Development of a sediment redox potential model for the assessment of postdepositional metal mobility

A numerical model was developed to simulate the vertical profile of the redox potential in benthic sediments. The benthic sediments were subdivided vertically into six zones, each with different microbial and chemical reactions: aerobic respiration, denitrification, manganese reduction, iron reduction, sulfate reduction, and methanogenesis. Microbial degradation of organic matter and subsequent chemical reactions of interest were formulated using stoichiometric relationships and considering the vertical advective/dispersive transport in the sediments. The kinetics of utilization of the different electron accepters during the biodegradation of the organic matter were described by a Monod-type formulation. Eleven coupled differential equations were derived and solved interactively utilizing an iterative multistep numerical method. The model input parameters include the rate of solid deposition, concentrations of electron accepters in the water overlaying the sediments, activities of the benthic fauna, and molecular diffusion. The model simulates the redox potential as well as eleven chemical constituents in the sediments, three solids (particulate organic matter, manganese oxide, and iron oxide), and eight dissolved species (oxygen, nitrate, sulfate, ammonia, dissolved manganese, dissolved iron, sulfide, and methane). The model demonstrated that accurate estimates of the flux of primary electron accepters and donors from the overlying water to the benthic sediments is important to determine the redox conditions in sediments. Bioturbation and the rate of pore-water infiltration are processes that have a major influence on this flux.