Major ion chemistry of groundwater in the Sangong River Watershed, Northwestern China

Abstract

Hydrochemical investigations were conducted to determine the chemical composition of groundwater in the Sangong River Watershed (SRW), Northwestern China. Thirty-two groundwater samples were collected from different wells to monitor the water chemistry of various ions. The results of the chemical analysis indicate that the groundwater in the area is generally neutral to slightly alkaline and predominantly contains Ca²⁺ and Mg²⁺ cations as well as HCO₃ ⁻ and SO₄ ²⁺ anions. The Na⁺+K⁺ concentration increases from the alluvial–diluvial fan to alluvial plain. TDS values are becoming larger along the direction of the groundwater flow in general because long residence time and evaporation. The contribution of Na⁺+K⁺ to the major cations which have been affected by water–rock interactions has great uncertainty. High positive correlations were obtained among the following ions: SO₄ ²⁻–Mg²⁺, SO₄ ²⁻–Na⁺+K⁺, Cl⁻–Mg²⁺, Cl–Na⁺+K⁺, Mg²⁺–Na⁺+K⁺ and SO₄ ²⁻–Cl⁻. The dominant hydrochemical facies is Ca²⁺–Mg²⁺–HCO₃ ⁻, which is in relation with their interaction with the geological formations of the basin, cation exchange between groundwater and clay minerals and anthropogenic activities. The saturation index (SI) indicates that the gypsum-halite dissolution reactions occur during a certain degree of rock weathering. Almost all SI values of anhydrite are less than zero which shows that it remains unsaturated with respect to these minerals. That is to say, those carbonate mineral phases may have influenced the chemical composition of the study area. According to the Wilcox, US Salinity Laboratory and residual sodium carbonate (RSC) classifications, most of the groundwater samples in the study area are suitable for irrigation.