Near-surface (<100 m) ground water in east-central Michigan basin is characterized by high total dissolved solids (100 to 53,000 mg/l). The source of the salinity and the cause of its distribution are unclear. The hypothesis investigated was that the source of the salinity is brines advecting or diffusing upward into near-surface meteoric water, and the cause of the salinity distribution is the slow flushing of water in the argillaceous sediments by recent meteoric water. δ18O, δD, δ13C, δ34S, 3H, and selected geochemical variables were studied in the glacial-drift and bedrock aquifers composing the ground-water system.

The following isotopic ranges were found: δ18O, 6.93‰ to -18.46‰ δD, -47.3‰ to -137.4‰ δ13C, -6.26‰ to -20.65‰ and δ34S, +3.56‰ to +67.44‰. δ18O and δD plot along the meteoric-water line, and most are extremely light. δ13C is skewed to lighter values, and δ34S becomes heavier with decreasing dissolved SO4 concentration.

The results are interpreted to indicate that the saline ground water is a mix of modern-day, meteoric water and water that recharged the aquifers at a time when the climate was much cooler. The cause of the salinity distribution is the slow flushing of the aquifers by present-day recharge water. The source of the salinity could be the upward diffusion or advection of Michigan basin formation water, but this has not been confirmed. Microbial reduction of SO4 is occurring, affecting the concentrations of SO4 and HCO3 and the values of δ13C and δ34S.

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