This study focuses on uranium (U) in surface and groundwaters in Boreal Europe (Sweden, Finland, Russia). Data from recently completed regional hydrogeochemical surveys and from site-specific studies were combined, in order to enhance the current understanding of U behaviour in the catchments and water bodies of these northerly latitudes. Over Precambrian areas (dominated by igneous and metamorphic rocks) the aqueous U concentrations in general increased in a downward direction, i.e. from stream waters to overburden groundwaters to bedrock groundwaters, and they were correlated with the U abundance in the surrounding overburden (mainly glacial till). Over Phanerozoic areas (dominated by terrigene deposits containing or composed of limestone) the aqueous U concentrations were, in contrast, unrelated to overburden U concentrations and strongly correlated with dissolved Ca and HCO3 concentrations. There is thus an overall geochemical and hydrochemical control, respectively, related to the underlying lithology. At geologically specific and local sites there is a range of correlations and control mechanisms of aqueous U. From acid sulphate soils, occurring abundantly on coastal plains, runoff below pH 4.0 is enriched in U (up to 55 μg/l) most likely due to oxidation of U(IV) minerals followed by subsequent limited sorption of U(VI) in the acidic environment. In a studied black shale setting, characterized by high U concentrations (up to >200 ppm), U levels increased in groundwater (up to 200 μg/l) and surface water (up to 80 μg/l) as the conditions changed from reducing to oxidizing. In an unmineralized granitic setting, proposed as a repository for spent nuclear fuel, elevated U concentrations in surface waters (up to 25 μg/l) reflect a regional stream-hydrochemical anomaly and in bedrock groundwaters (up to >100 μg/l), most likely mobilization of uranyl from U-rich fracture coatings. In the Baltic Sea, which has unique brackish water, the ratio of U to Cl is similar to that in the oceans but contrasting near-coastal U trends exist, characterized by an inverse relationship between U and Cl concentrations. These coastal-water anomalies are most likely caused by high U levels in inflowing streams, and possibly to some extent submarine discharge of U-enriched waters.

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