Effects of Gas Ebullition on Cohesive Sediment Resuspension and Cap Stability

Sediment resuspension can significantly contribute to the mass transport of sediment-bound contaminants into the overlying water column. This study considers the influence of gas ebullition (due to microbial activity) on resuspension rates as observations indicate that fine-grained sediments may be lifted into the water column in the bubble wakes. Capping aims to minimize contaminant migration to the water column by reducing sediment resuspension rates. In addition to the influence of gas fluxes on resuspension rates for uncapped cohesive sediment beds, this experimental investigation considers the effectiveness of two types of caps, sand and AquaBlok®. The investigation involved experiments conducted on sediments (d₅₀=10 μm) collected from the Anacostia River in Washington DC. Gas ebullition fluxes were selected based on values reported in the literature. The experimental results for the zero gas flux case indicated that the resuspension increases as the square of the bed shear stress level above a threshold or critical shear stress required to initiate resuspension. With regards to the ebullition experiments, it was observed that air migration occurs as a series of discrete bubbling events through semi-permanent channels in the sediment bed associated with a cyclical buildup and release of gas pressure within the sediment. A significant amount of sediment was carried in the wakes of released bubbles. When the same conditions were studied with sediment caps applied over the bed surface, no sediment resuspension was observed over the duration of the experiments with the sand cap. AquaBlok® cap heaved up and ruptured if the pressure head differential across the cap reached a level on the order of 20–25 cm.