Modeling Hg reactive transport in soil systems using HP1

We developed a reactive transport model for simulating mercury (Hg) fate and transport in the unsaturated zone, in order to gain insight into the environmental fate of Hg originating from contaminated ssoil. The code was implemented using the module HP1 (Jacques et al., 2006; 2008), which couples HYDRUS-1D (Šimůnek et al., 2008) for water and solute transport with PHREEQC (Parkhurst and Appelo, 1999) for geochemical reactions. The main processes accounted for in the model are Hg aqueous speciation (including complexation with dissolved organic matter (DOM), humic and fulvic acids (HA and FA), and thiol groups), Hg sorption to solid organic matter (SOM, while also distinguishing between humic and fulvic acids and thiol groups), dissolution of solid phase Hg (e.g., cinnabar HgS(s)), dissolution of Hg non-aqueous phase liquid (NAPL), sunlight-driven HgII reduction to Hg0, Hg0 diffusion in the gas phase and volatilization, and DOM sorption to soil minerals. A sensitivity analysis shows that the most sensitive parameters determining Hg leaching or retardation are the equilibrium constant for Hg sorption to humic and fulvic acids (i.e., the most abundant sorption sites in SOM), the initial concentration of Hg, and the concentration of DOM in the soil solution.