Hydrochemistry of flowback water from Changning shale gas field and associated shallow groundwater in Southern Sichuan Basin, China: Implications for the possible impact of shale gas development on groundwater quality

The worldwide expansion of shale gas production and increased use of hydraulic fracturing have raised public concerns about safety and risks of groundwater resources in shale gas extraction areas. China has the largest shale gas resources in the world, most of which are located in the Sichuan Basin. Shale gas extraction in the Sichuan Basin has been increasing rapidly in recent years. However, the potential impact on shallow groundwater quality has not yet been systematically investigated. In order to evaluate the possible impact of shale gas extraction on groundwater quality, we present, for the first time, the hydrochemistry and Sr isotopic data of shallow groundwater, as well as flowback and produced water (FP water) in the Changning shale gas field in Sichuan Basin, one of the major shale gas fields in China. The Changning FP water is characterized by high salinity (TDS of 13,100–53,500 mg/L), Br/Cl (2.76 × 10⁻³) and ⁸⁷Sr/⁸⁶Sr (0.71849), which are distinguished from the produced waters from nearby conventional gas fields with higher Br/Cl (4.5 × 10⁻³) and lower ⁸⁷Sr/⁸⁶Sr (0.70830–0.71235). The shallow groundwater samples were collected from a Triassic karst aquifer in both active and nonactive shale gas extraction areas. They are dominated by low salinity (TDS of 145–1100 mg/L), Ca-HCO₃ and Ca-Mg-HCO₃ types water, which are common in carbonate karst aquifers. No statistical difference of the groundwater quality was observed between samples collected in active versus nonactive shale gas extraction areas. Out of 66 analyzed groundwater, three groundwater samples showed relatively higher salinity above the background level, with low ⁸⁷Sr/⁸⁶Sr (0.70824–0.7110) and Br/Cl (0.5–1.8 × 10⁻³) ratios relatively to FP water, excluding the possibility of contamination from FP water. None of the groundwater samples had detected volatile organic compounds (VOCs). The integration of geochemical and statistical analysis shows no direct evidence of groundwater contamination caused by shale gas development.