Natural yield of thermal waters in the Teplice nad Bečvou, their hydraulic connection with Bečva river and discussion of location of recharge area and discussion of location of recharge area
Abstract
Abstract Information on recharge and flow area of thermal waters is commonly limited as these waters are normally accessible for study only in discharge area. The same is true for thermal waters in Teplice nad Bečvou Spa, flowing from hypogenic karst in Paleozoic limestones, deeply buried under greywackes, and partly also under Carpathian Flysch Nappes.
Increasing interest in hydrogeology of this area in last years has been due to the presence of the Hranice Abyss, currently the deepest underwater cave in the world, and secondly due to the planned dam construction across the Bečva River, only several kilometers upstream from the Spa (Geršl 2016, Geršl ---- Konečný 2018, Sracek et al. 2019, Vysoká et al. 2019, Klanica et al. 2020). The objectives of this contribution are as follows: i) to estimate the natural yield of thermal water; ii) to delineate river segments which are hydraulically connected to karst aquifer in surroundings of the Spa wells based on their hydraulic response to the Bečva River oscillation; iii) to localize the recharge area of thermal water.
The total yield of thermal water was estimated from the change of electric conductivity (EC) of the Bečva River, where a portion of thermal water directly flows. EC was measured along riverbanks during exceptionally low flow conditions in November 2018.
Major inflows of thermal water to the Bečva River were localized by thermal camera (Figs. 1, 2; Table 1). Reevaluation of all available data from archive wells enabled to delineate the thermal water occurrence in the discharge area and define boundaries with plain cold groundwaters (Fig. 1).
Based on the measured EC and the mixing equation, the natural yield of thermal waters is 14 l/s, which is close to the results of the collective pumping test in the Spa. Based on the natural yield and temperature of thermal water and the comparison to terrestrial heat flux, the extent of the flow area of thermal waters is at least 11 km2, but more likely several tens of km2.
Ideas of some previous authors that source of thermal water is derived from the Bečva River several km upstream of the Spa thus can be excluded (c.f. Švajner 1982). It is also unlikely that dominant part of thermal water is recharged via carbonate outcrops of the Maleník Hill due to their small extent.
Flysch rocks act as regional aquitard hosting gas reservoir not far from the study area. Most likely, the recharge area of thermal water is located in the Odra Hills in wider surroundings of Potštát town to the North, where active flow was observed in greywackes in depth of 1300 m below the surface.
Greywackes are underlain by limestones. This is in agreement with O and H stable isotopes of thermal waters, which are depleted in heavy isotopes indicating higher altitude of recharge area (Sracek et al 2019, Vysoká et al 2019).
Water level fluctuation of the Spa wells with thermal water can be relatively well predicted from fluctuation of the Bečva River level (Figs. 3-5). Based on the elevation of the river water level along its stream course (Table 2), the studied wells R1, R3, HV301 are hydraulically connected to a river segment spanning between 1.2 km above and 0.5 km below the river gauging station, thus in clos spa surroundings.
Inflows of thermal water to the river traceable by escaping CO2 bubbles were observed in the same river segment in the past. Hydraulic effect of river on wells is traceable as far as 400 m from the riverbed also in areas which have been recently occupied by cold groundwater (Fig. 1).
This shows that groundwater table is very flat, and limestones are strongly karstified in wider river surroundings.