Particulate Organic Matter Retention by Sandy Sediments

The hyporheic zone (HZ) is the volume of streambed sediments where surface water and groundwater mix. The mixing of ground and surface water geochemical constituents including organic matter and the presence of large microbial populations create an important location for biogeochemical activity. Large and periodic fluctuations in river stage such as those associated with operation of hydroelectric dams, may enhance downstream hyporheic exchange by reversing the direction of flow to/from the stream on a regular basis. Solute transport in hyporheic zones is affected by river regulation; however, little is known about the transport of particulate organic matter (POM) under these conditions. We performed a laboratory experiment to simulate the transport and retention of POM under conditions simulating flow reversal in the hyporheic zone. Here, POM is created by inoculating artificial river water with algae obtained from the Hanford Reach of the Columbia River. POM transport experiments are then conducted in columns packed with sandy sediments; these data are then used to calibrate a model describing fundamental processes controlling its transport in saturated sediments. Models results reveal that filtration, sorption to sediment, and sorption to previously retained POM are important mechanisms of POM retention in sandy, saturated sediments. Additionally, upon flow direction reversal, POM is readily mobilized leading to effluents concentrations above the initial input concentrations. These findings indicate that river stage changes may change POM storage in the hyporheic zone depending on the type and magnitude of flow variation.