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.

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A comprehensive analysis of the hydrogeological conditions in the westernmost part of the Bulgarian Danube Plain is presented. During the Neogene and Quaternary period this area is subjected to continuous uplift. As a result, river valleys are deeply incised into the terrain and all shallow aquifers are well drained in the hydrological network. This process determines the specific regime and dynamics of the groundwater, characteristic of the identified West Moesian uplifted hydrogeological system. The Quaternary aquifer is the most abundant in the system. It is formed in four riparian lowlands – two along the Timok River (Bregovska and Baley-Kudelin) and two along the Danube River (Novoselska and Vidin Lowlands) (Figs 1, 7, 8). The hydraulic properties of the aquifer are very good: Transmissivity T = 800-1000 m2/d and hydraulic conductivity K = 60-80 m/d. Several Ranney water collector wells extracting up to 140 L/s have been built in the Vidin lowland. Potential groundwater resources in this lowland are estimated at about 14 m3/s, with 13 m3/s being infiltrated from the Danube River. Second in importance in the hydrogeological system is the Sarmatian aquifer. It consists of two layers: upper – limestone and lower – sandy limestone (Figs 2 and 3). The waters are used for local water supply by capturing karst springs. The results of monitoring observations on the chemical composition of groundwater during the period 1980-1991 are presented in diagrams (Figs 4, 5, 6, 9, 11, 12, 13). Groundwater, being the only water source in the area, must be carefully protected from industrial and agricultural pollution.