Analysis of characteristics and distribution differences of emerging contaminants in wastewater from metro maintenance depots based on non-targeted techniques

This study employed high-resolution mass spectrometry （HRMS）-based non-target screening to systematically identify the composition and spatial distribution characteristics of emerging contaminants （ECs） in wastewater from three functional zones of a metro maintenance station： the storeroom （S1）， the office and residential area （S2）， and the final discharge outlet （S3）. A total of 417 contaminants were detected， covering eight major categories including industrial materials， pharmaceuticals， pesticides， and natural products. Among them， 48 substances were identified with Level 1 confidence based on spectral matching. Pesticides exhibited the highest detection frequency and concentration levels， representing the primary source of contaminant load. Semi-quantitative concentration heatmaps of 24 pesticides revealed significant spatial variation： S2 exhibited the highest number and concentration of contaminants， reflecting pollution from landscaping and vector control activities in the residential area； S1 and S3 showed relatively lower levels， indicating dilution， migration， and attenuation processes within the system. Representative pesticides such as bifenox displayed clear concentration gradients across sampling points， suggesting multiple transport mechanisms such as surface runoff， hydraulic transfer， and sorption. These findings highlight the complexity and diversity of EC sources in urban transportation infrastructure wastewater， underscore the necessity of prioritizing pesticides in regulatory management， and provide fundamental data for understanding the environmental behavior of ECs and informing water environment risk assessment.