AN EXPERIMENTAL STUDY TO DETERMINE THE RATE AND DEPTH OF PORE-WATER AND PASSIVE-CONTAMINANT EXCHANGE BETWEEN A POROUS GRANULAR BED AND OVERLYING CHANNEL FLOW

To better resolve the controls on rates and depths of penetration of solutes and fine particle-bound contaminants into relatively coarse-grained streambeds, we conducted experiments in a paddle-driven, re-circulating flume to examine the penetration of a passive dye tracer into an initially flat, porous bed at velocities ranging from 10 cm s-1 to 70 cm s-1. The porous bed comprised 1.5-mm diameter glass beads, and in a third of the experiments the bed was covered with a 1-mm mesh to prevent bedforms from occurring, even when flow velocity was sufficient to mobilize the bed. Dye was initially introduced into a flow of known average speed and turbulence intensity. Subsequently, visual measurements of the depth of dye penetration with time were made through the sidewall of the flume. Independent measurements of penetration depths and rates using in-bed lysimeters and changes in another tracer (salt) in selected experiments validated the more expedient approach of visual assessment of dye penetration. Upon comparing our results with those of previous studies we find that i) at all velocities, the dye penetration front comprises irregular lobes and clefts, with lobes typically developing and moving fastest under minor topographic highs of the bed, ii) solute and passive-contaminant infiltration to shallow depths in a stationary bed is well described on average by a diffusive exchange rate that scales predictably with turbulence intensity, bed grain size and bed porosity, and iii) this exchange rate is diminished in mobile beds relative to that expected for stationary beds of identical grain size and subject to similar total shear stress.