Formatted Title
Identification and Enhancement of Naturally-Occuring In Situ Aerobic Metabolic Biodegradation of 1,4-Dioxane
Background/Objectives
Shallow groundwater at a former warehouse was impacted by 1,4-dioxane and volatile organic compounds (VOCs) from wastewater discharges to a septic tank and associated leach field during pack parts cleaning operations. Historical concentrations of 1,4-dioxane at the Site were reported up to 60,000 micrograms per liter (µg/L). A phytoremediation pilot study and source mass removal activities had reduced the maximum 1,4-dioxane concentration to 11,000 µg/L in 2021; however, a pilot study expansion is warranted to increase the rate of contaminant concentration decline and the degree of contaminant mass flux reduction at the property line. 1,4-Dioxane is a probable human carcinogen, is an emerging contaminant in groundwater at many military and industrial contaminated sites, and currently has limited in situ remedial options. Numerous laboratory and pilot-scale studies provide evidence that 1,4-dioxane can undergo aerobic metabolic biodegradation; therefore, in situ bioremediation options were evaluated.
Approach/Activities
To support the development of future additional remedial options for the Site, natural attenuation processes were first examined. It was determined that the naturally-occurring microbial population, Pseudonocardia dioxanivorans BERK-1, in the Site aquifer is capable of degrading 1,4-dioxane in situ if sufficient oxygen is present (where dissolved oxygen [DO] is greater than approximately 1 milligram per liter [mg/L]). In addition, observation of degradation process in laboratory systems and isotope enrichment downgradient of the source areas in compound specific isotope analyses (CSIA) indicate the occurrence of natural degradation of 1,4-dioxane at the site. A data gap investigation was performed to evaluate vertical and spatial connections of 1,4-dioxane concentrations and aquifer characteristics.
Results/Lessons Learned
The results of the data gap investigation provide evidence that naturally-occurring aerobic microbiological destruction of 1,4-dioxane is occurring in the shallow, oxic portion of the aquifer but not in the deeper, anoxic zone. The confirmed relationship between aerobic geochemical conditions and lower 1,4-dioxane concentrations indicated that introduction of atmospheric air to the subsurface will increase DO, stimulate biodegradation, and accelerate the destruction of 1,4-dioxane. Pilot study expansions in the second half of 2023 have included installation of a novel engineered aeration trench at the source area and a downgradient TreeWell® phytoremediation system. Preliminary testing of the aeration trench system is anticipated for early 2024, with full-scale startup to follow. Cellular-connected environmental monitoring systems providing oxidation-reduction potential and water level data will be utilized for real-time system monitoring. Data from initial system operations will be presented at the conference.