Stream-subsurface exchange of zinc in the presence of silica and kaolinite colloids

The mobility of sorbing contaminants in surface waters often depends strongly on associations with sediments, including both fine suspended particles and stationary bed sediments. Hydrodynamic flow coupling causes an exchange of dissolved and suspended substances between streams and underlying pore waters (hyporheic exchange). As a result, the fate of many pollutants is expected to be greatly influenced by the flux of colloids and contaminants across the stream-subsurface interface and the interactions of both types of substances with the bed sediments. Herein, we present experimental results on the stream-subsurface exchange of zinc in the presence of colloidal silica and kaolinite in a laboratory flume. We also apply a process-based theoretical model to predict the coupled transport of colloids and reactive solutes. Model input parameters were obtained using independent batch and column experiments. Zinc immobilization in the bed was found to be significantly greater in the presence of kaolinite than in the presence of colloidal silica. Model predictions indicated that there were two distinct reasons for the greater zinc immobilization in the presence of kaolinite: zinc sorbed more strongly to kaolinite than to silica, and kaolinite particles also deposited more readily in the streambed than did silica colloids. Model simulations were found to be highly dependent on the colloid size distribution. When the colloids had a bimodal distribution, colloidal-phase contaminant transport occurred primarily on the finer fraction, but bulk colloid deposition was dominated by removal of the coarser fraction.