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Gimeno, M. J., Tullborg, E. - L., Nilsson, A. - C., Auqué, L. F., & Nilsson, L. (2023). Hydrogeochemical characterisation of the groundwater in the crystalline basement of Forsmark, the selected area for the geological nuclear repositories in Sweden. Journal of Hydrology, 624, 129818.
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.
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Gómez, P., Garralón, A., Buil, B., Turrero, M. J., Sánchez, L., & Cruz, B. de la. (2006). Modeling of geochemical processes related to uranium mobilization in the groundwater of a uranium mine. Science of The Total Environment, 366(1), 295–309.
Abstract: This paper describes the processes leading to uranium distribution in the groundwater of five boreholes near a restored uranium mine (dug in granite), and the environmental impact of restoration work in the discharge area. The groundwater uranium content varied from \textless1 μg/L in reduced water far from the area of influence of the uranium ore-containing dyke, to 104 μg/L in a borehole hydraulically connected to the mine. These values, however, fail to reflect a chemical equilibrium between the water and the pure mineral phases. A model for the mobilization of uranium in this groundwater is therefore proposed. This involves the percolation of oxidized waters through the fractured granite, leading to the oxidation of pyrite and arsenopyrite and the precipitation of iron oxyhydroxides. This in turn leads to the dissolution of the primary pitchblende and, subsequently, the release of U(VI) species to the groundwater. These U(VI) species are retained by iron hydroxides. Secondary uranium species are eventually formed as reducing conditions are re-established due to water–rock interactions.
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Grodek, T., Enzel, Y., Morin, E., Jacoby, Y., Dahan, O., Benito, G., et al. (2008). Flood hydrology, flood routing, paleohydrology and the estimation of water resources along the shallow alluvial aquifers of the Kuiseb River, Namibia. In Abstracts – Israel Geological Society (Vol. 2008, 37).
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Grozeva, N. G., Radwan, J., Beaucaire, C., & Descostes, M. (2022). Reactive transport modeling of U and Ra mobility in roll-front uranium deposits: Parameters influencing 226Ra/238U disequilibria. Journal of Geochemical Exploration, 236, 106961.
Abstract: Uranium reserve estimates in ore deposits can be significantly impacted by 226Ra/238U disequilibria arising from the differential mobility of uranium and radium during groundwater transport. 1D reactive transport models were developed to investigate the long-term effects of retention processes (UO2(am) precipitation, U(VI) and Ra sorption on smectite, Ra co-precipitation with barite) on the repartitioning of 238U and 226Ra during formation of roll-front type deposits. Analytical solutions to radioactive decay chains were used in complement to examine the influence of geochemical parameters, including fluid 234U/238U activity ratios and α-recoil loss, on 226Ra/238U disequilibria in uranium ores. Model results demonstrate that smectite and barite can produce 226Ra/238U ratios \textgreater1 at low uranium contents and may explain 226Ra/238U disequilibria occurring in altered rock up- and downstream of roll-front deposits. The capacity of these phases to take up Ra and generate 226Ra/238U disequilibria depends on both mineral contents and groundwater compositions, and is thus expected to be site-specific. Simulations of ore deposits that advance downstream with time demonstrate the formation of stronger 226Ra/238U disequilibria, as expected, in the downgradient side or nose of the ore, reflecting both younger mineralization ages and the presence of active uranium precipitation. Whether disequilibria are positive or negative with respect to secular equilibrium, however, depends on the 234U/238U activity ratio in the fluid from which uranium minerals precipitate. Smaller hydraulic conductivities are shown to generate a narrower range in 226Ra/238U activity ratios with distance, and may explain the occurrence of disequilibria in the limb ore that are less pronounced than those in the nose. Furthermore, the ability of α-recoil loss to decrease 226Ra/238U activity ratios at secular equilibrium may account for negative disequilibria in high grade ores. The South Tortkuduk uranium deposits (Kazakhstan) are subsequently used as a case study to identify the processes and parameters that may contribute to 226Ra/238U disequilibria at this site. Variations in multiple parameters, including clay contents, barite contents, and mineralization ages, are found to reproduce measured 226Ra/238U activity ratios in the roll-front ore. Prioritization of these parameters will necessitate field measurements targeting both groundwater fluids and the host rock. Results from this study will ultimately aid geologists in building appropriate hydrogeochemical data sets to more efficiently locate and exploit uranium ore deposits.
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Hall, S. M., Gosen, B. S. V., Paces, J. B., Zielinski, R. A., & Breit, G. N. (2019). Calcrete uranium deposits in the Southern High Plains, USA. Ore Geology Reviews, 109, 50–78.
Abstract: The Southern High Plains (SHP) is a new and emerging U.S. uranium province. Here, uranyl vanadates form deposits in Pliocene to Pleistocene sandstone, dolomite, and limestone. Fifteen calcrete uranium occurrences are identified; two of these, the Buzzard Draw and Sulfur Springs Draw deposits, have combined in-place resources estimated at about 4 million pounds of U3O8. Ore minerals carnotite and finchite are hosted in dolomite at the Sulfur Springs Draw deposit, with accessory fluorite, celestine, smectite/illite, autunite, and strontium carbonate. Host carbonate at the Sulfur Springs Draw deposit is ∼190 ka and mineralization mobilized as recently as 3.8 ka. Ash collected near the deposit is 631 ka and erupted from the Yellowstone caldera complex. The Triassic Dockum Group that contains sandstone-hosted uranium deposits throughout the region and underlies the SHP is a potential source for uranium and vanadium. Regional uplift and dissection reintroduced oxygenated groundwater into the Dockum Group, mobilizing uranium. Additional uranium may have been contributed to groundwater by weathering of volcanic ash in Pliocene and Pleistocene host rocks. The locations of the uranium occurrences are mostly in modern drainage systems in the southeast portion of the SHP. Modelling of modern groundwater in the SHP carried out in a parallel study shows that a single fluid could form carnotite through evaporation, and that fluids of the requisite composition are more prevalent in the southern portion of the SHP. The southeastern portion of the SHP hosts more uranium occurrences due to a variety of factors including (1) upward transport of groundwater and connectivity between source and host rock, (2) higher uranium and vanadium content of groundwater, (3) higher rates of groundwater recharge in this region to drive the mineralizing system, and (4) shallower groundwater facilitating surface evaporation. Ongoing erosion of host rocks challenges preservation of deposits and may limit their size.
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