Characteristics of polycyclic aromatic hydrocarbons and potential ecological risks of surrounding soil in typical shale gas extraction sites in southwest China

Shale gas, a crucial transitional energy source, can alleviate energy security challenges but may also pose severe environmental threats. Current research on the soil ecological risks near shale gas extraction sites is relatively limited. Meanwhile, existing studies primarily focus on the quantification of target compounds, with a lack of bioassays, making it difficult to accurately assess the actual toxicity of soil samples. This study conducted a soil risk assessment in a major shale gas production area within the Longmaxi formation in southwest China. We collected soil samples from five extraction sites and assessed acute toxicity to the soil organism Folsomia candida. The levels of polycyclic aromatic hydrocarbons (PAHs) within these samples were measured. No significant mortality was observed to Folsomia candida, with PAH concentrations ranging from 76.7 ng·g−1 to 567 ng·g−1 dry weight, and a median value of 300 ng·g−1 dry weight—levels comparable to those in agricultural areas. Source analysis revealed that the primary sources of the soil PAHs originated from biomass and coal combustion. In various shale gas extraction sites, the levels of PAHs show a decreasing trend with increasing distance from the sampling points to the shale gas extraction sites. Notably, concentrations of specific three-ring and four-ring PAH compounds, such as phenanthrene, benzo[b]fluoranthene, and benzo[k]fluoranthene, significantly decreased with increasing distance. The risk assessment results suggested a low ecological risk from conventional shale gas extraction, with minimal impact on the surrounding soil environments. This study revealed the potential ecological risks to soils surrounding typical shale gas extraction sites in southwest China, contributing to a comprehensive assessment of the environmental impacts of shale gas development and providing a scientific basis for policy-making.