A physicochemical model for colloid exchange between a stream and a sand streambed with bed forms

Fine sediment exchange between a stream and the surrounding subsurface infl uences downstream contaminant transport and stream ecology. Fundamental mod els for this exchange were developed on the basis of (1) the hydraulics of bed form-driven advective pore water flow and (2) subsurface colloid transp ort processes. First, a model was developed to predict the advective flow i nduced in a sand bed by stream flow over bedforms. The resulting "pumping" exchange rate was calculated based on the streamflow conditions, bed form g eometry, and bed depth. The pumping exchange of suspended sediment was then calculated by superimposing advective transport and particle setting in th e bed and including the effect of physicochemical filtration by bed sedimen t. The filtration coefficient approach was used to predict the reduction in the concentration of transported particles. Both settling and filtration c ause colloids to be trapped in stream beds, producing a higher net exchange rate relative to conservative solutes. When transported particles are comp letely trapped in a single pass through the bed, the exchange calculation i s simplified because only the particle flux to the bed must be considered. In this case, the net exchange rate may be adequately represented by an eff ective piston velocity (flux/concentration) or loss rate to the bed in the advection-dispersion equation for the stream. Solute and colloid exchanges are predicted by the models without the use of fitting coefficients; only m easurable hydraulic and particle parameters were used as model inputs. Simu lations are presented which show the effect of stream parameters, settling, and filtration on net particle exchange. This fundamental approach to mode ling stream-subsurface exchange potentially has great utility for understan ding and predicting the transport and fate of reactive substances in stream s.