GEOLOGIJA 48/1, 141–152, Ljubljana 2005
The use of natural tracers in the study of the unsaturated zone of
a karst aquifer
Uporaba naravnih sledil pri {tudiju nezasi~ene cone kra{kega vodonosnika
Branka TR^EK Geolo{ki zavod Slovenije, Dimi~eva ul. 14, 1000 Ljubljana, Slovenija, e-mail:branka.trcek@geo-zs.si
Key words: karst aquifer, unsaturated zone, natural tracers, experimental field site Sinji vrh (SW Slovenia)
Klju~ne besede: kra{ki vodonosnik, nezasi~ena cona, naravna sledila, poskusno polje Sinji vrh (JZ Slovenija)
Abstract
The unsaturated zone of a karst aquifer was investigated in a catchment area of the Hubelj spring (SW Slovenia). The monitoring of flow and natural tracer transport was undertaken with the intention to study the recharge, storage and discharge processes of the karst aquifer, mixing processes, residence times and transport phenomena. The results point out the significance of effects of the fast preferential flow – epiflow. This flow is the main factor controlling contaminant transport towards the aquifer saturated zone.
Kratka vsebina
V zaledju izvira Hubelj (JZ Slovenija) so potekale raziskave nezasi~ene cona kra{kega vodonosnika. Vzpostavil se je monitoring toka in prenosa naravnih sledil, katerega namen je bil {tudij procesov napajanja, uskladi{~enja in praznjenja kra{kega vodonosnika, procesov me{anja, zadr‘evalnih ~asov ter mehanizmov prenosa snovi. Rezultati so opozorili na vplive hitrega prednostnega toka – epitoka. Le-ta je glavni faktor za prenos in {irjenje onesna‘enja preko nezasi~ene cone do zasi~ene cone vodonosnika.
Introduction
Groundwater of karst aquifers is a very important source of drinking water supply in Slovenia. In order to protect the karst water from pollution it is necessary to investigate the behaviour of contaminants and thus to understand better natural factors that control its behaviour. However, the heterogeneity of karst aquifers makes it difficult to quantify and predict the movement of groundwater and contaminants through and/or between different aquifer zones. Concerning this the study of flow and solute
transport mechanisms have been undertaken in a karst aquifer in the catchment area of the Hubelj spring (South-Western Slovenia) (Fig. 1) with the intension to answer several open questions connected with problems which result from the duality of the aquifer recharge, storage and discharge processes. This duality is reflected in the fast concentrated flow through the karst conduit network and the relatively slow diffuse flow from the low permeability rock blocks.
The research was focused on the study of mechanisms that cause flow and solute transport from an upper unsaturated zone
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Branka Tr~ek
that was investigated in an artificial tunnel 10 to 20 m below the surface (Fig. 1). This study based on natural tracers that are an important tool for investigating flow systems, mixing processes, residence time and connected storage properties of groundwa-ter, water quality monitoring, dilution and biodegradation processes and transport phenomena.
Description of the study area
The catchment area of the karst spring Hubelj (Fig. 1) is estimated to 50-80 km2 (Tri{i~, 1997). This region belongs to the high karstic plateau Trnovski gozd with the mean altitude of 900 m a.s.l. The plateau consists of carbonate rocks that are covered by shallow soils (10 -50 cm) of low water holding capacities (22-142 mm) and high infiltration rates (Mati~i~ , 1997). The average annual precipitation amount is 2450 mm, while the average annual air temperature is 7-9 °C.
The highly karstified carbonate, consisted mainly of the Jurassic limestone, is bounded by the Norian-Rhaetian dolomite in the north and by the Eocene flysch beds in the south and in the east (Fig. 1). The tectonic structure of the research region is complicated. The predominant tectonic elements are complex overthrusts cut by a dense system of subvertical faults (J a n e ‘ et al., 1997). The Av~e fault is the most important regional fault crossing the study area. Near the
thrust lines carbonate rocks are finely crushed and ground and, consequently, less permeable.
