Assessment of Contaminant Loss from Proposed Lower New Bedford Harbor CAD Cell

摘要

A contaminant-loss assessment, including both short-term losses (prior to capping) and long-term losses (following capping), was performed for a proposed 650 ft × 650 ft × 47 ft confined aquatic disposal (CAD) cell to contain 335,000 cubic yards of sediment in lower New Bedford Harbor. Short-term losses result from displacement of CAD cell water contaminated by resuspension and stripping of mechanically dredged material during placement from a split hull barge, release of contaminated porewater by consolidation of the dredged material, diffusion from the exposed dredged material, and turbulent diffusion of contaminated CAD cell water. Long-term losses result from diffusion from the capped dredged material and consolidation of the dredged material. Shortterm contaminant losses resulting from placement operations are predicted to be about 0.08% of the total PCBs and 0.03% of the total copper placed in the CAD cell. Capping with a 3-foot sand layer is sufficient to provide long-term isolation of the water column from the contaminants in the dredged sediment. Without consideration of burial, contaminant breakthrough through the cap at a concentration of 0.01% of the in situ porewater contaminant concentration is predicted to take 1800 years.