Abstract:Fine sediment deposition in streambeds can reduce pore water fluxes and the overall rate of hyporheic exchange, producing deleterious effects on benthic and hyporheic ecological communities. To increase understanding of the factors that control the reduction of hyporheic exchange by fine sediment deposition, we conducted experiments in a laboratory flume to observe changes in the rates of solute exchange and kaolinite clay deposition as substantial amounts of kaolinite accumulated in the streambed. Two long-term experiments were conducted, with durations of 14 days and 29 days. Use of a laboratory flume system allowed steady stream flow conditions to be maintained throughout both experiments, and alternating injections of known quantities of kaolinite and a sodium chloride tracer were used to assess the effect of clay accumulation on hyporheic exchange directly. In the first experiment, there was no bed sediment transport and kaolinite deposition formed a highly clogged near-surface layer that greatly reduced hyporheic exchange. Application of a fundamental model for advective hyporheic exchange indicated that the effective permeability and porosity of the streambed decreased substantially during the course of the experiment. In the second experiment, the kaolinite was prepared with different surface properties to be more mobile, and the experiment was conducted with a small degree of bed sediment transport. As a result, no distinct clogged layer developed, and the rate of hyporheic exchange was found to remain approximately constant throughout the experiment (29 days). These results indicate that increasing fine sediment loads, e.g. those that occur from changes in land use, can have substantially different impacts on hyporheic exchange and associated ecological processes depending on the stream flow conditions, the rate and frequency of bed sediment transport, and the extent of interaction of the introduced fines with bed sediments.
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.
Contaminant transport in streams can be significantly modified by both stream−subsurface exchange and the presence of colloidal particles, but the interaction of these effects is not well understood. Exchange with the hyporheic zone exposes contaminants to surface−chemical reactions with streambed sediments, while colloidal particles have a large reactive surface area that allows them to carry pollutants that would otherwise be transported primarily as dissolved species. A new theoretical model is developed to predict the role of colloids in mediating advective contaminant exchange between streams and streambeds. Bedform-induced pumping theory is applied to model physical transport, and colloid filtration and reversible contaminant sorption are used to calculate the local distributions of colloids and contaminants within the streambed. Residence time functions of both colloids and contaminants in the bed are then used to link contaminant concentrations in the pore water and streamwater. Model simulations indicate that, under conditions of low colloid filtration and strong contaminant sorption to colloids, contaminants are mobilized by colloids and there is less retention of contaminants in the streambed. This is the case of “colloid-facilitated contaminant transport” commonly considered in groundwater transport. On the other hand, when colloid filtration is high and contaminants still sorb strongly to colloids, contaminant mobility decreases and there is greater contaminant retention in the streambed. We term this case “colloid-impeded contaminant transport”. Thus, we find that a variety of contaminant transport behavior can occur depending on the concentration and mobility of suspended particles in the system and the relative affinity of contaminants for colloids and other solid phases.
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