Uranium in groundwaters: Insights from the Leinster granite, SE Ireland

Abstract

Uranium concentrations in groundwaters from >100 private wells in the Caledonian Leinster Granite Batholith of SE Ireland range from a few μg/l to >300 μg/l. Approximately 10% of the wells exhibit uranium concentrations that exceed the current WHO provisional guideline of 30 μg/l and a further 10% exceed the original WHO guideline value of 15 μg/l. New whole-rock geochemical analyses and laboratory batch leaching experiments confirm the granite as the ultimate source of the groundwater uranium, but carbonate-bearing glacial deposits that overlie the granite play an important role in facilitating the generation of soluble and mobile anionic uranyl carbonate complexes. Whilst the granite has average uranium concentrations broadly similar to other Irish and British Caledonian granites, it is enriched locally in uranium, with whole-rock values up to c. 30 mg/kg. Uraniferous zircon, apatite, monazite and associations with secondary Fe-bearing oxide minerals were detected in drillcore whole-rock samples that had the highest uranium concentrations, but uraninite was not detected. Broad positive correlations between groundwater uranium concentrations and bicarbonate alkalinity and electrical conductivity are found in both the sampled groundwaters and in laboratory leachates of the granite and surficial glacial deposits. Leachates from the granite had approximately an order of magnitude higher uranium concentrations compared with those from the overlying limestone-bearing glacial tills. (234U/238U) ratios for the groundwaters and the leachates always exceed unity, indicating preferential leaching of loosely bound radiogenic 234U from mineral surfaces or grain boundaries into the groundwaters, rather than wholescale dissolution of the primary uraniferous minerals in the granite. Strontium isotopes ratios in the groundwaters constrain the balance between limestone and granite-derived Sr, and by extension the sources of dissolved Ca. These data indicate that calcite dissolution is the main source of dissolved calcium, highlighting the role of weathering of overlying limestone-bearing glacial tills during recharge in determining the geochemistry of the groundwaters. Geochemical modelling indicates that in the oxidising groundwaters of the study area U6+ is dominant, in the form of UO2(CO3)3−4 at the high pH values (7.49–7.69) of waters from wells overlain by limestone-bearing till. By contrast, in a small number of wells overlain by granite-bearing till, modelling indicates that the dominant species is UO2(CO3)2−2 reflecting their lower pH (6.22–6.30). Overall, carbonate species play a major role in uranium solubility and mobility in these granite-hosted groundwaters.