Overexploitation hazards and salinization risks in crucial declining aquifers, chemo-isotopic approaches

Bastam region is overusing its groundwater resources stocks. The groundwater levels, and hydrogeochemical data during a 14-year period (2002-2016) and isotopic composition (2016) of the 22 groundwater samples were investigated to determine the hydrogeological regime of Bastam Plain and identify the geochemical processes that control the groundwater quality. This plain is situated in a semi-arid region of northern Iran, mainly recharging from the carbonate Mountains in the north and southwest. In general, overexploitation of the groundwater due to low precipitation and changing precipitation regime has destructive effects on the hydrogeological setting and flow regime of Bastam crucial aquifer, including the decline of the groundwater level with rate of 0.9 m/year, water storage deficit of 17.32 Mm³/year and consequently, the intrusion of saline water towards the aquifer from the western and eastern salty plains. Based on the EC values, the groundwater samples divided into three groups of fresh waters with EC < 1000 μS/cm and Ca-Mg-HCO₃ water type which are located in the recharge zones, semi-saline waters (1000 < EC < 2500 μS/cm) with Na-Mg-Cl-SO₄ type at the center and saline waters with EC values more than 2500 μS/cm and Na-Cl type in the eastern and western parts of the plain. A few groundwater samples (group 2) fall on the mixing line between fresh and saline water in Piper diagram during the saline water intrusion, but most of the samples depart from this mixing line, indicating water-rock interactions in this area, resulting in deficit of Na⁺ and surplus of Ca²⁺ concentration. The fresh water samples cluster along the local meteoric water line in δ¹⁸O and δ²H relationships, suggesting an integrative and rapid recharge with meteoric water. Hydrochemical and isotopic characteristics indicate that invasion of the saline water resulted from halite dissolution with minor evaporation in the surrounding salty playas is the main origin for the increasing salinity in the Bastam aquifer. The salinization risks limit the groundwater withdrawals from Bastam aquifer in some areas, threatening the future sustainable development of the region.