RADIUM IN THE JORDAN SANDSTONE AQUIFER (MINNESOTA): NEW INSIGHTS FROM ISOTOPIC MEASUREMENTS OF GROUNDWATER AND AQUIFER SOLIDS

Naturally-occurring radium-226 levels in the Cambrian Jordan sandstone aquifer (JDN) in southeastern Minnesota vary across two orders of magnitude (0.1 – 11 pCi L^-1) and many wells exceed EPA standards in the Twin Cities metropolitan area (TCMA). Local enrichment of ²²⁶Ra is associated with geographic trends and hydrogeologic setting. Unaffected waters show conditions expected in pure quartz sand aquifers (²²⁶Ra ≤ 1 pCi L^-1, ²²⁸Ra/²²⁶Ra ≈ 1), whereas high-²²⁶Ra waters exceed EPA drinking water standards (²²⁶Ra to 11 pCi L^-1, ²²⁸Ra/²²⁶Ra ≤ 0.5). This implies that high ²²⁶Ra and low ²²⁸Ra/²²⁶Ra are associated with solid-phase enrichments of ²²⁶Ra’s precursors in the ²³⁸U series consistent with the higher mobility of U than Th in groundwater systems. Low-Ra waters are commonly found in oxic, nitrate- and ³H-bearing subcropping sections of the aquifer and in isolated subcrops of JDN confined beneath thick tills in the western TCMA. Preliminary γ-spectrometric analysis of well cuttings from four wells in the TCMA indicates that ²²⁶Ra varies spatially and vertically by a factor of up to ~3, ²²⁸Ra is consistently near detector background, and ²²⁸Ra/²²⁶Ra occupies the same range as water samples.

Indirect evidence supports paleohydrologic fluxes and/or ingrowth from U deposition for anomalous ²²⁶Ra activities: (1) Similar enrichments of ²²⁶Ra > 3 pCi L^-1 with ²²⁸Ra/²²⁶Ra < 0.5 in the vertically distinct St. Peter and Jordan quartz sandstone aquifers and the Galena and Prairie du Chien carbonate aquifers; (2) the ~80 km N-S trend of ²²⁶Ra enrichment in the Prairie du Chien-Jordan subcrop at the extent of the Des Moines lobe tills south of the TCMA; and (3) a localized zone of ²²⁶Ra enrichment along the same N-S trend in the subcropping Galena carbonate aquifer. However, γ-spectrometric analysis of ²³⁸U activity in well cuttings in the TCMA is on the about same order as ²²⁶Ra, which could imply a more stable history for U-series radionuclides than geographic trends suggest. Additional solids analysis may clarify the nature of these radionuclide variations. Given the similarity of ²²⁸Ra/²²⁶Ra between rock and water samples, the observed Ra levels in groundwater are best explained by Ra release from locally-enriched aquifer solids and relatively inefficient adsorption of Ra in the anoxic, Mn- and Fe-reducing conditions of the aquifer.