Surface water and groundwater hydraulics of lowland karst aquifers of Estonia

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: Salajo similar to e (SKS) and Tuhala karst system (TKS), and their interaction with the hosting karst aquifer. Highresolution 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.

Automated summary

Karst aquifers are an important source of domestic groundwater in Estonia. This study focused on two lowland karst systems and their interaction with the hosting karst aquifer. The research found that the hydrology and hydraulics of these systems were influenced by factors such as evapotranspiration, snow deposition, and snowmelt floods. The solute transport characteristics varied significantly due to differences in the structure and maturity of karst porosity.