Field Testing of Activated Carbon Mixing and In Situ Stabilization of PCBs in Sediment
Hydrophobic organic compounds such as polychlorinated biphenyls (PCB) and dichlorodiphenyltrichloroethane (DDT) associate with fine-grained, organic-rich sediment material, serving as a contaminant reservoir in shallow estuarine and coastal regions from which fish and bottom-dwelling organisms could accumulate toxic compounds that may be passed up the food chain. These contaminants may be of more or less concern depending on how weakly or strongly they are sorbed to sediment organic matter. In SERDP-funded research with sediment from Hunters Point Naval Shipyard in San Francisco Bay (ER-1207), the Stanford team found that the PCBs and polycyclic aromatic hydrocarbons (PAH) tend to accumulate preferentially in coal-derived and char particles where the compounds may be strongly bound. Building on these observations, this project demonstrated a new technology for contaminated sediment management where activated carbon (AC) is mixed into sediment. In the field demonstration, large-scale equipment was used to mix GAC into the sediment. Investigators tested whether PCBs were repartitioned into the AC by assessing the reduction in biological uptake of these compounds and reduction in their leaching into the overlying water.
Objectives of the Demonstration
Prior laboratory studies and a preliminary field pilot-scale study showed that the addition of AC to sediment contaminated with PCBs significantly reduced the chemical and biological availability of PCBs. The objective of this project was to demonstrate and validate an innovative treatment for in situ stabilization of PCBs in sediment under field conditions at Hunters Point Naval Shipyard. Specific objectives included the following:
- Demonstrate and compare the effectiveness, in terms of AC application and ease of use, of two large-scale mixing technologies.
- Demonstrate that AC treatment reduces PCB bioaccumulation in field tests.
- Demonstrate no significant sediment resuspension and PCB release after the large-scale mixing technologies are used.
Using two commercial equipment devices, AC was successfully incorporated into test plots to a nominal 1 ft depth at a dose of 2 to 3% depending on sampling locations. This was verified by the increases in total organic carbon and black carbon contents in AC-amended sediment. In situ 28-day semi-permeable membrane device (SPMD) uptake studies showed 50-66% reductions in PCB uptake in AC-amended test plots depending on AC dose. In situ bioassays with the bent-nosed clam, Macoma nasuta, also showed the effectiveness of AC treatment, although the in situ bioassay results were sometimes confounded by field conditions resulting from newly deposited sediment, heat stress, and shallow burrowing depth. To overcome these factors, ex situ bioassays with M. nasuta were conducted with field sediment in the laboratory, which showed about 50% reduction in PCB bioaccumulation with a 2% AC dose.
Field-exposed AC retained a strong stabilization capability to reduce aqueous equilibrium PCB concentrations by as much as 95%, depending on AC dose, for the duration of the 18-month field study. This was demonstrated also in long-term, SPMD exposure tests lasting more than 7 months. The time series test results showed the AC continually reduced SPMD uptake of PCBs, achieving reductions ranging from 76% for tetra-chloro PCBs to 42% for hepta-chloro PCBs. A strong AC-dose response effect was observed for both aqueous equilibrium PCB concentrations and M. nasuta PCB bioaccumulations. Neither PCB resuspension from the test plots nor adverse impacts to indigenous amphipods and benthic community was observed during the entire assessment period. Overall, the AC treatment did not impact macro-invertebrate benthic community composition, richness, or diversity.
This project completes the first field demonstration of sorptive amendment to sediment to reduce PCB exposure and risk. Overall, this study indicates that in situ AC amendment to contaminated sediments can provide a suitable, cost-effective method for reducing contaminant exposure to the water column and biota where the source of the PCB is from within the sediment. Additional mixing during or after AC deployment, sequential AC deployment or greater AC dose, or reducing AC particle size will improve overall effectiveness. Cost analyses of the demonstration showed that scaling up the AC treatment method at Hunters Point Naval Shipyard would result in total costs that may be 75% less than for dredging and disposal.