Radium isotopes to trace uranium redox anomalies in anoxic groundwater

Ra-223, Ra-224, Ra-226, and Ra-228 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 mu/L) aqueous uranium (U-aq) at 400-650 m depth. Ra isotope data is interpreted alongside previously reported Rn-222, U-234, and U-238 data, as well as PHREEQC geochemical modelling and uranium mineralogy. A novel, [Ra-223/Ra-226] 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 [Ra-223/Ra-228](corr) (where "corr" subscript refers to a correction applied to compare [Ra-223/Ra-228](GW) to the more commonly reported [Ra-226/Ra-228](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 [Ra-223/Ra-228](co)(rr) and [Ra-224/Ra-228](GW) also suggest that U(VI)(aq) precipitates as UO2+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.