Petric.indd 171 � AB STRA CT For the adequate protection of karst springs it is important to understand the functioning of karst systems in their re- charge areas. Besides tracer tests with artifi cial tracers, tracings with natural tracers, such as temperature, are a very useful research tool, especially in complex karst systems with various types and sources of recharge. Detailed mon- itoring of water levels and temperatures of the Malenščica karst spring, as well as of the intermittent Cerknica Lake and Kotliči spring in its recharge area, indicated that temperature is a good tracer in winter and summer periods, es- pecially when the Cerknica karst polje is fl ooded. High or low air temperatures infl uence the temperature of water, which sinks at the Cerknica polje and fl ows rapidly through karst channels toward the springs. Detailed analysis of selected events enabled calculation of the velocity of groundwater fl ow toward the Malenščica spring under different hydrological conditions. The estimated values are in accordance with the velocities previously defi ned in this area, based on several tracer tests with artifi cial tracers. From the results, it is possible to assess the transport velocities of contaminants under different hydrological conditions. This is important information for the protection of the Malenščica spring, which is captured for the public water supply. Keywords: karst spring, water temperature, recharge, Malenščica, Slovenia Water temperature as a natural tracer – a case study of the Malenščica karst spring (SW Slovenia) � Janja Kogovšek and Metka Petrič Karst Research Institute SRC SASA, Titov trg 2, 6230 Postojna, Slovenia; (kogovsek@zrc-sazu.si, petric@zrc-sazu.si) doi: 104154/gc.2010.14 Geologia Croatica 63/2 171–177 6 Figs. 1 Tab. Zagreb 2010 Geologia CroaticaGeologia Croatica 1. INTRODUCTION Karst aquifers are very vulnerable to various sources of pol- lution. To protect them properly, it is important to understand the characteristics of groundwater fl ow in their recharge ar- eas. Different research methods are used in the studies of karst systems, one of the most often applied methods being tracer tests. Artifi cial tracers proved to be very useful in re- search of underground water connections, but in addition, tracings with natural tracers are becoming a very important tool for the study of the functioning of karst springs. A case- study of the Malenščica karst spring in south-western Slo- venia is described here, in which water temperature was tested as a possible natural tracer. Groundwater temperature can be used as a tracer to re- veal the regional structure of a groundwater fl ow system (BONACCI, 1997). It is a parameter that carries information on the hydraulic and thermal conditions in the karst interior (GENTHON et al., 2005). It can be considered as a non- conservative tracer due to the exchanges of temperature be- tween water and rock along the underground fl ow. When recharge water enters the aquifer system and fl ows down- gradient through the conduits, the initial temperature differ- ence will decrease due to the heat-exchange at the water-rock interface. The degree of heat exchange is controlled by the thermal characteristics of the rock and the conduit charac- teristics, such as conduit length, cross-sectional area, and fl ow velocity (RENNER & SAUTER, 1997). Water tends to Geologia Croatica 63/2Geologia Croatica 172 establish equilibrium temperature with the surrounding rock and sediments through conductive and advective processes (DOGWILER & WICKS, 2005). Additionally, in complex karst aquifers, signifi cant changes of temperature under a variety of hydrological conditions are a consequence of the infl ows of water with different characteristics from various parts of the recharge area. In several studies, measurements of fl ow and temperature in karst springs were used to defi ne the physical characteris- tics of large scale conduits in a karst aquifer (RENNER & SAUTER, 1997; HÜCKINGHAUS et al., 1997; LONG & GILCREASE, 2009). A more detailed study of the heat trans- port modes occurring in an observed karst aquifer was carried out by BUNDSCHUH (1997). Relationships between air and water temperatures were used to classify stream-groundwater interactions (O’DRISCOLL & DE WALLE, 2006). The Malenščica