A model for contaminant and sediment transport via gas ebullition through a sediment cap

A model was developed for contaminant release from sediments as a result of methane gas ebullition. Material balances on the sediment and overlying water yielded two ordinary differential equations for the contaminant levels in each of these compartments. Integration of the model equations simulated the transient concentration in each compartment. The influence of a cap layer on the attenuation of contaminant release was also modeled. The model was verified by experimental data on a laboratory column in which gas was introduced at a steady rate into sediments. The model was used to calculate the contaminant flux into the air for both capped and uncapped sediment under simulated field conditions. It was also used to investigate the effect of total suspended sediment (TSS), gas flux, and sand cap on the contaminant flux into the air. In an uncapped sediment, the resuspension of sediment and associated contaminants dominated the flux, while in capped sediment, where direct resuspension was eliminated, a sand cap can be an effective barrier for contaminant transport from sediments, even in the presence of gas ebullition. Sensitivity analysis suggested that the cap thickness and its organic carbon content are two important parameters that determine the cap effectiveness in controlling the contaminant flux into the atmosphere.