Formatted Title
Sequential Anaerobic and Aerobic Bioaugmentation for Commingled Groundwater Contamination of Trichloroethene and 1,4-Dioxane
Background/Objectives
Chlorinated solvents, notably trichloroethene (TCE), and the cyclic ether stabilizer 1,4-dioxane (dioxane), have been frequently detected commingling in contaminated aquifers. With the discovery of reductive dehalogenation, anaerobic bioremediation has emerged as a feasible and economical alternative for in situ treatment of chlorinated solvents, particularly TCE. However, two issues have been frequently reported at sites where anaerobic bioremediation is implemented, underscoring the need for effective solutions. First, once TCE is reduced, dehalogenation of cDCE and VC occurs at a slower pace in the field, conducive to the prolonged occurrence of these toxic degradation byproducts. The other issue is the concurrence of trace levels of dioxane, an anthropogenic cyclic ether used for stabilizing chlorinated solvents. In this present study, we designed and demonstrated a sequential treatment strategy that can effectively reduce TCE first by SDC-9 under anaerobic conditions and then oxidize dioxane and other persisting cVOCs by Azoarcus sp. DD4 under aerobic conditions.
Approach/Activities
Groundwater and bedrock core samples were collected from a site located in central New Jersey in April 2017. This site was operated by a gas company and has been historically impacted by TCE contamination in a deep bedrock aquifer up to 61 m below the ground surface (BGS). Four anaerobic treatments were prepared, including (1) killed control (I-KC), (2) live control (I-LC), (3) bioaugmentation with SDC-9 (I-SDC), and (4) bioaugmentation with SDC-9 and sulfate amended (I-SDC-SO4). After the removal of TCE, two sets of anaerobic treatments, bioaugmentation of SDC-9 without amendment of sulfate (I-SDC) and bioaugmentation of SDC-9 with the amendment of sulfate (I-SDC-SO4), were selected for sequential treatment of dioxane via aerobic cometabolism by DD4.
Results/Lessons Learned
Here we developed a sequential anaerobic and aerobic treatment strategy effective to mitigate the co-contamination of TCE and dioxane, particularly when dioxane is present at ppb levels relevant to many impacted sites. After the primary anaerobic treatment by a halorespiring consortium SDC-9, TCE was effectively removed, though lingering less-chlorinated metabolites, vinyl chloride (VC) and cis-dichloroethene (cDCE). Subsequent aerobic bioaugmentation with Azoarcus sp. DD4, a cometabolic dioxane degrader, demonstrated the ability of DD4 to degrade dioxane at an initial concentration of 20 μg/L to below 0.4 μg/L and its dominance (~7%) in microcosms fed with propane. Even better, DD4 can also transform VC and cDCE in tandem, though cDCE and VC at relatively high concentrations (e.g., 1 mg/L) posed inhibition to propane assimilation and cell growth of DD4. Mutagenesis of DD4 revealed group-2 toluene monooxygenase and group-5 propane monooxygenase are responsible for cDCE and VC co-oxidation, respectively. Overall, we demonstrated the feasibility of a treatment train combining reductive dehalogenation and aerobic co-oxidation processes in tandem to not only effectively clean up prevalent co-contamination of TCE and dioxane at trace levels but also mitigate persistent products (e.g., cDCE and VC) when complete reductive dehalogenation of less-chlorinated ethenes occurs slowly in the field.