spring is an interesting example of a spring fed by primary recharge (autogenic), in the karst mas- sif (deep karst water), and secondary recharge (allogenic), along the karst poljes (surface water in the intermittent lake and sinking streams). Both types of infl ows mix within the karst system and their contribution to the spring discharge varies according to the hydrological conditions. The aim of the present study was to compare the water temperatures at various locations within the studied karst system in order to assess the relationship between the main contribution areas within the catchment of the Malenščica spring under differ- ent hydrological conditions. 2. CHARACTERISTICS OF THE STUDY AREA The Malenščica spring is captured for the water supply of approximately 21,000 inhabitants in the municipalities of Postojna and Pivka. It is situated at the southern border of the Planina karst polje (Fig. 1). According to data for the pe- riod 1961–1990, its lowest discharge was 1.1 m3/s, with mean discharge of 6.7 m3/s, and maximum discharge at 9.9 m3/s. The recharge area of the spring was defi ned on the known geological and hydrogeological conditions, and the results of tracer tests. Its central part is the karst massif of Javorniki and Snežnik, in which underground fl ow is domi- nant. The area is composed of Cretaceous carbonate rock, mostly limestone. On the eastern and northern sides of Ja- vorniki there is a string of karst poljes that are distributed in a SE-NW direction, of which the Cerknica polje is the larg- est. The oldest rocks in this part are Upper Triassic dolomites between the Planina and Cerknica poljes. Jurassic limestone and locally dolomite form the north-eastern boundary of the sequence of karst poljes. Quaternary alluvial sediments were deposited on karst poljes. At high water, these are fl ooded and intermittent lakes are formed. At the north-western boundary of the Cerknica polje, the Cerkniščica and Stržen surface streams, which fl ow along the polje, sink and fl ow underground mostly toward the Rak and Kotliči springs in Rakov Škocjan. Both springs recharge the Rak River, which fl ows on the surface for 2 km. The Rakov Škocjan area is composed of Cretaceous limestone, which is covered along the stream with Holocene sediments. Rak sinks underground again in the Tkalca Cave and fl ows toward the Malenščica and Unica springs at Planina polje. Due to the great extent and complex structure of the catchment, the proportions of the different infl ows change in time with the different hydro- logical conditions. A variety of geological, geomorphological, speleologi- cal and hydrogeological research has been carried out in the described study area. The results improved the understand- Fi gu re 1: Hydrogeological map of the study area. Legend: 1. Malenščica spring, 2. Unica spring, 2a. Plani- na Cave-Pivka branch, 2b. Planina Cave-Rak branch, 3. Tkalca Cave (ponor of the Rak River), 4. Kotliči spring, 5. Cave system Zelše (spring of the Rak river)–Karlovica (ponor on the Cerknica polje), 6. Postojna Cave, 7. Karst aquifer, 8. Fissured aquifer, 9. Porous aquifer, 10. Very low permeable rocks. Kogovšek and Petrič: Water temperature as a natural tracer – a case study of the Malenščica karst spring (SW Slovenia) Geologia Croatica 173 higher than in the infi ltrated water in the karst massif, which reaches equilibrium with the rock temperature at a depth of approximately 100 m. This was proven by long-term con- tinuous measurements of water temperature in the vadose zone of the Postojna Cave (KOGOVŠEK, 2009). In the un- derground system, these two types of recharge interact, and the temperature at the spring may refl ect the relationship be- tween them. This important infl uence of hydrological condi- tions should be considered in the interpretation of the data obtained, but we should be aware that some other impacts, which are very diffi cult to detect and explain, also infl uence the characteristics of temperature at the spring. The fi rst systematic observations of water temperature of the Malenščica spring were carried out from June 1999 to the end of 2000 using a Gealog S Logotronic data-logger, and in 2001 with an ISCO 6700-Sonde YSI 600. Analysis of the data obtained indicated the important infl uence of the in- termittent