Predicting the performance of activated carbon-, coke-, and soil-amended thin layer sediment caps

In situ capping manages contaminated sediment on-site without creating additional exposure pathways associated with dredging, e.g., sediment resuspension, and potential human exposure during transport, treatment, or disposal of dredged material. Contaminant mass is not immediately removed in sediment capping, which creates concerns over its long-term effectiveness. Groundwater seepage can also decrease the effectiveness of in situ capping. This study compares the effectiveness of commercially available sorbents that can be used to amend sand caps to improve their ability to prevent contaminant migration from the sediments into the bioactive zone. Amendments evaluated include coke, activated carbon, and organic-rich soil. The properties relevant to advective-dispersive transport through porous media (sorption, porosity, dispersivity, and bulk density) are measured for each material, and then used as inputs to a numerical model to predict the flux of 2,4,5-polychlorinated biphenyl (PCB) through a sand cap amended with a thin (1.25-cm) sorbent layer. Systems with and without groundwater seepage are considered. Isolation times provided by the sorbent layers increased with increasing sorption strength and capacity (activated carbon≫coke≈soil≫sand). The effective porosity, dispersivity, and bulk density of the sorbent layer had little effect on cap performance compared to sorption strength (K_f). In the absence of seepage, all sorbents could isolate PCBs in the underlying sediment for times greater than 100years and would be effective for most cap applications. With groundwater seepage (Darcy velocity=1cm/day), activated carbon was the only sorbent that provided contaminant isolation times greater than 60years. Long isolation times afforded by sorbent-amended caps allow time for inherently slow natural attenuation processes to further mitigate PCB flux.