Modeling the transport and reaction of trace metals in water-saturated soils and sediments

A model has been formulated of the equilibrium speciation, kinetic reaction, and transport of trace metals in the presence of biodegradation of organic substrates in saturated porous media. Kinetics of various processes (biodegradation, chemical reactions, and precipitation and dissolution of minerals) together with transport processes (advection, bioturbation, and diffusive/dispersive mixing) are quantified in a set of coupled mass balance equations (for the organic substrate, electron acceptors, reduced species, and trace metals). These steady state, one-dimensional equations are discretized using a second-order-accurate finite difference approximation. ApE is estimated at each node in the domain on the basis the concentrations calculated and the half reaction for the dominant terminal electron acceptor at that location. The dynamic model is coupled iteratively to a modified version of the U.S. Environmental Protection Agency's MINTEQA2, which calculates equilibrium chemical speciation (including aqueous speciation, adsorption, and precipitation of minerals) at each node of the domain. The primary dependent variables are the total dissolved concentrations of the aqueous species together with the solid concentrations of the minerals. To demonstrate that this formulation can simulate biodegradation using reaction rates consistent with published values, simulations are compared to data from the sediment pore waters of a small lake. Simulations are presented of the transport and reaction of arsenic in lake sediments to illustrate how this model can be used to evaluate trends in trace metal mobility as affected by various water quality parameters through their influence on the biogeochemistry of natural systems.