Cerknica Lake on the spring, but the lake temper- atures were not regularly measured. In order to improve un- derstanding of the functioning of the studied karst aquifer, instruments were installed in 2007 to measure data on the watercourses in the catchment of the spring and its water capture. Data was collected on water levels, discharges, elec- trical conductivity and temperature in 30-minute intervals from autumn 2007 to spring 2009. These parameters were measured with an ISCO 6700 with the YSI 600 sonde and 750 Area-Velocity Module at the Malenščica spring, and with TD divers (Van Essen, a Schlumberger company) at other locations. Analysis of the temperature and water level data (or discharge) of the Malenščica and Kotliči springs, were compared with the Cerknica Lake. Data for the latter was provided by our colleague Janez Turk. Air temperature data at the Postojna meteorological stations was obtained from the Environmental Agency of the Republic of Slovenia. 4. RESULTS The comparison between air temperatures and temperatures of the Cerknica Lake indicated that water temperature reacts ing of the functioning of this karst system. An important pro- portion of recharge with primary infi ltration on the Javorniki massif was proved by tracer test on the Poček military train- ing area (KOGOVŠEK, 1999). Underground water connec- tions between the Malenščica spring and the ponors at the north-western border of the Cerknica polje, and ponors in Rakov Škocjan were also confi rmed by several tracer tests (HABIČ, 1987b). The infl ow from the Cerknica part of the recharge area greatly depends on the hydrological conditions, which were not registered in the above mentioned tracer tests. In May 2008 a tracer test was carried out with the injec- tion of uranine in a ponor at the north-western border of the Cerknica polje (GABROVŠEK et al., 2010). Under the con- ditions of a relatively stable discharge of the Malenščica spring of 6.4 m3/s, the main direction of groundwater fl ow towards the Kotliči spring and further toward the Malenščica spring was proven. Lower concentrations were later detected in the Rak branch of the Planina Cave. The dominant appar- ent fl ow velocity, (regarding the fi rst peak of the break- through curve) toward the Kotliči spring was assessed at 109 m/h, and from there to the Malenščica spring as 99 m/h. Analyses of calcium and magnesium concentrations were carried out simultaneously with the tracer test. Comparison of Ca/Mg ratios of the Rak (2.5), Kotliči (4.5) and Malenščica (6.5) springs indicates an important part of the recharge of the Malenščica spring is from the Javorniki karst aquifer, which is mainly composed of Cretaceous limestone. Detailed monitoring of the Ca/Mg ratios between 1999−2001 (KOGOVŠEK, 2004) proved comparable values of Ca/Mg ratios: 1.6 for Rak, 4.8 for Kotliči, and 6.5 for Malenščica springs for similar hydrological conditions. 3. METHODS The study is based on the fact that the waters that recharge the spring have different water temperature characteristics. Surface fl ows are much more infl uenced by the air tempera- ture and the changes of water temperatures are signifi cantly Fi gu re 2: Comparison betwe- en air temperature and the temperature of the Cerknica Lake. Geologia Croatica 63/2Geologia Croatica 174 to the changes of air temperature with a particular time lag. The best fi t in the comparison of the periods of increase and decrease of temperature was obtained by the calculation of the 10 day moving average of air temperatures, which means that the lake temperature is conditioned by the average tem- perature of the 10 preceding days (Fig. 2). High oscillations of water temperature of the Malenščica spring were detected by measurements in the period from June 1999 to December 2001. The highest recorded temper- ature was 19.2ºC on July 24, 1999, and the lowest 1.7 on January 21, 2001. These extremes were reached in periods when the Cerknica polje was fl ooded and with contempora- neous high or low air temperatures (Table 1). Between 1999−2001, the minimum discharges of the Malenščica spring were recorded at the end of August and September (Fig. 3). The lowest discharge of 1.97 m3/s was recorded on September 2, 2001, when the water temperature was 8.4 ºC. Under such hydrological conditions in the