Terminal electron accepting processes in the alluvial sediments of a headwater stream

Chemical fluxes between catchments and streams are influenced by biogeochem ical processes in the groundwater-stream water (GW-SW) ecotone, the interfa ce between stream surface water and groundwater. Terminal electron acceptin g processes (TEAPs) that are used in respiration of organic C in anoxic env ironments may have a strong effect on nutrient dynamics and water chemistry . Concentrations of oxidized and reduced forms of terminal electron accepte rs (dissolved O-2, NO3-, Fe2+, SO42-, and CH4) were measured in networks of vertically nested wells installed beneath the surface stream and in the ne ar-stream aquifer of a headwater catchment. Tracer addition experiments wer e conducted in surface and groundwater environments of a Ist-order montane stream to characterize hydrologic fluxes between the stream and aquifer, an d to quantify ecosystem retention of terminal electron accepters (NO3- and SO42-) in the GW-SW ecotone. Sulfate retention was evident in both hyporhei c and groundwater environments. Distribution of important redox sensitive s olutes varied predictably with changing hydrologic residence time of water in the GW-SW ecotone. Results suggest a strong hydrologic control of TEAPs and ecosystem retention of biologically important solutes in the GW-SW ecot one related to characteristics of GW-SW mixing and residence time of water in the hyporheic zone.