Coupled Reactive Transport Modeling and Isotopic Tracer Integration for Quantifying Heavy Metal Attenuation Kinetics in Riparian Vadose Zone Sediments

Abstract

Anthropogenic heavy metal contamination of riparian vadose zone sediments represents a persistent threat to groundwater quality and fluvial ecosystem integrity. This study presents a coupled reactive transport modeling (RTM) framework integrating stable isotopic tracers (δ66Zn, δ114Cd) with multi-phase geochemical partitioning algorithms to quantify attenuation kinetics of zinc, cadmium, and lead in heterogeneous alluvial sediment profiles. Column leaching experiments were conducted under controlled redox gradients, and outputs were calibrated against PHREEQC-3 geochemical simulations. Results demonstrate that organo-mineral co-precipitation at redox transition zones accounts for 61–74% of total metal immobilization, with kinetic rate constants exhibiting a strong pH dependency (R² = 0.94). The proposed framework improves predictive accuracy for contaminant breakthrough modeling by 38% over conventional Kd-based approaches, providing a robust tool for vadose zone remediation design.