As the work of Luther and others have shown, Fe and Mn are key metals in natural waters because their redox cycling tends to involve the cycling of organic matter and because the oxides of these elements are important carriers of other trace elements. Process-oriented and site-specific studies have resulted in much progress in understanding the behavior of these two elements. However, we are not yet to the point where one can use the hydrological, landscape and chemical characteristics of a river system to make a reasonable prediction of the concentrations of Fe and Mn. Using results from our surveys of trace elements in the Yukon River Basin as well as the Mississippi and other rivers, simple models have been developed to explain dissolved and colloidal Fe and Mn concentrations throughout this system. To a first order, Mn (which is mainly found in the dissolved phase) is largely controlled by the availability of reducing sources. Colloidal Fe is generally correlated with dissolved Mn, a result consistent with the more rapid precipitation kinetics of Fe than Mn. The behavior of dissolved Fe can be generally reproduced by a simple model balancing organic complexation versus FeOOH solubility control, both of which are pH-dependent. These models allow predictions of how changing landscapes (due, for example, to climate change) could affect the concentrations of Fe and Mn.

Towards a model of dissolved and colloidal Fe and Mn in rivers

245th National Meeting (2013)

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