃^- were similar to those for rhodamine WT and Br^-, which suggests that NO₃^- and thus ⁹⁹Tc are not significantly attenuated by reduction in the hyporheic zone. Propane concentrations approached steady-state values after concentrations of the other tracers peaked. This result and the consistent decrease in TCE/⁹⁹Tc ratios with increasing distance downstream suggest that volatilization is the primary mechanism of TCE attenuation along Little Bayou Creek.