Mixing of deep basinal brines and glacial meltwater inferred from major ion chemistry, stable isotopes and noble gases in the Saginaw aquifer, Michigan

Water samples were collected from the shallow Saginaw aquifer in southern Michigan and analyzed for major ion chemistry, stable isotopes and noble gases. Major ion composition of Saginaw aquifer waters indicate at least 3 geochemically distinct water types: CaMgHCO₃, NaHCO₃ and NaCl. In particular, samples closer to the recharge area are predominantly of the CaMgHCO₃ type and evolve into a NaCl dominant water type towards the discharge area. Comparison of measured Cl^- and Br^- from the Saginaw aquifer water samples with deep basinal brines suggests that high salinities near the discharge areas are the result of mixing of fresh water and deep basinal brines transported through upward cross formational flow. An almost linear trend observed between Na⁺, Mg²⁺, K⁺ and Br^- for the Saginaw water samples near the discharge area and deep basinal brines further suggests upward cross formational flow and subsequent mixing. Stable isotope measurements for Saginaw water samples yield large isotopic variations, with values ranging from -17.48 to -8.49 for δ¹⁸O and from -124.03 to -59.17 for δD. All samples lie close to the global and local meteoric water line indicating that they originated as meteoric recharge. Stable isotopic composition of Saginaw groundwater samples were also compared with measured values of local rainfall and snow samples. These two end members vary between -23.7 to -20.1 for δ¹⁸O and -173.7 to -147.5 for δD in rainfall and -6.8 to -2.2 for δ¹⁸O and -35.4 to -11.1 for δD in snow with all Saginaw samples falling in between. Based on mean values for these two end members, the amount of recharge derived from snow melt for samples closer to the recharge area is ≤48% while samples closer to the discharge area point to glacial recharge contribution of up to 74%. Measured neon concentrations in excess of air-saturated water equilibrated at recharge area conditions also display large variations with values ranging between 9.6% and 125% for ΔNe. In particular, samples closer to the recharge area have less than 45% excess neon while samples closer to the discharge area have greater than 90% excess. The latter result suggests glacial meltwater addition either due to the ability of ice to accommodate Ne or recharge conditions that favor excess air. Thus, both stable isotopes and noble gas composition suggest that NaCl type waters found in the Saginaw discharge area were likely derived from a time period when climate was cooler than present and significant recharge occurred from glacial meltwater. The observed negative correlation between excess neon and δ¹⁸O and δD respectively, highlights the potential of excess neon as a tracer of glacial melt water in groundwater systems.