Geochemical and hydrologic dynamics in evaporative groundwater-dominated lakes of glaciated Montana and North Dakota

Evaporative groundwater-dominated lakes have higher evaporation than precipitation and little surface water inflow or outflow, raising salinity over groundwater levels. The glaciated semi-arid Great Plains contain numerous such lakes, exhibiting a range in salinity up to saturated brines ({dollar}>{dollar}200 g/L). Because groundwater is the source of solutes, coupled fluid-chemical mass-balance modeling elucidates lake-groundwater interaction. In two typical lakes (a fresh marl lake, 4-7 g/L, pH 9.3; a hypersaline playa, 200-240 g/L, pH 9.6), authigenic carbonates (aragonite, dolomite, hydromagnesite) and sodium-sulfate salts (mirabilite, thenardite), respectively, are dominant mineral precipitates from groundwater. These two types may occur in hydraulically interconnected series.; Composition of groundwater is different from and less saline than lake water. Lake-water evolution may be modeled as the product of evaporation from shallow groundwater (possibly with some admixture from intermediate-depth groundwater) under slight carbonate-supersaturation as observed in marl lakes (pCO{dollar}sb2{dollar} = 10{dollar}sp{lcub}-3.49{rcub}{dollar}, SI{dollar}sb{lcub}rm aragonite{rcub}{dollar} = +0.4). Observed Na, SO{dollar}sb4{dollar}, and Cl in lake waters concentrate conservatively; Mg deviates from a conservative trend, suggesting that carbonate reactions like dolomitization (observed locally) remove it from solution. Contribution from deep ({dollar}>{dollar}300 m) groundwater is likely small. The causal relationship between lake chemistry and groundwater source allows inferences to be drawn from lake water chemistry regarding aquifer and depth of circulation.; {dollar}sp{lcub}210{rcub}{dollar}Pb dating of marl yields a post-1960 sedimentation rate of 0.8-1.0 kg/m{dollar}sp2{dollar}/year, higher than before European settlement (1910). The recent increase is attributed in part to eolian transport resulting from tillage and drought, but may also reflect changes in groundwater recharge. The period from 1920 to 1940 was marked by especially high rates of eolian transport of detrital silicate and carbonate minerals (quartz, feldspar, clays, calcite, dolomite).; Observed salinity response to recent drought varied between lakes. Coupled fluid-chemical mass balance modeling shows that factors influencing response include amplitude and period of climatic fluctuation, volume and shape of lake basins, lake outflow characteristics, and sensitivity of recharge to climatic perturbation. Typical lake volume and outflow characteristics impart non-linear response to even simple climatic forcing. Similar models incorporating hydrologic elements may be useful for interpretation of Holocene climatic variations from sediment cores.