Establishing in-situ water quality criteria for assessing ecological risk of labile As in the Three Gorges Reservoir during low-water level period

The Three Gorges Reservoir (TGR) is divided into three hydrological zones due to hydrological regulation and intricate hydrodynamic conditions. Such spatial heterogeneity further aggravates the spatial differentiation of trace elements. This study explored the spatial distribution patterns, dominant driving factors, and ecological risks of labile arsenic (As) across different hydrological zones of the TGR during the low-water period. Furthermore, novel in-situ water quality criteria for labile As were established by coupling the Diffusive gradients in thin films (DGT) with species sensitivity distribution models. The results revealed that the concentrations of labile As exhibited a clear spatial trend and were influenced by hydrodynamic conditions. Mantel tests and partial least squares path modeling demonstrated that hydrodynamic conditions served as the core co-regulators of the spatial distribution of labile As. Based on the species sensitivity distribution (SSD), graded short-term (Grade I–III: 8.31, 83.09, 340.00 μg L −1 ) and long-term (Grade I–III: 0.50, 4.98, 49.80 μg L −1 ), WQC for labile As were derived. The TGR met Grade I for both criteria, indicating low ecological risk. Probabilistic risk assessment further confirmed that labile As posed negligible toxic effects on aquatic organisms. The findings and proposed criteria provide a robust scientific foundation and technical reference for refined water quality management of large reservoir ecosystems during low-water level period.