Examination of hydrological, chemical, and biological gradients along the hyporheic zone of an upland agricultural stream in East Tennessee

The hyporheic zone, an area of subsurface mixing of stream and groundwater experiences bi-directional movement of water and solutes that reduces pollutant loads in the water. Research of the hyporheic zone in an upland pasture along Hickory Creek in East Tennessee identified patterns of nutrient transformation in both the horizontal and vertical planes. Two transects of shallow groundwater wells were installed to examine the horizontal gradient, and vertical gradients were studied using suction lysimeters installed at three depths near each shallow groundwater well and peepers installed in the stream. Analyses for oxidation-reduction potential, pH, nitrate-N, ammonia-N, sulfate-S, sulfide, chloride, ferric iron, ferrous iron, total organic carbon, biochemical oxygen demand, chemical oxygen demand, fecal conforms, fecal streptococci, and total solids were conducted on the shallow groundwater, while the suction lysimeter samples and peeper samples were analyzed for nitrate-N, sulfate-S, chloride, and total organic carbon. Additionally, soil samples were collected from around each well and were analyzed for microbial biomass, dehydrogenase activity, L-asparaginases, acid and alkaline phosphomonoesterases, arylsulfatases, soil moisture content, and soil pH. Composite samples from Hickory Greek were analyzed for nitrate-N. sulfate-S, chloride, and total organic carbon concentrations. Piezometer nests located at each shallow groundwater well and at two intermediate locations indicated the occurrence of a wet and a dry season. Statistical analyses indicated that nitrification potential decreased with increasing distance from the stream while denitrification potential increased with increasing distance from the stream; organic concentrations (biochemical oxygen demand, chemical oxygen demand, and total organic carbon) increased with distance from the stream edge; both the oxidation-reduction potential and the pH decreased as distance from the stream edge increased; and ferrous iron concentrations increased with increasing distance from the stream. Significant trends were noted for microbial biomass and soil moisture content, with both increasing with distance from the stream. Vertically, there was an increase in denitrification potential with depth, a decrease in sulfate-S concentration with depth, and an increase in total organic carbon with depth. Two main points were confirmed by the research, 1.) oxidation and reduction processes were the primary means of constituent transformations and not microbial mineralization, and 2.) the lengths of the water flow paths and the retention times of the water in the hyporheic zone were important factors in determining the ability of the hyporheic zone to provide treatment.