The research was centered at three areas of the observed aquifer: the recharge area, the upper unsaturated zone and the discharge area. The latter was investigated in the Hubelj spring and the first two at the Experimental field site of Sinji vrh, which is located 600 m above the spring (Figs. 1, 2).
Hubelj is one of the biggest Slovene karst springs. Its mean discharge is 3 m3/s, the minimum one is 0.2 m3/s, while the maximal one is 59 m3/s (J a n e ‘  et al., 1997).
The upper unsaturated zone was investigated in an artificial tunnel which represented a natural laboratory for studies of chemical and stable isotopic composition of the seepage water, and with it the drainage system of the tunnel cover. The tunnel lies 5 to 25 m below the surface at 825 m mean altitude (Fig. 2). This area consists of the limestone of Lias-Dogger age (^en~ur Curk & Veseli~, 1999; ^en~ur Curk, 2002; Tr~ek, 2001). Since the Experimental field site of Sinji vrh belongs to a region of the Av~e fault, the rock is fractured, broken and crushed near the fault lines.
Methods and techniques
Monitoring of flow and natural tracer transport was performed during the period from 1999 to 2000. The saturated zone outflow was monitored in the Hubelj spring,
Figure 1. Study area.
Slika 1. Raziskovalno obmo~je.
The use of natural tracers in the study of the unsaturated zone of a karst aquifer
143
Figure 2. Longitudinal section of the artificial tunnel Sinji vrh with sampling points for isotopic and
chemical analyses.
Slika 2. Vzdol‘ni prerez umetnega rova Sini vrh z vzor~evalnimi mesti za izotopske in kemijske
analize vode.
while the precipitation and the upper unsa-turated zone seepage water were monitored near the tunnel entrance and at six tunnel sampling points (SVR-1, SVR-2, SVR-3A, SVR-3B, SVR-4 and SVR-7) respectively (Fig. 2). Continuous measurement of water balance and of physico-chemical water parameters (pH and electroconductivity) were carried out to obtain basic information on the study area. Furthermore, monthly water sampling for analysis of 18O, 13C, dissolved organic carbon (DOC) and alkalinity composition was undertaken to obtain additional information about mixing processes and groundwater residence times.
It could be said that the chemical composition of karst groundwater is quite uncomplicated, seeing that bicarbonate, calcium and sometimes magnesium species prevail. The most important processes that affect the chemical and isotopic composition of the total inorganic carbon in groundwater are a) the dissolution of CO2 in the unsaturated zone and b) the dissolution of carbonate minerals in unsaturated and saturated zones. The partial pressure of CO2 is the main factor that controls these two processes.
Alkalinity is a conservative tracer, because changes of the carbonic acid concentration do not have influence on it. Alkalinity is defined as an excess of base cations over acid anions (S i g g et al., 1992). Thus, it is a measure for a system reaction. Lower the alkalinity value, lower is the dissolution of carbonate minerals in water, which results into lower bicarbonate ion concentration. On the other hand this reflects lower carbonic
acid concentration as the consequence of lower partial pressure of soil CO2. It could be recapitulated that all these relationships express the seasonal variation of the discussed parameter.
Precipitation washes out of the soil also DOC, which originates from decomposition of the organic material in the upper soil horizon (Brooks et al., 1999). The DOC concentration depends besides on the soil properties also on the vegetation type, aquifer hydraulic characteristics and the unsatura-ted zone depth. The DOC concentration is usually between 10 and 100 mg/l in the soil, but it selectively decreases with the aquifer depth. Therefore, the values of the DOC concentration vary between 0.2 and 10 mg/l in groundwater, but they are frequently lower than 1-2 mg/l (Clark & Fritz, 1997; Kendall & McDonell, 1998).
Significant quantities of DOC are washed out of the soil during the major precipitation events and snowmelt (Hongve, 1999). Hence it follows that the dissolved organic carbon is a potential tracer of the fast preferential flow in the karst aquifer. It was reported that a) DOC is a very important tool for the flow and solute transport studies of groundwater with residence time of at least one month (Batiot et al., 2000), and that b) high DOC concentrations in the aquifer deeper parts result from long residence time (Hongve , 1999).
