CHEMICAL EVOLUTION OF SHALLOW GROUNDWATER ALONG THE NORTHEAST SHORE OF MONO-LAKE, CALIFORNIA

We attempted to quantitatively discriminate between hypothesized sourc es and geochemical processes responsible for the chemical evolution of shallow groundwater along the northeast shore of Mono Lake, an alkali ne saline lake located in a hydrologically closed basin in east centra l California. Shallow groundwater samples from 17 sites perpendicular and 11 sites parallel to the lakeshore were analyzed for major ions. T he shallow groundwater contains remnant solutes from higher lake stand s, which are transported toward the lake by lateral flow and surface r unoff. The flow system appears to be segregated into two different reg ions: a concentrated, highly saline groundwater underlying much of the northeast shore and pockets of more localized lower-salinity groundwa ter. The saturation stare of the groundwater with respect to certain m inerals was determined, and simulations for both evaporative concentra tion of inflow and mixing of lake water with inflow coupled with miner al precipitation were performed. Solute trends in the shallow groundwa ter result primarily from the degree of mixing with historical lake wa ter; however, evaporative concentration and redissolution cycles along with various chemical fractionation mechanisms including Ca, Mg, and Na carbonate precipitation, sulfate reduction, ion exchange, and poten tially Mg silicate formation are also important controls.