RECHARGE AND RETENTION TIME STUDY OF A PARTLY KARSTIFIED AREA OF BOC (EASTERN SLOVENIA) USING HYDROGEN, OXYGEN AND CARBON-ISOTOPE COMPOSITION AS NATURAL TRACERS

The study is focused on the dolomite-limestone drinking water aquifers in the Boc massif, as well as on the andesite-aquifer containing mine ral water in the vicinity of Rogaska Slatina. The catchment area is li mited and both drinking and mineral waters are discharged from the sam e source. The increasing use of deeper aquifers means that natural spr ings and shallow wells have become sporadic. Consequently new techniqu es of investigating recharge and aquifer capacity are required which c an augment classical hydrogeological methods. Current research into th e mineral and drinking wafer aquifers in the area of Rogaska Slatina i s based on measuring the isotopic composition of light elements, (H, C and O) as natural tracers. It can be concluded that all the groundwat ers investigated are typically infiltrated meteoric water. The drinkin g waters are generally young and were infiltrated up to about ten year s ago. The isotopic composition of oxygen is similar to recent precipi tation (delta(18)O = -9.3 +/- 1 parts per thousand) and the drinking w aters contain tritium. Tt was found that exploited mineral waters rech arged aquifers during colder periods; they are only partly mixed with younger water as can be seen from the isotopic composition of oxygen a nd corrected C-14 dating, which puts the mean ages at between around 1 00 and 8,000 years. With regard to the ''nuclear period'' (1960-64) wi th abnormally high tritium activities of precipitation, all the waters examined can be divided into at least three main infiltration groups depending on their measured tritium content: around 35 years old (> 80 T.U.), older (> 10 T.U.) and younger (10 to 60 T.U.). Detailed dating is possible following the above classification. Isotope exchange betw een rocks and water is negligible and therefore very deep circulation at the temperature conditions above 80 degrees C: does not occur. Diss olved inorganic carbon (DIG) in the drinking waters is the result of e quilibrium reactions between carbonates and organically produced CO2 ( delta(13)C = - 14.5 +/- parts per thousand), while the high concentrat ions of DIC (delta(13)C = + 3 +/- parts per thousand) and CO2 observed in the mineral waters are generated by low-temperature decarbonatizat ion processes and indicate the deep origin of CO2, from where gas migr ates into mineral water aquifers. Correlation analyses between the par ameters studied are performed. Useful conclusions concerning water cir culation and the capacities of aquifer reservoirs are described which support the future optimal pumping of mineral and drinking water al th e limited catchment area of Rogaska Slatina and Boc.