PORE-WATER EXCHANGE AT THE SEDIMENT-WATER INTERFACE

Interaction between a stream-bed and overlying channel flow affects rates and depths of hyporheic contaminant transport. To better understand roles of sediment properties and mobility, among other factors, in these processes, we conducted experiments in a re-circulating flume examining penetration of a dye tracer into a porous bed at velocities ranging from 10 cm/s to 70 cm/s. The porous bed comprised 1.5-mm diameter glass beads and, in selected experiments, was covered with a 1-mm mesh to prevent development of bedforms, even when flow speed was sufficient to mobilize the bed. Dye was introduced to and instantaneously mixed within a flow of known average speed and turbulence intensity. Visual measurements of the depth of dye penetration into the porous, initially clear-water porous bed with time were then made through the sidewall of the flume. In selected experiments, independent assessments of contaminant penetration into the bed using in-bed lysimeters and changes in dilute salt concentration validated the more expedient approach of visual assessment of dye exchange. Related experiments were undertaken to examine the importance of turbulence-driven versus buoyancy-driven exchange between an initially fresh-water bed and an overlying salt-laden flow. Upon comparing our results with those of previous studies we find that i) at all velocities, and under nominally flat and rippled, mobile beds alike, the dye penetration front exhibits an irregular, lobate geometry, ii) average pore-water exchange at the sediment-water interface scales predictably with turbulence intensity, bed grain size and bed porosity, and iii) that the presence of significant, unstable buoyancy under environmentally-relevant conditions (initial salinities of up to 10 ppt in flows of up to 30 cm/s and overlying an initially freshwater bed) result in bed-penetration rates of passive tracer that are enhanced up to ten-fold over the rate observed for analogous, neutral buoyancy conditions.