Abstract

An updated ground-water flow model of Cache Valley has been created using MODFLOW. This numerical model was based on a conceptual model developed from seven hydrostratigraphic cross sections constructed from drillers’ logs. These cross sections show that the most productive aquifer, termed the principal aquifer, is located in the southeastern portion of the valley and is overlain by two continuous confining layers. The principal aquifer consists of alluvial fan and deltaic sands and gravels deposited by streams draining the Bear River Range, while the two confining layers correlate with the lacustrine deposits of the Bonneville and Little Valley lake cycles.

Calibration of the numerical model indicates that subsurface recharge to the principal aquifer from the surrounding bedrock, as well as subsurface discharge to the surrounding bedrock from an unconfined aquifer overlying the two confining layers, may occur. Two predictive simulations were run with increased pumping from the principal aquifer of 34 cubic feet per second (1.0 cubic meter per second), the first using the average annual precipitation and the second using the lowest annual precipitation measured from 1984 to 1997. The increased pumping was largely offset by an increase in subsurface recharge for both simulations. During the first simulation, discharge to springs, streams, evapotranspiration and from the unconfined aquifer all remained unchanged, suggesting that the confining layers may serve as an effective barrier to flow between the unconfined and principal aquifers. During the second simulation, discharge to springs, streams, evapotranspiration and from the unconfined aquifer all decreased, suggesting that droughts may have a greater impact on the ground-water system than additional pumping.