Identification and hazard prioritization of hydrophobic organic chemicals in flowback and produced water particles:...

Hydraulic fracturing flowback and produced water (HF-FPW) has raised significant concerns owing to its potential impact on aquatic organisms and human health. Understanding the chemical composition of HF-FPW is crucial for developing appropriate management and remediation strategies. Herein, we performed nontarget screening on hydrophobic organic chemicals in the particulate phase of FPW (P-FPW) using gas chromatography-Orbitrap mass spectrometry coupled with cheminformatic analysis. In total, 5807 features were discovered, with 209 annotated with positive confidence levels, which were further classified based on their chemical taxonomy and functional use information. We found that benzenoids dominated the chemical class, followed by hydrocarbons. The annotated chemicals were classified into fragrances, catalysts, antimicrobials, antioxidants, and other classes. Chemical overlap across countries (China vs. Canada) and wells was observed, with most chemicals reaching peak intensity within 24 or 48 h after the initial flowback and gradually decreasing. Approximately two-thirds of the identified or annotated chemicals have not been previously reported or included in existing HF-related databases, indicating expanded chemical coverage by the current screening workflow. A Toxicological Priority Index (ToxPi) scheme, which integrates chemical properties, ecological toxicities, and in vivo exposure potentials, was adopted to prioritize chemicals for further evaluation. Seven chemicals were proposed as prioritized compounds, of which the ester derivative of perfluorobutanoic acid, octacosyl heptafluorobutyrate (confidence level 2), exhibited the highest ToxPi score. Notably, most prioritized substances have limited toxicological data and are beyond the routine monitoring of the HF industry, highlighting significant gaps in our understanding of HF-related chemical content and environmental risk associated with water management.