Underlying riparian lithology controls redox dynamics during stage-driven mixing

Abstract

The interactions between surface water-groundwater (hyporheic) exchange and sediment lithology influence oxidation–reduction (redox) conditions near river-aquifer interfaces. Redox dynamics have implications for subsurface nutrient transformations, but are difficult to assess spatially and temporally using traditional geochemical observations. This study applied continuous, three-dimensional monitoring of redox potential in a shallow riparian aquifer to assess the geochemical response to sediment lithology and hydrogeologic variables. We mapped a series of cross-sectional redox contour plots from throughout the aquifer over a storm event, which captured the dynamics of redox conditions during stage-driven mixing. Results show that redox potential increased by over 400 mV at some locations following river water infiltration during the storm, while persistent zones of high potential endured along preferential, high-conductivity flow paths. Conditions within high-conductivity (>1 cm/s) sediments took longer to recover (up to 30 days) following storms than those within finer, low-conductivity (6.9 × 10−5 cm/s) sediments (days to hours), however, attributed to less organic matter driving nutrient transformations. Dynamic surface water-groundwater interactions intermittently perturb redox conditions, but the underlying sediment lithology ultimately governs the spatial and temporal dynamics of aquifer redox conditions. These novel observations provide fundamental insights into hyporheic exchange and riparian geochemical dynamics broadly applicable to hydrogeology and biogeochemistry.