AN EXPERIMENTAL STUDY OF BUOYANCY-DRIVEN PENETRATION OF A PASSIVE TRACER FROM CHANNEL FLOW AND INTO AN UNDERLYING POROUS BED

Mass exchange between stream channel flow and an underlying bed of porous sediment exerts control on transport rates and fate of contaminants in a watershed. Specifically, buoyancy-driven penetration of relatively dense flows into beds with relatively fresh porewater is a potentially important but as yet poorly constrained process of mass exchange at the sediment-water interface active in estuarine environments during tidal incursions and in rivers during high discharges when flow are heavily-laden with suspended fine sediment. We report data from laboratory experiments designed to explore the balance between turbulence-driven and buoyancy-driven mass exchange between flow and underlying bed in a new re-circulating, paddle-driven flume with a porous, initially-freshwater bed of 1.5-mm diameter glass beads. In experiments using different speeds flow velocities and initial salinities, we find that i) visual assessment of tracer dye penetration through the transparent flume sidewall effectively tracks the penetration front of salt solution into a porous bed, and ii) a straightforward dimensional analysis of the processes under consideration successfully collapses our observations in an efficient, informative and usable form. Key findings of this analysis include i) that under conditions of relatively neutral buoyancy, rates of tracer penetration into a porous bed are proportional to bed shear stress and diminish with depth in the bed, and ii) 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-1 and overlying a porous, 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. In addition to the environmental implications of our findings, we note that some previous experimental studies looking at mass exchange at the sediment-water interface have used salt as a passive tracer, and thus may have included unintended and unaccommodated buoyancy effects.