Numerical study on co-transport of natural colloids and Am(III) in the vadose zone sediments

Natural mobile colloids strongly facilitate the subsurface transport of radioactive Americium(III) (Am(III)), considerably extending the transport distance of Am(III) in porous media. Assessing the long-term transport behavior of colloidal Am(III) and its potential subsurface risk is critical for the safety evaluation and emergency management of radioactive disposal sites. In this study, laboratory column experiments combined with numerical simulations were conducted to investigate colloid-facilitated Am(III) transport in unsaturated porous media. A one-dimensional unsaturated column model was constructed using Hydrus-1D to inversely determine key retention and transport parameters, followed by a parameter sensitivity analysis to quantitatively evaluate the influencing intensity of hydrodynamic and geochemical factors. On this basis, an unsaturated disposal site model was established to predict the environmental transport characteristics of colloidal Am(III) under varied leakage and rainfall scenarios. Sensitivity analysis identified highly sensitive parameters such as the medium dispersion (0.141 ≤ S ≤ 0.258) and the saturated volumetric water content (-0.165 ≤ S ≤ -0.139), as well as low sensitivity parameters such as the saturated permeability coefficient (-0.044 ≤ S ≤ -0.012) and medium density (-0.0030 ≤ S ≤ -0.0026). The scenario simulation results indicated the vertical transport depth of colloidal Am(III) in the vadose zone increased with increasing time, rainfall intensity, and leakage amount. Short term heavy rainfall (100-300 mm/d) remarkably accelerated downward transport of colloidal Am(III), leading to a maximum breakthrough depth of 0.60-1.26 m without reaching the groundwater table. Nevertheless, long-term leakage under Case S1 (> 224 years) or large volume accidental leakage under Case S3 (> 7.66 m³) may eventually lead to groundwater contamination, posing severe threats to subsurface environmental safety.