Characterizing hazardous substances of shale gas wastewater from the upper Yangtze River: A focus on heavy metals and organic compounds.

In the last decade, rapid shale gas exploration in upper Yangtze River ecological zone in China has led to increasing concerns about the environmental impact of shale gas wastewater (SGW). However, our understanding of the types of potential hazardous substances of SGW remains limited. In this study, eight SGW samples from three shale gas regions in upper Yangtze River: the Sichuan Basin, the Guizhou Plateau, and the Three Gorges Area were collected, and their general water quality, trace metals, and organic compounds were comprehensively analyzed. Our in-depth analysis detected 55 kinds of trace heavy metals, with 24 exceeding detection limits. Most of them were of the concentration below 100 μg/L. Concentrations of primary pollutants, including Cd, Cr, As, Pb, and Ni, remained below Integrated Wastewater Discharge Standard (GB 8978-1996), indicating minimal environmental risk. The organic analysis identified 45 to 104 kinds of volatile and semi-volatile organic compounds in SGW samples from different regions. SGW samples from the Sichuan Basin exhibited a balanced proportion of aliphatic and aromatic compounds, with oxygen and nitrogen-substituted heteroatomic compounds prevailing, while SGW samples from the Guizhou Plateau and the Three Gorges Area were dominated by aromatic compounds, particularly hydrocarbons. Several organic substances exhibited high response strengths across multiple SGW samples, including isoquinoline, dibenzylamine, 2,4-di-tert-butylphenol, 1,2,3,4-tetrahydro-naphthalene, and 1,2,3,4-tetrahydro-6-methyl-naphthalene. The Globally Harmonized System (GHS) of Classification and Labelling of Chemicals classified most high-response organics as high acute and chronic aquatic hazards. Our findings indicate that high salinity and a variety of high-risk organic pollutants, rather than heavy metals, are the primary pollutants in SGW, underscoring the urgency of safety management of SGW.