The Department of Defense (DoD) has hundreds of sites contaminated with chlorinated solvents, which represents a large remediation liability. 1,4-Dioxane is increasingly recognized as a challenging contaminant at sites where 1,1,1-trichlorethane (1,1,1-TCA) was released to soil and groundwater. Dioxane can form persistent plumes that require ongoing treatment. While these large plumes may contain relatively low concentrations of 1,4-dioxane (e.g., less than 100 µg/L), sites with concentrations greater than the health-based drinking water standards continue to involve active remediation. Although no federal drinking water standards have been established to date, the US Environmental Protection Agency has established an “action level” of 3 µg/L for 1,4-dioxane. State drinking water guidance limits have been put in place by various states which include values as low as 0.25 µg/L in New Hampshire.

1,4-Dioxane is not easily treated. Ex situ advanced oxidation processes (AOPs) are the most developed approach for 1,4-dioxane treatment. Because of high operation and maintenance (O&M) costs associated with AOPs, successful deployment of in situ approaches would grant remedial project managers a far more flexible and cost effective remedial approach. Traditional in situ chemical oxidation (ISCO) is not a solution to persistent plumes because the reactants are relatively short‑lived.

The overall objective of this project was to demonstrate the use of slow-release chemical oxidants to destroy 1,4-dioxane and chlorinated volatile organic compounds (CVOCs) in groundwater in situ. Specific quantitative performance objectives concerned effectiveness; sustainability/longevity; and oxidant transport, which were met.

Slow-release chemical oxidant cylinders match the contaminant destruction rate to the contaminant transport rate with a sustainable, simple, and low O&M approach. Chemical oxidant (i.e., sodium persulfate or potassium permanganate or mixtures thereof) embedded in a slow-release wax formulation “cylinder” can be emplaced in groundwater wells, using a funnel and gate (F&G) configuration, permeable reactive barrier (PRB), or directly installed into boreholes. The oxidant/paraffin mixtures were designed to allow oxidant to gradually diffuse into the groundwater and slowly oxidize dioxane and CVOCs. They are slowly consumed and persist sufficiently to result in 1,4-dioxane destruction as a dilute plume migrates through the treatment zone created by these cylinders.

The demonstration was conducted at Naval Air Station North Island Operable Unit 11 with the system consisting of two boreholes containing sodium persulfate cylinders without any activators. A pump was used to extract groundwater and to promote a controlled hydraulic aquifer because of the low ambient gradient at the site. The extracted water was then injected into a downgradient reinjection well. Samples upgradient and downgradient of the oxidant cylinders were collected and analyzed to evaluate technology performance. A treatability study was also conducted prior to the field demonstration.

Maximum contaminant destructions (99.3% and 99.0% for 1,4-dioxane and total CVOCs, respectively) exceeded the performance objective of 90%. The upgradient 1,4-dioxane and total CVOC concentrations were 20,000 µg/L each. The downgradient 1,4-dioxane concentration was 140 µg/L. 1,1-DCE was reduced from 7,600 to < 33 µg/L; 1,1-DCA was reduced from 2,200 to 110 µg/L; cis-1,2-DCE was reduced from 7,900 to 75 µg/L; and TCE was reduced from 2,700 to 15 µg/L. Sodium persulfate concentrations decreased in an exponential pattern over time with 31% and 9% predicted to be remaining after 6 and 12 months, respectively. 1,4-Dioxane and CVOC removals were ≥ 99% after 119 days, corroborating high contaminant destruction for extended time periods even when oxidant concentrations may be variable or declining. Thus, the criterion of contaminant destruction effectiveness being maintained for greater than 4 weeks was exceeded.

Capital and operating costs were estimated for a hypothetical site approximately 400 ft in length and 100 ft in width, with a treatment thickness of 20 ft ranging between 20 and 40 feet below ground surface (bgs), a 1,4-dioxane concentration in groundwater of approximately 10,000 µg/L, and a groundwater velocity of approximately 5 feet per day (ft/day). Various remediation scenarios were evaluated. A PRB with persulfate cylinders had a total project cost of $2.9 million, which is less than an F&G $3.7 million. It was also less than AOP at $4.3 million and a PRB with periodic manual injection of the same mass of aqueous sodium persulfate at $6.2 million. The results indicate that persulfate cylinders in a passive PRB configuration may potentially result in significant cost saving over traditional approaches.

Technology selection should keep in mind the intended use of slow-release oxidant cylinders – passive and long-term treatment of contaminated groundwater. Applicable contaminants include those that are capable of being oxidized by chemical oxidants that are released by the oxidant cylinders. At this site, 1,4-dioxane was demonstrated to be oxidized by unactivated persulfate. It may or may not be oxidized at sufficient rates at other sites and engineering, treatability, or pilot studies should be conducted. Other contaminants including CVOCs and petroleum hydrocarbons, such as benzene and methyl tertiary butyl ether (MTBE), are also potentially applicable.

Other technologies that should be considered are pump and treat and in situ bioremediation. The technology selection process conducted as part of a feasibility study will consider effectiveness, implementability and cost. The most common applications include passive PRBs or F&G systems as alternates to pump and treat. The ultimate goal of utilizing the slow-release oxidant cylinders should be to treat the aquifer rather than water in monitoring wells. Therefore, consideration should be made prior to deploying the oxidant cylinders in monitoring wells.

Design of a remediation system using slow-release oxidant cylinders must consider cylinder spacing; changeout frequency; groundwater velocity; contaminant plume width, depth, and length; reaction kinetics of the released oxidant with target contaminants as well as natural oxidant demand in the aquifer; the potential for density driven flow; and the optimal configuration (e.g., PRB vs. F&G). The oxidant cylinders are commercially available off the shelf from Carus Corporation. Equipment for suspending cylinders in wells or reactive gates are not standardized and will require engineering design and possible custom fabrication.

Evans, P.J., P. Dugan, D. Nguyen, M. Lamar, and M. Crimi. 2019. Slow-release permanganate versus unactivated persulfate for long-term in situ chemical oxidation of 1,4-dioxane and chlorinated solvents. Chemosphere, 221:802-811.