CONCEPTUAL HYDROGEOLOGICAL MODEL OF THE OGOSTA RIVER FLOODPLAIN (WESTERN BALKAN, BULGARIA) AND ITS APPLICATION FOR PREDICTING OF GROUNDWATER CONTAMINATION WITH ARSENIC

The region of the Western Balkan is characterized by its complex geology and the presence of polymetallic ores. Iron and base metal (Pb-Ag) deposits are located in the upper valley of the Ogosta River near the town of Chiprovtsi. The local Fe-ores are rich in arsenic and copper, while the Pb-Ag ores contain much zinc and cadmium. The former mining and ore-processing activities have caused widespread environmental contamination with arsenic and heavy metals. Geological mapping, geophysical surveys, drilling, water monitoring and experimental infiltration tests were performed within the alluvial aquifer in the floodplain of the Ogosta River in order to assess and forecast the groundwater pollution. These resulted in delineating the spatial extent of the aquifer, its boundary conditions, hydraulic characteristics and interactions with other hydrogeological units, as well as the position of the water table. Special attention was paid to the vadose zone and its hydraulic characteristics with regard to flow and pollutant transport. The obtained results together with data on pollutant concentration in soil, alluvial deposits, river water and groundwater will allow establishing relevant predictive models of water flow and arsenic migration, in both the saturated and unsaturated zones.