RADIUM ISOTOPE RESPONSE TO AQUIFER STORAGE AND RECOVERY: EXAMPLE OF A MINNESOTA SANDSTONE AQUIFER

Radium (Ra) differs from other trace elements of health significance (e.g. As) by its behavior represented by a balance of physical (alpha recoil) and chemical (e.g. adsorption) processes. To evaluate the practicality of aquifer storage and recovery in the Cambrian Mt. Simon sandstone aquifer of Minnesota, an aquifer that naturally yields levels of radium often requiring treatment prior to use, a pilot injection study was conducted on behalf of three cooperating cities in the northwest Twin Cities Metropolitan Area. Here we report results of a supplemental radium investigation conducted in logistical coordination with the pilot study. 63 × 10⁶ gal (2.4 × 10⁵ m³) of groundwater from other sources were injected over ~4.5 months and stored for ~3 months after the end of injection. Subsequent recovery of 52 × 10⁶ gal (2.0 × 10⁵ m³) over ~2 months resulted in total storage time of 3-9 months. The recovered water resembled the injected water in terms of nonreactive ions such as Cl^-, which implies that only minor mixing of native groundwater with injected water occurred. However, the radium content of the recovered water exceeded what would be expected from mixing of injected water (²²⁶Ra+²²⁸Ra < 2 pCi/L) with native groundwater (²²⁶Ra+²²⁸Ra > 6 pCi/L). During recovery, short-lived ²²⁴Ra activity stabilized within hours, whereas long-lived ²²⁶Ra and ²²⁸Ra increased gradually for two months (0.7-2.2 and 1.0-2.3 pCi/L, respectively), approaching the maximum contaminant level of 5 pCi/L ²²⁶Ra+²²⁸Ra. The rapid increase of ²²⁴Ra, and ²²⁴Ra/²²⁸Ra ratios >1 after the first two hours (1.5-2.6), are consistent with alpha recoil (release of Ra from the α-decay of solid-phase Th) as a major Ra-releasing mechanism because ²²⁴Ra is replaced more rapidly than the longer-lived ²²⁸Ra. Decreasing ²²⁸Ra/²²⁶Ra during recovery (1.8-1.0) could be interpreted as a recoil effect due to half-life differences and/or as an indicator of mixing between injected water and native groundwater. Overall, activities of ²²⁶Ra and ²²⁸Ra in recovered water are influenced by alpha recoil during storage and/or mixing of high-Ra native groundwater into the injected water. These results imply that longer storage duration than documented here may yield higher levels of ²²⁶Ra+²²⁸Ra in recovered water.