Characterization of the Karst water regime in the Danube catchment area (Bulgaria)

The purpose of study is to estimate the role of karst springs in the formation of the flow of the Bulgarian rivers that are right tributaries of the Danube River. The study area includes the region from the Danube River to the main ridge of the Balkan (Stara Planina), representing a major water divide that separates the Black Sea catchment area from the Aegean one. The eastern border represents the watershed between the Danube and the Black Sea hydrological zones. From a geological point of view, the northern part of the area is located on the Moesian platform and the southern part belongs to the Fore Balkan and Balkan areas where various types of rocks of different geologic age outcrop. In some of them, there are conditions for the formation of karst water. In the northern part of the area they form distinct aquifers that gradually sink to the north; this is so called "platform" type of karst. In the southern mountainous part there are numerous karst basins. The most significant of karst springs are included in the national groundwater monitoring network. The hydrographs of karst springs are analyzed in view of the specific features of karstification. To classify the studied springs with respect to their regime, several indicators are used. Furthermore, the role of karst waters in the river runoff of the Bulgarian tributaries of the Danube River is assessed