Hydrogeochemical and isotopic evidences of the underlying produced water intrusion into shallow groundwater in an oil production area, Northwest China

The extensive use of fossil fuels (e.g., oil) poses a hidden danger to groundwater quality. However, inorganic pollution has received limited attention compared to organic pollution. In this study, the potential contaminant sources to shallow groundwater were investigated using hydrochemical (e.g., major and trace elements) and isotopic (delta 2H and delta 18O) methods at an oil field, northwest China, with emphasis on the identification of produced water (PW; oil production -related water) intrusion. The results showed that the groundwater samples can be chemically and isotopically classified into two groups: Group A (severely polluted) and B (slightly or nonpolluted). The hydrochemical characteristics of Group A were similar to that of PW, with a comparable Na+/Cl- ratio and elevated levels of Na+, Ca2+, Cl-, Br-, Sr, Ba, Li, B and total volatile organic compounds (TVOCs; volatile and semi -volatile) concentration, but lower HCO3- and SO42- contents, and depleted delta 2H and delta 18O, which was not suitable for drinking. Groundwater salinity sources involve mineral dissolution (silicate, carbonate and evaporite), cation exchange and anaerobic microbial degradation, as well as deep PW intrusion (especially in Group A). The Cl mixing model showed that PW contributed 13.63-27.78 % to Group A, supported by the results of the isotope mixing model based on delta 2H and delta 18O (24.43-33.29 %). An overall pollution conceptual model involves three modes: fracturing, surface infiltration, and groundwater lateral runoff. This study validates the effectiveness of Na, Cl, Br, Sr, Ba, Li and B as favorable tracers for monitoring PW intrusion.