Radium isotopes to trace uranium redox anomalies in anoxic groundwater

Abstract

223Ra, 224Ra, 226Ra, and 228Ra isotopes have been measured in groundwaters from depths ranging 50–900m in fractured crystalline bedrock (Forsmark, Sweden) to understand the reason for elevated (up to 150μg/L) aqueous uranium (Uaq) at 400–650m depth. Ra isotope data is interpreted alongside previously reported 222Rn, 234U, and 238U data, as well as PHREEQC geochemical modelling and uranium mineralogy. A novel, [223Ra/226Ra]GW-based approach (where brackets and “GW” subscript refer to expression of an activity ratio measured from groundwater) to groundwater residence time estimation shows that elevated [Uaq] is most common in Holocene-age groundwaters of marine origin. Although these groundwaters are geochemically reducing, the [223Ra/228Ra]corr (where “corr” subscript refers to a correction applied to compare [223Ra/228Ra]GW to the more commonly reported [226Ra/228Ra]GW) suggest that they interact with U-rich pegmatites containing Proterozoic- and Palaeozoic-age Ca-U(VI)-silicate minerals, which are undersaturated in the present groundwaters. Local aqueous U(VI) can be stabilized in Ca2UO2CO30 complexes at pe-values as low as −4.5 but is susceptible to reduction after a modest decrease in pe-value, alkalinity, or Ca concentration. The [223Ra/228Ra]corr and [224Ra/228Ra]GW also suggest that U(VI)aq precipitates as UO2+X at the interface between marine and non-marine groundwaters. From these data, local [Uaq] is proposed to be governed by on-going water-rock interaction involving old U(VI)-minerals.