Heterogeneous processes affecting trace contaminant distribution in estuaries: The role of natural organic matter

Our objective in writing this paper is to frame the issues pertaining to the role of natural organic matter in affecting the speciation and transport (fate) of trace contaminants, which include hydrophilic trace metals and hydrophobic trace organics, within estuaries. In particular, we focus on one aspect of the problem: the partitioning of trace contaminants between organic and inorganic particles, colloids and solution. This paper is developed along three main lines. First, we review the literature with respect to trace metal sorption by metal oxides in the presence of organic ligands. Second, we examine the role of colloidal organic matter in regulating the estuarine behavior of trace contaminants. The focus of this portion of the paper is on the trapping of trace metals within colloidal and particulate organic matter and a re-examination of the ‘particle concentration effect’. Third, we propose a new conceptual model (the ‘percolator’ model) which links several processes (sorption, diffusion, coagulation) and which serves as a framework for evaluating the trapping of trace metals and organic matter within estuarine sediments. One conclusion derived from simulations of the diffusive flux of DOC from sediments is that the magnitude of sediment/water interfacial shear stress does not affect DOC flux until the shear stress is sufficient for the onset of bed erosion. The consequence is that interactions between DOC and sediment materials become the controlling factors in regulating the diffusive flux of DOC.