1. Jože PEZDIČ Department of Geology, NTF, University of Ljubljana, Aškerčeva 12, SI-1000 Ljubljana, Slovenia Key words: Slovenian Karst, karst hydrogeology, stable isotopes, oxygen, hydrogen, ISOTOPE EFFECTS IN KARST INFILTRATION MATTRIX

2. Pezdič J.: Isotopes in Karst 1 INTRODUCTION Karst phenomena were studied in more detail because of their scientific interest as well as because of the necessities of humans living in the karst regions. Water is of greatest interest. In karst landscapes there is practically no surface stream water and aquifers are located inside the fracture zones in carbonate rocks. In such conditions water infiltrates very quickly through the fractures of various sizes into the ground to reach the water table after a certain time. The understanding of this percolation through the unsaturated zone was the goal of our studies in the Slovenian epikarst.

3. Pezdič J.: Isotopes in Karst 2 SPECIAL TERMS AND DEFINITIONS Some special terms are applied: water waves, sponge, seasonal effect, drainage channels and fractural flow. They can be used very successfully as secondary definitions in studies of the epikarst water drainages or aquifers. ANALITICAL METHOD The isotope composition of oxygen, deuterium and tritium in water was measured at long term intervals in precipitation, different vertical recharge locations as well as in differently fractured zones. The mass spectrometer Finnigan MAT 250 with accuracy ± 0.1 ‰ for δ 18 O and ±0.5 ‰ for δD was used as analytical equipment.

4. Pezdič J.: Isotopes in Karst 3 PRECIPITATION It is well known that the most important step in examination of water cycle are precipitations, what in our case means their isotopic composition. Across the Slovenian karst region mean annual values for δ 18 O range from –6.5 to –9 ‰, and those for deuterium from –43 to –62 ‰. It was found that according to meteoric line there are influences of continental as well as Mediterranean climate. So, the d constant in equation of meteoric line varies from 10.5 to nearly 12, which is the characteristic value for the Mediterranean region. Particulate precipitation can differ from –3 ‰ in summer showers to –17 ‰ in snow of the coldest part of the wintertime.

5. Pezdič J.: Isotopes in Karst 4 INFILTRATION Vertical percolation or leakage is the most favored way of water infiltration in karst conditions. Karst is composed mainly of limestone that is more or less fractured and corroded. The soil is composed of limestone and organic matter residua and has an important role in infiltration and karstification. Moreover, the soil is the product of water aggressiveness. The aquifer body is unsaturated for the most time. The isotope characteristics and diversities of water transport in such unsaturated matrix is the aim of the current discussion.

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7. Pezdič J.: Isotopes in Karst 6 TRANSPORT The precipitation are partly evaporating at the surface and are included into transpiration processes. The remaining meteoric water (about 60 % in the semi- mediteranian climate) is for a short time captured in the soil. There it becomes saturated with carbon dioxide that makes it aggressive to carbonates. The kinetics of further infiltration depends upon the systems of fractures and porosity of the limestone. The varieties of infiltration range from leaking through very thin capillaries to percolation through wide karst channels. In these mixed infiltrated waters the isotope composition is more and more homogenized. But the final composition is differently expressed in any particular type of the aquifer, and depend most often upon its dimensions.

8. Pezdič J.: Isotopes in Karst 7 DRAINAGES The method for efficiently following the water cycle is the isotope hydrogeology. In karst conditions we found that the average retention time of water is approximately half a year. This is usually expressed as the seasonal effect. The underground water is really highly homogenized but about 10-20 % of the precipitation isotope signal still remains. For the vertical percolation study the small catchments are of principal importance. Several caves with thin cover not more than 10 to 100 m thick were investigated. It was found that the summer showers and snow melting in spring are the best tracers because of their greatest isotope compositoin diference compared to aquifer average.

9. Pezdič J.: Isotopes in Karst 8 SPECIFIC KARST AQUIFERS Because at these occasions a great quantity of the water infiltrates into aquifer in a short time, and we can speak about a kind of the “water wave”. In addition, at these periods the aquifers are also highly unsaturated, and events are short enough to be followed completely. The examined covers are mostly fine fractured with only several somewhat larger fractures. Taking into account the fact that at the beginning of a shower or a snow melt the soil as well as the karstic aquifer are unsaturated, only little of isotopically homogenized water remains. Considering also the fact that soil and carbonate aquifer are composed of thin drainage units, the lamellar flow is possible.

10. Pezdič J.: Isotopes in Karst 9 FIELD EXPERIENCES Our studies in the Planinska jama cave confirmed this presumption. Comparison with other studies indicates that in such conditions right at the beginning all empty volume of pores and fractures is filled with new water, which then pushes out the amounts of stored water. The fractural flow is established. In this manner, such aquifer functions as a sponge: if it is slightly wet, it first soaks a great quantity of water and then releases it. Additionally to Planinska jama cave some examples from the cave systems of Postojna jama, Škocjanska jama, Taborska jama as well as from springs such as Mrzlek, Malne and Divje jezero have been evaluated.