MODELING THE TRANSPORT AND REACTION OF TRACE-METALS IN WATER-SATURATED SOILS AND SEDIMENTS

A model has been formulated of the equilibrium speciation, kinetic rea ction, 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 d issolution 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, electro n accepters, reduced species, and trace metals). These steady state, o ne-dimensional equations are discretized using a second-order-accurate finite difference approximation. A pE is estimated at each node in th e domain on the basis the concentrations calculated and the half react ion 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 equilibr ium chemical speciation (including aqueous speciation, adsorption, and precipitation of minerals) at each node of the domain. The primary de pendent variables are the total dissolved concentrations of the aqueou s species together with the solid concentrations of the minerals. To d emonstrate that this formulation can simulate biodegradation using rea ction rates consistent with published values, simulations are compared to data from the sediment pore waters of a small lake. Simulations ar e 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.