USE OF DISSOLVED GAS RECHARGE TEMPERATURES AND TRITIUM-HELIUM AGE DATES TO DETERMINE SOURCES OF RECHARGE, DELINEATE FLOWPATHS, AND QUANTIFY GROUND WATER TRAVEL TIMES IN AN INTERMOUNTAIN BASIN-FILL AQUIFER, NORTHERN UTAH VALLEY, UTAH

Ground water from three confined basin-fill aquifers is the primary source of municipal water in northern Utah Valley, an area experiencing rapid population growth. Increased well withdrawals coupled with recent drought conditions have caused ground-water levels to decline to their lowest recorded levels. The effects of withdrawals and modifications to the current hydrologic system are not well understood, and calibrated heads and fluxes based on a previous regional-scale numerical ground-water flow model are likely non-unique because of the scarcity of hydrogeologic data in the recharge areas and the high degree of uncertainty in basin discharge rates. Estimated primary sources of recharge to the northern Utah Valley aquifers are seepage from natural and irrigation channels along the mountain front (35%) and subsurface inflow of ground water from the adjacent mountain block (55%). Dissolved gases are currently being used to refine a conceptual model and calibrate a numerical model in the valley aquifers. Despite the complexities of mixing inherent in large volume wells with long screened intervals, noble gas recharge temperatures and tritium-helium age dates are capable of quantifying fractions of recharge from particular sources and ground-water travel times in northern Utah Valley. Preliminary results indicate that nearly all ground water in the confined aquifers contains some fraction of recharge derived from the mountain block and that much of the ground water in the upper two confined aquifers in the eastern half of the valley is less than 50 years old. These data are combined with major ion chemistry to further delineate ground- water flow paths, quantify residence times, and highlight areas of ground water composed primarily of recharge from distinct (mountain front vs. mountain block) sources.