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

Chemical fluxes between catchments and streams are influenced by biogeochemical processes in the groundwater-stream water (GW-SW) ecotone, the interface between stream surface water and groundwater. Terminal electron accepting processes (TEAPs) that are used in respiration of organic C in anoxic environments may have a strong effect on nutrient dynamics and water chemistry. Concentrations of oxidized and reduced forms of terminal electron accepters (dissolved O-2, NO3-, Fe2+, SO42-, and CH4) were measured in networks of vertically nested wells installed beneath the surface stream and in the near-stream aquifer of a headwater catchment. Tracer addition experiments were conducted in surface and groundwater environments of a Ist-order montane stream to characterize hydrologic fluxes between the stream and aquifer, and to quantify ecosystem retention of terminal electron accepters (NO3- and SO42-) in the GW-SW ecotone. Sulfate retention was evident in both hyporheic and groundwater environments. Distribution of important redox sensitive solutes 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 ecotone related to characteristics of GW-SW mixing and residence time of water in the hyporheic zone.