Water movement in the zone of interaction between groundwater and the Columbia River, Hanford Site, Washington

A two-dimensional model that simulates flow pathlines in a vertical cross section oriented perpendicular to the Columbia River has been developed for a location at the Hanford Site. Hydraulic head data from wells and the adjacent river were used to calculate flow direction and velocity in hourly increments for an entire seasonal cycle. River stage cycles extend through a range of several meters, thus exerting a strong influence on water movement in the zone of interaction. By including a fluctuating river stage at the river boundary (center of channel), the model showed that landward of and beneath the shoreline, flow pathlines within the aquifer are deflected downward. The region immediately beneath the shoreline is strongly influenced by river water that infiltrates during high river stage. On the river side of the shoreline, groundwater discharges upward into the river channel, with pathlines converging in the riverbed relatively close to shore. If the model is run assuming a constant, average river stage, these movement features are not represented, thus demonstrating the need to include transient boundary conditions when a fluctuating river stage influences the zone of interaction between groundwater and surface water. The model provides information that supports a variety of applications, including monitoring strategies, contaminant transport models, risk assessments, remedial action design, and compliance requirements for remedial actions.