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

Abstract

Abstract. Fine sediment exchange between a stream and the surrounding subsurface influences downstream contaminant transport and stream ecology. Fundamental models for this exchange were developed on the basis of (1) the hydraulics of bed form-driven advective pore water flow and (2) subsurface colloid transport processes. First, a model was developed to predict the advective flow induced in a sand bed by stream flow over bedforms. The resulting "pumping" exchange rate was calculated based on the streamflow conditions, bed form geometry, and bed depth. The pumping exchange of suspended sediment was then calculated by superimposing advective transport and particle settling in the bed and including the effect of physicochemical filtration by bed sediment. The filtration coefficient approach was used to predict the reduction in the concentration of transported particles. Both settling and filtration cause colloids to be trapped in stream beds, producing a higher net exchange rate relative to conservative solutes. When transported particles are completely trapped in a single pass through the bed, the exchange calculation is 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 effective piston velocity (flux/concentration) or loss rate to the bed in the advectiondispersion equation for the stream. Solute and colloid exchanges are predicted by the models without the use of fitting coefficients; only measurable hydraulic and particle parameters were used as model inputs. Simulations are presented which show the effect of stream parameters, settling, and filtration on net particle exchange. This fundamental approach to modeling stream-subsurface exchange potentially has great utility for understanding and predicting the transport and fate of reactive substances in streams.

Hyporheic exchange of solutes and colloids with moving bed forms

“…This paper will present a new model for fine particle exchange with a sand bed covered by moving dunes and ripples, i.e., for the case where stream velocities are sufficiently high to produce bed-load sediment transport with downstream motion of dune-shaped bed forms. This extends the range of applicability of the models presented in our previous work [Packman et al, 2000a]. We also report the results of flume experiments with moving bed forms, and demonstrate that the model yields good predictions of exchange when the bed form celerity is sufficiently high.…”

“…Colloid filtration is the physicochemical process whereby suspended particles become attached to stationary sediment grains in a porous medium. In flume experiments we observed that particle settling and filtration in the bed caused suspended clay to behave very nonconservatively [ We also previously observed that pumping transport of colloids to the deep bed will often result in nearly all colloids becoming trapped [Packman et al, 2000a[Packman et al, , 2000b. Thus it will also be assumed that exchanged particles are completely removed from suspension when they penetrate below the zone of active bed sediment transport.…”

“…Streamlines for this velocity distribution are given by Packman et al [2000a]. The dynamic head variation depends on the stream velocity, bed form height, and stream depth.…”

“…We previously presented a model for the pumping exchange of suspended particles [Packman et al, 2000a], which will serve as the basis for our new combined pumping/turnover model for particles. The colloid pumping model superimposes particle transport behavior on basic advective pumping.…”

“…The goal of our work is to clarify the processes that control the delivery of fine sediments to the streambed and the mechanisms for colloid capture, retention, and release by sand-sized bed sediments. In previous publications we illustrated that stream-subsurface exchange processes can produce trapping of fine particles in the streambed [Packman and Brooks, 1995], outlined the methodology needed to conduct controlled colloid-transport experiments in recirculating flumes [Packman et al, 1997], presented a model for fine particle capture due to stationary bed forms [Packman et al, 2000a], and showed that the model could predict the results of flume experiments on kaolinite exchange with a stable sand bed in a laboratory flume [Packman et al, 2000b]. This paper will present a new model for fine particle exchange with a sand bed covered by moving dunes and ripples, i.e., for the case where stream velocities are sufficiently high to produce bed-load sediment transport with downstream motion of dune-shaped bed forms.…”