Evaluation of Hydrochemical Characteristics of Groundwater and Salinity Evolution in Semnan Plain

Semnan plain with arid and semi-arid climate is situated in southern flank of Alborz range and north of great desert of Central Iran. In this study the result of analyses of major ions in 35 deep wells and qanats and oxygen-18 and deuterium stable isotopes in 5 wells and one sample of the river water have been used to access the chemical characteristics of groundwater. Hierarchical cluster analysis (HCA) has been used for classifying the water samples. The results show that the samples can be grouped into three main categories: (1) waters with highest bicarbonate content and lowest TDS, (2) waters with highest sulfate content and (3) chloride waters with maximum salt content. Checking the spatial distribution of the samples shows that the samples of each group are situated in a specific area; indicating similar affecting processes on water quality in each group. Mineral saturation index calculated with the PHREEQC software showed that the samples are super-saturated with respect to the carbonate mineral and under-saturated to saturated with respect to the gypsum and anhydrite and under-saturated with respect to the halite. Based on isotopic analysis, correlation coefficient and ion ratios, chloro-alkaline index and Gibbs plot of the chemical composition of the groundwater in Semnan plain are controlled by the intrusion of Abgram salt water and salt dissolution, groundwater residence time, silicate weathering, cation exchange, evaporation and agricultural return flow.