Geochemical evolution of ground water in Smith Creek Valley—a hydrologically closed basin in central Nevada, U.S.A

Smith Creek Valley is a hydrologically closed basin in which ground water is recharged by subsurface inflow from sorrounding mountains and infiltration of streamflow into alluvial-fan deposits near the mountains. Ground water is discharged by evapotranspiration from shallow ground-water areas in the central part of the basin. Dominant ions in the dilute recharge water are Na, Ca and HCO 3. Dissolved solids concentration increases during flow through the basin-fill sediments, with Na becoming increasingly dominant. In the discharge area, a bare-soil playa sorrounded by phreatophytic vegetation, ground-water salinity, dominated by Na and Cl, increases markedly. The main processe controlling geochemical evolution of ground water in the basin-fill aquifer were identified using major-ion chemistry, mass-balance calculations, thermodynamic calculations, stable isotopes, and mineral identification. These processes are: (1) dissolution of volcanic tuff and tuff-derived basin-fill deposits; (2) cation exchange of Ca and Mg in the water for Na in clay minerals; (3) weathering of plagioclase to montmorillonite; (4) precipitation of zeolite minerals; (2) concentration of dissolved constituents by evapotranspiration; (6) dissolution of Cl and SO 4 evaporative salts; (7) precipitation of calcite.