حسین عابدیان

The use of isotopes, especially stable environmental isotopes, is very important in investigating the water cycle and water resources studies. Preparation of the local meteoric water line and its equation in a region is used to determine the origin of the air mass affecting the region and is used as a basis for a more detailed study of water resources, because by maintaining the initial characteristics of precipitation as an input to the groundwater aquifer, the subsequent changes that occur in the groundwater aquifer will be traceable.

The studied area is located in Chaharmahal and Bakhtiari province and in Chelgerd, Farsan and Ardal cities, and in terms of structure, it is a part of high Zagros (shredded or eroded). The major part of the study area is located in the large Karun basin and part of it is located in the Gavkhouni basin.
In this research, in order to investigate the isotopic composition of precipitation, a total of 38 rain samples and 7 snow samples were collected from 9 stations at different altitudes with monthly frequency in the 2017-2018 in the form of cumulative and precipitation event in the rainy season. In order to determine the isotopic values of the collected samples, they were sent to Mesbah Energy laboratory and the isotopic composition of oxygen and hydrogen of all samples was determined by a laser spectroscopy.
By examining the composition of oxygen 18 and deuterium isotopes of precipitation samples in hydrological year 2017-2018, the local meteoric water line for the middle Zagros study area in the Zardkoh Bakhtiari area has been drawn and compared with the global meteoric water line and the Meditrranean meteoric water line. In addition, in order to establish a relationship between precipitation isotopes and regional weather conditions, the correlation of altitude, temperature and precipitation parameters on stable isotopes of oxygen 18 and deuterium of event precipitation samples from different altitudes has been investigated. Also, in order to do more detailed studies and determine the origin of moisture masses, the correlation between the amounts of oxygen 18 and deuterium isotopes with deuterium excess and electrical conductivity was also investigated.

By examining the isotopic results obtained from the collected precipitation samples, for the first time, the atmospheric water isotopic line of the middle Zagros region in the Zardkoh Bakhtiari was obtained with the equation δD =7.1068 δ18O+12.364. The local meteoric water line of the region had differences with global and Meditrranean meteoric water line due to the occurrence of evaporation and transpiration from precipitation. The changes of δ18O value in meteoric precipitation in the region are from -12.92‰ to -1.07‰ (average -6.38‰) and δD value is from -80.16‰ to 7.51‰ (average -33.58‰).
Comparing the isotopic content of precipitation in the range with altitude and amount of precipitation indicates that precipitation with richer isotopic composition occurred in areas with lower altitude and amount of precipitation. For every 100 meters change in altitude in this area, about 0.49 and 2.16 permil decrease in δ18O and δD isotopic values, respectively, and for every 20 mm increase in precipitation, about 1.3 permil decreases in δ18O isotopic value and about 10.1 of a permil decrease in the amount of deuterium occurs.
Also, the increase in temperature has led to the enrichment of oxygen 18 and deuterium isotopes in the study area. So that the changes of δ18O and δD of the precipitation samples of the middle Zagros range are about 0.9 and 0.4 permil per 5 degrees centigrade of temperature change, respectively
Also, the amount of electrical conductivity (EC) of precipitation samples has a direct relationship with the amount of oxygen 18 and deuterium isotopes of these samples, so that with an increase of 20 microsiemens per centimeter of electrical conductivity, the amount of oxygen 18 and deuterium isotopes of precipitation samples increase by about 1.45 and 9.40 permil, respectively.
In the conducted studies, the relationship between deuterium excess and isotopic values of precipitation samples was also investigated. deuterium excess of precipitation is usually affected by relative humidity and atmospheric temperature of the source of moisture, which is why it is considered an important indicator in determining the source of moisture. If the cloud humidity drops below 100%, deuterium excess occurs. The increase in the amount of deuterium excess and the decrease in isotopic values of δ18O and δD of precipitation in the study area also indicate the main origin of moisture masses from Mediterranean regions.

Physicochemical and isotopic parameters of springs are one of the important parameters in hydrogeological studies, because they represent the primary characteristics of precipitation and the history that groundwater flow in the karst. In order to investigate the flow system in the karst of Jahrom-Asmari formation in the area of Sangvil anticline in Chaharmahal & Bakhtiari provience, discharge, hydrochemical and environmental isotope data of springs, boreholes, precipitation and river were studied and it was determined that the karst flow type of the range is mainly dependent on the joint and fracture and the flow type in the range is diffuse-conduit and diffuse. Since the use of isotope studies in completing hydrogeological studies and detection the type of flow is useful, along with other parameters, Based on the studies carried out in this research, the Meteoric water line in the Sangvil anticline was obtained as the relationship = 6.3 δ18O + 14.8 δD with a correlation of 96%.The average amount of oxygen 18 in meteoric water and water resources is equal to -4.56 and -7.06 permil, respectively, and the average amount of deuterium in meteoric water and water resources is equal to -13.82 and -32.06 permil, respectively. in the present study, the study of environmental isotopes showed that the anticline aquifer has negative values compared to the local meteoric water line, and high volume precipitation and recharge snow determines the isotopic composition of the anticline aquifer. In this way, precipitations with low volume and high isotope composition penetrate less into the aquifer.

