Salinization of reservoirs in regions with exposed evaporites. The unique case of Upper Gotvand Dam, Iran.

Construction of the great Upper Gotvand Dam in the Karun River, Iran, with an impoundment capacity of 4.5 billion cubic meters, has resulted in an environmental disaster: accumulation of 66.5 million metric tonnes of dissolved salt in the reservoir and a dramatic increase in the salinity of the reservoir water up to 200 g/L. This paper aims to identify and assess the main sources of the salinization of the reservoir water integrating multiple data: (1) geological and geomorphological evidence; (2) continuous vertical records of electrical conductivity at 11 stations along the reservoir; (3) total dissolved solids and major ion concentrations of 108 water samples at 58 sampling stations; (4) δ18O, δ2H in 35 water samples and δ37Cl in 16 water samples from the reservoir and surrounding rivers and springs. Geological and geomorphological evidence, hydrogeochemical data and isotopic signature reveal that halite dissolution is the main cause of salinization in the Gotvand Reservoir. The results show that salt dissolution in the Gachsaran Formation (especially at Anbar Ridge), which was under-estimated in the study phase of the project, has provided about 86 per cent of the dissolved salt accumulated in the reservoir over a 2-year period since the impoundment of the reservoir in 2011.