ponors at the north-western edge of the Cerknica polje, only a small amount of the Cerkniščica River sinks, and the Stržen River does not reach the ponors, so the infl ow from the Cerknica area has only a minor infl uence on the Malenščica spring. Therefore the temperature of the spring refl ects primary re- charge from the Javorniki karst aquifer. At low waters in Au- gust 2003 (the spring of the Rak River was dry), the Ca/Mg ratio of the Malenščica and Kotliči springs was practically the same (approximately 4.5) (KOGOVŠEK, 2004). This indicates that under such conditions both springs are mainly recharged from the Javorniki area, where more permeable fi ssures and conduits are activated, and their contribution dominates over that from the less permeable parts of the karst system. In August and September 2000, the discharge of Malenščica did not fall below 3.28 m3/s, but its low temper- ature of 8.6 ºC indicates that infl ow from the Cerknica area was very low. During low waters in September 1999, when the mini- mum yearly discharge of 3.41 m3/s was reached, a somewhat higher temperature of 9.4 ºC indicated a more signifi cant in- fl ow of warmer water from the Cerknica area. This was the end of a long, very hot summer period of high waters, which did not occur in the following two years. At the beginning of June 1999 the temperature of the Malenščica spring had already reached 18ºC. From June 6 to July 1, 1999, the temperature decreased to 13ºC in condi- tions of minor oscillations of discharge around 7 m3/s, that indicates the gradual decrease of infl ow from the Cerknica area. On July 10, 1999, the discharge started to increase, reached the maximum value above 8 m3/s, remained at that level for 12 days, and then started to decrease (Fig. 4). The temperature of the Malenščica spring increased rapidly, and even daily oscillations of temperature were registered. The highest temperature of 19.2ºC lasted for 7 hours on July 24, 1999. With a further decrease in discharge, the temperature was also decreasing as a consequence of a reduction of the recharge from the Cerknica area and an increased proportion of the contribution from the Javorniki karst aquifer. Evi- dently the temperature of the sinking water in the Cerknica polje was very high, and further fl ow through well developed karst channels was rapid, so that no signifi cant cooling of a relatively large amount of fl owing water occurred along the underground path. The discoveries described above were additionally con- fi rmed by observations between 2007−2009. At very low wa- ter when the discharge of the Malenščica spring decreased toward 2.9 m3/s on March 9 in 2008 (Figs. 5 and 6), the tem- perature of the spring slowly increased, (even though the air temperature and consequently the lake temperature were very Fi gu re 3: Recorded tempera- tures and discharges of the Malenščica spring from June 1999 to December 2000. Dot- ted line borders a selected in- terval presented in Fig. 4. Table 1: Minimum and maximum temperatures of the Malenščica and Kotliči springs occuring during fl oods in the Cerknica polje. Malenščica spring Kotliči spring Hydrological year Tmax (°C) Tmin (°C) Tmax (°C) Tmin (°C) until 19.9.1999 19.2 (24.7.99) 20.9.1999–30.9.2000 2.3 (26.12.99) 31.9.2000–1.9.2001 14.4 (7.5.01) 1.7 (21.1.01) 4.9.2007–17.10.2008 17.7 (17.8.08) 5.0 (16.2.08) 22.7 (3.7.08) 2.6 (22.2.07) from 17.10.2008 1.7 (2.1.09) 15.3 (17.5.09) 2.0 (5.1.09) Total period 19.2 1.7 22.7 2.0 Kogovšek and Petrič: Water temperature as a natural tracer – a case study of the Malenščica karst spring (SW Slovenia) Geologia Croatica 175 low), reaching 8.1ºC. This indicates that the amount of infl ow from a very cold Cerknica Lake was very low. A slightly larger infl uence of the lake was observed in the Kotliči spring. A reverse situation was observed in autumn 2008. The temperature of the Cerknica Lake increased during the sum- mer to 25ºC, and the Malenščica spring reached 17.7ºC. In September and in the beginning of October the lake temper- ature was still above 20ºC, while in the Malenščica spring, the temperature decreased to 9ºC, parallel with the decrease of discharge to 1.6 m3/s on October 17, 2008. Again this can be explained by an increased amount of recharge from the Javorniki karst aquifer. During lengthy fl ood