Controls on Spatial Variability of Uranium in Sandstone Aquifers

Controls on the spatial variability of uranium (U) in sandstone aquifers were investigated by water sampling and geophysical logging of wells in the Jacobsville Sandstone in Michigan’s Upper Peninsula. 235U/238U isotope ratios in well water are consistent with naturally occurring U, likely derived from U-enriched strata, which make wells in oxidized, alkaline aquifers at risk for supplying water with elevated U concentrations. Twenty-nine percent of 218 wells completed in the Jacobsville Sandstone have U concentrations above the U.S. Environmental Protection Agency’s maximum contaminant level (MCL), which is 30 µg/L. Although no regional trends are apparent, neighboring wells tend to have similar U concentrations, except where topography results in significant differences in well bottom elevations. Selected water wells in the Jacobsville Sandstone were logged with a gamma ray spectrometer to determine whether the spatial variability of ground water U concentrations correlates with the distribution of U in the sandstone. Not all wells with relative U enrichment in the sandstone produce water with U above the MCL, indicating that the influence of U enrichment in aquifer material may be modified by other hydrogeochemical factors. Ground water age dating may help explain variations in well water U concentrations by identifying water-leaching U for varying periods of time. These results may have implications for interpreting controls on the spatial variability of naturally occurring contaminants in ground water in other areas. This study supports the general utility of U isotope data to investigate rock water interactions.