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In the XX century many drillings for exploration and exploration of oil and gas were conducted in Northern Bulgaria. Up to 1500 boreholes with a depth of several hundred meters to more than 5000 m have been drilled on its territory so far. During drilling, aquifers with different hydrochemical properties have been registered. The aim of the present study is to assess the current ecological status of the old oil boreholes in Central Northern Bulgaria. As a result of the field activities a catalogue with information about its locations, actual state and physical-chemical characteristics has been created. There are many boreholes that have been abandoned and not liquidated and others that have been compromised. It has been estimated that most of them do not pose a risk to the environment and also that some of them are used or can be used for different purposes. Only a few artesian boreholes, with high total dissolved solids (TDS) of the water or high concentration of some chemical components, constitute a risk. There is also a possibility of mixture of groundwater from different aquifers as a result of the poor condition of the drill pipes. [ABSTRACT FROM AUTHOR]

The Dobrudzha Coal Deposit is characterized by complex geological, tectonic and hydrogeological conditions. The formed aquifers and complexes, on the one hand, hinder the possible exploitation of the coal deposit, on the other hand they are important for drinking water supply in the area. The aim of the present study is to develop a 3D hydrodynamic model in the area of the deposit to elucidate the dynamics of the flow processes, the hydraulic connections between the different hydrogeological units and the possibilities for water and mass exchange between them. The model has been generated with Modflow computer software. Various options have been developed, varying the boundary conditions and the water conductivity of the main fault disturbances in the area. With each of the developed model solutions, quantitative assessment of the water balance elements has been made and three-dimensional solutions for the flow field are obtained. The obtained results were used for the preparation of prognosis for the occurrence of negative processes of quantitative and qualitative impact on the groundwater and ecological consequences for the upper aquifers in the eventual development of the Dobrudzha Coal Deposit. [ABSTRACT FROM AUTHOR]

Oil and gas exploration in Bulgaria has been mainly conducted in Northern Bulgaria. Up to 2500 boreholes have been drilled on the territory of the country so far with a depth of several hundred meters to more than 6000 m. During drilling, groundwater with different hydrochemical properties, often with high total dissolved solids (TDS) and contents of a number of elements, have been registered. After the completion of the drillings, many of them have not been liquidated, as in some of the boreholes artesian water occurred. This is the case of a borehole near the Village of Rasovo, where TDS is higher than 23,6 mg/l and the temperature is 36 °C. The artesian water forms a pond with high concentration of iodine, bromine and other components. The aim of the present study is to evaluate the range of this leakage, if there is a risk of contamination of the soils, surface and shallow groundwater around the borehole, and to estimate the changes of its quality and quantity characteristics. The obtained results allow us to make an assessment of the impact of the artesian water on the environment. It is concluded that it is necessary either to prevent the leakage from the borehole or to start exploitation of the water. [ABSTRACT FROM AUTHOR]