Geological and hydrogeological controls on possible transboundary groundwater flow between Bulgaria and Greece

A major issue in groundwater and surface water management and monitoring is transboundary groundwater flow between neighbouring countries. The subject of our study is the assessment of the groundwater flow across the state border between Bulgaria and Greece, which is characterized by complex natural features. We present a study on the factors that control the groundwater flow and formation, such as lithological composition, geological structure, topographic features, surface water network, etc. It was determined that, in most sections along the state border, the probability of transboundary groundwater flow is rather small, due to the position of the surface and groundwater divides. In the areas where the state border follows a ridge with fissured groundwater system, there are no prerequisites for transboundary flow. Groundwater flow is likely to occur only in the marble-dominated karst areas, as well as in the Quaternary or Neogene–Quaternary aquifers in river valleys that host porous groundwater bodies. The probability of transboundary groundwater flow could increase in response to certain human activities.