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

Abstract

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 acceptors (dissolved O2, 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 1st-order montane stream to characterize hydrologic fluxes between the stream and aquifer, and to quantify ecosystem retention of terminal electron acceptors (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.