Date of Award:

2003

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Thomas E. Lachmar

Abstract

A groundwater model of Cache Valley was created using MODFLOW. Steady-state calibration of the model demonstrated that recharge to the lower confined aquifer may occur along the margin of the valley that borders the Wellsville Mountains and the Bear River Range. Steady-state calibration also showed that discharge from the unconfined aquifer may occur along the eastern and western margins of the valley in both the Utah and the Idaho portions of the valley. Two simulations were run with increased pumping of 3 5 cubic feet per second (1 cubic meter per second) from the principal aquifer. The first simulation was run with the average annual precipitation value of 1.2 feet per year (0.36 meters per year), while the second was run with a less than average annual precipitation value of 1 foot per year (0.3 meters per year). The first simulation produced very little change within the unconfined aquifer. The discharge from the groundwater system through springs, seepage to streams, evapotranspiration, and general head boundaries remained unchanged with the increase in discharge through pumping. This indicates that the two continuous, confining layers that blanket the valley may serve as a barrier to groundwater flow between the unconfined and lower confined aquifer. The increased pumping within the principal aquifer did not stimulate increased recharge along the western margin of the valley. This indicates that true steady-state conditions were not achieved in the amount of time that the model had indicated. During the second simulation, decreased recharge to the groundwater system through infiltration of precipitation caused a decrease in discharge from the groundwater system through seepage to streams, springs, evapotranspiration, and general head boundaries. The increased pumping within the principal aquifer also did not stimulate increased recharge along the western margin of the valley. As with the first simulation, this indicates that true steady-state conditions were not achieved in the amount of time that the model had indicated. A sensitivity analysis of the model concluded that the hydraulic conductivity of the two continuous, confining layers that blanket the valley proved to have a relatively substantial impact on the water levels in the confined aquifers. The sensitivity analysis also showed that altering the vertical hydraulic conductivity of the lower confined aquifer produced minimal head changes.

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