USE OF DISSOLVED GASES AND OTHER ENVIRONMENTAL TRACERS FOR EVALUATING SOURCES OF RECHARGE TO THE LITTLEFIELD SPRINGS COMPLEX, NORTHWESTERN ARIZONA

The Littlefield Springs complex includes more than 200 springs, discharging approximately 6,000 cubic meters per hour along a 10-km reach of the Virgin River above the confluence with the Colorado River at Lake Mead. Understanding the source of this water is important for evaluating salinity control in the Virgin River Basin. Constituents used to evaluate recharge sources included major-ion chemistry, F^-, Br^-, ^87/86Sr, d²H, d¹⁸O, ¹⁴C, ³H, ⁴He, SF₆, CFCs, and noble gases. Results suggest that water emanating from Littlefield Springs is a binary mix of older groundwater in the regional carbonate aquifer and modern (post-1950s) seepage from the Virgin River. Corrected ¹⁴C ages of groundwater emanating from Littlefield Springs range from 6,000 to 9,000 years. Yet large amounts of terrigenic helium (up to 2.5 x 10 ^-5 cm³STP/g) and low ³He (R/R_a values as low as 0.08) indicate that the component of groundwater from the regional carbonate aquifer is even older and may have recharged during the Pleistocene. The distribution of infiltration of precipitation using the Basin Characterization Model, along with hydrogeologic cross-sections and regional carbonate aquifer hydraulic gradients, indicates recharge likely occurs in the Clover and Bull Valley Mountains to the north rather than the local Virgin Mountains. This high-altitude recharge is supported by relatively cool noble-gas recharge temperatures (less than 13 to 17¢ªC) and isotopically depleted d²H and d¹⁸O. CFCs and ³H indicate that the modern fraction of water emanating from Littlefield Springs is likely stream seepage along the Virgin River Gorge. Terrigenic SF₆ prevented the use of this trace atmospheric gas for dating of groundwater discharging from Littlefield Springs; similarly, large amounts of terrigenic ⁴He precluded ³H/³He dating. While several methods were used to evaluate the relative fractions of sources to the springs, comparison of CFC-12 and CFC-113 atmospheric mixing ratios provided the most robust estimates, indicating that about 40% of Littlefield Springs comes from Virgin River seepage and has an apparent travel time of about 27 ± 3 years. These conclusions suggest that removal of sources of higher salinity to the Virgin River upstream of the Gorge may also reduce salinity of water discharging from Littlefield Springs into the Virgin River.