Many groundwater plumes containing 1,4-dioxane and chlorinated solvents are not being treated to unrestrictive use and require ongoing management. Passive technologies that are capable of providing low-cost and sustained plume management are a pressing need. Slow-release chemical oxidant cylinders are a potential solution to this challenge. Wax cylinders containing sodium persulfate, potassium permanganate, or a mixture of these two oxidants, can be used to intercept a groundwater contaminant plume as a permeable reactive barrier or in a funnel and gate configuration. However, questions exist regarding the effectiveness of this technology. For example, it is not well documented whether the technology is capable of effectively reducing concentrations of 1,4-dioxane and chlorinated solvents in groundwater. Additionally, engineering design parameters such as minimum cylinder spacing are poorly understood.
The Department of Defense Environmental Security Program (ESTCP) funded CDM Smith to conduct a demonstration of this technology and document its cost and performance. The demonstration was conducted at Operable Unit 11 at Naval Air Station North Island, San Diego, California. At this location 1,4-dioxane and chlorinated solvents are present downgradient of the source area at concentrations approaching 10 mg/L each. The demonstration involved simulating a PRB, comparing technology performance to pre-established quantitative performance objectives, and comparing technology cost to various alternatives including pump and treat.
Results demonstrated that use of sodium persulfate oxidant cylinders promoted destruction of 1,4-dioxane and chlorinated solvents (1,2-dichloroethene, 1,1-dichloroethane, cis-1,2-dichloroethene, and trichloroethene) by more than 99% compared to a goal of 90%. Addition of activators was not required to promote chemical oxidation by sodium persulfate. The cylinders continued to release oxidant for at least 6 months and up to one year. Groundwater geochemistry downgradient of the reactive zone returned to natural upgradient conditions. Various cost-scenarios were evaluated and demonstrated the lifecycle cost of the technology was less than pump and treat using advance oxidation or manual periodic injection of aqueous chemical oxidant solutions even when cylinder spacing was as close at 2.5-feet. This study represents the first systematic evaluation of slow-release chemical oxidant cylinders and demonstrated the technology is capable of cost-effective passive plume management.
Dr. Pat Evans, CDM Smith