The objective of this project was to demonstrate and validate activated carbon (AC) in situ wetland remediation technologies that have been designed to sequester contaminants in wetlands without adversely impacting the ecology of these systems. Remediation of wetlands soils impacted by contamination presents unique challenges because of the desire to preserve hydric soil structure and the presence of sensitive ecological receptors. Traditionally, wetland remediation has relied on physical removal and off-site disposal of hydric soils, which can destroy habitat and create restoration challenges. This project demonstrated a less aggressive, more sustainable, and cost-effective remediation approach.
The field demonstration was performed at Canal Creek, Aberdeen Proving Ground (APG), Maryland. Specific objectives included: (1) evaluate the ability of AC to reduce the bioavailability of (and risks associated with exposure to) polychlorinated biphenyls (PCBs) in wetland habitats at the Canal Creek site using a variety of AC delivery systems; (2) provide cost and performance data; (3) obtain regulatory agency and trustee acceptance; and (4) generate and disseminate lessons learned. Performance objectives were generally achieved, although there is some uncertainty in the final results.
The in situ remediation technology evaluated in this project used engineered sequestration agents containing AC to reduce the bioavailability and toxicity of PCBs in hydric soils. Sequestration agents were mechanically deployed over the surface of a wetland and allowed to naturally integrate into the surface layer of the hydric soil through natural mixing processes (i.e., bioturbation, tidal cycles, root mixing, etc.). Incorporation of sequestration agents into the biologically active zone (BAZ) increases the partitioning of PCBs to the bulk phase and limits PCB bioavailability to benthos.
The field demonstration monitored the performance of three potential AC remediation technologies: two pelletized AC products (AquaBlok® and SediMite™), a powder activated carbon (PAC) slurry (referred to as the Slurry Spray), and an engineered manufactured soil cover system (referred to as the Sand control). Untreated control plots (Control) were used for comparative control purposes. The goal of this approach was risk reduction, not mass removal; therefore, performance was gauged through post-treatment evaluation of reduction in PCB bioavailability.
The efficacy of the technologies on the sequestration of PCBs was assessed via evaluations of PCB pore water and tissue residue concentrations (pre- and post-treatment, and relative to control plots). In addition, the partitioning of PCBs from hydric soils to pore water, and from hydric soil to benthic macroinvertebrate tissue was also evaluated. Ecological monitoring was conducted to assess the extent to which the treatment technologies impacted wetlands vegetation and benthic macrofauna. The uptake of nutrients by plants was also measured for each of the treatment types.
Remediation effectiveness was assessed by measuring changes in the bioavailability of PCBs and bioaccumulation of PCBs through pore water sampling and laboratory bioaccumulation testing. Average concentrations of PCBs in pore water generally decreased following treatment within the Slurry Spray and AquaBlok® treatment plots; however, only the post-treatment results for AquaBlok® were statistically lower than pre-treatment levels. AquaBlok® and Slurry Spray post-treatment pore water concentrations were statistically significantly lower than the post-treatment Control plots. PCB concentrations in benthic tissue generally decreased following treatment within the Sand control, Slurry Spray, and AquaBlok® treatment plots, but only the post-treatment results for AquaBlok® were statistically lower than the pre-treatment levels. Post-treatment tissue concentrations from all four treatments were also arithmetically lower than the post-treatment Control, but only the AquaBlok® and Slurry Spray results were statistically lower than the post-treatment Control plots.
No adverse effects were observed on the benthic infaunal population at the demonstration site, although ecological conditions were such that this metric provided only limited data. No adverse effects on plant community composition or nutrient uptake were observed.
Cost performance analysis suggests that remedial costs typically would range from $60,000/acre to $200,000/acre, which may be 20% to 60% less, on average, than more aggressive remedial approaches.
While the overall project findings suggest that additions of AC can sequester PCBs, the field demonstration results were not conclusive in demonstrating effective reductions in bioavailability. The results of the field demonstration indicate that additional monitoring may be necessary to demonstrate that in situ active remediation by AC can be effective in sequestering hydrophobic organic compounds in contaminated wetland sediments.
The technologies evaluated are cost-effective, and equipment to deploy amendment products in wetland settings is readily available and easily adapted to the task. Challenges in technology delivery were noted during cold weather.
The in situ technologies are best suited for use in wetland habitats where: habitat disruption should be minimized; desirable flora or fauna might be harmed by traditional remedial excavation methods; the cost of excavation and disposal are not commensurate with the level of risk reduction desired; and access to the wetland system (e.g., infrastructure improvements) for sequestration delivery and long-term monitoring are not cost-prohibitive.