Surface water and groundwater hydraulics of lowland karst aquifers of Estonia

• Published in: Journal of hydrology (Amsterdam) 2022-07, Vol.610, p.127908, Article 127908
• Main Authors: Koit, Oliver, Mayaud, Cyril, Kogovšek, Blaž, Vainu, Marko, Terasmaa, Jaanus, Marandi, Andres
• Format: Article
• Language: English
• Subjects: 2018 drought; Estonia; Groundwater recharge; Hydraulics; Karst; Tracer test
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/j.jhydrol.2022.127908

•Studied karst systems are hydraulically linked to the hosting karst aquifers.•The greatest event sensitivity observed during autumn and snowless winter periods.•A typical water year with a snowmelt spring flood showed ambiguous CCF response.•Flat topography and shallow vadose zone contribute to complex baseflow interaction.•Matrix-conduit gradient inversion occurs during high flow events. In Estonia, karst aquifers supply roughly a third of annually abstracted domestic groundwater. Karst systems recharged by allogenic humic-rich streams originating from mire-dominated watersheds are common in Lower Estonia (lowlands). This study aimed to characterize the hydrology and hydraulics of two binary lowland karst systems: Salajõe (SKS) and Tuhala karst system (TKS), and their interaction with the hosting karst aquifer. High-resolution water level, discharge, specific conductance time series data combined with tracer test results were analyzed to assess the two systems. The temporal effect of evapotranspiration, snow deposition and snowmelt floods contributed to the variability in recharge event sensitivity of studied watersheds and aquifers, resulting in time-varying CCF strength between precipitation and discharge. The greatest event sensitivity was observed during the autumn and snowless winter seasons. The celerity and solute transport were the greatest in the preferential flow paths of both karst systems, however, solute transport characteristics varied significantly because of differences in structure and maturity of karst porosity. While the dilute allogenic recharge flushed the karst systems occasionally, the contribution of high-SEC groundwater baseflow was significant. During flood events, the matrix-conduit hydraulic gradient could be inverted, resulting in dilute floodwater being pushed into the hosting aquifer from the karst system.