A numerical model to estimate sediment oxygen levels and demand

A numerical model that simulates sediment oxygen levels and demand (SOLD) w as developed and tested against field data. The model accounts for changes in sediments due to advection, diffusion, chemically and biologically media ted redox reactions, and other chemical transformations. The microbial degr adation of organic matter and the interactions of the subsequent chemical c onstituents are described using appropriate stoichiometric relationships. S ediment oxygen demand (SOD) is computed from fluxes of oxygen and reduced c ompounds across the sediment-water interface. To test the the model, simula tions were compared against published data obtained from lake sediments. Th e model results displayed reasonable agreement with the measurements. The m odel was used to examine some of the major factors that affect the sediment oxygen demand. Results show that although fluxes of methane and ammonia in crease with an increasing C rain rate, the oxygen nux remains almost consta nt for the simulations conducted. For an increase in bioturbation and/or te mperature, however, all fluxes increase significantly. The oxygen concentra tion at the sediment/water interface also affects the SOD significantly, es pecially the fraction of the total SOD that is due to the nux of oxygen int o the sediments. The fraction of the SOD that is due to the oxidation of am monia increases with increasing oxygen concentration at the sediment/water interface, while the fraction associated to the methane release and its oxi dation decreases slightly because more organic C is oxidized aerobically. T he contribution of reduced manganese, iron, and sulfide to the SOD are rela tively low, due to their precipitation with other chemical species.