The stable isotope of 18O is an ideal conservative tracer under low temperature conditions (Clark & Fritz, 1997). After the infiltration  of  precipitation only  physical
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Branka Tr~ek
processes such as diffusion, dispersion, mixing and evaporation alter the groundwater 18O isotopic composition (?18O). Hence, the sampled water ?18O together with hydrome-tric data provides information on the movement and mixing of water masses on condition that the isotopic composition of each water masse significantly differs.
The seasonal variation of the precipitation ?18O represents an input signal that may be used for the groundwater dating. Namely, precipitation infiltrates into the soil and recharges the aquifer, where it is mixed with the pre-stored groundwater. This results in the input signal attenuation indicated in a lowering of the isotopic variation amplitude. Owing to different mixing and homogenisation stages, groundwater has different ?18O throughout the aquifer and with that different amplitude of the isotopic seasonal variation. These differences can be applied for the determination of groundwater residence time seeing that the longer the residence time, the lower is the amplitude of the groundwater isotopic seasonal variation.
Although the stable isotope of 13C is not a conservative tracer, it can trace the carbon sources and the reactions for a multitude of inter-reacting organic and inorganic species. The groundwater 13C composition (?13C) gives information on the flow type and solute transport. It provides an insight into the water geochemical evolution, rock types in the flow paths surroundings and the recharge area conditions (Hoefs, 1997; Urbanc , 1993; Pezdi~ , 1999; Tr~ek, 1997). It is well known that ?13C is about 0 ‰ in carbonate rocks, -25 ‰ in the soil CO2 (similarly as in plants) and -7 ‰ in the atmospheric CO2, respectively (Kendall & McDonell , 1998; Pezdi~ , 1999).
?13C varies like the alkalinity, but inverse proportionally. The higher values of ?13C usually occur during the winter. The deviations from general trends may result from hydrological events (e.g. storm events).
The characteristics of the isotopic composition of natural substances are described in numerous publications (e.g. Clark & Fritz, 1997; K endall & McDonnell, 1998). Measurements of the stable isotopic composition of substances are conventionally reported in terms of a relative value d,
?x (‰) = (Rx/Rst – 1) · 1000         (1)
where Rx is the isotope ratio (e.g. 18O/16O and 13C/12C) in the substance X, Rst is the isotope ratio in the corresponding international standard substance, and d is expressed in parts per thousand.
Laboratory measurements
Most of water samples were analysed for d18O at the GSF-Institute of Groundwater Ecology in Germany (the first measurements were performed at the Josef Stefan Institute in Ljubljana), with a standard analytical error of ± 0.05 ‰.
The DOC composition of water samples was measured at the same institution. The standard analytical deviation was ± 0.05 mg/l.
?13C of water was analysed in the laboratory of the Jo‘ef Stefan Institute in Ljubljana with the standard analytical error of ± 0.15 ‰.
The alkalinity analyses were performed by me in the laboratory of the Geological Survey of Slovenia. The titration method was used according to instructions from the book Standard Methods for the Examination of Water and Wastewater (Greenberg et al.,1992).
Statistical processing of data
Data were statically analysed with box-plots, which are a very useful and concise graphical tool for summarising the distribution of data sets (Fig. 3). Boxplots visually illustrate a) the mean of the data (the median – the centre line of the box), b) the vari-
Figure 3. Description of the boxplot. Slika 3. Opis {katlastega diagrama (o - oddaljena vrednost, - <J» zelo oddaljena vrednost).
The use of natural tracers in the study of the unsaturated zone of a karst aquifer
145
ation or the spread (interquartile range – the box height), c) the skewness (quartile skew – the relative size of box halves), and the presence or absence of unusual values (outside and far outside values) (H e l s e n & H i r s c h , 1992). They are even more useful for comparing these attributes among several data sets. In the case of normally distributed data, the outside values occur fewer than once in 100 times and far outside values fewer than once in 300000 times, respectively (H e l s e n & H i r s c h , 1992). If the occurrence of these values is more frequent than the expected, then this is an indication that data may not originate from a normal distribution.
