Effects of Low-Permeability Layers in the Hyporheic Zone on Oxygen Consumption Under Losing and Gaining Groundwater Flow Conditions

Recent studies at the watershed scale have demonstrated the dominant role that river bedforms play in driving hyporheic exchange and constraining biogeochemical processes along river corridors. At the reach and bedform scales, modeling studies have shown that sediment heterogeneity significantly modifies hyporheic flow patterns within bedforms, resulting in spatially heterogeneous biogeochemical processes. In this work, we summarize a series of flume experiments to evaluate the effect that low-permeability layers, representative of structural heterogeneity, have on hyporheic exchange and oxygen consumption in sandy streambeds. In this case, we systematically changed the geometry of the heterogeneities, the surface channel flow driving the exchange, and groundwater fluxes (gaining/losing) modulating the exchange. The flume was packed with natural sediments, which were amended with compost to minimize carbon limitations. Structural heterogeneities were represented by continuous and discontinuous layers of clay material. Flow patterns were studied using dye imaging through the side walls. Oxygen distribution in the streambed was measured using planar optodes. The experimental observations revealed that the clay layer had a significant effect on flow patterns and oxygen distribution in the streambed under neutral and losing conditions. Under gaining conditions, the aerobic zone was limited to the upper sections of the bedform and thus was less influenced by the clay layers that were located at a depth of 1-3 cm below the water-sediment interface. We are currently analyzing the results with a numerical flow and transport model to quantify the reactions rates under the different flow conditions and spatial sediment structures. Our preliminary results enable us to show the importance of the coupling between flow conditions, local heterogeneity within the streambed and oxygen consumption along bed forms and are expected to improve our ability to model the effect of stream-groundwater interactions on nutrient cycling.