Modeling EDTA-facilitated cadmium migration in high- and low-permeability systems using MODFLOW and RT3D

Cadmium (Cd) has impacted groundwater resources and can pose a serious threat to human health and the environment. Its fate in groundwater is complex and challenging to predict, as it is affected by adsorption to sediments, complexation with aqueous phase ligands, and variations in hydraulic conductivity. In this study, a 2D reactive transport model based on MODFLOW and RT3D is used to simulate published experimental results of cadmium migration without and with EDTA present in a flow cell containing high- and low-permeability zones (i.e., HPZs and LPZs). The model is then extended to conceptual flow cells with more complex LPZ configurations. Simulation results generally match the experimental data well, and analysis of experimental and simulated Cd effluent concentration profiles shows that EDTA enhances Cd removal from LPZs relative to water alone. Simulation results indicate that faster Cd removal is due to EDTA complexation with adsorbed Cd in LPZs, which enhances its solubilization and subsequent back diffusion. Lastly, simulation results show that with increasing LPZ heterogeneity more Cd is retained in flow cells, and EDTA is more effective in enhancing Cd removal relative to water alone; these results are attributed to more LPZ-HPZ interfaces that enhance Cd mass transfer into LPZs during contamination, and enhance EDTA mass transfer into LPZs to promote cleanup. Overall, the results highlight the promise of using EDTA to remove Cd from heterogeneous sites, but caution is advised due to model simplicity and lack of consideration of changes in solution pH, redox potential, or competing cations.