| Summary: | <p><sup>223</sup>Ra, <sup>224</sup>Ra, <sup>226</sup>Ra, and <sup>228</sup>Ra 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<sub>aq</sub>) at 400–650 m depth. Ra isotope data is interpreted alongside previously reported <sup>222</sup>Rn, <sup>234</sup>U, and <sup>238</sup>U data, as well as PHREEQC geochemical modelling and uranium mineralogy. A novel, [<sup>223</sup>Ra/<sup>226</sup>Ra]<sub>GW</sub>-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<sub>aq</sub>] is most common in Holocene-age groundwaters of marine origin. Although these groundwaters are geochemically reducing, the [<sup>223</sup>Ra/<sup>228</sup>Ra]<sub>corr</sub> (where “corr” subscript refers to a correction applied to compare [<sup>223</sup>Ra/<sup>228</sup>Ra]<sub>GW</sub> to the more commonly reported [<sup>226</sup>Ra/<sup>228</sup>Ra]<sub>GW</sub>) 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<sub>2</sub>UO<sub>2</sub>CO<sub>3</sub><sup>0</sup> 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 [<sup>223</sup>Ra/<sup>228</sup>Ra]<sub>corr</sub> and [<sup>224</sup>Ra/<sup>228</sup>Ra]<sub>GW</sub> also suggest that U(VI)<sub>aq</sub> precipitates as UO<sub>2+X</sub> at the interface between marine and non-marine groundwaters. From these data, local [U<sub>aq</sub>] is proposed to be governed by on-going water-rock interaction involving old U(VI)-minerals.</p>
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