Application of hydrochemistry and stable isotopes (δ34S, δ18O and δ37Cl) to trace natural and anthropogenic influences on the quality of groundwater in the piedmont region, Shijiazhuang, China

Because of their frequent association with man-made products, increasing SO42− and Cl⁻ concentrations are often important indicators of deterioration of groundwater quality. The integrated use of hydrochemistry and multi-isotopic approaches (δ³⁴S, δ¹⁸O and δ³⁷Cl) was applied in Shijiazhuang area, China, to characterize SO42− and Cl⁻ sources; and to evaluate factors, including natural processes and anthropogenic activities, that affect groundwater quality. In the plain area, the SO42− concentrations of the surface water and shallow groundwater (mean 2.42 mmol/L and 2.06 mmol/L, respectively) were generally higher than those of the deep groundwater (mean 0.69 mmol/L). The natural waters in the mountain area (mean 4.36 mmol/L) had higher SO42− contents than those in the plain area (mean 1.73 mmol/L). The shallow groundwater samples collected in the southern part of the plain area had higher Cl⁻ concentrations than other samples. The SO42− in natural waters showed variable δ³⁴S and δ¹⁸O values, ranging from +8.7‰ to +21.7‰ and from +5.9‰ to +11.4‰, respectively. The δ³⁷Cl values fell in a narrow range of −0.22‰ to +0.64‰ with a mean value of +0.14‰. The hydrochemistry of samples is in part controlled by water-rock interaction processes, but also influenced by anthropogenic activities. The variations of δ³⁴S and δ¹⁸O values indicate gypsum, oxidation of inorganic sulfide minerals, and detergents are the primary sources of sulfate in the samples. Bacterial SO42− reduction is an important reaction affecting the SO42− concentrations in the deep groundwater of the plain area. The plot of δ³⁷Cl and 1/Cl⁻ combined with the δ³⁷Cl information of potential Cl⁻ sources suggests that the dissolved Cl⁻ in natural waters is mainly sourced from dissolution of Cl-bearing minerals, import of wastewater, and precipitation. Given the above, the elevated concentration of SO42− and Cl⁻ in the groundwater could be mainly attributed to mining activities in the mountain area and input of wastewater from domestic and industrial activities.