Construction of dams on limestone formations, however morphologically, has an appropriate form; but usually causes seepage from the reservoir to the downstream. Also, the construction of a dam on calcareous-karst formations has the risk of over seepage from the reservoir, and the existence of the karst leads to complicating this issue. Therefore, various methods of hydrogeological analysis are needed to determine the condition of karstification of reservoir and dam axis in the karstic areas. Beheshtabad dam is in investigation studies stage and approximately situated at the far end of the anticline axis named Sangwill, which composed of dolomitic limestone with roughly 700 meters thickness. The right side of the reservoir is mainly in direct contact with this karstic formation. Hydrogeological analysis for determining the reservoir’s seepage in right side are including measurement of water level and boreholes hydrochemical analysis, measuring springs discharge and hydrochemical condition of springs and environmental isotopic studies. The results showed that karstic aquifer’s water level on the right side of the reservoir has very similar fluctuations with a difference between the minimum and maximum of 2.5 to 3 meters. The water level from the northern limb with a level of about 1625 decreases towards the spring in the southern limb with a level of about 1595 and a gradient of about 0.007. Also, spring discharge in the sought limb has the same fluctuation with three coefficients of recession curve. Recession curve coefficient α1 in the order of 10-2 indicates the intermediate flow (diffuse-conduit) make up 15% of the flow and the coefficients α2 and α3 in order 10-3 indicate the diffuse flow accounts about 85% of the aquifer flow. Hydrochemical and isotopic parameters of springs and boreholes also have low fluctuations. These variations for anions and cations are less than 20% and for Ec and acidity are less than 5%. The amount of environmental isotopes in springs and boreholes does not show any significant variations so that the standard deviation obtained from monthly acquired data is less than 1.2 per mil for δD and less than 0.2 per mil for δ18O. The amount of environmental isotopes shows that the source of springs is about 2300 meters and composed of heavy rainfall and snow in two limbs. Also, the isotopic composition of downstream springs is the average of the amount of isotopic value boreholes in the north and sought limbs showing that springs catchment area in left side (SPL) and right side (SP5) is recharged from both limbs of the anticline. Finally, the results of the studies show that the Jahrom-Asmari karstic aquifer on the right side of the reservoir is an aquifer with diffuse to diffuse-conduit flow. The aquifer is recharged through two anticline’s limb and mainly through the system of joints and fractures. The groundwater flows from the northern limb to the southern limb and is discharged into the springs downstream. In such a condition, the hydraulic connection between the anticline’s northern limb with the southern limb is exists. This connection is formed at the time of reservoir formation, between the reservoir and the lower springs of the dam axis with a higher gradient (around 0.037). Therefore, by the dam impoundment, the major seepage current in right abutment of the reservoir in the north limb of anticline towards sought limb and finally discharge in springs downstream of the dam axis

