Application of dissolved gases concentration measurements, hydrochemical and isotopic data to determine the circulation conditions and age of groundwater in the Central Sudetes Mts

Abstract

In the Central Sudetes, the areas of sedimentary Permian and Carboniferous rocks are characterized by small resources of groundwater and are much less understood than aquifer systems occurring in the crystalline and Mesozoic sedimentary rocks. The environmental tracer studies conducted in a double sampling cycle during changing conditions of groundwater inflow can significantly facilitate the understanding of hydrogeological conditions in areas of the complicated geological structure. Therefore, in order to identify the hydrogeological conditions of study area, measurements of geogenic and anthropogenic gases (He, Ne, Ar, CFC-11, CFC-12 and SF6), determinations of isotopic composition of groundwater (δ18O, δ2H and 3H) and the hydrogeochemical data (including selected REEs, thorium and uranium) were used. The research, which was conducted both in natural groundwater flow conditions and in conditions of flow disturbed by the continuous exploitation of the groundwater, enabled, among other things, a direct determination of the rate of maximum decomposition of CFC-11 and CFC-12 at depths of 50–450 m in the clastic rocks. It ranges from 189 to 315·10−12 g·L−1·yr−1 for CFC-11 and from 110 to 198·10−12 g·L−1·yr−1 for CFC-12. The research conducted also indicates that in the study area the vertical or nearly vertical groundwater flow may have significance comparable to the lateral groundwater flow. This results in an intense mixing of groundwater which leads to the local occurrence of groundwater containing the ions and gases in concentrations characteristic of the deep aquifers in the studied area, with the simultaneous occurrence of ions and gases of anthropogenic origin. The results of measurements on CFCs, SF6 and noble gases indicate a significant inflow of young groundwater to the whole freshwater zone. An occurrence of a geogenic SF6 source, which is most probably associated with volcanic rocks, was also found. Calculated the groundwater ages indicate that the research area is dominated by groundwater of age less than 1 ka, and mainly in the range of 0.05–0.25 ka. The results of modeling of the influence of the variability of U and Th concentrations on the He in situ production within the aquifer (PHe) indicate, that for groundwater with He concentrations of 10−6–10−8 cm3 STP g−1, i.e. modern groundwater, the He in situ production within the aquifer is actually negligible, so it may be omitted in the calculation of their age. Considering the fact that, in the study area which is terrain with elevated uranium concentration there is no significant effect of PHe on groundwater dating using the helium method, it can be concluded that the dating results obtained with this method in such areas can be fully comparable with those obtained for areas with lower concentrations of uranium. It was also found that mobile ions of uranium and suspension of thorium compounds occurred in the groundwater are not a significant source of He in situ production within the studied aquifers.