Potential in situ approaches for remediation of contaminated sediments

Sediments that contain hydrophobic organic contaminants (HOCs), such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and metals can be toxic, may bioaccumulate in the aquatic environment, and represent a persistent threat to humans and aquatic ecosystems. The most common and in-use technologies for treatment of contaminated sediments are environmental dredging and monitored natural recovery (MNR) and/or non-reactive sand caps. Environmental dredging creates challenges including identification of disposal facilities or the design and construction of confined disposal facilities, controlling resuspension, and minimizing post dredging residual contamination. Installation of sand caps has its own set of challenges. This technology if not designed and placed correctly can cause advective flow that might result in rapid breakthrough of contaminants depending on geo-chemistry and characteristics of the sediments. To date, MNR has received limited acceptance as a solution for contaminated sediments. Implementation of in situ treatment technologies has been discussed as potential alternatives to dredging, capping, or MNR. Solidification/stabilization of sediments using deep soil mixing is one of the technologies that may prove useful for in-situ stabilization of sediments. This technique can be used as an interim remedial action prior to dredging or capping. Studies conducted on the Passaic River have shown a potential successful application for cement deep soil mixing (CDSM) for solidification/stabilization of the sediments that can result into chemical fixation of contaminants. A significant increase in sediment strength using solidification techniques can occur by using 7% cement content in CDSM slurry mixture under ambient conditions. The percentage of cement must be modified in the presence of high organic content (>20%) due to retardation effects of organics on hydration of cement mixture. Currently, in-situ treatment methods such as reactive capping technologies are under intense research for their effective potential and long term use. The reactive materials such as activated carbon, organoclay, apatite, gypsum, zeolite, zero-valent iron etc., can be used for treatment of contaminants in sediments. These reactive materials can be directly mixed into the sediment or could be used for in situ capping by covering the sediment using geotextile mat or loose reactive material. Research has been conducted to develop a multi-symptom remedy for sediments that are contaminated with both metals and HOCs. The solution includes developing a composite active capping system consisting of apatite minerals, sorbent materials (activated carbon and organoclay), all embedded within an engineered geotextile mat. The use of geotextile mat provides uniform placement of reactive material in defined proportions. These geotextile mats impregnated with reactive material are called reactive capping mats. The reactive capping mat which is thin layer cap that isolates contaminated sediment bed from the overlying surface water eliminates contaminant transport mechanisms such as bioturbation, scouring and uprooting of macrophytes and provides reactivity to reduce contamination associated with diffusive and advective flux.