Stabile isotope methods used to characterise the relation between karst water and surface water with examples from Albania
Romeo Eftimi Tirana,
Abstract: Karst aquifers are characterised by high heterogeneity created and organised by groundwater flow. The classical study methods such as bore holes, pumping tests, and point observations give important data but cannot be extended to the entire aquifer. However the stabile isotope methods could be applied in basin wide areas including surface water like rivers, lakes and sea and different aquifers like karstic or gravelly ones. Some study examples from Albania shown in this paper demonstrate the successful application of the isotope methods for the solution of the following problems: a) recharge of Poçemi springs by the Vjosa River; b) the recharge of Ohrid Lake by the Prespa Lake and c) the identification of recharge sources of Bistrica karst spring.
Key words: stable isotope methods, evaporation line, recharge sources, mixing analyses, relation karst water – surface water
Karst aquifers have complex and original characteristics which make them very different from the other aquifers in their high heterogeneity created and organised by groundwater flow (Bakalowicz 2005). The high heterogeneity and low predictability of karst aquifers make karst water investigation often a difficult task. The classical study methods such as bore holes, pumping tests, and point observations give important data but cannot be extended to the entire aquifer. However the stabile isotope methods could be applied in basin wide areas including surface water like rivers, lakes and sea and different aquifers like karstic or gravelly ones.
Particular contribution for the application of the environmental isotope methods in groundwater studies, including karst aquifers, has the International Atomic Energy Agency (IAEA). Nowadays the isotope methods are organic part of the regional and detailed hydrogeological investigations as demonstrated by innumerable papers published in much known specialised magazines such as Ground Water, Journal of Hydrology, Hydrogeology Journal etc.
Most important of stable isotopes used for solving of the hydrologic problems appear to be oxygen-18 and deuterium expressed as δ18O and δD (Bradley at al. 1972, IAEA 1981, 1983, 1989, IAEA-IHLS 2004). There are two important characteristics which make stable isotopes very useful for these applications, namely: a) the distinct isotopic composition of the waters from different geographic origins (mainly different elevations) or hydrologic nature, and b) the conservation of the isotopic content of the water in the groundwater bodies (Gat and Dansgaard, 1970). In a given region, the δ-values of precipitation at higher altitudes is generally more negative (more depleted on isotopes). Gradients for δ18O between 0.15 and 0.5‰/100m and the gradients for δD between about 1.5 and 4.0‰ are considered to be typical (IAEA 1981). Many problems of the groundwater origin depend on the altitude effect. Conservation of the isotopic content is realized as the lesser interaction of isotopes with soil and rock material, as long as temperatures are not too high.
In many cases, the isotopic composition of meteoric groundwater is found to match the mean composition of precipitation over the recharge area to a fair approximation. Two processes could change the isotopic composition of the surface and ground waters, namely a) the mixing and b) the evaporation. Many practical hydrologic problems could successfully be resolved if during the study clear evidence is obtained about the change in isotopic composition of the water due to the mixing or evaporation (IAEA 1981, 1983; Clark & Fritz 1997, Mazor 1972, Aggarwal et al. 2005).
Fig.1: Location of the study areas: Poçemi springs; 2. Prespa and Ohrid Lakes; 3. Bistrica spring; (green colour shows the karst aquifers)
Three study examples from Albania, shown in this paper, demonstrate the high efficiency of the isotope methods for the solution of the following problems: 1) recharge of Poçemi springs by the Vjosa River; 2) the recharge of Ohrid Lake by the Prespa Lake, and 3) the identification of recharge sources of Bistrica karst spring. Figure 1 shows the location of the study areas
Recharge of Poçemi springs from Vjosa River
Construction of a dam 110 m high was proposed on Vjosa River, in south–central Albania, to produce electricity. The future lake would be developed east to the dam site widely contacting with high karstified rocks. A group of springs, named Poçemi springs, issuing from the Kremenara karstic massive (Dhame 1987), approximately 1.5 km north to the proposed dam site, in downstream direction of the future lake (Fig. 2).
Poçemi springs issuing at elevation 41 m above sea level and their average discharge is 1.7 m3 /s. The total karst groundwater resources of the Kremenara basin result into 14.7*106 m3 or 0.46 m3 /s, equal to 27% of the total discharge of Poçemi springs. This result supports the hypothesis about the existence of another additional recharge source for Poçemi springs.
