General characteristics of karst basins in Bulgaria

Due to the complex geological structure and variable relief in Bulgaria, numerous karst basins are formed. According to Boyadjiev (1964) there are more than 170 karst basins. Each of them is characterized according to its location, recharge and drainage conditions, with inherent regime and different vulnerability to human impacts. In Figure 1 the main outcropping karst collectors in the country are presented in a light grey colour. The dots in the figure locate the karst springs that will be discussed.

The following classification for the main karst basins in Bulgaria is proposed.

Karst basins related to well-defined aquifers

These are related to carbonate formations of the Moesian platform. The important part of the formations is saturated, and aquifers are mainly uncon-fined. The confined aquifers are developed where these formations are covered. Due to the large volume of accumulated groundwater, the springs draining them are characterized by relatively stable discharge. The most important karst springs in Bulgaria, the Devnia springs, are of this type.

Karst basins in the lower parts of the country related to block carbonate bodies

The major portion of these is below the local erosion base. This type of karst basin is distributed mainly in the intermediate hydrogeological structure zone. The recharge is mainly due to precipitation, and the groundwater is karst-fissured and fissure-karstic. The karst basin in Palaeogene limestone near the town of Chirpan, and some others in the Zemen passage, belong to this type. The regime of karst springs is influenced by relatively low transmissivity of the medium and low volume of accumulated groundwater in the saturated zone. Due to the smooth relief, the soil layer is the most important factor that governs its regime.

Large karst basins with open karst and relatively smooth relief

The recharge is mainly due to rainfall. In most cases these karst basins are in the thick vadose (unsatu-rated) zone and insignificant phreatic zone. Variations of the discharge are related to areal recharge. Several karst basins formed in shallow synclines in Fore-Balkan (southern part of the Moesian platform) belong to this type (Gabare, Krushuna and Musina springs).

Mountain karst basins with exceptional rainfall recharge

These are formed in the Balkan and Rila-Rhodopes massif zones. They are characterized by a large difference in altitude between recharge and discharge zones and large diversity. The spring outlets appear in the lowest part of the karst basins in contact with low-permeability rock formations. Most of them are perennial, with high variation in discharge. The characteristic feature for such springs is the presence of a local saturated zone near the spring issue (for example Bistretz spring).

Mountain karst basins with river recharge

These karst basins do not differ from the previous type from the geological or geomorphological point of view. The main difference is the available permanent river recharge that leads to their specific regime. Iskretz, Glava Panega and Matnitza springs are of this type.

High mountain karst basins with important snowmelt recharge

These are karst basins with areal recharge distributed at high altitude above 2000 m in which snow cover is present up to early summer and in some parts all through the year. These conditions are of primary importance to the regime of karst springs. A typical karst basin of this type is the Razlog karst basin located in the region of north Pirin.

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