In Situ Measurements of Hyporheic Flow and Biogeochemical Reactions across a Wide Range of Flow and Sediment Conditions in Aquatic Systems

Hyporheic exchange and solute mass transfer between surface waters and sediments are key processes controlling biogeochemical reactions in rivers, streams, floodplains, lakes, and wetlands. These processes have been difficult to predict outside of simplified conditions of laboratory flumes due to the variety of mass transfer processes that potentially are involved (e.g. diffusion, interfacial slip flow, bedform-scale advective pumping, turbulence penetration, and head potential-driven groundwater exchange). In situ sampling technologies that operate over a broad range of flow and sediment conditions are needed to overcome predictive limitations. The USGS MINIPOINT sampler has been tested in widely varying hydrologic and sediment conditions ranging in surface water flow velocities (0.03 to 30 cm s^-1), grain type (silt, sand, gravel, and organic sediment), median grain size (<0.06 to 2 mm), sediment porosity (0.2 to 0.9), organic content of sediment (0.2 to 80%), hydraulic conductivity of sediment (10^-5 to 10^-2 cm s^-1), and groundwater specific discharge (undetectable to 10^-3 cm s^-1). Measurements of oxygen reduction, denitrification, and manganese oxidation across these widely varying hydrologic and sediment conditions are suggestive of common patterns in the environmental and biogeochemical factors controlling the depth of flux, turnover time, significance of reactions in aquatic sediments, and effects on the chemistry of downstream waters.