The investigation was based on the assumption that isotopic contents of local precipitation, Poçemi springs and of Vjosa River are different. Seven sampling points were chosen (Fig. 2): five issues of Poçemi springs (Nr 1 to 5); Vjosa River (Nr 6) and a spring of local recharge issuing at elevation about 40 m above Vjosa Rivera level (Nr 7). Sampling collection started in June 1986, and continued until May 1987. Temperatures, electric conductivity and pH of the sampled surface or groundwater were measured in the field. The samples were analyzed for the major ions in the laboratory of the Hydrogeological Service and for Oxigen-18 and Deuterium in the Laboratory of Isotope Section of IAEA (Table 1).
Figure 3 shows the seasonal variations in the oxigen-18 content of the Poçemi springs, the Vjosa River and the spring of local recharge. The oxygen-18 content of the K O S O V A F Y R O M G R E E C E A L B A N I A 2 1 3 z Poçemi springs is similar and homogenous at -7.85‰ vs SMOW. The springs represent a well mixed aquifer. The average oxygen-18 value of the Vjosa River is -8.22‰ vs SMOW. The seasonal fluctuation of Vjosa River is plotted against the isotopic composition (Fig. 4). It is seen that the Vjosa River and the Poçemi Springs have similar isotopic composition, whereas the locally recharged spring is heavier in isotopic composition (δ18O = -6.51‰ vs SMOW).
Fig. 2: Geological Map of Kremenara karst basin and location of sampling points. 1. Gravel; 2. Clay (terra rossa); 3. Siltstone and sandstones; 4. Stratified and massive limestone; 5. Spring; 6. Gallery with water; 7. Project dam site; 8. Elevation in m a.s.l.
Table 1. Mean values of some selected chemical data and of oxygen-18 of the water sampling points T – temperature; C – conductivity; σ – standard deviation; VR – Vjosa River; LS – Local spring.
As seen in Table 1 the Vjosa River is characterized by relatively high SO4 concentration. The Vjosa River and the Poçemi springs have more or less similar chemical composition, whereas the chemical composition of the locally recharged spring is quite different. The similarity of the chemical composition of two different waters is in general considered as an indication of their possible similar origin (Mazor 1985).
Fig.3: Variation in oxygen-18 content of the Vjosa River, Poçemi springs and a local spring of local origin.
Fig.4.Seasonal fluctuations and mean values of 18O content from investigated water samples of the Poçemi area.
Fig. 5. Variation in SO4 ion content of the Vjosa River, Poçemi springs and of the local spring.
Particularly indicative are the seasonal variations of the SO4 ion content in the observed water points. The SO4 seasonal fluctuations are bigger in the Vjosa River and smaller in the local spring, while the Poçemi springs stay in between. These data, as isotopic data did, suggest that the recharge of the Poçemi springs most probably is not of local origin but originates from another source with higher SO4 content like Vjosa River. The same could be concluded also from the graphics showing the relation between δ18O and SO4 contents and of the water conductivity (Fig.6 and 7). On both graphics the Poçemi springs stay on the mixing line of Vjosa River with the local spring which represents the Kremenara massif karst water.
Fig.6: Relationship between δ 18O and sulphate concentration in Vjosa River, Poçemi springs and local recharge (local spring).
Fig. 7: Relationship between δ 18O and electrical conductivity in Vjosa River, Poçemi springs and and local recharge (local spring).
Table 2: Estimated recharge sources of Poçemi springs
Based on simple two-component mixing analyses, the contribution of Poçemi spring recharge sources is estimated (Table 2). The results of the estimation using isotopic content (Akitie et al. 1989) and SO4 concentration (Eftimi & Dhame 2006) stay in a very good harmony (Table 2), and also fit very well also with the results provided by the balance calculations (Eftimi and Dhame (1990).
Recharge of Ohrid Lake by Prespa Lake
The Small and Big Prespa Lake and the Ohrid Lake waters are shared by Albania, FYR of Macedonia and Greece and constitute a common hydraulic system (Fig. 8). The Prespa Lake elevation is 850 m a.s.l. while the Ohrid Lake is at 695 m a.s.l. Their surfaces are 274 km² and 348 km² respective. Both lakes are separated by the Mali Thate – Galichica karst massif highest peak, 2287 m a.s.l.. Big graben structures, that of Prespa Lake on the east, and that of Ohrid Lake on the west delimit both sides of the Mali Thate karst massif, geologically representing a horst (Fig. 6). On the Ohrid lakeside, in the Albanian-FYROM borderland the big karst springs of St. Naum and Tushemisht are situated, which in total discharge about 255*106 m3 /year (8.1 m3 /s). Additional quantities of water drain into the lake as sub-lacustrine springs. Cvijic (1906) was the first to formulate the hypothesis that Prespa Lake recharges St. Naum and Tushemisht springs at Ohrid lakeside.
