Assessment feasibility of in-situ capping and contaminant mobility in NAPL-contaminated sediments

Abstract

In-situ capping (ISC) has been successfully used in numerous sites to separate contaminants such as polycyclic aromatic hydrocarbons (PAHs) in the sediment from the overlying water. The effectiveness of ISC over extremely soft sediment contaminated with nonaqueous phase liquids (NAPLs), however, is less well understood. Such sediments exhibit low bearing capacity and may be subject to significant consolidation increasing the potential for NAPL and dissolved contaminant expression in water. A series of experiments were conducted on lagoon and surge pond sediments from a site in south Louisiana contaminated with varying degrees of NAPL to predict the ability to cap the sediments without sediment failure and with containment of the NAPL. The methodology employed may prove useful in other sediment capping projects. The results indicated that placement of a cap would result in intermixing in cap sediment interface, and penetration of NAPL and dissolved contaminants into the cap layer due to consolidation. Sediments with higher NAPL content (up to 30% NAPL) exhibited a greater degree of intermixing of sediment and cap materials and greater penetration of contaminants. High NAPL content sediments were also more likely to be resuspended by the act of capping leading to contamination of the cap layer. The study demonstrated that traditional sand capping technology is feasible to cap NAPL contaminated sediment but identified the nature and extent of problems to be expected when substantial NAPL is present.