Modelling coupled water flow, solute transport and geochemical reactions affecting heavy metal migration in a podzol soil

Many or most subsurface pollution problems at the field scale involve such simultaneous processes as water flow, multicomponent solute transport, heat transport and biogeochemical processes and reactions. Process-based models that integrate these various processes can be valuable tools for investigating the mobility of a wide range of inorganic and organic contaminants subject to different hydrologic and geochemical conditions. The HP1 reactive transport simulator, obtained by weak coupling of HYDRUS-1D and PHREEQC-2, was developed and designed to address multicomponent geochemical transport processes in the vadose zone. In this paper we discuss a hypothetical HP1 application involving the transport of major cations and heavy metals in a soil during transient flow over a period of 30 years. Results show that variations in water content and water fluxes can significantly influence the speciation, and thus the mobility and availability, of elements. Decreasing water contents near the soil surface lowered pH of the soil solution and produced new cation exchange equilibrium conditions. The upward transport of Cl during summer due to increased evapotranspiration, and subsequent accumulation of Cl near the soil surface, caused an increase in the total aqueous Cd concentration because of the formation of Cd–Cl complexes. Coupled reactive transport codes for the unsaturated zone, such as HP1, are promising tools to unravel the complex interaction between soil physical and biogeochemical processes for all kinds of problems, including the impact of natural processes and antropogenic activities on soil evolution.