Excess N and denitrification in hyporheic porewaters and groundwaters of the San Joaquin River, California.

The San Joaquin River (SJR) in California is purported to receive high nitrate loadings from surrounding agricultural lands through both surface and groundwater inputs. To investigate the potential removal of nitrate (NO) from surface and ground water sources, the spatial variations in dinitrogen (N) gas concentrations and direct measurements of sediment denitrification potential (DNP), with amended NO and carbon (C) treatments, were investigated in the summer along a 95-km reach of the San Joaquin River. Excess N in hyporheic porewaters ranged from <0.1 to 8.65 mg L and was significantly higher in porewaters from the 1.3 m (ground water source) versus 0.3 m (mixed surface and ground water) depths. In deep groundwater wells (3-7 m), median excess N concentration was 5.39 mg L (range = <0.1-14.6 mg L). Excess N concentrations were inversely correlated with dissolved oxygen and NO concentrations suggesting denitrification as an important process in the dominantly anaerobic sediments. Hyporheic porewater NO concentrations exceeded the detection limit of 0.01 mg L in only 20% of the hyporheic porewaters, in spite of high NO concentrations measured in both surface waters (mean = 2.25 mg N L) and surrounding groundwaters. Sediment DNP rates averaged 253 and 297 μg N kg hr for NO amended, and NO + C amended sediments, respectively, supporting the prevalence of denitrification in hyporheic sediments. Our results indicate that the hyporheic/riparian zones act as an anoxic barrier to nitrate transport from regional groundwater and as a location to remove NO from surface waters exchanging with the hyporheic zone.