EVALUATING NATURAL ATTENUATION OF CONTAMINANTS ALONG A FIRST-ORDER COASTAL PLAIN STREAM

Little Bayou Creek, a first-order stream in McCracken County, Kentucky, receives discharge of ground water contaminated by past activities at the U.S. Department of Energy´s Paducah Gaseous Diffusion Plant. Natural attenuation of trichloroethene (TCE) and technetium-99 (99Tc) along the channelized reach of the creek below the contaminated springs was studied for five consecutive seasons (from January 2002 through January 2003). Bromide, nitrate, and rhodamine WT were injected as slug tracers of baseflow; simultaneously, propane was continuously injected. Tracer concentrations were monitored at two downstream locations. Contaminant concentrations were also monitored and stream flow was gaged prior to tracer injection. Slug tracer peaks tended to coincide for each test, with travel times highest in October and lowest in June. Propane concentrations tended to plateau except in August and at the farther downstream site in October. Simulations with the OTIS-P code showed that solute transport in the stream is predominantly one-dimensional, with insignificant secondary storage in the stream and the hyporheic zone. TCE is attenuated along the studied reach by dilution and volatilization, which increased with ambient temperature and with discharge. TCE volatilization coefficients calculated from modeled values of the first-order loss coefficient for propane are close to the coefficients calculated from TCE concentrations. 99Tc appears to be attenuated only by dilution. Rhodamine WT data suggest that hydrophobic sorption of TCE is insignificant, while nitrate analyses indicate that TCE and 99Tc are not transformed by reduction. These results are consistent with the lack of secondary storage, since sorption and reduction would be likely to occur in low-flow regions within the stream bed and banks.