Results and discussion
The most important results of the ?18O monitoring are the age estimations of the sampled groundwater (Tr~ek, 2002). Differences between the seasonal variation of ?18O in precipitation and in groundwater we-
re taken into consideration. Because there is just the 2-year series of data for the study area precipitation, the data of the nearest compatible precipitation station (Ljubljana) were used. Polynomial trends (amplitudes and their phase shifts), linear trends and annual averages of sampled water ?18O were compared, which is summarised in Figure 4. Because of the typical ?18O seasonal variation and its positive linear trend, the average residence time of the SVR-7, SVR-4, Hubelj and SVR-3A groundwater should be less than 5 years. On the other hand the average residence time of the SVR-3B, SVR-1 and SVR-2 groundwater should be longer, owing to the untypical ?18O seasonal variation and its negative linear trend. The estimations of the groundwater average residence time for single sampling points are: 3 months for the SVR-7 groundwater, 9 months for the SVR-4 groundwater, 2-3 years for the Hubelj spring, 4-5 years for the SVR-3A gro-undwater, 5-6 years for the SVR-3B ground-water and at least 10 years for the SVR-1
Za      annual average of precipitation S18O (1984-98 in Ljubljana, 2000 at Sinji vrh) /
letno povpre~je §18O padavin (1984-98 v Ljubljani, 2000 na Sinjem vrhu) ____    polynomial trend of precipitation S18O / polinomni trend S18O padavin
------    linear trend of precipitation S18O / linearni trend S18O padavin
•      average of sampled water S18O in 2000 / povpre~na S18O vzor~ene vode leta 2000
Figure 4. Trends of average annual ?18O of precipitation compared with average annual ?18O of groundwater sampled in the catchment area of the Hubelj spring.
Slika 4. Primerjava povpre~ne letne ?18O padavin in povpre~ne letne ?18O vzor~ene podzemne vode v
zaledju Hublja.
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Branka Tr~ek
Figure 5. Boxplots of the dissolved organic carbon concentration of monthly water samples.
Slika 5. [katlasti
diagrami
koncentracije
raztopljenega
organskega ogljika v
mese~nih vzorcih
vode.
and SVR-2 groundwater (it can be one or more decades longer).
The major characteristics of the sampled water dissolved organic carbon (DOC) are presented in Figures 5 and 6. It could be noticed in Figure 5 that DOC values of SVR-1, SVR-2, SVR-3A, SVR-3B and the Hubelj
spring base flow waters (i.e. older ground-water) range similarly, from 0.1 to 2 mg/l. Their means are 0.47, 0.54, 0.8, 0.43 and 0.59 mg/l, respectively. The one of the SVR-3A water is somewhat higher, which probably reflects the faster groundwater dynamics. On the other hand, the DOC ranges of the
Figure 6. Time-trend of the dissolved organic carbon concentration of monthly water samples. Slika 6. ^asovni prikaz koncentracije raztopljenega organskega ogljika v mese~nih vzorcih vode.
The use of natural tracers in the study of the unsaturated zone of a karst aquifer
147
Figure 7. Boxplots of the alkalinity of monthly water samples.
Slika 7. [katlasti diagrami alkalnosti v mese~nih vzorcih vode.
younger groundwater of SVR-7 and SVR-4 are wider, particularly the former, which reflects the intensive groundwater dynamics. The corresponding means are somewhat higher with regard to the others, 0.78 and 0.94 mg/l respectively, but yet similar to the one of the SVR-3A water.
The occurrence of the outlying and far-outlying values is distinctive for all sampled waters, except for the one of SVR-2 (Fig. 5). These values might result from the fast preferential flow. The flow effects are noticed in Figure 6, such as in the summer and autumn 1999 and in July 2000. During these periods the preferential flow had to wash out of the soil increased amounts of organic carbon compounds, which resulted in the increased DOC concentration of gro-undwater. The highest concentrations were measured in the SVR-7 water (Fig. 6). Hence, the recharge of he sampling point SVR-7 should be linked with the fastest drainage paths of all, which is in accordance with the shortest residence time of its water.
