Quantity and quality of groundwater discharge in a hyper-saline lake environment, Great Salt Lake, Utah, USA

Previous studies in Great Salt Lake (GSL) suggest that unmeasured sources of selenium (Se) may enter the lake via groundwater discharge. A fiber-optic distributed temperature sensing (FO-DTS) survey was performed in the south arm of GSL during 2010 to identify near-shore areas of groundwater discharge. The FO-DTS data found there are consistent cold-water temperature anomalies with measured seepage rates between 0.01–2.37 cm/day. The CRP survey also identified three continuous layers in the subsurface: (1) a top conductive layer 2 m thick with resistivity of 0.5 ohm-m; (2) a middle 3-4 m thick resistive layer with resistivity about 4-5 ohm-m; and (3) a bottom conductive layer with a resistivity about 0.5-1 ohm-m. The middle layer is likely mirabilite (Na₂SO₄·10 H₂O), a hard impermeable salt, which appears to decrease in thickness in a W-NW direction towards the lake. Water-level observations indicate the aquifer is semiconfined and supports the CRP data showing a thinning or disappearance of the salt layer. The positive seepage suggests the mirabilite may not be as impermeable as it seems or the top 1 m of the sand above the hard salt layer is acting as a shallow aquifer which transmits shallow groundwater discharge. The R/Ra values in the groundwater are <1 and the tritium concentrations (1.2-2.0 TU) in the groundwater suggests the water has components of old and young groundwater. Geochemical modeling shows that mirabilite saturation index values for the groundwater samples were all slightly >0, indicating the water is at equilibrium with respect to mirabilite. The open water of GSL along the south shoreline had Se concentrations (1.9 µg/L) 3-4 times higher than 4 open water sites in the south arm of GSL (0.5 µg/L) measured during a 2006-2008 study. Geochemical modeling was performed on a groundwater sample collected 2 km north of a mine tailings pond containing elevated arsenic (As) and low Se concentrations. The modeling results suggest that under reducing conditions, As-bearing minerals are mobilized while Se-bearing minerals will likely precipitate out of solution possibly explaining why the shallow groundwater below the hard salt layer have low concentrations of Se (0.9-2.3 µg/L).