SPATIAL PATTERNS OF REDUCING CONDITIONS AND MASS FLUXES OF DISSOLVED SPECIES IN A STREAMBED AND THEIR RELATIONSHIP TO GROUNDWATER DISCHARGE PATTERNS

An investigation of the hydrogeology, contaminant distribution, and geochemistry along a 60 m reach of a river was undertaken to determine the transport and fate of a 45 m wide tetrachloroethene groundwater plume as it discharged from a sand aquifer through a streambed. The site was characterized using the Waterloo profiler, mini-profiler, conventional and driveable multilevel samplers, ground penetrating radar surveys, streambed temperature mapping, drivepoint piezometers, and soil coring and testing. Of the 470 interstitial water samples collected from the streambed to define the plume, about 337 were analyzed for inorganic parameters to determine the geochemistry and redox conditions within the streambed. Investigations showed large spatial variations in reducing conditions (aerobic to methanogenic) that were strongly correlated with the magnitude and direction of the water flux in the streambed. In high discharge areas (>200 L/m²d), the discharging groundwater underwent little geochemical alteration as it quickly passed through the sandy streambed deposits and those areas were anaerobic to nitrate reducing. In moderate to low groundwater discharge areas, the slow moving interstitial water showed evidence of manganese, iron, and sulfate reduction and also methanogenesis. Lower fluxes were associated with more reducing conditions and with the presence of silt and clay deposits. Areas of hyporheic zone flow and no discharge locations were generally aerobic but methane was detected at some locations. Recharge locations were geochemically similar to hyporheic zone areas. Chemical mass fluxes out of the streambed were estimated and mapped using interstitial water concentrations at 80 locations and water discharge rates calculated at 380 locations. High nitrate mass discharges occurred at high groundwater discharge areas while high methane mass discharges occurred at low groundwater discharge areas. Water flux was a good indicator of geochemical conditions at this site because low water fluxes were caused by low hydraulic conductivity silts and clays that were also areas of reducing conditions because they had high mean organic carbon contents (foc 4.9%). This study shows that characterizing water fluxes may be a useful tool for predicting geochemical conditions and the spatial variability within a streambed.