conditions (from March to the be- ginning of May 2008, and December 2008 to March 2009) the water temperature was very uniform within the observed system (Cerknica Lake−Rakov Škocjan−Malenščica). Water temperatures of the Malenščica and Kotliči springs were compared in more detail over the short interval from March to April 2008 (Fig. 6). In the beginning, the discharges were low and recharge from the Javorniki area prevailed in both springs. After precipitation a greater infl uence of the infl ow from the Cerknica area was indicated by rapid changes of temperature. The reaction of the two springs is similar, but due to a larger distance and longer path of underground fl ow the changes at the Malenščica spring are delayed and reduced. Based on the assumption that the velocity of the transfer of temperature along the underground fl ow is con- sidered to be equal to the velocity of water fl ow, the positions of the peaks (maximum) or saddles (minimum) of the tem- perature curves of the Kotliči and Malenščica springs were compared. In this way the fl ow velocity in the karst system between the two springs was estimated at 145 to 215 m/h (at discharges of Malenščica from 5 to 9 m3/s). These values are comparable with fl ow velocities from 140 to 256 m/h, which were calculated as a result of 5 tracer tests performed in the area between Rakov Škocjan and Malenščica (HABIČ, 1987a). A lower fl ow velocity of 99 m/h was defi ned by the tracer test in May 2008 (discharge 6.4 m3/s, slowly decreas- ing). Unfortunately, only small changes of hydrological con- ditions and water temperatures were recorded during this tracer test, and it was not possible to accurately defi ne the peaks and saddles of the curves. Nevertheless, an approxi- mate comparison of both temperature curves indicates a sim- ilar range of velocity as was calculated based on the results of tracing with uranine in the same period. Daily oscillations of temperatures, recorded during high waters in the beginning of April 2008 (Fig. 6) are interest- ing. They were very regular at the Kotliči spring and slightly less distinctive at the Malenščica spring. The comparison of Fi gu re 4: Temperatures and discharges of Malenščica spring for a selected interval in the summer of 1999. Fi gu re 5: Recorded tempera- tures and discharges from Sep tember 2007 to April 2009. Dotted line borders a selected interval presented in Fig. 6. Geologia Croatica 63/2Geologia Croatica 176 their position confi rms the characteristics of groundwater fl ow in the Rakov Škocjan-Malenščica karst system as de- scribed above. On July 21, 1999, the temperature of the Malenščica spring (discharge 8 m3/s) was 17.8ºC, 21.5ºC at Kotliči, and 23ºC at the Cerknica Lake. Supposing that the temperature of the infl ow from the Javorniki karst aquifer is 8.5ºC (based on the measured temperatures at Malenščica at very low dis- charges), we assessed the amount of recharge from the Cer- knica area in the Malenščica spring to be up to 70%. Similar assessment for the end of December 1999 (discharge of Malenščica approximately 10 m3/s) resulted in calculation of an even higher amount. In March 2008, the temperature of the Cerknica Lake was 5.4ºC, and the Malenščica spring was at 8.1 ºC (discharge 3 m3/s). This indicates recharge from the Cerknica area at approximately 15%. Such calculations can give us a general idea about the different contributions to the recharge of the Malenščica spring, but as they do not take into account the changes of water temperature along the underground path due to the exchange with surrounding rock, they are only rough estimations. 5. CONCLUSION Propagation of temperature signals through the underground system in the catchment of the Malenščica spring was anal- ysed under different hydrological conditions. The results ob- tained indicate that temperature is a very good natural tracer of water fl ow in the study area of a very complex karst sys- tem, with well developed karst channels, especially in winter and summer periods when the Cerknica polje is fl ooded. Minimum and maximum temperatures of the spring were recorded in the periods of high water (dominated by recharge from the Cerknica area) and very low or high air tempera- tures. During low waters the recharge from the Javorniki karst aquifer prevails. A