The altitude effect of stable isotopes in meteoric waters (Bradley at al. 1972) was used to identify waters coming from different potential groundwater recharge sources of the study area. The local precipitation on the karst massif and Prespa Lake water is examined as some recharge potential source to Mali Thate – Galichica karst groundwater. The groundwater recharge area mean elevation is expected to be higher than the Prespa Lake elevation. Water bodies covered by the sampling program included: two lakes (Nr 1 and 2), Devoll River (Nr 11), Tushemisht springs (Nr 3 to 5), a local spring (Nr 6) and some springs in Bilishti valley (Nr 7 to 10). Sampling points locations are shown on the lakes Prespa and Ohrid area Hydrogeological Map (Fig. 8). For the interpretation the results from isotope measurements performed by Anovski et al (1991) were also used.
Fig. 8: Hydrogeological Map of the area of the lakes Prespa and Ohrid (compiled by R. Eftimi). Numbers show the sampling points. The arrows show the proven underground connection
Fig. 9 shows the correlation between mean δ1 😯 and δD values based on the results of isotope analyses from sampling points on the territory of Albania (Eftimi and Zoto 1997, Eftimi ar al. 2002) and of FYR of Macedonia (Anovski at al. 1991). The slope of the Local Meteoric Water Line (LMWL) is 8, which is the same as that of the World Meteoric Water Line (WMWL), but the deuterium excess d is 14 instead of 10 of the WMWL. Such “anomalous” values of intercept are characteristic for Eastern Mediterranean countries (Gat & Dansgaard 1970, Leontiadis et al. 1997).
The slope of LMWL for investigated lakes and springs is 5.4, which indicates that the water from the sampled points has been influenced by excessive evaporation relative to the input. In the present case, the low slope of δD-δ18O relationship of surface and groundwater is caused by the intensive evaporation of the Prespa Lake. The mixing at different proportions of both precipitations infiltrated into the karst massif and the Prespa Lake water is responsible for the isotopic composition of the springs falling in the evaporation line. The mixing end- members are Prespa Lake (indexes δ18O = -1.72‰ and δD Gravelly aquifer Karst limestone aquifer Sandstone aquifer Rocks practically without groundwater Gravelly aquifer 1 3 4 6 5 7 9 2 10 11 = -21.84‰), and the infiltrated in the karst massif precipitation represented by the point of interception of both lines in Fig. 7 (indexes δ18O = -10.20‰ and δD = -67.00‰). Environmental stable isotopes 2 H and 1 😯 demonstrate that the Prespa Lake recharge is bigger in Tushemisht Spring than in St. Naum Spring (Table 3).
ger in Tushemisht Spring than in St. Naum Spring (Table 3). As a result of the two-component mixing analysis it was concluded that the Tushemisht Spring is recharged at 53% (1.3 m³/s) by the Prespa Lake and at 47% (1.2 m³/s) by the precipitations infiltrated in the Mali Thate-Galichca karst massif (Eftimi and Zoto 1997). The contribution of Prespa Lake to the recharge of the St. Naun Spring is smaller; according to Anovski at al., (1991) it makes about 38% of the mean discharge of this spring. The total contribution of Prespa Lake to Tushemisht and St. Naun Springs is about 3.65 m³/s (115*106 m³/year), and the contribution of the precipitation is about 4.45 m³/s (140.3*106 m³/year).
Fig. 9: δ 18O – δ D plot of waters from the area of lakes Prespa and Ohrid
Table 3: The contribution of Prespa Lake to the recharge of karst springs at Ohrid lakeside (PL-Prespa Lake water, IP-Infiltrated precipitation in the karst massif)
The results of an artificial tracer experiment supported by IAEA showed a very complex groundwater circulation system in the Mali Thate – Galichica karst massif. Big differences of the measured maximum flow velocities (from 233 m/h to 3200 m/h) indicate the presence of differently developed karst conduits at small-scale distances (Amataj et al. 2005).
Identification of recharge sources of Bistrica karst spring
The goal of this research was t o investigate the mechanism of recharge of the Mali Gjere Mountain karst massif feeding the largest Albania’s spring, Bistrica. Mali Gjere Moutain karst massif is located in the south-eastern part of Albania, on the border with Greece. The total surface of the karst massif is 440 km 2 , mostly located in Albanian territory (about 400 km2 ). The highest peaks of Gjere Mountain are 1798 a.s.l. and the mean elevation is about 900 m a.s.l. The mountain crest is a natural surface water divide between the Drinos River located on the east, and Bistrica River basin located on the west. Gjere Mountain is an anticline (Fig. 10, 11) consisting of a Mesozoic carbonate sequence overthrown to Perm-Triassic gypsum and clay deposits, surrounded by Palaeogene and Neogene flysch formations. Only in G R E E C E the central-western side of the Gjere Mountain structure, from Jergucat village on the south to Dervician village on the north (distance 6.5 km) the flysch formation is missing and carbonate rocks contact the Drinos valley gravelly deposits.
