Abstract: Numerous groundwater analyses from the crystalline bedrock in the Forsmark area have been performed between 2002 and 2019, together with thorough geological, geophysical, and hydrogeological studies, within the site investigations carried out by the Swedish Nuclear Fuel and Waste Management Company. The groundwater samples have been taken from boreholes down to ≈ 1000 m and the analysis include major- and trace-elements, stable and radiogenic isotopes, gases and microbes. The chemical and isotopic composition of these groundwaters evidences the presence of non-marine brackish to saline groundwaters with very long residence times (many hundreds of thousands of years) and a series of complex mixing events resulting from the recharge of different waters over time: glacial meltwaters, probably from different glaciations of which the latest culminated some 20,000 years ago, and marine waters from the Baltic starting some 7000 years ago. Later, meteoric water and present Baltic Sea water have recharged in different parts of the upper 100 m. These mixing events have also triggered chemical and microbial reactions that have conditioned some of the important groundwater parameters and, together with the structural complexity of the area, they have promoted a heterogeneous distribution of groundwater compositions in the bedrock. Due to these evident differences in chemistry, residence time and origin of the groundwater, several groundwater types were defined in order to facilitate the visualisation and communication. The differentiation (linked to the paleohydrological history of the area) was based on Cl concentration, Cl/Mg ratio (marine component), and δ18O value (glacial component). The work presented in this paper increases the understanding of the groundwater evolution in fractured and compartmentalised aquifers where mixing processes are the most important mechanisms. The model proposed to characterise the present groundwater system of the Forsmark area will also help to predict the future hydrogeochemical behaviour of the groundwater system after the construction of the repositories for the nuclear wastes.

Abstract: This paper reviews critical features of basin-related uranium mineral systems in Australia. These mineral systems include Proterozoic unconformity-related uranium systems formed predominantly from diagenetic fluids expelled from sandstones overlying the unconformity, sandstone-hosted uranium systems formed from the influx of oxidised groundwaters through sandstone aquifers, and calcrete uranium systems formed from oxidised groundwaters flowing through palaeochannel aquifers (sand and calcrete). The review uses the so-called ‘source-pathway-trap’ paradigm to summarise critical features of fertile mineral systems. However, the scheme is expanded to include information on the geological setting, age and relative timing of mineralisation, and preservation of mineral systems. The critical features are also summarised in three separate tables. These features can provide the basis to conduct mineral potential and prospectivity analysis in an area. Such analysis requires identification of mappable signatures of above-mentioned critical features in geological, geophysical and geochemical datasets. The review of fertile basin-related systems shows that these systems require the presence of at least four ingredients: a source of leachable uranium (and vanadium and potassium for calcrete-uranium deposits); suitable hydrological architecture enabling connection between the source and the sink (site of accumulation); physical and chemical sinks or traps; and a post-mineralisation setting favourable for preservation. The review also discusses factors that may control the efficiency of mineral systems, assuming that world-class deposits result from more efficient mineral systems. The review presents a brief discussion of factors which may have controlled the formation of large deposits in the Lake Frome region in South Australia, the Chu-Sarysu and Syrdarya Basins in Kazakhstan and calcrete uranium deposits in the Yilgarn region, Western Australia.

Abstract: The concentration of the bioavailable uranium fraction (Ubio) at the European scale was deduced by geochemical modelling considering several definitions found in the literature and the FOREGS European stream waters geochemical atlas dataset to produce a Ubio baseline. A sensitivity analysis was performed using three thermodynamic databases. We also investigated the link between total dissolved uranium (Uaq) concentrations, speciation and global stream water chemistry on the one hand, and the lithology and ages of the surrounding rocks on the other. The more U-enriched the stream sediments or rock type contexts are, which tends to be the case with rocks containing silicates (4.1 mg/kg), the less U-concentrated the stream waters are (0.15 μg/L). Sedimentary rocks lead to slightly higher Uaq concentrations (0.34 μg/L) even if the concentration in sediment (Used) is relatively low (1.6 mg/kg). This trend is reversed for Ubio, with higher concentrations in a crystalline context. The mean estimated Ubio value ranges from 1.5.10−3 to 65.3 ng/L and can fluctuate by 3 orders of magnitude depending on the considered definition as opposed to by 2 orders of magnitude accountable to differences between thermodynamic databases. The classification of the water in relation to the two surrounding rock lithologies makes it possible to reduce the mean variability for the Ubio concentrations. Irrespective of the definition of Ubio considered, in 59% of cases the Ubio fraction represents less than 1% of Uaq. Several threshold values relating to Ubio were proposed, assuming knowledge only of the aqueous concentrations of the major elements and Uaq.