The Karun River waters in the Ambal ridge region (Zagros mountain Range, southwestern Iran): mixing calculation and hydrogeological implications

The Karun River is one of the most important waterways in Iran and the quality level of its waters is very important for all the communities living around it. The salinity of river waters increases and the water quality decreases while it passes through the four km long valley consist of Gachsaran geological formation (mainly gypsum and halit), in the Ambal ridge area. This ridge is located some 5 km upstream of the Gotvand dam (under construction), which in the future will be inundated. In this section of the river, water chemistry suddenly evolves from Ca-HCO 3 type with TDS of ~0.4 g/l to Na(Ca)-Cl(SO 4 ) type with TDS of ~1.5 g/l, possibly due to mixing, carbonates precipitation and dolomitization. These variations suggest that saline water mixed with the river water extensively in the Ambal ridge area. Two hypotheses are suggested for the origin of the saline waters: first, in situ dissolution of halite and gypsum units of exposed Gachsaran Formation of the nearby area could produce saline ground-water flow into the river, and second, the intrusion of Karun River water itself through fractures of evaporite layers at location, where it first comes in contact with the Ambal area and in consequence, dissolution of halite and gypsum by the water, which discharge into the river via a number of exposed and underwater springs in the ridge. Calculations indicate that the average annual saline water seepage required for the change of the chemical composition is ~0.24% of the total river flow rate. Water balance calculations suggest that the required catchment area for the saline ground-water flow should be >300 km 2 , much larger than the Ambal area itself. However, this excess area almost is exist in a closed by plain, the impermeable marl layers in the Gachsaran Formation restraint the flow of groundwater and suggest that the first hypothesis is less likely.