Saline groundwater in Mono Basin, California: 2. Long‐term control of lake salinity by groundwater

Abstract

Mono Lake is a perennial, closed‐basin lake that has existed for at least 700 kyr. This 46‐m‐deep alkaline lake's present concentration is about 90,000 ppm total dissolved solids. Well logs show that beneath part of the lake, saline groundwater of concentration > 18,000 ppm extends to the bottom of the basin fill aquifer (Rogers and Dreiss, this issue). Numerical simulations indicate that redistribution of the basin's solutes between the lake and underlying saline groundwater body, driven by late Quaternary lake level changes, may be responsible for the high present‐day solute content of Mono Lake. At low lake levels, the high lake concentration causes solute loss via free convection; this might occur more rapidly through faults or fractures. The saline‐fresh groundwater interface position reflects a balance between the saline groundwater density and the force of inflowing fresh groundwater discharging near the shoreline. At higher lake stages, the shoreline discharge zone moves toward the basin edge, and the unrestrained saline groundwater mass subsides, drawing solutes from the lake into the basin sediments. Falling lake levels again constrict the saline groundwater beneath the lake, forcing saline water into the lake and increasing its solute content. The sediment permeability below the lake is the major control on the solute transfer rate between the lake and the groundwater reservoir. Only the larger, longer‐term lake stage changes cause saline groundwater movement; depending on the assumed basin fill aquifer permeability, equilibration of the saline groundwater and lake solute content with lake stage changes requires hundreds to thousands of years. Simulations suggest that the historical conditions, where a more saline Mono Lake (50,000–90,000 ppm) overlies less concentrated groundwater (apparently ∼ 18,000 ppm), could be due to the still present impact of late Pleistocene lake high stands. Mono Lake's historical salinity data have a large scatter, but suggest a 5% decrease in the lake's solute content over the last 50 years. We estimated the diffusive solute flux into lacustrine sediments for this period from core data. The salinity data are consistent with a combination of solute diffusion into sediments and an equal, or greater, model‐predicted advective solute loss to groundwater.