THE INFLUENCE OF IRRIGATION INFILTRATION ON THE EVOLUTION OF GROUNDWATER CHEMISTRY NEAR FERNLEY, NEVADA

The Fernley irrigation area is in the lower Truckee River Basin, northwestern Nevada, about 20 miles upstream from terminal Pyramid Lake. In this semi-arid basin, natural recharge is estimated to be 600 acre-feet per year. Thousands of acre-feet of Truckee River water are used annually to irrigate the Fernley area. This irrigation water flushes salts from the approximately 100-foot thick unsaturated zone and mixes with native groundwater before being transported to the Truckee River. The mixed water ranges in total dissolved solids (TDS) from about 500 to 6,000 mg/L, thus adding water significantly higher in TDS to the river (generally less than 350 mg/L). The amount of mixing of native groundwater with irrigation water is estimated with NETPATH on the basis of major-ion chemistry and evironmental isotopes. A low flow survey was conducted in the lower Truckee River to estimate TDS loading from two sources.