GEOCHEMISTRY OF K-RICH ALKALINE LAKES, WESTERN SANDHILLS, NEBRASKA, USA

The Alkali Lakes region of the western Sandhills, Nebraska, has lakes that range in composition from freshwater to brine with TDS exceeding 250,000 mg/l. An unusual geochemical feature of these lakes is the con servative behavior of K with concentrations exceeding 1,900 mmoles/kg (86,000 mg/l). The lakes are dominantly Na-K-CO3-(SO4)-(Cl) and Na-K-S O4-CO3-(CI) Waters. Lakes occupy interdunal areas where there is littl e or no surface runoff. Groundwater primarily from locally derived pre cipitation is the principle source of water and solutes. This origin f or the source water contrasts with closed-basin saline lake complexes, where surface water from the adjacent areas flows directly into the l akes or recharges the groundwater system. The principle geochemical pr ocess controlling lake chemistry is evaporative concentration. Other p rocesses are operating, but to a lesser extent; these include mineral precipitation and dissolution and organically mediated sulfate reducti on. Geochemical mass-balance modeling indicates distinct differences i n the amount of water that is required to be evaporated to produce the observed lake compositions. These differences are related to the grou ndwater inflow to outflow ratio for individual lakes. This emphasizes that, although evaporation and related processes control the geochemic al evolution, the local hydrology of individual lakes regulates the ex tent to which these processes will proceed.