Nutrient-heavy metal risk linkages and their differentiated driving mechanisms in sediments of a subtropical drinking water reservoir, South China.

Whether nutrient enrichment is consistently associated with heavy metal ecological risks in drinking water reservoir sediments, and whether such associations are controlled by shared sources or sedimentary environmental mediation, remain poorly understood. This study investigated the Dawangtan Reservoir in South China using measurements of eight heavy metals and nutrients from 144 surface sediment samples, integrating the geo-accumulation index, potential ecological risk index (RI), positive matrix factorization (PMF), K-means clustering, and random forest-SHAP analysis. Results showed that: (1) the reservoir sediments were at moderate ecological risk, with a mean RI of 157.27, and Cd and Hg were the dominant risk contributors; (2) PMF identified five sources categories, including atmospheric deposition, traffic emissions, industrial discharge, agricultural non-point sources, and natural background. As and Cu were mainly associated with agricultural non-point sources, whereas Cd and Hg were independently controlled by industrial discharge and atmospheric deposition; (3) K-means clustering classified the sampling sites into "low nutrient-low risk" and "high nutrient-high risk" types, while RF-SHAP analysis identified pH and organic matter (OM) as the primary environmental factors differentiating the two types. Specifically, As and Cu showed positive associations with TN and TP, indicating source-mediated coupling through shared agricultural input pathways. In contrast, Cd and Hg showed weak site-level correlations with nutrients but were enriched in the high nutrient-high risk class, suggesting that their zonal co-occurrence may be associated with favorable sedimentary conditions, particularly high pH and OM, rather than direct nutrient-driven coupling. These findings indicate that nutrient-metal relationships in reservoir sediments are controlled by both source-related inputs and sedimentary environmental conditions. Therefore, coordinated nutrient-metal control is recommended for agricultural-source elements such as As and Cu, whereas Cd and Hg require independent source-specific management targeting industrial discharge and atmospheric deposition.