239-6 Isotopic Indicators (223Ra,224Ra,226Ra,228Ra,222Rn,87Sr/86Sr) of Radium Mobilization and Retardation In the Mt. Simon Aquifer, Minnesota

Tuesday, 7 October 2008: 9:15 AM
George R. Brown Convention Center, 352DEF
David S. Vinson1, James R. Lundy2, Gary S. Dwyer1 and Avner Vengosh1, (1)Division of Earth & Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, NC
(2)Minnesota Department of Health, St. Paul, MN
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 SO42-/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.