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 on the Hanford Site. Hydraulic head data from wells and the adjacent river were available to calculate flow direction and velocity in hourly increments for an entire seasonal cycle. The computer code Subsurface Transport Over Multiple Phases was used for flow calculations. River stage cycles extend through a range of several meters, thus exerting a strong influence on water motion in the zone of interaction. Flow pathlines from the aquifer are deflected downward beneath the bank storage zone. 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. 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 ground 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.