1- Introduction For a dam project in the karstic areas, the most important parameter is the hydraulic conductivity values which are necessary for determining the amounts of leakage from its reservoir and abutments However, secondary porosity and flow networks can cause heterogeneity and anisotropy in karst fields, leading to changes in hydraulic conductivity values and then significant differences in leak calculation values due to scale changes. Limestone formation in a regional scale is generally heterogeneous, and the heterogeneity of hydraulic conductivity (Karami 2002) usually specify the value of heterogeneity in karstic aquifers Kiraly (1975) investigated karstic aquifers and fractures in the Jura Mountains in Switzerland. He reported that the sub-local scale to the well-scale and the great permeabilities on the regional scale are related to karstic conduits and increase hydraulic conductivity. Rovey and Cherkauer (1995) measured the hydraulic conductivities of five hydrostratigraphic carbonate units at different scales and reported that the values of hydraulic conductivities show direct proportionality with the measurement scales. Sauter (2005) studied the various methods, sub-local to regional scales, for determining the permeability in a karstic environment. He mentioned that the various hydraulic conductivities in different scales could be due to the spatial organization and the degree of networking of the drainage system (Sauter 2005). Other hydrological and geological studies have investigated the relationship between scale with values of hydraulic conductivities in fractured rocks (Illman 2007) and sedimentary formations (Chapuis 2010, Galvão et al. 2016). The main objective of this study is an investigation of the scale effect on the amount of water leakage from the reservoir and the dam abutments in the karstic area. We selected the Beheshtabad for the case study of this issue. 2- Methodology Beheshtabad dam is double-arch dam with a height of 180 meters and the reservoir volume of 1050Í106 m3. The dam is situated on the Beheshtabad River with an approximately 33 m3/s flowrate. The right side of the reservoir is in contact with karstic limestone-dolomite of the Sangvil anticline named Jahrom- Asmari Formation with a thickness of about 700 meters. Determining the amount of leakage from the reservoir requires an accurate estimation of the hydraulic conductivity according to the contact scale with the karstic aquifer. The values of hydraulic conductivities have been measured in Jahrom-Asmari Formation of the right-side reservoir using various methods in three scales, sub-local, local and regional scales. We used the Slug and lugeon tests in sub-local scales and conducted simultaneous measurements of spring discharge and boreholes water levels such as pumping wells in the local scale. Also, hydraulic conductivity in the regional scale determined by Recession curve and Darcy method. Finally, the amount of leakage was calculated in different scales and compared based on the hydraulic conductivity values of different scales. 3- Findings The hydraulic conductivity has been calculated on a different scale with a related test for limestone aquifer on the reservoir’s right-side. The values of hydraulic conductivities are not the same in different methods, and it is in the range of 2.1×10-6 to 1.6×10-4 m/s (Table 1). The lowest hydraulic conductivity belongs to a slug test conducted on a sub-local scale, and the highest hydraulic conductivity is for recession curve and Darcy method in regional scale. Also, the spring discharge method and the water levels of boreholes as a pumping well in the local scale estimates the values of hydraulic conductivities. The calculated hydraulic conductivities on the regional scale are about 70 times higher than those for the sub-local scale, which is related to the effect of scale in the karstic environments. According to the results, experiments with a radius of greater than 500 meters will determine the equivalent volume of hydraulic conductivity to the karstic mass region. Table 1. Calculated hydraulic conductivity value using different methods Methods KMIN (m/s) KMAX (m/s) KRE (m/s) PACKER TEST 1.00E-05 1.00E-07 3.20E-06 SLUG TEST 1.10E-06 3.30E-06 2.10E-06 DARCY 1.00E-04 1.30E-04 1.20E-04 RORABAUGH 1.41E-04 3.60E-04 1.60E-04 MILANOVIC 4.10E-05 2.00E-04 8.00E-05 Seep 2D software determined the leakage value for the northern limb to the downstream and southern limb. The amount of leakage varies due to changes in values of hydraulic conductivities. The amount of leakage calculated based on the average reservoir cross-section with a limestone aquifer at sub-local, local and regional scales are 0.3, 3.9, and 5.4 to 8.1 m3/s, respectively. Such differences in the estimation of leakage are related to the scale effects on the heterogeneity of karstic areas. Hydrogeological studies indicate that the Beheshtabad reservoir is contacted regionally with the right-side limestone aquifer; therefore, the leakage amount should be modeled based on the regional hydraulic conductivity. According to the regional scale, hydraulic conductivity varies between 1.1×10-4 to 1.6×10-4 m/s. Therefore, the leakage amount from the dam reservoir will be between 6.3 m3/s and 8.1 m3/s (Fig. 1). Fig 1. Leakage amount based on regional scale hydraulic conductivity 4-Conclusion For determining the amount of dam reservoir leakage in the karstic areas, the most critical parameter is the value of hydraulic conductivity that changes with the measuring scale. When reservoir and abutment of the dam contacted with karst, some methods for determining the regional hydraulic conductivity such as dye tracing, the Darcy method, and the recession curve method are more accurate to estimate the leakage amount. If the reservoir contact is local, the pumping test is more accurate for determining the leakage value. Also, in the sub-local contact scale, lugeon, injection, and slug experiments are more accurate methods in terms of determining leakage amount. In karstic formations, the scale effect is one of the most important factors for reservoir considering in the stage of site selection of the dam. Therefore, the choice of the dam axis can be carried out in places where the reservoir and abutments of the dam are not in contact with the karst network as far as possible. References Chapuis, R.P., 2010. Permeability scale effect in sandy aquifers: A few case studies. Proceedings of the18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris Galvão, P., Halihan, T., Hirata, R., 2016. The karst permeability scale effect of Sete Lagoas, MG, Brazil. Journal of Hydrology 532, 149-162 Illman, W.A., 2006. Strong field evidence of directional permeability scale effect in fractured rock. Journal of Hydrology 319 (1-4), 227-236 Karami, G.H., 2002. Assessment of heterogeneity and flow systems in karstic aquifers using pumping test data. Ph.D. thesis, University of Newcastle Kiraly, L., 1975. Rapport surl’etat actuel des connaissances dans le domaine des caracteres physiques des roches karstiques. In Hydrogeology of Karstic Terrains (eds Burger A and Dubertret L). International Union of Geological Sciences, Series B, 3:53–67 Rovey, C.W., Cherkauer, D.S., 1995. Szxcale dependency of hydraulic conductivity measurements. Groundwater, 33(5), 769-780 Sauter, M., 2005. Scale effects of hydraulic conductivity in karst and fractured aquifers. Geophysical Research Abstracts, Accepted 28 April 2005, https://www.cosis.net/abstracts/EGU05/07748/EGU05-J-07748.pdf