The boxplots in Figure 7 illustrate the statistical characteristics of the sampled water alkalinity. Waters of SVR-1 and SVR-2 have by far the lowest mean alkalinity, 1.61 and 1.51 meq/l respectively, which reflect their lowest reactivity. Because of similarity of the alkalinity ranges and seasonal variations (Figs 7, 8), the dynamics of these waters should be similar too. Owing to longer residence time, the water should slowly seep through the upper unsaturated
zone and recharge these two sampling points.
In contrast to waters of the previous two sampling points, those of SVR-3A and SVR-7 have the highest mean alkalinity values, 2.31 and 2.35 meq/l respectively, and the widest, and also similar, ranges of the alkalinity (Fig. 7). In this the highest reactivity as well as dynamics of all waters are reflected, which could be observed also in Figure 8. Therefore these two sampling points should be recharged through the fast drainage paths. Concerning that, it should be stressed that the average residence time of SVR-3A and SVR-7 waters differs considerably, which indicates that the fast flow does not comprise only the younger water.
Waters not mentioned yet are located between the above-quoted pairs in Figures 7 and 8. Hence it follows that their reactivity and dynamics should be also in-between. Waters of SVR-4 and SVR-3B have similar alkalinity means, 2.15 and 2.13 meq/ l respectively, and ranges, but a different distribution of data. The Hubelj spring data range somewhat different from the previous two, but they are similarly distributed like those of the SVR-4 water. However, the mean value of the spring alkalinity, 2.31 meq/l, is the same as the one of the SVR-3A water.
At the end the attention should be given also to two pairs of waters, namely (a) the waters of SVR-7 and SVR-4, and (b) the waters of SVR-3A and SVR-3B. Waters of each pair have a similar average residence
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Branka Tr~ek
Figure 8. Time-trend of the alkalinity of monthly water samples. Slika 8. ^asovni prikaz alkalnosti v mese~nih vzorcih vode.
time, which differs for a definite shift. According to the interpretation of Figures 7 and 8, the two sampling points of each and every one pair should have the same catchment area, but their recharge should depend on the different dynamics leading to shifts of their average residence time. The hydrodynamic conditions under which the sampling point SVR-7 and SVR-4 or SVR-3A and SVR-3B were recharged by the water of the similar composition could be noticed in Figure 8.
The results of ?13C are illustrated in Figures 9 and 10. ?13C data of SVR-3A and SVR-7 waters with the highest dynamics range the least of all, and have the lowest means, – 14.28 and –13.5 ‰ respectively (Fig. 9). These mean values indicate the dissolution of carbonate minerals by carbonic acid, since they occur between ?13C of carbonate rocks and soil CO2 (Kendall & McDonell , 1998).
On the other hand, the oldest waters of SVR-1 and SVR-2 with the slowest dynamics have the widest ranges of ?13C (Fig. 9). Their mean values are –8.32 and –9.4 ‰ respectively, which is most probably the result of (a) isotopic exchange between the gro-undwater and the rock matrix, (b) degassing process, or (c) dissolution of carbonate rocks by stronger acids (Huneau & Blavoux, 2000; Kendall & McDonell , 1998).
In contrast to this is the explanation for the widest data ranges not so evident. With regard to the longest average residence time and the slow dynamics just the opposite would be expected - that ?3C of these waters would be the most homogeneous. Why this is not so?
Since the answer could not be found in the literature, I found my own explanation, which was discussed and approved by Professor Jo‘e Pezdi~ from the Ljubljana University. It presumes that the sampling points SVR-1 and SVR-2 are recharged by the laminar flow from a low permeability rock matrix. At times of intensive hydrological events, this matrix could be influenced by a flow breakthrough containing some portions of the event and soil water. These breakthroughs might be the reason for larger deviations of some ?13C values from the relatively homogeneous ?13C, which was verified by the analysis of Figure 10, and particularly by results of the short-term sampling during the storm event (Tr~ek, 2003).
In spite of all that, the discussed effect is not characteristic for ?18O of SVR-1 and SVR-2 waters, since the breakthrough water most probably does not differ much from the water previously stored in the rock matrix.
