The Skaistkalne area in Latvia is one of the places where karst processes in gypsum strata occurs. The Iecava and Memele rivers border the area with extensive surface karst features such as sinkholes and karst lakes. Earlier investigations suggested a hydraulic connection between the Iecava and Memele rivers exists via the karst conduits due to the water level (WL) difference in the rivers. A set of methods was performed to study the possible connection: dye tracer was applied in the Iecava river and its occurrence was visually observed at the karst lakes and Memele river; the current velocity was measured and discharge of rivers calculated at several profiles; surface water and groundwater composition was studied involving in situ measurements of water pH and electrical conductivity, water sampling and chemical analysis of the water samples on the content of sulphates, calcium and magnesium ions. A numerical finite element 3D groundwater flow model was developed to assess the impact of WL changes in rivers to groundwater flow. The study showed that there is direct hydraulic connection between the rivers – water from the Iecava river flows to the Memele river. The groundwater discharge to the Memele river varies seasonally, and more intensive groundwater discharge is observed during the high season.

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