Recharge, delayed groundwater-level rise and specific yield in the Triassic karst aquifer of the Kopa Mountain, in the Western Carpathians, Slovakia
Notable delay of more than 1 year (12.5–15.9 months) between precipitation recharge and groundwater-level upturn was recorded in the Triassic karstic aquifer of the Kopa Mt. (1,187 m asl) in the middle of the West Carpathians in Slovakia. The velocity of recharge transit (0.24–0.70 m/day) was affirmed by borehole hydrographs. The groundwater level here is deep (113–300 m below ground surface) but approximately evenly distributed at the same elevation (~467 ± 10 m asl). Dolomites extend over 78% of the surface area of the karstic reservoir, with a Cretaceous marly aquitard boundary on the perimeter and on its base. The whole area is recharged merely by precipitation and entirely discharged by springs and small surface streams. The water balance between recharged and discharged water, and the groundwater-level fluctuation (shift in time), enabled calculation of stored groundwater volumes. The estimated specific yield of the Triassic dolomitic aquifer was unusually high (7.6–23.4%, mean 16.1%). Karstic cavities were also widely found in the borehole logs of nine boreholes that were 87–490-m deep and drilled in Triassic dolomites and limestones along the axis of a planned highway tunnel. Groundwater level monitoring took place over 40 months, between June 2014 and October 2017. Typical seasonal water-level oscillations were absent in the karstic reservoir, but water-level change was relevant to delayed past recharge events. In a borehole situated in the Cretaceous marly aquitard, good seasonal correlation with effective precipitation and also tidal response was observed.