Mathematical model for arsenic transformation and transport in the saturated zone

Toxic metalloids of abandoned mines, such as arsenic, emerge as a major contaminant. Arsenic occurs in natural waters as arsenite (As3+) or arsenate (As5+). Arsenite is more soluble and toxic than arsenate. Under oxidizing conditions, the predominant form of inorganic arsenic is arsenate, and arsenite is more prevalent under reducing conditions. It is known that microorganisms play a critical role in both the direct reduction and oxidation of the arsenic species. The biogeochemical redox processes and transport behavior need to be coupled in a reactive transport model to predict concentrations of the toxic inorganic arsenic in aqueous phase. A new reaction module describing the fate and transport of inorganic arsenic species are developed and incorporated into the RT3D (Reactive Transport in 3-Dimensions) code. The simulation results of numerical example demonstrate that the proposed model can describe the arsenate reduction to arsenite and its subsequent transport. To verify the model, laboratory column experiments are conducted. The arsenic transport results obtained from the model are compared with the experimental results. The reactive transport model of arsenic will be useful to predict the mobilization of arsenic, the evolution of its contaminant plumes, and the fate and transport of arsenic species in groundwater systems.