Using the methods described by Turc (1954) and Kessler (1967) the effective infiltration of the mean annual precipitation recharge of Mali Gjere massif karst groundwater is estimated to equal to about 1175 mm/year (5.17*108 m3 /year, or 16.4 m3 /s). The evapotranspiration resulted to be about 573 mm/year (2.25*108 m3 /year, or 8.0 m3 /s) and the surface runoff is 175 mm (0,77*108 m3 /year, or 2.44 m3 /s). Most of the karst water resources of the Mali Gjere massif discharge in its western side, where Bistrica spring with mean discharge of 18.4 m3 /s issues, as well as some other springs, each having mean a discharge of about 100 l/s.
Fig. 10: Hydrogeological Map of Gjere Mountain karst massif area and the sampling points (Eftimi at al. 1985)
The largest spring on the eastern side of this massif is the temporary spring Viroi, flowing only about 8 to 9 months a year, and having maximal discharge of about 40 m3 /s. The third important spring is Lista (mean discharge of 1.7 m3 /s), situated on Greek territory. The entire Mali Gjere Mountain karst massif springs total discharge is estimated to be about 7.42*108 m3 /year, (23.6 m3 /s). As could be noticed, the total discharge of the springs of Mali Gjere karst massif, is about 2.26*108 m3 /year (7.2 m3 /s), corresponding to about 30 % larger than the calculated mean efficient precipitation infiltration into the massif (Eftimi at al. 2007).
Fig. 11: Hydrogeological cross-section of Gjere Mountain 1 – Gravelly fluvial deposits, 2 – Palaeogene-Neogene flysch deposits, 3- Palaeogene limestone, 4 – Cretaceous limestone, 5- Jurassic limestone 6 – Permian-Triassic gypsum and clay, 7 – Groundwater level, 8 – Perennial spring, 9 – Temporary spring, 10 – Main groundwater flow direction
Environmental isotope techniques were used here together with the hydrological and hydrochemical methods to verify the partial replenishment of the karst water resources of Mali Gjere karst massif by the gravely aquifer of the Drinos River. The formulation of this hypothesis takes into consideration the good hydraulic connection between the gravely and karst aquifers, as well as the natural groundwater slope to the Blue Eye Spring, (Fig. 11). Sample collection began in January 1988 and continued until December 1999, and some sporadic sampling was done during 1996. The sampling locations are shown in Figure 10 and the index values are presented on Table 4.
The isotopic composition of all the six outlets of Bistrica spring (nr1-6) is very similar and homogenous and the standard deviation of mean values for each outlet varies within 0.02-0.08 for δ18O and within 0.6-1.1 for δD values. The δ18O and δD values of Bistrica spring presented on Table 4 are the weighted values of six spring’s outlets.
The correlation function between mean δ18O and δD values of sampled points and (shown in Fig. 12) results in two equations (Eftimi 2009, Eftimi at al. 2007): δD = 7.29 δ 18O + 13.18 (r = 0. 99) (1)
δD = 2.28 δ 18O – 26.58 (r = 0. 99) (2)
Table 4. Mean isotopic composition of water samples of Gjere Mountain karst massif No – order number, SL – sampling location, E – elevation, MD – mean discharge, Nr – number of the measurements, DE – deuterium excess, T – temperature, EC – electric conductivity
Fig. 12: Relationship between δ18O and δD for waters of Gjere Mountain karst massif
Equation (1) could be considered a local meteoric water line and its slope is close to 8 of the WMWL. The deuterium excess is +18‰ instead of +10‰ of the global meteoric water line (Dansgard, 1964) as Albania belongs to the east Mediterranean anomalous zone concerning the deuterium axis (Gat and Carmi 1970, Gat and Dansgaard 1970). Equation (2) describes a mixing line; the mixing of infiltrated into the karst massif precipitation and of the groundwater of Drinos valley gravelly aquifer is responsible for the isotope composition of Bistrica spring.
