Geochemistry of Ground Water at and Near Fernley Wildlife Management Area, Lyon County, Nevada Michael S. Lico U.S. Geological Survey Water Resources Division Nevada District Office Room 227, Federal Building Carson City, Nevada 89701 Abstract The geochemistry of ground water in a shallow alluvial- lacustrine aquifer beneath the Fernley Wildlife Management Area and vicinity in northwestern Nevada was studied as part of the Department of the Interior irrigation-drainage program to determine sources of potentially toxic elements found in aquatic biota. The aquifer consists of unconsolidated silt and clay with some sand and gravel layers, and is composed of volcanic-lithic fragments (plagioclase, augite, and hypersthene in a felsitic groundmass), quartz, feldspars, micas, mafic minerals, and clay minerals. The ground water ranges from a dilute sodium-bicarbonate type water (dissolved solids, about 360 milligrams per liter) to a very saline sodium-chloride type (dissolved solids, about 16,000 milligrams per liter). The pH ranges from 7.5 to 8.7, and the Eh ranges from strongly oxidizing to slightly reducing. A geochemical-reaction model was constructed from water quality data, mineralogic data, and thermodynamic calculations. The model results indicate that the major-element aqueous chemistry is controlled by precipitation-dissolution reactions, exchange processes, and evapotranspiration. Calcite and montmorillonite are precipitating and gypsum and volcanic-rock fragments (as seen visually) are dissolving within the aquifer. Clay-mineral surfaces are adsorbing calcium and releasing sodium. The trace elements antimony, arsenic, boron, molybdenum, phosphorus, uranium, and vanadium probably are derived from dissolution of volcanic-rock fragments or surface coatings on aquifer material. Evapotranspiration concentrates the dissolved elements. These processes in combination account for the large increase in concentrations of dissolved solids and trace elements observed in the shallow ground water.