Impact of Clay Addition Size on Hyporheic Exchange During Bedform Migration

Hyporheic exchange is an ecologically important process that controls the exchange of nutrients between the water column and microbe-rich sediment bed in streams. Fine particle deposition has been shown to induce clogging and thus reduce hyporheic exchange flux (HEF) and stream-groundwater interactions. This phenomenon has primarily been studied with stationary bedforms. We conducted experiments to study the effect of fine particle deposition on clogging and HEF with mobile bedforms. The experiments were conducted in a 640-cm-long and 30-cm-wide recirculating flume, which was packed with homogeneous sand. Experiments involved measuring HEF and bed morphodynamics before and after addition of suspended kaolinite clay into the flume. Clay suspensions were added to the flume multiple times during each experiment. Three experiments were performed, with clay concentrations of 165, 410, and 660 mg/L, respectively. HEF was quantified using salt tracer additions, and porewater flow patterns were observed through the addition of a dye tracer to the overlying water. We found that the decrease in HEF is proportional to the cumulative amount of clay accumulation in the bed, while the reduction in the pore volume participating in HEF was sensitive to the magnitude of the clay injections. The pattern of clay deposition differed by magnitude of the clay injections, the smallest additions of clay resulting in deposition primarily as a layer below the maximum scour depth, while the two larger clay additions resulted in an expanded zone of accumulation within the active layer of bedform motion and scour. The presence of clay in the bed and water column did not have a noticeable effect on the measured bedform height properties. These experimental results are important for understanding the complexity of hyporheic exchange in rivers with bedform-induced hyporheic exchange flow.