PNNL

Abstract

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 motion in the zone of interaction. By including a fluctuating river stage at the river boundary, the simulation showed that beneath the bank storage zone, flow pathlines within the aquifer are deflected downward. Groundwater discharge upward into the river channel is focused relatively close to shore and the region immediately beneath the shoreline appears to be dominated by river water that infiltrated during high river stage. If the model is run assuming a constant, average river stage, these features are not represented, thus demonstrating the need to include transient boundary conditions when a fluctuating river stage influences the interface 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.