Sr isotopes, hydrogeologic setting, and water-rock interaction in the Mt. Simon Sandstone (Minnesota, USA)

We present new⁸⁷Sr/⁸⁶Sr ratios in groundwater from Proterozoic and Cambrian sandstone aquifers of southeastern Minnesota (USA), emphasizing the Mt. Simon aquifer. In Mt. Simon waters,⁸⁷Sr/⁸⁶Sr ranges from 0.7141 in the recharge area to 0.7085 at depth. Sampling of the overlying Franconia, Ironton, and Galesville aquifers similarly indicates⁸⁷Sr/⁸⁶Sr from 0.7132 in shallow conditions to 0.7086 at depth. Proterozoic sandstone aquifers in shallow bedrock conditions exhibit⁸⁷Sr/⁸⁶Sr of 0.7103–0.7119. Sr sources to the Mt. Simon include: (1) more radiogenic Sr derived from overlying Cretaceous rocks and/or Pleistocene glacial deposits that incorporate Precambrian crystalline rocks and Cretaceous sedimentary rocks; and (2) less radiogenic Sr associated with Na-bearing waters, possibly from plagioclase weathering. Overall, the observed⁸⁷Sr/⁸⁶Sr and correlation with carbon-14 activity record the Sr system’s sensitivity to groundwater evolution from the recharge area to deep, slow-circulating areas and contributions from multiple Sr sources associated with hydrogeologic setting.