1,2-Dibromoethane (EDB) is biodegradable under anaerobic conditions via reductive dehalogenation by Dehalococcoides spp. and is susceptible to abiotic transformation by hydrogen sulfide (H₂S) and iron sulfide (FeS). The compound also has been reported to biodegrade aerobically in surface soils, to be susceptible to aerobic cometabolism by methanotrophic and propanotrophic bacteria, and to support the growth of a mycobacterium. Despite reports of its biodegradability, however, EDB from leaded fuels is present in groundwater at thousands of sites in the United States at concentrations far exceeding its maximum contaminant level (MCL) more than 20 years after its use was banned.

The objectives of this project are to evaluate, quantify, and verify rates of EDB natural attenuation in groundwater and determine if biostimulation can be used to enhance EDB degradation rates when monitored natural attenuation (MNA) appears insufficient to protect critical receptors. The project will use compound-specific stable isotope analysis (CSIA) to estimate EDB degradation rates in groundwater at a Department of Defense (DoD) site under widely differing geochemical conditions, ranging from highly reducing to oxic. These data will provide critical insights into the extent to which this compound naturally biodegrades in situ via anaerobic biological pathways, abiotic mechanisms, and aerobic biodegradation. The plume-wide isotope values, geochemical parameters, and concentration data will be combined with a groundwater flow model to generate and validate an MNA model for EDB at the site. The project also will test the efficacy of using aerobic co-metabolic (or anaerobic) biostimulation to enhance EDB removal where natural attenuation processes are insufficient.

CSIA relies on the fact that carbon exists in the form of two stable isotopes that can be distinguished based on their molecular weight by using mass spectroscopy. Furthermore, biological and abiological processes favor the consumption of one isotope (¹²C) over the other (¹³C), so consumption of the carbon-containing chemical (e.g., by biodegradation) can be evaluated by measuring the ratio of the two isotopes. During this project, the isotopic fractionation of carbon (¹³C/¹²C) in EDB during biodegradation and abiotic degradation will initially be quantified and calibrated in the laboratory using dehalogenating bacterial cultures, aerobic cultures shown to cometabolize EDB, and reactive iron species (e.g., iron minerals, Fe^o and FeS), respectively. Once calibrated, the CSIA method will be used to evaluate and confirm transformation of EDB at a selected field site. This analysis aids in evaluating the natural attenuation of the chemical in the field. In addition to the MNA analysis, a field demonstration of biostimulation for enhancing EDB degradation to achieve MCL levels will be performed. The demonstration will involve a single well push-pull test with the addition of either propane plus oxygen (aerobic co-metabolism) or sodium lactate (anaerobic dehalogenation), depending on which approach is most effective in laboratory pretesting. Both CSIA and contaminant concentration changes will be used to confirm in situ EDB degradation.

This demonstration will provide DoD with a widely applicable in situ remediation approach for EDB and useful parameters and tools for assessing natural attenuation, and supporting MNA as a remedy, for EDB at DoD sites. The approaches demonstrated are expected to significantly reduce O&M costs at sites where pump-and treat-systems operate or will operate to remove EDB from groundwater. Moreover, this demonstration will provide a validated in situ treatment method for DoD sites that currently, or are found in the future to be, contaminated with this fuel additive. (Anticipated Project Completion - 2018)

Koster van Groos, P.G., Hatzinger, P.B., