CONCEPTUAL HYDROGEOLOGIC FRAMEWORK AND SIMULATION OF THE SHALLOW UNCONFINED AQUIFER IN CACHE VALLEY, UTAH

The hydrogeologic framework of the shallow unconfined aquifer in the Utah portion of Cache Valley was evaluated and simulated to provide a better understanding of aquifer ground-water flow, water volume, and its susceptibility to contamination. The revised conceptual framework used in this study is based on the analyses of shallow drill holes, water-well reports, and previous published geochemistry data for the shallow aquifer and surface water. The conceptual model considers ground-water flow in the shallow aquifer to be restricted by a lower continuous, low-permeability layer that correlates to lake-bottom deposits of Lake Bonneville, which occupied Cache Valley from about 25,000 to 13,000 years ago. The shallow unconfined aquifer in Cache Valley is composed of delta and lake deposits that grade into finer grained lake-bottom deposits, and have been reworked at the surface by alluvial processes. Seepage measurements and the geochemistry of water in the rivers and reservoir in the valley indicate these surface waters gain water from the shallow aquifer system. Recharge to the shallow unconfined aquifer is by infiltration of direct precipitation on the valley floor, seepage of canals that divert water from rivers that flow across Cache Valley, unconsumed irrigation water, and underflow from Quaternary deposits along the margin of the aquifer. Discharge from the aquifer system is by seepage to rivers and reservoirs in the valley, from springs, and as evapotranspiration. A two-layer, steady-state, finite-difference model was used to simulate the shallow ground-water flow in the shallow aquifer system. Model parameters consisted of recharge, spring discharge, evapotranspiration, discharge to rivers and reservoirs, and underflow. Model calibration was accomplished by varying the model parameters through trial-and–error to determine a best-fit match between simulated and measured values of head and a simple water budget. The model is most sensitive to recharge.