Identifying Candidate Persistent, Mobile, and Toxic (PMT) and Very Persistent and Very Mobile (vPvM) Substances in Shale Gas Drilling Fluids by Combining Nontarget Analysis and Machine Learning Model

Abstract

Shale gas extraction has raised environmental concerns on regional water resources. Horizontal drilling is a process in which drilling fluids containing complex organic and inorganic chemicals are intensively applied. Accidental spill and improper disposal of drilling fluids and related wastes might pose risks to surrounding groundwater environment. Given regional ground water quality, persistent, mobile, and toxic (PMT) and very persistent and very mobile (vPvM) substances should be of particular attention. However, recent research rarely focused on chemical compositions of drilling fluids, and the harmful PMT/ vPvM substances in drilling fluids remain unknown. In this study, we utilized a nontarget screening strategy to detect and identify the organic compounds in drilling fluids collected in southwest China. Specifically, a total number of 371 compounds were detected in drilling fluids, and the main fraction of the compounds was alicyclic compounds. Later, an original machine learning model developed by us was applied to identify the candidate PMT/vPvM substances among the detected organic compounds. Our study identified 29 candidate PMT/vPvM substances, thus providing a list of prioritized substances for early warning and risk assessment of regional groundwater contamination.