Quantity and quality of groundwater discharge in a hypersaline lake environment

Geophysical and geochemical surveys were conducted to understand groundwater discharge to Great Salt Lake (GSL) and assess the potential significance of groundwater discharge as a source of selenium (Se). Continuous resistivity profiling (CRP) focusing below the sediment/water interface and fiber-optic distributed temperature sensing (FO-DTS) surveys were conducted along the south shore of GSL. FO-DTS surveys identified persistent cold-water temperature anomalies at 10 separate locations. Seepage measurements were conducted at 17 sites (mean seepage rate = 0.8 cm/day). High resistivity anomalies identified by the CRP survey were likely a mirabilite (Na2SO4·10H2O) salt layer acting as a semi-confining layer for the shallow groundwater below the south shore of the lake. Positive seepage rates measured along the near-shore areas of GSL indicate that a ∼1-m thick oolitic sand overlying the mirabilite layer is likely acting as a shallow, unconfined aquifer. Using the average seepage rate of 0.8 cm/day over an area of 1.6 km2, an annual Se mass loading to GSL of 23.5 kg was estimated. Determination of R/Ra values (calculated 3He/4He ratio over the present-day atmospheric 3He/4He ratio) <1 and tritium activities of 1.2–2.0 tritium units in groundwater within and below the mirabilite layer indicates a convergence of regional and local groundwater flow paths discharging into GSL. Groundwater within and below the mirabilite layer obtains its high sulfate salinity from the dissolution of mirabilite. The δ34S and δ18O isotopic values in samples of dissolved sulfate from the shallow groundwater below the mirabilite are almost identical to the isotopic signature of the mirabilite core material. The saturation index calculated for groundwater samples using PHREEQC indicates the water is at equilibrium with mirabilite. Water samples collected from GSL immediately off shore contained Se concentrations that were 3–4 times higher than other sampling sites >25 km offshore from the study site and may be originating from less saline groundwater seeps mixing with the more saline water from GSL. Additional evidence for mixing with near shore seeps is found in the δD and δ18O isotopic values and Br:Cl ratios. Geochemical modeling for a water sample collected in the vicinity of the study area indicates that under chemically reducing conditions, arsenic- (As) bearing minerals could dissolve while Se-bearing minerals will likely precipitate out of solution, possibly explaining why the shallow groundwater below and within the mirabilite salt layer contains low concentrations of Se (0.9–2.3 μg/L).

Raw project data is available by contacting [e-mail]

为探究大盐湖（GSL）的地下水排泄情况并评估地下水排泄作为硒（Se）来源的潜在重要性，开展了地球物理和地球化学调查。针对沉积物/水体界面的下方进行了连续电阻率剖面（CRP）和光纤分布式温度传感（FO-DTS）调查。FO-DTS调查在GSL南岸10个不同位置识别出持续的冷水温度异常。在17个地点进行了渗流测量（平均渗流速率=0.8 cm/天）。通过CRP调查识别出的高电阻率异常很可能是由作为浅层地下水半限制层的芒硝（Na2SO4·10H2O）盐层引起的。沿GSL近岸区域测得的正渗流速率表明，覆盖在芒硝层之上的约1米厚的生核状砂层很可能充当一个浅层、无限制的含水层。利用0.8 cm/天的平均渗流速率和1.6 km²的面积，估计GSL每年硒的质量负荷为23.5 kg。在芒硝层内及其下方的水中，R/Ra值（计算为3He/4He比值相对于现今大气中3He/4He比值的比值）小于1，以及1.2–2.0个三氚单位的活动性，表明区域性和局部地下水流动路径向GSL排泄的汇聚。芒硝层内及其下方的水中，其高硫酸盐盐度来源于芒硝的溶解。从芒硝层下方的浅层地下水中溶解的硫酸盐样品的δ34S和δ18O同位素值几乎与芒硝核心材料的同位素特征一致。使用PHREEQC计算的水样饱和指数表明，水与芒硝处于平衡状态。从GSL岸边立即采集的水样中硒浓度比研究地点25 km以外的其他采样点高3–4倍，可能源自与GSL更咸的水混合的较少盐度地下水渗漏。在δD和δ18O同位素值以及Br:Cl比值中发现的额外证据表明了与近岸渗漏的混合。在研究区域附近采集的水样进行地球化学建模表明，在化学还原条件下，含砷（As）矿物可能会溶解，而含硒（Se）矿物可能从溶液中沉淀出来，这或许可以解释为什么在芒硝盐层下方和内部的浅层地下水中硒浓度较低（0.9–2.3 μg/L）。原始项目数据可通过联系[e-mail]获取。