Remains the description of ?13C of SVR-3B and SVR-4 waters and of the Hubelj
The use of natural tracers in the study of the unsaturated zone of a karst aquifer
149
spring base flow. Since the ranges and the means of these data lie just between those previously discussed, the composition of these waters presumingly reflects the average geochemical and hydrodynamic conditions (Figs 9, 10).
The information of Table 1, which presents the water balance of the tunnel sampling points in 2000, additionally contributes to presented results: 1) the estimations of
Figure 9. Boxplots of the ?13C of monthly water samples.
Slika 9. [katlasti diagrami ?13C v mese~nih vzorcih vode.
sampled water average residence time are indirectly verified by the sapling points volume, 2) it is evident that sampling points SVR-1 and SVR-2 are mostly recharged in the winter-spring period, 3) similar percentages of a) the SVR-3B, SVR-4 and SVR-7 water volume, and of b) the precipitation volume are noticed in each season, which reflects the piston effect, 4) the comparison between the water volume portions relating
Figure 10. Time-trend of the ?13C of monthly water samples. Slika 10. ^asovni prikaz ?13C v mese~nih vzorcih vode.
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Table 1. Water balance of the tunnel sampling points in 2000. Tabela 1. Vodna bilanca vzor~enih vod v rovu v letu 2000.
year/leto 2000		SVR-1	SVR-2	SVR-3A	SVR-3B	SVR-4	SVR-7	Precipitation/ padavine (mm)
annual volume/ letni volumen (l)		4.7	5.1	10.5	13.5	10.1	10.7	2493.2
velocity (m/year)/ hitrost (m/leto)		0.47	0.51	1.05	1.35	1.01	1.07	
winter-spring vol. / zimsko-pomlad. vol.	(%)	76	66	21	34	31	30	27
summer vol./ poletni vol. (%)		14	0	59	16	14	17	19
autumn-winter vol./ jesensko-zimski vol.	(%)	10	34	20	50	55	53	54
max./maks. vol. (%)		Mar.	Jan.	Jul.	Nov.	Oct.	Oct.	Nov.
to the storm event points out a) the greatest dynamics of SVR-3A, which should most probably result from the epiflow, and reflects that b) the laminar flow had to recharge SVR-1 and SVR-3B, while c) SVR-4 and SVR-7 had to be partially recharged also by the faster flow.
It is presumed that the artificial tunnel near Sinji vrh penetrates the epikarst zone as well as the lower unsaturated zone, while the space between them belongs to the so-called transitional area.
The characteristics of waters of sampling points SVR-7 and SVR-4 indicate the epikarst zone. The water that has been previously stored in the aquifer higher areas should mostly recharge the two sampling points. Owing to the piston effect, this water should be discharged to the sampling points through the fast drainage paths.
The characteristics of waters of sampling points SVR-1 and SVR-2 demonstrate that the tunnel most probably penetrates the lower unsaturated zone in this area. The sampling points should be recharged by the laminar flow from the tiny fractured rock blocks.
The characteristics of waters of sampling points SVR-3A and SVR-3B indicate the transition area. Both sampling points should be recharged from the same catchment, only that the former should be recharged by the epiflow and the latter by the diffuse flow from the epikarst zone. It is presumed that the epiflow is linked to larger tectonic fractures that might be connected with a shaft.
Conclusions
It could be summarised that data of natural tracers monitoring provided a) an insight into the sampled water dynamics and reactivity and b) the estimations of sampled water average residence times. The significance of effects of the fast preferential flow-epiflow and of the duality of recharge processes was pointed out.
The results are in agreement with the epikarst hypothesis which presumes that an important part of the karst aquifer recharge rapidly arrives into the karst aquifer conduit network from the epikarst zone. The epikarst zone plays the role of a Faraday cage with respect to lower aquifer parts, resulting in the water concentration and storage (Kiraly et al., 1995). Epiflow is the main factor controlling contaminant transport towards the aquifer saturated zone.
The presented results could improve the karst aquifer vulnerability characterisation and thus contribute to the development of karst areas.
