Birgitta Backman

Uranium in surface and groundwaters in Boreal Europe

ABSTRACT 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.

Natural geochemical concentrations and fluxes of Cu, Th and U in Finland

Concentrations of U, Th and Cu were studied as a part of the coordinated IAEA (International Atomic Energy Agency) research project on ‘The use of selected safety indicators (concentrations; fluxes) in the assessment of radioactive waste disposal’. Uranium and Th served as analogues of radionuclides in the waste matrix (spent fuel) and Cu is a construction material in the Finnish disposal approach in granitic rock. Release and migration of these components through the geosphere to the biosphere is thought to be caused and influenced by corrosion, oxidation and dissolution and subsequent transport and retardation processes in groundwater in hydraulically conducting rock fractures. The weathering rates for Cu, U and Th were calculated on the basis of the weathering rate of base cations and the concentrations of these elements in the parent soils. The mean weathering rate for Cu in till soils is 0.21 mg m−2 a−1. The mean values of estimated weathering rates for Th and U are 0.17 mg m−2 a−1 and 0.054 mg m−2 a−1, respectively. Analytical data and flux measurements from groundwater monitoring of selected springs are used to calculate the quantity of dissolved elements discharging from the aquifer. Typical fluxes for five small springs were 0.023–0.46 kg Cu km−2 a−1 and 0.003–0.43 kg U km−2 a−1. The estimated fluxes of Cu and U in headwater streams were calculated on the basis of concentration data and the rate of stream flow. The estimated fluxes in 30 streams are 0.44–1418 g/month for Cu and 0.08–285 g/month for U. From the point of view of the use of natural elemental concentrations and fluxes as natural safety indicators in the evaluation of nuclear waste repository, the basis of this complementary or alternative concept has been corroborated by the results of this work. Elemental concentration data can be better understood in relation to the underlying geology and the prevailing geochemistry and by correlation to observed elemental fluxes. The comparison of the spatial distribution of elements in different matrices gives a qualitative estimation of the dominating fluxes. Widespread correlation between different compartments indicates mobile element fluxes from the geosphere to the biosphere. Also fixation in sinks, in particular, for U was observed. The geochemical cycles are dominated by near-surface weathering processes. Fluxes are also controlled by small-scale, local groundwater flow patterns; contributions from greater depths were found to be negligible.

A hydrogeological model for groundwater management of a shallow low-lying coastal aquifer in southern Finland under climate change

A shallow low-lying coastal sand aquifer in southern Finland is vulnerable to the climate change and human activities. Under future climate change, a rise in sea-level would cause some parts of the aquifer and the water intake well to be under seawater. This, together with the predicted increase in precipitation, would enhance groundwater recharge and raise the water table, consequently contributing to the potential deterioration of groundwater quality or potential flooding in the low-lying aquifer area. An information on geological and hydrogeological characteristics of the aquifer for the climate change adaptation plan including the possible new locations of water intake wells was needed. This study aimed to construct a three-dimensional geological model and evaluate heterogeneity of the aquifer to provide a geological framework for groundwater flow model and the assessment of groundwater vulnerability. The methods used consist of a stochastic-geostatistical approach incorporated with groundwater flow model to predict the distributions of the superficial layers of a heterogeneous aquifer and to identify the distributions of the aquifer medias (sand and gravel) as well as groundwater flow system. In addition, the LiDAR-based digital elevation model was utilized to define the flood prone areas under the climate change scenarios. The three-dimensional geological model provides a better characterization of the heterogeneity of the aquifer and improved reliability of subsequent groundwater flow model and vulnerability assessment in the aquifer area. The proposed new locations of water intake wells and the results of the study provided useful information for local authorities for groundwater management in future.