Factors of formation of the chemical composition of groundwater of the Oligocene aquifer in the western part of the Middle Ob hydrogeological basin

Purpose and relevance of the work. The constantly growing demand for fresh groundwater in the region of intensive oil production and potential anthropogenic impact on groundwater necessitates the forecasting of qualitative and quantitative indicators of their chemical composition. The purpose of the study is to determine and analyse the effect of conditions and factors on the formation of spatial and temporal patterns of change in the chemical composition of groundwater of the Oligocene horizon in the western part of the Sredneobsky hydrogeological basin. The research methods included the analysis of physiographic, geological, hydrodynamic and anthropogenic factors and assessment of their influence on the changes in the indicators of the chemical composition of groundwater of the Oligocene horizon. The results of the work and their application. As a result of the study it was determined that mineralisation and the degree of enrichment of organic matter in groundwater horizon varies over the area in a regular way in accordance with the physiographic zonality and geological–structural plan of the area. The peculiarities and changes in the lithological and mineral composition of water–bearing rocks are the factor of formation of the variegated basic composition of groundwater and increased concentrations of silicon, total iron, ammonium and manganese in it, as well as the regularities of changes in these indicators over the area. Change of groundwater dynamics under water withdrawal is the factor influencing water mineralisation due to change of water exchange character. Violation of the natural hydrogeochemical regime determines extreme values of mineralisation and chloride type of water at local sites. The results of the study can serve as a basis for predicting groundwater quality indicators of the Oligocene horizon in the study area. Conclusions. The peculiarities of the chemical composition of groundwater of the Oligocene horizon are caused by the joint action of physiographic, geological, geodynamic, and anthropogenic factors, the main of which are the confinement to the edge part of the West Siberian plate, as well as anthropogenic changes in the natural hydrochemical regime of groundwater in the process of their exploitation.