Prospective modelling with Hydrus-2D of 50 years Zn and Pb movement in low and moderately metal contaminated agricultural soils

Results of detailed modeling of in situ redistribution of heavy metals in pedological horizons of low and moderately metal contaminated soils, considering distinctly different long-term land use, are scarcely reported in literature. We used Hydrus-2D software parameterized with abundant available local soil data to simulate future Zn and Pb movements in soils contaminated by metallurgical fallout in the 20th century. In recent work on comparing different modeling hypotheses, we validated a two-site reactive model set with adjusted chemical kinetic constant values by fitting the 2005 Zn and Pb concentration profiles in soils, with estimated 1901-1963 airborne Zn and Pb loads (Mallmann et al., 2012a). In the present work, we used the same approach to simulate 2005-2055 changes in Zn and Pb depth-distribution and soil-solution concentrations, comparing two hypotheses of chemical equilibrium: i) the validated two-site model (one site at equilibrium and the other involved in kinetic reactions with pore water) set with adjusted kinetic EDTA extraction constants, and ii) a non-linear one-surface site adsorption equilibrium model. Simulated transfers were found generally lower and more realistic when using the two-site model. Simulations showed that consistent Zn redistribution and loss occurred in the moderately contaminated soil until 2055, i.e., more than one century after the main metal deposition, but negligible in low contaminated soils. Transfer of Pb was small in the three soils and under both hypotheses. In 2055, simulated Zn outflow concentrations remained under threshold values for drinking water