MODELING OF GROUNDWATER FLOW AFFECETD BY ARTIFICIAL RECHARGE IN THE WEBER RIVER BASIN, NORTHERN UTAH

A numerical model of the aquifer system in the Weber Delta area, located between the Wasatch Mountains and The Great Salt Lake in northern Utah, has been prepared to evaluate the effectiveness of Weber River Basin Aquifer Storage and Recovery project. The USGS Modflow model, represents an area exceeding thirty square miles and six geologic layers, including two major regional aquifers, the Delta aquifer and the Sunset aquifer. The model uses transient flow conditions to illustrate a steady decline in both aquifers in last fifty years and to allow predictive simulations. The model reflects declined water levels in last fifty years from between twenty feet in the west part of the model to eighty feet in the east part of the modeled area. Artificial recharge ponds are located in the area characterized by the largest declines of water table. Predictive simulations indicate changes in water levels at various rates of artificial recharge. Most of the flow occurs from the Wasatch Mountains toward the Great Salt Lake, so that no flow boundaries have been placed along the northern and southern boundaries of the modeled area. The model uses general head boundary to simulate artificial recharge where downward flow to the main aquifer water table is delayed by a local perched aquifer. Rewetting modules in conjunction with general head boundaries allowed to simulate recharge across the perched aquifer. The shape of the “recharging mound” in the perched aquifer was calibrated based on microgravity data. Water levels in the main aquifer, recently about 232 feet below the ground surface, after increasing by approximately one foot at the beginning of the artificial recharge, remained at this elevation for a period of nine months after the artificial recharge was shut down. The model is calibrated using water levels from various periods of time in last fifty years when ground water withdrawals has increased from five acre-feet per year to about thirty acre-feet per year. The model includes all available data on aquifer properties, historic water levels and components of the hydrologic budget. The model also shows effects of extended drought conditions in years 1997-2004 on water levels in main aquifers.