I. Stanev, S. Vesselinov. Vertical zoning of the underground hydrosphere in the Bulgarian part of the Lower Danube artesian basin. The vertical zoning of the underground waters in the southern part of the Lower Danube artesian basin has been studied from the view point of the co-existence of both elisional and intimately related to them infiltration water-bearing systems. The hydrogeological stratification is made on the basis of defined and described marker (peak) surfaces of higher mineralization of the underground waters which enable to divide the underground hydrosphere into four hydrogeological levels - E1, E2, E3 and E4. The boundaries between the individual levels are marked by three maxima o f higher mineralization of the underground waters which define the marker (peak) surfaces F1, F2 and F3. The two minima of lower mineralization cover the middle parts of the hydrogeological levels E2 and E1. The uppermost marker surface F1 in the hydrogeological level E1 is followed upwards by a gradual decrease in the mineralization towards the surface which is due to the infiltration influence, and the lowermost marker surface F3 – by a decrease of mineralization downwards in the hydrogeological level E4 as a result of dehydration of clays (inversion).
Particular attention is paid to the character of marker surfaces of higher mineralization which form the hydrogeological structures (positive, negative, inclined). They have a concrete hydrogeological meaning which is directly related to the generation and accumulation of hydrocarbons in view of their prospecting.