Little Bayou Creek is a first-order tributary to the Ohio River in McCracken County, Kentucky. The stream receives inflow from an aquifer contaminated by past waste disposal activities at the Paducah Gaseous Diffusion Plant (PGDP) a US Department of Energy uranium enrichment facility and Superfund site. Contaminants include trichloroethene (TCE), a suspected carcinogen, and technetium-99 (⁹⁹Tc), a radionuclide. The present work involves identifying processes that may naturally attenuate contaminants. I hypothesize that (1) these processes of natural attenuation include (a) dilution, sorption, and volatilization for TCE and (b) dilution for ⁹⁹Tc and (2) dilution and volatilization of TCE vary seasonally.

The proposed study will include seasonal monitoring of the surface and subsurface discharge in the creek. Surface discharge will be gauged by the cross-section method (Rantz, 1982), which involves stretching a measuring tape across the stream, perpendicular to the direction of the flow. Measurement of depth and flow velocity of the vertical section across the stream channel by flow meter at multiple points permits calculation of the stream discharge at that location. In order to account for hyporheic-zone flow and to monitor natural attenuation of TCE and ⁹⁹Tc, seasonal tracer tests will be run. Br^-, a conservative tracer, will be introduced into the stream as a slug of NaBr and used to measure total stream flow (including underflow) at the gauging points. This will also give an estimation of the attenuation of ⁹⁹Tc by dilution, as this radionuclide tends to occur as the TcO4^- ion under aerobic conditions. Br^- concentrations will be measured by ion-selective electrode. Rhodamine WT dye will also be injected with the NaBr slug to visually delineate the tracer plume. Because the meta isomer of Rhodamine WT is prone to hydrophobic sorption to sediment, the breakthrough curve for the dye will be compared with those of Br- to infer the extent of TCE sorption to the stream sediments. Rhodamine WT concentrations will be measured by fluorometry. Following addition of Br^- and the dye, propane (C₃H₈) will be constantly bubbled into the stream as a non-conservative tracer to simulate the attenuation of TCE by in-stream volatilization. The volatilization values of the propane will be compared with a standard ratio to estimate the actual volatilization rate of TCE. Propane concentrations will be analyzed by gas chromatography with a flame ionization detector to measure concentration and hence to determine the gaseous exchange rate of propane in the stream with the surrounding air.