Sr isotopes, hydrogeologic setting, and water-rock interaction in the Mt. Simon sandstone (Minnesota, USA)
We present new 87Sr/ 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 indicates 87Sr/ 86Sr from 0.7132 in shallow conditions to 0.7086 at depth. Proterozoic sandstone aquifers in shallow bedrock conditions exhibit 87Sr/ 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 observed 87Sr/ 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. © 2010 Taylor & Francis Group, London.