Acknowledgement
I would like to acknowledge Professor Miran Veseli~ and Professor Jo‘e Pezdi~ of the University in Ljubljana who served as mentors for the Hydrogeology and Isotope Science respectively and Professor Klaus-Peter Seiler and Wilibald Stichler from the GSF-Institute of Groundwater Ecology (Germany) for chemical and isotopic analyses.
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Uporaba naravnih sledil pri {tudiju
nezasi~ene cone kra{kega vodonosnika
(povzetek)
Kra{ki vodonosniki so pomemben vir kakovostne vode v Sloveniji, zato jih je treba varovati pred onesna‘enjem in prekomernim izkori{~anjem. Glede na to, postaja {tu-dij toka in prenosa snovi v kra{kem vodono-sniku vse pomembnej{i. Bil je tudi predmet raziskav, ki so potekale v zaledju kra{kega izvira Hubelj (sl.1). Pozornost je bila namenjena {tudiju mehanizmov, ki povzro~ijo tok in prenos snovi iz nezasi~ene cone vodono-snika.
V raziskavo so bila vklju~ena opazovanja a) zasi~ene in b) nezasi~ene cone vodonosni-ka. Prva se je opazovala na izviru Hublja, ki predstavlja glavni iztok vode iz kra{kega vo-donosnika, druga pa na poskusnem polju Sinji vrh, okoli 600 m nad izvirom Hublja (sl. 1). Nezasi~ena cona vodonosnika se je pro-u~evala v umetnem rovu, 5 do 25 m pod povr{jem (sl. 2).
Raziskovalna metodologija je temeljila na naravnih sledilih, vklju~ujo~ monitoring stabilnih izotopov (18O in 13C) in kemijskih parametrov (raztopljen organski ogljik -DOC in alkalnost) v podzemni vodi, vzor~e-ni v rovu in na izviru Hublja, ter v padavinah, vzor~enih na poskusnem polju Sinji vrh. Monitoring toka in prenosa naravnih sledil je potekal v letih 1999 in 2000 z namenom, da se prou~i procese napajanja, us-kladi{~enja in praznjenja kra{kega vodo-nosnika, procese me{anja, zadr‘evalne ~ase podzemne vode ter mehanizme prenosa snovi.
Razlike med amplitudami sezonskega nihanja izotopske sestave kisika (?18O) v padavinah in v podzemni vodi so omogo~ile oceno povpre~nega zadr‘evalnega ~asa vzor~ene podzemne vode:
• okoli 3 mesece za vodo SVR-7,
• okoli 9 mesecev za vodo SVR-4,
• 2-3 leta za bazni tok Hublja,
• 4-5 let za vodo SVR-3A,
• 5-6 let za vodo SVR-3B,
• najmanj 10 let, lahko pa tudi eno ali ve~ dekad dalj{i, za vodo SVR-1 in SVR-2 (sl. 4).
Sliki 5 in 6 prikazujeta glavne DOC v vzor~enih vodah. Na prvi sliki opazimo, da imajo vode SVR-1, SVR-2, SVR-3A, SVR-3B in baznega toka Hublja (starej{e vode) podoben razpon vrednosti. Tudi njihove sred-
nje vrednosti so podobne; nekoliko vi{jo ima le voda SVR-3A, kar verjetno odseva hitrej{o dinamiko podzemne vode. Po drugi strani sta razpona vrednosti parametra mlaj{ih vod SVR-7 in SVR-4 precej {ir{a, zlasti prve, kar odseva intenzivno dinamiko podzemne vode. Glede na druge, sta tudi srednji vrednosti teh vod ustrezno vi{ji, vendar podobni, kot je tista vode SVR-3A.
Nastopanje oddaljenih in zelo oddaljenih vrednosti (sl. 3) je zna~ilno za vse vzor~ene vode, razen za vodo SVR-2 (sl. 5), kar je lahko posledica prednostnega toka, katerega vplive zasledimo na sliki 6 (o~itni so npr. poleti in jeseni 1999). V teh obdobjih so morale padavine sprati ve~je koli~ine organskih snovi iz tal, kar je povzro~ilo povi{anje koncentracije DOC v podzemni vodi. Naj-vi{je koncentracije so bile izmerjene v vodi SVR-7. Glede na to mora biti napajanje tega vzor~nega mesta vezano na najhitrej{e dre-na‘ne poti izmed vse, kar je v skladu z oceno najkraj{ega zadr‘evalnega ~asa vode SVR-7.
