Coupled stream-subsurface exchange of colloidal hematite and dissolved zinc, copper, and phosphate.

Previous studies have shown that stream-subsurface exchange has important implications for both colloid and reactive solute transport in streams because it increases the opportunity for the interaction of stream-born substances with bed sediments. We executed a series of laboratory flume experiments to study the coupled stream-subsurface exchange of hematite and the sorbing solutes zinc, copper, and phosphate. A fundamental process-based transport model was applied to analyze the experimental results. We found that hematite had a significant effect on the transport of all ions tested. In addition, hematite mobility was substantially modified by the presence of these solutes. Batch and column experiments showed that the difference in hematite mobility was a direct effect of zinc, copper, and phosphate sorption to the hematite surface. Sorption substantially modified the hematite surface charge and subsequently hematite filtration behavior. These results suggest that the behavior of contaminants cannot be analyzed separately from colloids in surface and groundwater systems because surface-chemical processes can cause their behavior to be coupled. In particular, our results show that general and specific interactions between contaminants and iron oxide particles can alter colloid mobility, perturb natural fine particle dynamics, and either favor or disfavor contaminant mobility.