The chemical composition of the groundwaters of the active water exchange zone of the northern part of the Kolyvan-Tomsk folded zone

This work describes the chemical composition of the groundwater of the active water exchange zone of a moderately moist climate area on the example of the northern part of the Kolyvan-Tomsk folded zone. Administratively, the study area is located in the southeastern part of Western Siberia, in Tomsk Oblast and takes the Tom-Yaya interfluve. In the geological and hydrogeological studies regarding the territory of interest there are two structural stages. The upper stage of unconsolidated Mesozoic-Cenozoic deposits accommodates stratified pore water; the lower stage of an ancient Paleozoic basement has crack water of weathering zone and tectonic disturbances. Groundwater of the upper and lower stages is separated by clay waterproof rocks of weathering crusts having a regional distribution. The material for the studies was provided by data obtained in the course of field work in the period from 1992 to 2013. Permanently waters were investigated in the Problem Research Hydrogeochemical Laboratory of National Research Tomsk Polytechnic University. In the interpretation and compilation of chemical analyses 601 sampling points in the groundwater of unconsolidated deposits and 138 sampling points of groundwater bedrock were used. It has been shown that the tested waters are representative leaching groundwater having total salinity up to 1 g/l, a neutral or weakly alkaline pH and preferably calcium hydrocar-bonate composition. They are characterized by low average content of chlorides, sulfates and sodium. Micro composition of groundwater has a wide range of components with concentrations often elevated. The source of enrichment is often a natural factor: enrichment and mineralization of rocks and zones of faults. It was also noted that, along with the natural factor in the formation of the chemical composition of groundwater, there is an anthropogenic effect. In this case, there may be a simultaneous presence in a water sample of a number of components with high and even very high concentrations, not only in relation to the background content, but also in comparison with the MAC for drinking water. In general, the hydrogeochemical characteristics of groundwater of the upper and lower stages are expressed in a rather low differentiation of mineralization, and the monotony of anion-cation composition -bicarbonate water with a predominance of the role of calcium. In almost all waters (except anthropogenic) the amount of HCO ₃ , Ca and Mg + ions is generally about 90 % of the water-dissolved salts, which meets the continental genesis of sedimentary rocks and the origin of water infiltration. In general, the chemical composition of the groundwater of the studied landscape zone (zone of southern taiga) is typical for waters not exposed to the processes of continental salinization. Concentrations of many components in the waters of the Paleozoic formations are comparable to the concentrations of the same substances in the waters of the Meso-Cenozoic sediments. This is probably due to the presence in most cases of the hydraulic connection between the aquifer complexes in both stages. This work was supported by grant RFBR № 14-05-31095-mo^, № 13-05-98070-r_sibir_u and № 13-05-00062 А

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