Ecological and geochemical characteristic of natural waters within the influence limits of the Dobrivlyany gas condensate field (Precarpathia)

A study of natural waters within the influence of the Dobrivlyany gas condensate field, located in the Dobryany and Dobrivlyany area village councils of the Stryi district of the Lviv Region of Ukraine, was conducted. The purpose of the work is to evaluate the geochemical indicators of natural waters based on our own research, to find out the role of natural and man-made factors in the formation of the chemical composition of waters. The objects of the research are surface waters and groundwaters within the influence of the Dobrivlyany gas condensate field. Research results. In the research area, as well as in the Bilche-Volytsia zone in general, there are unfavorable conditions for active water exchange and the formation of fresh infiltration waters, since the upper part of the geological section is characterized by clayey Neogene deposits. The depth of penetration of fresh hydrocarbonate-calcium waters here does not exceed 70 m. The small thickness of the zone of active water exchange is one of the indicators of difficult water exchange in the subsoil, therefore, favourable conditions for the preservation of hydrocarbon deposits. As a result of exploratory work, the Dobrivlyany gas condensate field was opened in 2016. During 2016–2019, 7 boreholes were drilled, and industrial gas inflows were received from horizons LD-9, LD-12, and N1kr+K2. In tectonic terms, the deposit is confined to the northwestern part of the Kosiv-Ugersko subzone of the Bilche-Volytsia zone of the Carpathian Foredeep. Observation of the macro component composition of groundwater from the wells of the Dobrivlyany gas condensate field indicates the stability of the hydrodynamic conditions of the deposit and, accordingly, the low variability of their geochemical characteristics. Direct hydrogeochemical zonation is clearly manifested in the increase in water mineralization with depth: for the LD-9 horizon at depths of 800–840 m, it varies within the range of 34.74–48.55 g/dm3, for LD-12 at depths of 995–1010 m – 33.82–73.70 g/dm3 and for N1kr+K2 at 1131–1158 m – 67.49–100.31 g/dm3. The content of Br and J doubles with depth. All waters are of the chloride-calcium type (according to Sulin), and genetic indicators vary within narrow limits (rNa/rCl – 0.82–0.91; Cl/Br – 216–315; rSO4 ∙ 100/rCl – 0.003–0.5) and indicate their thalassogenic sedimentogenic origin. The analysis of research results showed that underground waters belong to a single hydrodynamic system, and are characterized by the same type of water according to their chemical composition. According to the qualitative characteristics of groundwater from an ecological borehole and a well in the Dobrivlyany village remain clean and meets regulatory requirements for drinking water. At the same time, water from a well in Vivnya village is characterized by a high content of nitrates (2.28 maximum permissible concentration), which is caused by the location of the pig complex “Halychyna-Zakhid” LLC on the outskirts. Increased content of organic substances in the waters of two wells of the Railiv village was also established, in which the permanganate oxidizability reaches 1.08–1.24 maximum permissible concentration, and, according to Hygienic requirements for drinking water intended for human consumption, is unfit for consumption. The waters of the Kolodnytsia River are chloride-hydrocarbonate sodium-calcium (calcium-sodium) in composition, while the waters of the Stryi River and the Zhizhava River are calcium bicarbonate.