**3. Hydrogeology and hydrology of the North Lithuanian karst region**

The North Lithuania karst region is located in the eastern part of the Baltic Artesian Basin. The active water change zone is up to 270 m thick and includes aquifers in the Quaternary and in the Upper Devonian (the Istras‐Tatula, Kupiskis‐Suosa and Sventoji‐Upninkai) forma‐ tions. This series of aquifers is underlain by the 60–100 m thick regional aquitard of the Narva Formation. The main source for recharge of the Istras‐Tatula and Kupiskis‐Suosa aquifers is shallow groundwater and surface water. All aquifers are being exploited to a different extent for drinking and domestic purposes and for industrial needs in this region [4, 11]. The infiltra‐ tion rate of precipitation influences variations in level and chemical composition of ground‐ water. Ground collapse affects the Quaternary cover and permits ready recharge of surface water into the Upper Devonian aquifers. The intensive karst zone is referred to the areas with intensive water circulation in open gypsum systems [4, 11, 12]. Water of the aeration zone mostly infiltrates through vertical fissures until it reaches either horizontal channels or imper‐ meable soil layer. A large amount of precipitation is collected in the bottom of the sinkholes, which are open holes or filled with permeable deposits [7].

In the karst region, most of the sinkholes are dry. Only during spring floods, they collect and temporarily retain atmospheric water or groundwater. In the course of time, the newly occur‐ ring sinkhole becomes shallow as a result of sedimentation processes and transforms into small bogs. As a result of a very good hydraulic link between the surface and groundwater, part of sediments is eliminated with the ground run‐off [3]. Karst lakes and dry sinkholes make natural drainage systems through which precipitation, snow melting and overland flow access gypsum strata of the Lower Devonian [2, 12].

Karst rivers are recharged mainly through the precipitation. A low part of run‐off is formed by the karst groundwater. Many rivers in the region cut upper part of the karst rocks and aquifers [4]. The groundwater discharge into the karst rivers of the region makes up from 25 to 40%, whereas in non‐karst rivers it comprises only 8–16% of the annual run‐off. The run‐off of karst rivers in drought periods always exceeds the run‐off of non‐karst rivers [11].
