PNNL

Abstract

Hydrologic exchange flows (HEFs) across the river-aquifer interface have important implications for contaminant plume migration and biogeochemical processes in the river corridor. Yet little is known about the hydrogeomorphic factors that control HEFs dynamics along river reach. Here, we developed a 3-D groundwater model for the free-flowing Hanford Reach (~75 km) of the Columbia River to quantify hydrologic exchange fluxes across the riverbed and to simulate the flow and transport in the river corridor encompassing multiple geomorphic features under dynamic hydrologic forcing. Our results revealed that HEFs showed a strong spatiotemporal pattern as well as interannual variabilities with wet years yielding larger exchange volume than dry years. In general, the river was losing during late spring to early summer when the river stage was high, and river was gaining in fall and winter when river stage was low. The magnitude and timing of river stage fluctuations controlled hot moments of HEFs (times of high exchange flux rates), with sharp changes in river stage magnitude causing large flux rates. River channel sinuosity and thickness of permeable geologic layer together controlled the locations of HEFs hot spots (areas with higher exchange flux relative to their surroundings) with the latter played a dominant role. Dam-induced daily river fluctuations drove high frequency exchange across river-aquifer interfaces and resulted in greater overall exchange volumes as compared to those under weekly smoothed river fluctuations. Our results showed that upstream dam operations enhanced the exchange between geochemically and biologically distinct river water and groundwater and had significant implications to the behavior of groundwater contaminant plumes in the river corridors of large regulated river systems.