Several laboratory and field studies have evidenced that colloids (e.g., ferric oxide particles) can significantly enhance the transport of plutonium (Pu) in groundwater. However, understanding the mechanisms of Pu mobility and evaluating Pu’s anomalous (non-Fickian) transport remain important yet highly challenging issues under the synergistic effects of fractured flow and goethite-mediated transport. Building on prior experimental data concerning Pu sorption/distribution and goethite-facilitated Pu migration in fractured granite, this paper focuses on modeling of the co-transport of tritium, Pu, and goethite colloids within the HYDRUS-1D framework. The study provides three fundamental advances. First, it provides the first experimental and simulated demonstration of a critical goethite colloid concentration threshold (Cg = 1 mg/L) that maximizes Pu mobility in fractured granite, revealing a non-monotonic relationship between colloid concentration and Pu transport regimes. Second, as an improvement in utility beyond existing continuum-scale approaches, it presents a novel colloid-mediated Pu transport model integrating mobile-immobile water (MIM) and attachment-detachment kinetics. This model uniquely represents both physical (fracture flow) and chemical (colloid-Pu interactions) non-equilibrium transport processes, and well fits measured anomalous breakthrough curves for both tritium and Pu. Third is the mechanistic discrimination of Pu intrinsic- versus pseudo-colloid transport. Specific mechanisms, such as size exclusion and Taylor dispersion, enable pseudo-colloids to migrate at near-water velocity, contrasting with considerable retention of intrinsic colloids. Additionally, effects of five optimized parameters on the anomalous breakthrough curves are quantified by sensitivity analysis. These findings redefine predictive capabilities for colloid-facilitated radionuclide transport in fractured systems, with direct implications for safety assessments of geological repositories.
Anomalous transport of colloidal goethite and plutonium in single granite fracture experiments: Modeling and evaluation
Subjects: Colloid