Numerical modelling of arsenic release in groundwater under reducing environment

A mathematical model is developed to predict the arsenic release in groundwater under reducing environment. The transport part of the model solves the transient convection dispersion differential equations. The kinetic sub-model describes the heterotrophic metabolisms of several groups of micro-organisms. To model a complete redox sequence (aerobic bacteria or denitrifiers, Mn(IV)-reduction, Fe(III)-reduction, respiration bacteria of Fe-As compounds, and As(V) reduction), five functional bacterial groups are defined. Microbial growth is assumed to follow Monod type kinetics. Substrate consumption and release of metabolic products is coupled to microbial growth via yield coefficients and stoichiometric relations. The exchange between the different phases (mobile/pore water, biophase, and aquifer material) is also considered in this approach. Results from a soil column experiment are used to verify the simulation results of the model. Arsenic concentrations demonstrate positively covariation with the concentration of iron and manganese but correlate negatively with the concentration of nitrate. The result of this model demonstrated that growth rate of manganese and iron-reducing bacteria depends on the availability of Mn(IV) and Fe(III). A simultaneous reduction of Fe(III) and As(V) is observed.