Characterization of surface-water quality in the S-Line Canal and potential geochemical reactions from storage of surface water in the Basalt aquifer near Fallon, Nevada

The Fallon basalt aquifer serves as the sole source of municipal water supply for the Lahontan Valley in west-central Nevada. Principal users include the City of Fallon, Naval Air Station Fallon, and the Fallon Paiute-Shoshone Tribe. Pumpage from the aquifer increased from about 1,700 acre-feet per year in the early 1970's to more than 3,000 acre-feet per year in the late 1990's, and has been accompanied by declines in water levels and changes in water quality. Storage of surface water in the basalt may mitigate the effects of pumpage, but may cause undesirable changes in water chemistry. In May 2001, the U.S. Geological Survey began a study, in cooperation with the Fallon Paiute-Shoshone Tribe, to characterize the surface-water quality of the S-Line Canal, a likely source of water for augmenting recharge. Because arsenic concentrations in ground water of the basalt aquifer exceed drinking water standards, the potential for arsenic release to artificial recharge was explored by using geochemical modeling. Model results suggest that arsenic release may increase concentrations to levels that could limit the use of artificial recharge. Field-based experiments are needed to evaluate the underlying model assumptions.