HETEROGENEITY IN RIVER-GROUNDWATER MIXING, MICROBIOLOGY, AND GEOCHEMISTRY IN AN ALPINE RIVERBED DURING BASEFLOW

The hyporheic zone, where river water and groundwater mix, has a large influence on river water quality. Hyporheic mixing varies spatially, which leads to gradients in microbial activity, community composition, and pore water chemistry. To assess how spatial changes in hyporheic mixing influence microbial community structuring and geochemical conditions, we quantified hyporheic flow using vertical head gradients, heat tracing and sampled pore water for chemistry at 100 locations at high resolution (~3 m) along a 200 m meander of East River, Colorado (USA). Additionally, potential aerobic respiration rates were quantified at 50 locations using resazurin assays. Preliminary results at a small subset of locations revealed increased rates of aerobic respiration in streambed locations where oxic river water downwells. We expect high-resolution sampling to exhibit similar trends, where river-groundwater mixing should vary across streambed morphology and impact hydrobiogeochemical processes. Our findings will provide better understanding of how hydrology influences biogeochemical conditions in the hyporheic zone during baseflow conditions, which is expected to lengthen with climate change.