Theme: 01. Remediation Technology Innovations

Topic Area: 01f. Emerging Remediation Technologies

Formatted Title:

Principles and Novel Application of Carbon Sequestration for Treatment of Dioxins in a Wetland

Background/Objectives:

Polychlorinated dibenzo-p-dioxins (PCDDs, “dioxins”) are a common class of industrial pollutants. Dioxins bioaccumulate in the food web and sorb strongly to soil organic matter and activated carbon. Sequestration of dioxins in activated carbon applied to sediment or soil limits dioxin bioavailability and is an emerging treatment to stabilize dioxins in situ and reduce ecological risk. Commercial products and formulations are designed for specific applications, such as sequestration of dioxins in submerged sediments. Powdered or granular activated carbon alone can be a viable treatment for soils, but distribution and mixing in soil is a design challenge, and application methods can damage soil, vegetation, and habitat.

Dioxins in stormwater runoff contaminated a 14-acre wetland in a side channel of the Columbia Slough near Portland, Oregon. The silt-loam soil is seasonally flooded and commonly wet. Grasses and shrubs provide high-value habitat for a variety of mammals, amphibians, birds, reptiles, and invertebrates. Dioxin TCDD Toxicity Equivalency Quotient concentrations in 13 sampling units ranged from 165 to 4,580 nanograms per kilogram (ng/kg), indicating unacceptable risk to ecological receptors. Dioxin concentrations in soil samples from 12 to 18 inches below the ground surface (bgs) were 10 to 100 times lower than in shallower samples from 0 to 6 inches bgs, indicating limited vertical mobility of dioxins. The objectives of the full-scale project were to reduce ecological risk while preserving the wetland.

Approach/Activities:

This project demonstrates an innovative regulatory approach and a novel and cost-effective field-scale method to land-apply activated carbon to treat dioxins in soil. ERM conducted a site investigation, risk assessment, and feasibility study, and implemented an innovative remedy consisting of spray application of granular activated carbon to the wetland soil.

The level II risk assessment developed site-specific risk-based cleanup levels to protect ecological receptors. The feasibility study selected land-application of activated carbon slurry as the primary cleanup alternative. The feasibility study considered several commercial activated carbon products. Literature review and collaborations with the Oregon DEQ identified a target 3 percent dry weight application of activated carbon in the upper 6 inches of soil where dioxin concentrations were highest and ecological exposures would occur.

ERM engineered a novel slurry-based delivery system to spray-apply activated carbon to the wetland surface. Bioturbation incorporates surface-applied activated carbon to the target 6-inch depth. Although carbon isotherms indicate that a small volume of carbon could sorb the dioxin mass, 64,000 pounds of activated carbon were applied to attain the 3 percent dry weight application criterion to achieve contact.

Results/Lessons Learned:

Field observations and calculations demonstrated application of granular activated carbon at a minimum 3 percent dry-weight application rate. Given adsorption theory and the literature review in the feasibility study, the remediation work plan specified uniform distribution of activated carbon at the 3 percent loading rate and passive mixing of the activated carbon into the soil profile as the only performance criteria. Sampling of dioxin in porewater and additional biological sampling were not required. Performance monitoring demonstrates that bioturbation has distributed the activated carbon into the soil profile. Regulatory closure of the site is underway. Collaboration and knowledge sharing with the agency were instrumental to the project success.

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Christopher Berg