Spatial and temporal variability in seepage between a contaminated aquifer and tributaries to the Ohio River

Although ground water discharge can limit plume migration and transfer contaminants to streams, interactions among ground water, rivers, and tributaries in contaminated watersheds have received relatively little attention. We used multiple methods to delineate seepage along Little Bayou and Bayou Creeks, tributaries to the Ohio River in McCracken County, Kentucky, from July 1996 through July 1998. The Paducah Gaseous Diffusion Plant (PGDP) lies between the creeks. Trichloroethene (TCE) and technetium-99 plumes within the underlying Regional Gravel Aquifer (RGA) extend several kilometers from PGDP toward the river. Both creeks tend to gain flow where they are incised into the RGA or contiguous strata in the Ohio flood plain. Bayou Creek also gains flow upstream of PGDP; other reaches of both creeks tend to lose flow. Local storms, river floods, and seasonal dry periods caused temporary changes in seepage rates and reversals in hydraulic gradients. Gaining conditions were indicated by seeps, springs, and boils, by upward hydraulic gradients from bank wells and bed piezometers to the stream, and by mixing models using chloride and oxygen-18. Mixing models and downward hydraulic gradients from the stream to wells indicated losing conditions. Annual ranges of stream, bed, and bank temperatures tended to be narrower, bed and bank temperatures in summer and early autumn tended to be cooler, and maximum values of specific discharge measured by seepage meters were greater along gaining than along losing reaches. Estimates of specific discharge from stream gauging and one-dimensional flow modeling did not conclusively identify losing and gaining reaches, but absolute values of those estimates fell within the range of seepage-meter measurements. Contaminants discharging to Little Bayou Creek were diluted downstream by uncontaminated ground water. Volatilization, biodegradation, or sorption probably removed TCE from stream water. These results indicate that discharge to tributaries can limit seepage of contaminants to rivers.