Gas Seep-Induced Solute Transport into Submerged Sediments

We investigated the downward migration of dye concentration mediated by air bubbles rising through a bed of sediment. The porous bed was composed of three samples of fire-dried quartz sands with coarse, medium and fine grains. The experimental setup is a rectangular container filled with a saturated sediment column overlaid by a dyed water layer. Air is injected centrally from the bottom of the container. The quantification of the total concentration uptake inside the sediment layer has been carried out experimentally using image processing technique. The spread of dye concentration from the overlying water into the sediment layer showed two distinct patterns, i.e. a conically developing shape for fine sand and horizontal front propagation for the medium and coarse ones. Based on the physical processes involved we introduce a novel pressure boundary condition for numerical simulation of the concentration uptake within the sediment layer that mimicked well the experimental observations. Furthermore, we obtained two empirical relations for the total concentration uptake as well as the dye propagation area as a function of the injected air. This finding plays a significant role for a quantitative estimation of downward flux of organic and particulate matter in the vicinity of gas seeps.