Effect of Sediment Gas Voids and Ebullition on Benthic Solute Exchange.

The presence of free gas in sediments and ebullition events can enhance the pore water transport and solute exchange across the sediment-water interface. However, we experimentally and theoretically document that the presence of free gas in sediments can counteract this enhancement effect. The apparent diffusivities (Da) of Rhodamine WT and bromide in sediments containing 8-18% gas (Da,YE) were suppressed by 7-39% compared to the control (no gas) sediments (Da,C). The measured ratios of Da,YE:Da,C were well within the range of ratios predicted by a theoretical soil model for gas-bearing soils. Whereas gas voids in sediments reduce the Da for soluble species, they represent a shortcut for low-soluble species such as methane and oxygen. Therefore, the presence of even minor amounts of gas can increase the fluxes of low-soluble species (i.e., gases) by several factors, while simultaneously suppressing fluxes of dissolved species.