Isotopic Indicators (223Ra,224Ra,226Ra,228Ra,222Rn,87Sr/86Sr) of Radium Mobilization and Retardation In the Mt. Simon Aquifer, Minnesota

The Cambrian Mt. Simon sandstone aquifer overlies Proterozoic basalt and Mid-Continent Rift sediments and constitutes a major water resource in Minnesota. Thirteen samples from wells open only to the aquifer lie along two transects converging in the Twin Cities area. ⁸⁷Sr/⁸⁶Sr decreases significantly from the recharge area (0.7120-0.7141, n=2) to the basin center where it underlies younger aquifers (0.7085-0.7097, n=11) and where Pleistocene recharge times are common. These waters contain enriched Br^- over seawater Br^-/Cl^- (~2x10^-3), indicating contributions from diluted brine.

All except the recharge-area wells exceed the radium MCL (median 12.0 pCi/L, range 2.9-27.7 pCi/L). Significant short-lived Ra (²²³Ra & ²²⁴Ra) suggests that (1) local water-rock interaction, rather than long-distance transport, contributes Ra; and (2) residence time alone does not cause Ra accumulation, although younger waters exhibit lower Ra activities. Although Ra activity is not well correlated with dissolved solids, we observe increasing ²²⁸Ra/²²⁶Ra activity ratios with increasing dissolved solids. This coincides with flowpaths from (1) recharge waters (²²⁸Ra/²²⁶Ra=0.44-0.82) to: (2) low-TDS, bicarbonate-dominated waters (²²⁸Ra/²²⁶Ra=0.97-1.55); and (3) waters significantly higher in sulfate and/or chloride (²²⁸Ra/²²⁶Ra=1.01-2.67). In sulfate-bearing waters, up to 40% of Ra is complexed as RaSO₄⁰. ²²⁸Ra/²²⁶Ra over 2 is usually associated with high SO₄^2-/Cl^- and barite supersaturation. Variation in ²²⁸Ra/²²⁶Ra in water may be caused by: (1) variation in the Th/U ratio of sandstone and/or (2) slow Ra incorporation into barite and the different decay constants of Ra isotopes. Thus, barite precipitation removes ²²⁶Ra most effectively because of the slow rate at which ²²⁶Ra is generated by recoil. That slow coprecipitation could influence ²²⁸Ra/²²⁶Ra implies inefficient adsorption to clays, Mn oxides, and/or Fe oxides. Inefficient Ra retardation is also suggested by low ²²²Rn/²²⁶Ra (median 69, range 7-618). Also, redox processes may be important at preventing Mn and Fe oxide precipitation and/or maintaining barite undersaturation by sulfate reduction.