The infiltrated precipitation into the karst massif is represented by the values of interception of both lines (Fig. 12), which practically coincides with the isotope composition of Viroi temporary spring, nr 14, δ18O = – 7.99‰ and δD = – 44.90‰. The groundwater recharged by the Drinos River gravelly aquifer groundwater is affected by the relative δD and δ18O enrichment. It seems that the mean recharge area of the Drinos River catchment has lower elevation than that of the Gjere Mountain karst massif. The isotope composition of gravelly aquifer groundwater is represented by the Jergucat borehole δ18O = – 6.80‰ and δD = – 42.20‰. Based on simple two-component mixing analyses, the Drinos Valley gravely aquifer is estimated to contribute about 30 % to the replenishment of the Bistrica spring. This is equal to 5.52 m3 /s or 1.74*108 m3 /year.
It is considered that the mean recharge altitudes (Mra) of the small local springs number 9 and 12 practically coincides with their corresponding elevations. Data from these springs are used to define the altitude effect (Fig. 13), and to predict the Mra from the δ18O values of the springs based on the following equation (3):
Fig. 13: δ18O versus altitude of springs
The altitude effect as defined b y equation (3) is -0.32 %o in δ18O-values per 100 m. According to literature data the altitude effect for δ18O %o – values vary from 0.15 %o to 0.5 %o per 100 m Payne et al., 1978; IAEA 1981; Leontiadis at al. 1997). Using the relation presented in Figure 13 the Mra in the study area esult as below: Bistrica – 800 m; Vrisi – 500 m; Kardhikaqi – 515 m and Viroi 915 m.
For the identification of Bistrica springs recharge sources the resulted relations of SO4 concentrations versus the δ18O concentrations proved to be very indicative (Fig. 14). Bistrica spring (nr 1-6) and Kardhikaq spring (nr 11) are on the contact line between carbonate rocks and Upper Triassic clayey-gypsum deposits. They are characterized by similar sulphate concentration, but show of significant differences in isotope concentrations and temperature.
A careful examination of the Figure 14 enables us to understand the different origin of sulphate ions at both springs. Bistrica spring lies on the mixing line, where end- members are precipitation infiltration into the karst massif and the groundwater of Drinos valley gravelly basin. But Kardhikaq spring lies far from this mixing line. This allows us to make some considerations about the high sulphate concentration of Bistrica spring, which seems to be related to the high sulphate concentration i n D rinos valley gravelly aquifer groundwater. As for Kardhikaq spring, which is recharged mainly by precipitation infiltration, the sulphate ion source could be the gypsum of Triassic deposits. Figure 15 shows the areal distribution of the sulphate ion concentration in the gravelly aquifer of Drinos river valley. The mean sulphate concentration of Drinos River in summer, at entering Albanian territory varies about 350-400 mg/l, but values up to about 650-700 mg/l are also measured.
As seen on Figure 15 the sulphate concentration in the southern and central part of the investigated valley is about 200 to more than 300 mg/l, but rapidly decreases in north direction, which is also the gravelly aquifer groundwater flow direction. In the northern part of the valley the sulphate concentration become less than 50 mg/l. The high sulphate concentration in the southern and central part of the valley is caused by the intensive infiltration of Drinos River water into the gravelly aquifer (Eftimi 2009). In the central part of the valley, in the sector between Jergucat and Sofratik villages, the enriched in sulphates gravelly aquifer groundwater seeps into Gjere Mountain karst massif.
Sulphate ion is used as a neutral one to estimate the mixing proportions contributing to Bistrica spring using as end-members the Viroi spring (point 14) for the karst massif groundwater, and the Jergucat borehole (nr 20) for the Drinos River gravelly aquifer. The sulphate concentrations of two mixing components used for the calculations are 47.0 mg/l for Viroi spring and 300 mg/l for the Drinos valley groundwater.
Fig. 14: δ18O versus sulphate concentration for waters of Mali Gjere karst massif
Fig. 15: Concentration of SO4 ion in gravelly aquifer of Drinos River Valley.
Table 5 Estimated recharge sources of Bistrica spring.
Based on simple two-component mixing analyses an estimation was made that the contribution of the Drinos valley gravely aquifer consist about 35 % of mean discharge from Bistrica spring. As seen in Table 5, the results obtained by the isotope and hydrochemical environmental tracer methods, as well as those derived by the balance method are well comparable and stay in a very good harmony
The environmental isotope methods are successfully applied for the identification of recharge sources in some karst areas of Albania. The altitude effect on stabile isotope of oxygen and hydrogen isotopes and the modification of isotopic content of ground and surface water by mixing or evaporation, appear to be the key factors for solving the problem in concern. Combined application of environmental isotope and hydro-chemical methods provide to be very useful evidence about the recharge sources of karst aquifers.
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