LONG-TERM CHANGES IN URANIUM ISOTOPES ACTIVITY IN THE VENDIAN GROUNDWATER OF THE MEZEN SYNECLISE
The studies aimed to determine the trends in the uranium isotopic composition of groundwater at its long-term exploitation and the possibilities of using this information to predict groundwater quality and to search for mineral resources. The paper determined the groundwater residence time in the aquifer based on uranium isotope data. As a result, we have found that boreholes located at the front of the stratal oxidation zone of sandstones of the Vendian-period Padun suite (within the Neogene-Quaternary Northern Dvina depression) had continuous increase in the activity of uranium isotopes as compared to the original ones. This increase is associated with redistribution of hydrogenic uranium in oxidizing conditions and with its precipitation, when these conditions change into the reducing ones. At the same time, according to the calculations, the concentration of uranium in sediments may exceed the background ones by the factor of 2 or 3. The resulting patterns can be used in searching for hydrogenic uranium ore occurrences in palaeovalleys of sedimentary basins. Wells associated with overlying horizons usually have decreased activity of uranium isotopes. This trend can be used in justifying the exploitation of mineral and drinking water. The best groundwater, in terms of radiology, is found in the sediments of the Vendian-period Mezen suite, due to reducing conditions for uranium in this formation.