ROLE OF COAL LAYERS FOR THE FORMATION OF CHEMICAL COMPOSITION OF THE UPPER PONTIAN AQUIFER, NORTHWEST BULGARIA.

The Upper-Pontian aquifer is one of the most important in Northwest Bulgaria. It is built of sands with a thickness of up to 100 m that outcrop on the surface at the periphery of the Lom depression. The aquiferous sands are overlain by a lignite coal layer up to several meters thick. In order to assess the influence of coal on the chemical composition of groundwater, an experiment has been carried out in laboratory. In view of the leading role of rainwater, the experiments were conducted with real rainwater. This water was mixed with crushed coal taken from the productive layer of the Lom Coal Basin. The solution was periodically shaken to ensure good interaction between the water and coal. In the period of 8 days, pH and water conductivity were measured periodically for six water samples. The results show that the water after contact with coal rapidly (within 2-3 hours) changes its pH and stabilizes around 5.6-5.8 values. In the beginning, the electrical conductivity increases very fast, then this process is delayed over time up to 2000 μS/cm. Most macro-components show increase in their content. The changes in concentrations of potassium, nitrates, nitrites, chlorine and phosphates are relatively less expressed. The most pronounced increase is observed for sulfates, which are one of the main sources of pollution in this case. The oxygen demand shows significant increase. For most of the micro-components there were no significant changes in their concentrations except for manganese and strontium. The dynamics of the existing patterns of elements in aqueous solutions, as well as the saturation indices of various mineral phases, have been calculated by the ion-associative method using a computer program Visual MINTEQ ver.3. [ABSTRACT FROM AUTHOR]