Hydrologically Driven Simulation of Heavy Metal Transport and Transformation in Watersheds: Modeling Frameworks, Emerging Challenges, and Mechanistic Insights Based on the SWAT Model

Watershed-scale simulation of heavy metal transport and transformation is an important tool for pollution source identification, environmental risk assessment, and watershed management decision-making. Existing hydrological and water quality models provide essential process-based foundations for heavy metal transport simulation, including runoff generation, soil erosion, sediment transport, and channel routing. However, limitations remain in the coupling of geochemical reactions, representation of metal speciation, response to extreme events, and separation of multiple uncertainty sources. Based on literature records from the Web of Science Core Collection, this study integrates a PRISMA-based screening procedure, CiteSpace bibliometric analysis, comparative assessment of representative hydrological and water quality models, and an AHP—MCDA semi-quantitative evaluation method to systematically review modeling frameworks for watershed-scale heavy metal transport and transformation. The SWAT model and its extended module SWAT-HM are further used as representative cases to analyze the coupling mechanisms among hydrological processes, sediment processes, and heavy metal transport and transformation. The results indicate that the SWAT model has relatively high overall applicability in terms of hydrological process representation, sediment/erosion process representation, and heavy metal coupling basis, making it suitable for long-term pollution load assessment in medium- and large-scale watersheds. However, for urban storm runoff, high-frequency water quality responses, surface water–groundwater exchange, and detailed reactive transport processes, other models or coupled modeling frameworks are still required. The SWAT-HM model links dissolved, particulate, and sediment-associated heavy metal transport with runoff, erosion, and sediment transport processes, but its representation of complex speciation transformation, valence state changes in redox-sensitive elements such as As and Cr, and colloid-facilitated transport remains simplified. Future research should move toward an integrated modeling system that coordinates mechanisms, data, and uncertainty, with particular emphasis on breakthroughs in multi-scale dynamic coupling, element-specific parameter systems, dynamic pH–Eh modules, and standardized uncertainty decomposition workflows, thereby improving model interpretability, predictive reliability, and management relevance.