detailed analysis of selected water waves enabled cal- culation of the velocities of groundwater fl ow toward the Malenščica spring for different hydrological conditions. Ve- locities obtained are comparable with those calculated on the results of tracings with artifi cial tracers. Information about the fl ow velocities at different hydrological conditions is im- portant for the protection of the springs captured for the wa- ter supply, because it enables assessment of the characteris- tics of the transfer of contaminants which differ signifi cantly according to the hydrological conditions. One of the main sources of pollution in karst areas is waste water from urban areas. In the settlements on the Cerknica polje, sewage wa- ter is only partly cleaned in an old water treatment plant with insuffi cient capacity, and it endangers the quality of the Malenščica spring. Although some interesting results were obtained by the research described here, the limitations of the research tool should not be disregarded. In order to verify and improve the findings we applied other research methods in the Malenščica study area, and the data obtained will be used in further studies. REFERENCES BONACCI, O. (1987): Karst Hydrology.– Springer-Verlag, Berlin, 184 p. BUNDSCHUH, J. (1997): Temporal variations of spring water tempera- tures in relation to the extents of the heat transport modes occurring in the karstifi ed lower Gypsum-Keuper aquifer (Karnian, southern Germany).– In: JEANNIN, P.-Y. (ed.): Proceedings of the 6th Con- ference on Limestone Hydrology and Fissured Media, La Chaux- de-Fonds, Switzerland 1997, 129–132. DOGWILER, T. & WICKS, M.C. (2005): Thermal variations in the hy- porheic zone of karst stream.– Speleogenesis and Evolution of Karst Aquifers, 3, 1–11. GABROVŠEK, F., KOGOVŠEK, J., KOVAČIČ, G., PETRIČ, M., RAVBAR, N. & TURK, J. (2010): Recent results of tracer tests in the catchment of the Unica River (SW Slovenia).– Acta carsologica, 39/1, 27−37. GENTHON, P., BATAILLE, A., FROMANT, A., D’HULST, D. & BOURGES, F. (2005): Temperature as a marker for karstic waters hydrodynamics. Inferences from 1 year recording at La Peyrére cave (Ariège, France).– Journal of Hydrology, 311, 157–171. HABIČ, P. (1987a): Raziskave kraških izvirov v Malnih pri Planini in zaledja vodnih virov v občini Postojna [Study of the karst spring in Malni near Planina and catchments of water resources in the Mu- nicipality of Postojna – in Slovenian].– Unpubl. report, Karst Re- search Institute, Postojna, 58 p. HABIČ, P. (1987b): Water tracing test on the karst watershed among Id- rijca, Vipava and Ljubljanica.– Acta carsologica, 16, 105–118. HÜCKINGHAUS, D., LIEDL, R. & SAUTER, M. (1997): Character- ization of karst aquifers by heat transfer.– In: JEANNIN, P.-Y. Fi gu re 6: Recorded tempera- tures and discharges for a se- lected interval in the spring 1999. Kogovšek and Petrič: Water temperature as a natural tracer – a case study of the Malenščica karst spring (SW Slovenia) Geologia Croatica 177 (ed.): Proceedings of the 6th Conference on Limestone Hydro logy and Fissured Media, La Chaux-de-Fonds, Switzerland 1997, 145– 148. KOGOVŠEK, J. (1999): New knowledge about the underground water drainage in the Northern part of Javorniki Mountains (High Karst).– Acta carsologica, 28/1, 161–200. KOGOVŠEK, J. (2004): Physico-chemical properties of waters in the Malenščica recharge area (Slovenia).– Acta carsologica, 33/1, 143–158. KOGOVŠEK, J (2009): Characteristics of water fl ow through the karst va- dose zone.– Unpubl. PhD Thesis, University of Nova Gorica, 215 p. LONG, A.J. & GILCREASE, P.C. (2009): A one-dimensional heat-trans- port model for conduit fl ow in karst aquifers.– Journal of Hydro- logy, 378, 230–239. O’DRISCOLL, M.A. & DE WALLE, D.R. (2006): Stream-air tempera- ture relations to classify stream-ground water interactions in a karst setting, central Pennsylvania, USA.– Journal of Hydrology, 329, 140–153. RENNER, S. & SAUTER, M. (1997): Heat as a natural tracer: Charac- terisation of a conduit network in a karst aquifer using temperature measurements of the spring water.– In: GÜNAY, G. & JOHNSON, I. (eds): Karst Waters & Environmental Impacts, Balkema, 423−431. Manuscript received December 23, 2009 Revised manuscript accepted February 25, 2010 Available online June 28, 2010