Oxygen Isotopes in Groundwater Discharge Recession Curves of Several Karstic Springs in the Brezovske Karpaty Mts., Slovakia.

Peter Malik* & Juraj Michalko
*Geological Survey of Slovak Republic
Mlynska dolina 1, 817 04 Bratislava, Slovak Republic
malik@gssr.sk, tel: ++421 2 59375416, fax: ++421 2 54771940

Brezovske Karpaty Mts., neighboring the northern (Slovakian) part of the Vienna Basin, are built mainly by Middle and Upper Triassic dolomites and limestones, are formed into small forested hills. During hydrogeological investigations in this area, groundwater quality of five karstic springs was monitored in 20-day intervals in 1987 - 1989. Groundwater chemical composition was very stable during the whole observation period, and only small deviations were observed. On the contrary, spring discharges seem to show various types of regime changes during the snow-melting period. However, although sampled for ¹⁸O/¹⁶O isotope composition, the data were never used to discuss the groundwater origin. GNIP (Global Network of Isotopes in Precipitation by IAEA) d ¹⁸O vs. SMOW data were used for comparison, samples taken since 1961 in the Vienna Hohe Varte station, situated some 100 km from the investigated area. For this purpose, only winter (October - April) part of the precipitation should be taken into account, as the real evapotranspiration data show that groundwater in the area is generated merely by snow melt or heavy rains in cold periods. Isotope composition of the outflowing groundwater was plotted vs. spring discharges, resp. various stages of the spring's recession curve - flatter parts for groundwater of the small fissures of the rock mass, steeper parts for groundwater circulating in more opened fissures or karst conduits. Genetic consequences should serve for more effective groundwater protection of already exploited springs.