T211 Sources, Transport, and Fate of Trace and Toxic Elements in the Environment

Tuesday, 2 November 2010
1:30 PM - 5:30 PM
Room 207

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

Paper number: T211-13
Program: Technical Sessions
Time: 4:20 PM – 4:35 PM

Authors

Abstract

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.