Lastnosti alkalnosti in izotopske sestave 13C (?13C) vzor~enih vod so prikazane na slikah 7 – 10. Podatki obeh parametrov nudijo podobne informacije. Srednje vrednosti in razponi parametrov vod SVR-1 in SVR-2 odsevajo najni‘jo reaktivnost in podobno, glede na zadr‘evalni ~as po~asno, dinamiko podzemnih vod. V nasprotju s prej{njimi pa odsevajo srednje vrednosti in razponi parametrov vod SVR-3A in SVR-7 najvi{jo reaktivnost in najhitrej{o dinamiko podzemnih vod izmed vseh. Pri tem je treba poudariti, da se oceni povpre~nega ~asa vod SVR-7 in SVR-3A precej razlikujeta med seboj, torej ni nujno, da je hiter tok povezan le z mlaj{imi vodami. Vode SVR-4, SVR-3B in baznega toka Hublja nastopajo med zgoraj omenjenimi pari, zato verjetno odsevajo vmesne geokemi~ne in hidrodina-mi~ne pogoje.
Namenimo pozornost {e paroma vod s podobnimi zadr‘evalnimi ~asi: vodam a) SVR-7 in SVR-4 ter b) SVR-3A in SVR-3B. Glede na interpretacijo vseh merjenih parametrov imajo najverjetneje vzor~na mesta vsakega posameznega para isto napajalno zaledje, razlikujejo pa se po dinamiki procesa napajanja, kar vodi do zamikov povpre~nih zadr-‘evalnih ~asov vod.
Sinteza vseh podatkov ka‘e na mo‘nost obstoja epikra{ke cone v kra{kem vodono-
152
Branka Tr~ek
sniku zaledja Hublja. Predvideva se, da umetni rov pri Sinjem vrhu predira na za~etku epikra{ko cono, na koncu spodnjo nezasi~eno cono, vmes pa tako imenovano prehodno ob-mo~je, kar podpirajo tudi informacije, ki jih nudi bilanca vzor~enih vod v tabeli 1.
Lastnosti vod SVR-7 in SVR-4 odsevajo epikra{ko cono. Kot posledica batnega efekta, naj bi se drenirala voda v vzor~ni mesti prek hitrih drena‘nih poti. Lastnosti vode SVR-1 in SVR-2 odsevajo spodnjo nezasi~e-no cono vodonosnika. Vzor~ni mesti najverjetneje napaja razpr{en laminaren tok iz drobno razpokanih blokov kamnin. Lastnosti vod SVR-3A in SVR-3B odsevajo prehodno obmo~je. Vzor~ni mesti naj bi imeli enako napajalno obmo~je, le da je napajanje SVR-3A v glavnem vezano na hitri prednostni tok - epitok, napajanje SVR-3B pa na razpr{en laminarni tok iz epikra{ke cone.
Monitoringa toka in prenosa naravnih sledil je omogo~ili vpogled v starostno strukturo, reaktivnost in dinamiko vzor~enih vod ter opozoril na dvojnost procesov napajanja kra{kega vodonosnika. Opozoril je tudi na epitok (hiter koncentriran tok po omre‘ju kra{kih kanalov, ki se drenira iz epikra{ke cone, vsebuje pa tudi vodo ponikalnic), ki je glavni faktor za prenos in {irjenje onesna‘e-nja preko nezasi~ene cone do zasi~ene cone kra{kega vodonosnika.
Rezultati so torej v skladu s tako imenovano epikra{ko hipotezo, ki predpostavlja, da pomemben del napajanja kra{kega vodo-nosnika izvira hitro in v koncentrirani obliki iz epikra{ke cone. Ta fenomen ima lahko pomembne posledice na transport onesna‘e-valcev in druge gospodarske probleme, ~e-sar ne smemo zanemariti pri varovanju kra{kih podzemnih vodnih virov.
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