Ground Water Chemistry Evolution Under Unsaturated Zone Sulfate Salt Dissolution in a Great Basin Lacustrine Aquifer, Western United States

Sulfate and chloride ions in combination with stable isotopes of water turned out to be a powerful tool to decipher the complex aquifer geochemistry in the Fernley Basin aquifer system, in the western Great Basin of the Western United States. The results permitted identifying the underlying hydrologic processes and conceptualizing a three-dimensional model of basin geochemistry. The primary causes of high total dissolved solids (TDS) in this lacustrine sediment aquifer system are dissolution of unsaturated zone Na-SO4 salts by infiltrating irrigation water. By irrigating an area of 2,185 hectares between 0.5 and 1.3 million metric tons of salts were washed into the underlying aquifer, increasing aquifer TDS from pre-irrigation values of about 600 mg/l to more than 5000 mg/l. Irrigation also raised the ground water table which adds to unsaturated zone salt dissolution. The high TDS ground water eventually discharges into the Truckee River, which is the only source of surface water for Pyramid Lake, a large hydrologically closed lake. At present most of the unsaturated zone salts beneath the irrigated areas have been dissolved. About 25 years' worth of water quality records from nine municipal wells show that the historically high TDS ground water is now gradually diluted by continued application of low TDS irrigation water. Due to the complex interaction of salt dissolution, mixing and dilution, the Fernley aquifer system is now characterized by three water types: 1) about 150 mg/l TDS mixed-cation- HCO3 river water; 2) 500 to 3000 mg/l TDS Na-Cl water and 3) 250 to 5000 mg/l TDS Na-SO4 water. Mixing of these three water types results in wide ranges of ground water chemistry. Mixing was identified between low TDS river water and high TDS Na-SO4 ground waters, and Na-SO4 and Na-Cl ground waters. Deep well samples from the central part of the basin indicate that the pre-irrigation ambient Na-Cl ground water type at depth has been mixed with Na-SO4 water in the upper 200 m of the aquifer, which is now discharging north into the Truckee River. A smaller source of high TDS water is ambient Na-Cl ground water flowing through a fault zone into the Truckee River at the northeastern basin periphery. Discharge of these two high TDS ground water sources are evident in the Truckee River's water chemistry during the low streamflow season.