Natural attenuation along a first-order stream recharged by contaminated ground water

Little Bayou Creek is an 11-km first-order stream in McCracken County, Kentucky, whose watershed includes the Paducah Gaseous Diffusion Plant (PGDP). Plumes of contaminated ground water exit the northern boundary of PGDP and pass under Little Bayou Creek. TCE and 99Tc have been detected in the creek, in a well installed along the west bank, and in springs feeding into the creek. In addition to effluent discharge from PGDP, Little Bayou Creek is sustained by ground-water inflow as discrete, fracture-controlled springs in the streambed or banks and via diffuse vertical seepage through the streambed. Hyporheic flow is variable, largely controlled by streambed topography, and apparently influenced by changes in ground-water flow paths.; Much of the inorganic stream chemistry falls within the range of values for surface water that are typical and adequate for aquatic ecology. However, contaminated ground water does impact stream chemistry, especially from the "spring reach," the section of stream lying between 3080 and 2730 m upstream of the mouth of the creek. During the study, east bank, west bank, and mid-channel springs within the spring reach contributed as much as 1201 mug/s TCE and 775 pCi/s 99Tc to the stream. TCE and 99Tc concentrations are higher in the spring waters than in the stream, and TCE concentrations are higher in the sediment pore waters of the streambed than in the water column.; Along the length of the stream receiving contaminated ground water, 99Tc is potentially attenuated only by dilution. Mechanisms potentially resulting in natural attenuation of TCE in Little Bayou Creek include sorption, dilution, volatilization, and biodegradation. Of these, volatilization and, to a lesser extent, dilution appear to be significant in the reduction of TCE concentrations in Little Bayou Creek. The length of the creek is such that TCE is largely volatilized before water from the stream enters Bayou Creek and the Ohio River. Volatilized TCE is transferred from the hydrosphere to the atmosphere, where it has an estimated half-life of 7 days before undergoing photocatalytic oxidation.