COUPLING STREAMBED TEMPERATURE AND GEOCHEMISTY TO UNDERSTAND THE INFLUENCE OF IN-STREAM OBSTRUCTIONS ON HYPORHEIC EXCHANGE IN A SEMI-ARID WATERSHED

In-stream flow obstructions and other streambed features enhance rapid hyporheic exchange. Spatially variable exchange creates physical and chemical gradients in the streambed, enhancing biogeochemical cycling in the hyporheic zone. Biogeochemical processes alter water passing through the subsurface, creating a unique chemical signature and influencing overall surface water chemistry. Here we quantify variability in hyporheic geochemistry and surface connectivity across three geomorphically heterogeneous reaches to assess the degree of biogeochemical cycling induced by streambed features.

Three 15- to 20-meter reaches of Red Canyon Creek, Wyoming, containing natural steps, an anthropogenic dam and/or a pool-riffle complex were instrumented with in-stream minipiezometers to sample the subsurface. Minipiezometers were screened 0.25 meters below the streambed in approximately 1 m. by 1 m grids at each site. Hydraulic gradient, pH and dissolved oxygen (DO) were measured at each piezometer and water was extracted for analysis of major ions and nutrients. Piezometers were replaced with temperature loggers at the same depth to record subsurface temperatures at 10-minute intervals for 96 hours.

Upstream of dams, results show muted diurnal temperature fluctuations in the streambed, reflecting limited influx rates of surface water, and anoxia (low DO paired with sulfate, manganese and iron reduction). Conversely, downstream pool-riffle complexes show oxia in the streambed (more DO and nitrification) and higher diurnal temperature fluctuations. Spatially variable biogeochemical cycling in the hyporheic zone, driven by morphological features, can be assessed by coupling streambed connectivity measurements (i.e. diurnal temperature fluctuations) with geochemistry and can facilitate understanding how hyporheic exchange influences overall stream chemistry.