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Regulations under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), the Resource Conservation and Recovery Act (RCRA), and their equivalents at the state level require cleanup of groundwater to strict numerical concentrations. However, current remedial technologies are often ineffective in eliminating in situ sources of contamination. Consequently, long-term containment is often required for plumes emanating from source zones. The primary challenge of long-term containment is that it can be both labor- and cost-intensive.
Pueblo Chemical Depot (PCD), Colorado, was constructed during World War II and built to serve as an ammunition and material storage and shipping center. Historical activity at PCD included demilitarization of expired munitions via washout operations conducted at Solid Waste Management Unit 17 (SWMU-17). Former washout ponds created groundwater plumes containing elevated concentrations of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and other energetic compounds and extending thousands of feet beyond the release area. In 1998, sediments associated with the former washout ponds were removed by excavation, but despite source excavation, the remaining soils are sustaining concentrations of RDX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), 2,4-dinitrotoluene (2,4-DNT), 2,4,6-trinitrotoluene (TNT), and 1,3,5-trinitrobenzene (1,3,5‑TNB) in groundwater. PCD is currently required by the Colorado Department of Public Health and Environment to clean up RDX concentrations in groundwater to less than 0.55 µg/L. Groundwater goals for other energetic compounds include 0.0885 µg/L for 2,4-DNT; 2.01 µg/L for TNT; 361µg/L for 1,3,5‑TNB; and 602 µg/L for HMX.
The focus of this demonstration was to define the viability of electrolytic reactive barriers (e-barriers) as an option for managing energetic compounds and other persistent contaminants in groundwater at U.S. Department of Defense (DoD) facilities. This effort included employing promising design improvements advanced under ESTCP project ER-200112 at PCD to develop an e-barrier as a new, low cost, containment technology.
E-barriers are founded on the principles of a permeable reactive barrier (PRB). Contaminants are carried through the reactive barrier via the natural flow of groundwater. Within the barrier, contaminants are degraded as they pass through titanium screen electrodes charged with low voltage DC current. Contaminants are sequentially exposed to electrolytic oxidation → reduction → oxidation → reduction. The primary appeal of e‑barriers has been the low power cost (cents/day/m2) and the potential to address contaminants that might otherwise be difficult to treat with existing technologies.
The e-barrier was located between two former washout ponds at PCD. Between 12 and 15 feet of sandy alluvium was encountered above the regionally extensive Pierre Shale formation. Groundwater was encountered in the lower five to seven feet of the alluvium. The average groundwater Darcy velocity was 250 ft/yr. Concentrations of RDX in groundwater have dropped from historic highs of approximately 400 μg/L to current levels of less than 10 μg/L. Similarly, concentrations of other energetic compounds have declined from past levels. The most recent data indicates concentrations of less than 1 μg/L HMX; 10-400 μg/L TNT; 10-40 μg/L 2,4‑DNT; and 300-3,000 μg/L 1,3,5-TNB.
No major problems were encountered during installation and operation of the e-barrier, and it is estimated that primary systems could be operated for a decade without replacements. Technologies that may compete with e-barriers include permeable bark mulch walls (ER-200426) and iron walls (ER-200223). Results from parallel ESTCP demonstrations suggest that these are likely to be even simpler to install and operate.
With respect to costs, e-barriers are more expensive than bark mulch walls and iron walls by a factor of three. Alternative assumptions could be employed to create a more favorable economic analysis. Unfortunately, it seems unlikely that this would lead to a scenario in which e-barriers could compete on a cost basis with bark mulch or iron walls. Combining implementation and cost results, it appears that the e-barrier’s niche is at sites where the limitations of bark mulch or iron wall would preclude their use. Considering this constraint and the limited number of RDX sites (approximately 20) identified through this project, at best there may be a handful of sites where e-barriers could be a viable technology for treating energetic compounds in groundwater.
Technology developed through advancement of the e-barrier is currently being adapted to other novel treatment technologies. These include aboveground systems for “point of use” groundwater treatment and in situ systems for oxygen delivery.