The Kowsar Dam is constructed on the Kheyrabad River at the northern limb of the Duck Anticline, close to its NW plunge, SW Iran. The dam is built on the karstic Asmari Formation, and the reservoir is in direct contact with this formation from the dam body to the upstream impermeable Gachsaran Formation. After reservoir impounding, several new springs emerged from the Asmari Formation, adjacent to the old small springs at the southern limb of the Duck Anticline. The discharge of these downstream springs was not reduced despite grout curtain treatment works. The main water leakage route is not below or through the grout curtain, as shown by considering rock permeability in pilot and check holes, cement consumption in grouting boreholes, borehole water levels, spring locations, and discharge of the dam galleries. The Duck Anticline is hydrogeologically connected to the adjacent Dill and Pahn Anticlines, comprising the Asmari Karst Aquifer, which is divided into three karst sub-aquifers. The general flow direction of karst water in each sub-aquifer was determined using hydrogeological analysis and water balance calculations. Two alternative models are proposed for water flow at the northern limb of the Duck Anticline. The main leakage route toward the downstream springs is most probably through a relict karst conduit system, developed along bedding planes of the Asmari Formation at the NW plunge of the anticline, according to the first proposed model. In this case, the water leakage can be significantly reduced by extension of the grout curtain further into the upstream impermeable Gachsaran Formation.

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