HYDROCHEMICAL CONDITIONS IN THE REGION OF DOBRUDZHA COAL DEPOSIT (NE BULGARIA)

Dobrudzha coal deposit is located in a tectonically disturbed zone in north-eastern Bulgaria. It has not been exploited till now due to the great depth of occurrence (1200-1400) m and very complicated geological and hydro geological conditions. Several storey arranged aquifers are discovered in the region. Each of them has a specific chemical composition depending on the intensity of water exchange and filtration properties of surrounding rocks. The water in karst aquifers (Sarmatian and Upper Jurassic-Lower Cretaceous) is of hydro carbonate calcium and calcium magnesium type and of low total dissolved solids (TDS), while for the other aquifers the TDS is significantly higher and the water chemical composition is different. This refers to the complexes situated between the karst aquifers as well as to the Carboniferous. The significant differences in the chemical water composition imply for an absence of vertical hydraulic connection between the aquifers. Speciation of different chemical elements in the groundwater is determined. Opportunities for dissolution or precipitation of the different mineral phases are evaluated. Studied characteristics differ significantly between aquifers and complexes due to changing hydrological and geothermal conditions in vertical direction. The obtained hydrochemical data for the Carboniferous and Upper Jurassic - Lower Cretaceous aquifer will be used as a base to predict the impact of future underground coal gasification in Dobrudzha coal deposit and CO2 storage on the hydrogeological conditions in the research area.