A reactive transport model for mercury fate in soil — application to different anthropogenic pollution sources

A one-dimensional model for simulating Hg fate and transport for variably saturated and transient flow conditions is presented. The model is developed using the HP1 code, which couples HYDRUS-1D for the water flow and solute transport to PHREEQC for geochemical reactions. The main processes included are Hg aqueous speciation and complexation, sorption to soil organic matter, dissolution of cinnabar and liquid Hg, and Hg reduction and volatilization. A test case is presented, assuming a hypothetical sandy soil profile and a simulation time frame of 50 years of daily atmospheric inputs. Mercury fate and transport are simulated for three different sources of Hg (cinnabar, residual liquid mercury or aqueous mercuric chloride), as well as for combinations of these sources. In the test case, Hg volatilization was negligible because the reduction of Hg2+ to Hg0 was inhibited by the low concentration of dissolved Hg. Hg leaching was mainly caused by complexation of Hg2+ with thiol groups of dissolved organic matter. Immobilization of Hg in the initially polluted horizon was enhanced by Hg2+ sorption onto humic and fulvic acids (which are more abundant than thiols). Potential benefits of the model for risk management and remediation of contaminated sites are discussed.