Radium isotopes to trace uranium redox anomalies in anoxic groundwater.

• 223Ra, 224Ra, 226Ra, and 228Ra isotopes were measured in groundwater as deep as 1000 m to understand the reason for elevated uranium in anoxic groundwater. • An approach to determine the residence time of Holocene-age groundwaters using radium isotope activity ratios was developed and tested. • Groundwater [223Ra/228Ra] activity ratios indicated that U aq is not related to U solid -enriched fractures. • Radium isotopes, when combined with geochemical modelling, provide insight into the temporal evolution of redox systems in deep groundwater. 223Ra, 224Ra, 226Ra, and 228Ra isotopes have been measured in groundwaters from depths ranging 50–900 m in fractured crystalline bedrock (Forsmark, Sweden) to understand the reason for elevated (up to 150 μg/L) aqueous uranium (U aq) at 400–650 m 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 [U aq ] 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 Ca 2 UO 2 CO 3 0 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 UO 2+X at the interface between marine and non-marine groundwaters. From these data, local [U aq ] is proposed to be governed by on-going water-rock interaction involving old U(VI)-minerals. [ABSTRACT FROM AUTHOR]