Abstract
The sequential extraction procedure is used to reconstruct the processes of redistribution of uranium isotopes in the mineral phases of the aquifer of the Vendian sandy-argillaceous deposits developed in the coastal territory of the European North of Russia. This aquifer has large resources of drinking and mineral groundwater which, however, are used in extremely limited quantities. This is due to the very complex nature of the hydrochemical conditions, and uranium-isotopic methods are used to clarify these conditions. The following mineral phases of core samples were characterized: adsorbed trace materials and carbonate minerals; ferrihydrate, amorphous minerals of Fe, Al and Si, and secondary U minerals; crystalline iron minerals; clay and some refractory minerals; and all remaining resistant minerals. The most resistant minerals, whose fraction ranges from 70.58 to 96.4%, have a minimum average uranium concentration of 0.47 ppm. This uranium is practically conserved in the rock, as a result of which the average ratio of its 234U:238U isotopes is close to equilibrium. In the remaining fractions, uranium is redistributed by groundwater along their flow lines from recharge areas in watersheds to areas of discharge into river valleys. Its maximum concentration (12.89 ppm) measurement is carried out by coprecipitation with iron hydroxides, then the uranium is adsorbed and precipitated with carbonates (9.14 ppm). The average 234U:238U is maximum in adsorbed trace materials and carbonate minerals (2.39 ± 0.36) and is close to that in fresh groundwater (2.8 ± 0.42). It is also increased in the amorphous minerals of Fe (1.53 ± 0.23). In general, there is a dependence of the 234U:238U activity ratio in rock on the degree of participation of groundwater in the deposition of hydrogenic uranium isotopes into the cracks and pores of these rocks. The results obtained contribute to the refinement of such a parameter as the retardation factor, which is necessary for understanding the processes of migration and concentration of uranium in sedimentary basins.
Highlights
The average uranium content in sedimentary rocks, such as clays, sandstones, and carbonates, ranges from 0.45 ppm to 3.7 ppm [1], but it is very actively redistributed in these partially permeable deposits in strictly defined directions and forms large rollfront accumulations on geochemical barriers
There is no doubt that the procedure for sequential extraction of uranium isotopes from mineral phases with reagents of different strengths [26,27,28,29,30,31] gives a more informative picture of the redistribution of uranium isotopes in the water–rock system
The distribution of uranium concentrations in various mineral associations of Vendian sedimentary rocks is rather uneven and as shown in previous works [32,33], this unevenness is mainly associated with the presence of relics of green-colored deposits, which were under reducing conditions for a long period of time
Summary
The average uranium content in sedimentary rocks, such as clays, sandstones, and carbonates, ranges from 0.45 ppm to 3.7 ppm [1], but it is very actively redistributed in these partially permeable deposits in strictly defined directions and forms large rollfront accumulations on geochemical barriers. To analyze the processes that control the distribution of nuclides in sedimentary basins, complex mathematical models have been developed using molar concentrations and activity of nuclides in groundwater and host rocks [17,18,19,20,21,22,23,24,25]; information on the distribution of uranium isotopes in the rock is obtained mainly by the method of its total digestion. The host rocks of sedimentary basins are characterized by a heterogeneous mineralogical composition and contrasting differences in the physicochemical properties of mineral formations, which determine the different migration abilities of uranium isotopes. There is no doubt that the procedure for sequential extraction of uranium isotopes from mineral phases with reagents of different strengths [26,27,28,29,30,31] gives a more informative picture of the redistribution of uranium isotopes in the water–rock system
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