The use of an integrated risk assessment to evaluate nutrient and heavy metal release in a Southern Chinese river...

The ecological risks and water-quality response mechanisms associated with complex sediment contamination in southern river-network systems remain poorly understood, posing challenges for precise pollution control. This study proposes a novel three-tier assessment framework that, for the first time, establishes a quantitative linkage between sediment pollution characteristics, release dynamics, and water quality response under varying hydrodynamic conditions. Taking the Nanshun-Lian'an polder river network in Shunde District as a case study, we developed a framework integrating pollution characteristics, release risk, and water quality response. Through vertical stratified sediment sampling, laboratory release experiments, and hydrodynamic disturbance simulations, the composite contamination characteristics of sedimentary TN/TP and Hg/Cd, their release risks, and the mechanisms driving overlying water quality responses were quantitatively evaluated. The results indicate pronounced spatial heterogeneity in sediment pollution. Industrial agglomeration areas were identified as extremely high-risk zones for Hg/Cd contamination (RI > 300, accounting for 15.2%), with bioavailable fractions comprising 63%-68%. Urban river reaches were dominated by high nutrient release risks (NPI = 1.85-10.67), where release rates in the 0-30 cm surface layer were 5-7 times higher than those in deeper layers. Agricultural tributaries exhibited concealed Cd risks, with exchangeable fractions accounting for 25%-26%. A critical flow velocity of 0.3 m s^-1 was identified as the threshold for sediment resuspension; beyond this, the release rates of NH₃-N and TN showed a significant positive correlation with flow velocity (R² = 0.78). Based on multiparameter clustering analysis, four sediment risk zones were delineated, and key management parameters-such as a dredging depth of 50 cm in extremely high-risk areas-were proposed. This assessment framework provides a systematic technical approach and scientific basis for precise risk identification, zoned management, and optimization of engineering parameters for sediment pollution control in southern river-network systems, with potential transferability to other deltaic river networks globally.