Exchange of adsorbing contaminants between streams and streambeds in the presence of colloidal particles

Considerable stream-subsurface exchange can be induced by the interaction of stream flow with bedforms and other topographical features on the streambed. Colloidal particles have a large reactive surface area that allows them to carry pollutants. Therefore, the fate of many pollutants depends to a large extent on the interfacial flux of colloids and contaminants to the streambed and the interactions of both types of substances with the bed sediment. In this study, a fundamental process-based stream-subsurface exchange model was developed to predict the coupled transport of colloids and contaminants. Laboratory flume experiments were conducted to demonstrate net contaminant transport behavior and to test model predictions using various combinations of silica, clay, and hematite colloids and sorbing contaminants including zinc, copper, and phosphate ions.; Model simulations indicate that two scenarios can occur when colloids have a significant effect on contaminant transport in streams: (1) Interaction of surface-active contaminants with mobile colloids tends to increase contaminant mobility. This is the case of “colloid-facilitated contaminant transport”; (2) sorption of contaminants to colloids that have a strong interaction with bed sediments tends to decrease contaminant mobility. We term this “colloid-impeded contaminant transport”. For both cases, higher colloid filtration results in more contaminants retained in the bed. However, increasing contaminant adsorption to colloids causes more contaminants to occur in a mobile suspended phase for the case of “colloid-facilitated contaminant transport”, and more contaminant retention in the bed for the case of “colloid-impeded contaminant transport”. When the interaction of contaminants with colloids is very strong, the contaminant movement will completely depend on the movement of colloids.; Flume experiment results show that more reactive colloids, smaller colloids, and higher colloid concentrations all can result in stronger effects of colloids on the transport of contaminants. Model predictions generally agree well with the flume experiment results, indicating that the model does a good job in representing the fundamental physicochemical processes that control the contaminant exchange in the presence of colloids. The model is very sensitive to the colloid settling and the adsorption of contaminants to colloids.