Evaluation of leakage potential from right abutment of Behestabad dam by hydrogeological investigations

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