Biogeochemical Dynamics of Metals in Surface Sediments

Philippe Van Cappellen School of Earth and Atmospheric Sciences, Georgia Inst. of Technology, GA 30332, USA

Yifeng Wang Sandia National Laboratories, Albuquerque, NM 87185, USA

The fate of trace metals in surface sediments is controlled by kinetic processes, including organic matter degradation, redox transformations, and mineral dissolution or precipitation reactions. These processes control the pH and redox conditions of the pore waters. They also control the rates of pore water metal release, and the availability of dissolved ligands, adsorption sites and solid host phases. Because the majority of those processes are irreversible, the quantitative description of metal dynamics in sediments should be based on a kinetic modeling approach that couples reaction rates to particulate and solute transport fluxes.

While many of the transport and reaction processes affecting the behavior of trace metals in sediments have been studied, in the field or in the laboratory, there have been few attempts to integrate them into comprehensive models. This paper describes a multicomponent sediment model consisting of a core reaction-transport routine for the major biogeochemical elements C, N, O, S, Fe and Mn, and a trace metal routine. The core routine calculates the distributions of all major redox and acid-base
pore water species, plus the distributions of the particulate trace metal carrier phases (organic matter, iron and manganese oxyhydroxides, carbonate minerals, and iron sulfide). With the information provided by the core routine, the trace metal routine simulates the cycling of trace metals between the pore solution and the main solid carrier phases.

Model applications include extraction of field-based reaction parameters and trace metal distribution coefficients, identification of dominant reaction pathways and sedimentary sinks, calculation of elemental budgets and benthic exchange fluxes, and development of diagnostic sediment quality parameters for toxic trace metals. As a sensitivity tool, the model can be used to study the responses of metal cycling in sediments to changes in internal and external forcings. The responses may no longer be intuitively predictable, because of the complex dynamics involved.