Reactive transport modeling of trace elements in the water column of a stratified lake: iron cycling and metal scavenging

A simple one-dimensional reactive transport model was developed to determine the importance of the dynamics of major redox species such as iron on the distribution of trace elements in aquatic environments. The model simulates the distribution of iron and lead in the water column of a stratified lake (Paul Lake, MI) assuming a quasi steady-state is reached. The reactions described in the model include oxidation of reduced iron to colloidal ferric iron at the oxic–anoxic transition in the water column, aggregation of colloidal iron to particulate iron, and reduction of colloidal and particulate iron in the deep anoxic waters. The model includes sedimentation of both colloidal and particulate species. The cycling of lead is described by adsorption on colloidal iron, transfer to the particulate phase by aggregation, and redissolution in the deep waters during reduction of colloidal and particulate iron. Generally, this model reproduces experimental data collected during three field trips from 1994 to 1996 well. However, these calculations clearly show that the processes regulating the cycling of iron and lead in the water column are more complex than what is generally admitted. Additional reactions not considered in the model affect the transformation of Fe, and thus affect the removal of Pb. These reactions have to be fully characterized in order to predict the chemical distribution of trace elements at oxic–anoxic transitions in the water column of aquatic systems.