Greater environmental risk of shale gas produced water from lacustrine than marine sources in Fuling shale gas field, China: Insights from inorganic compounds, dissolved organic matter, and halogenated organic compounds

Lacustrine shale gas represents a promising frontier in the future development of shale gas resources. However, research on the characterization of lacustrine shale gas produced water (SGPW) remains scarce. In this study, we characterized the geochemical properties of both marine and lacustrine SGPW (MSGPW and LSGPW) and assessed their dissolved organic matter (DOM) components using fluorescence EEM spectroscopy. Additionally, we employed Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to analyze halogenated organic compounds (HOCs) and non-HOCs in SGPW, as well as their transformations during storage in open impoundments. Pollutants in LSGPW generally had higher concentrations and greater fluctuations compared to those in MSGPW. Our findings from EEM spectroscopy and FT-ICR MS revealed that phenolic compounds may be important components of DOM in all SGPW. Moreover, the number of detected unique molecules in LSGPW was greater than in MSGPW. CHO or CHOS compounds dominated in non-HOCs, with LSGPW exhibiting generally higher DBE, modified aromaticity index (AI_mod), nominal oxidation state of carbon (NOSC), double bond equivalent minus oxygen per carbon ((DBE-O)/C) values, and lower H/C values compared to MSGPW, while unsaturated aliphatic compounds typically dominated in HOCs. Furthermore, we employed 37 transformation reactions that might occur during SGPW storage and found that oxygen addition and dealkyl group reactions were predominant, with these two types of reactions occurring more frequently in LSGPW than in MSGPW. LSGPW exhibited higher toxicity compared to MSGPW, with toxicity positively correlated with the concentrations of inorganic salts and organic substances with higher AI_mod, NOSC, and (DBE-O)/C. These findings contribute to a more comprehensive understanding of LSGPW, enabling the design and implementation of more rational disposal measures to effectively mitigate its potential environmental risks.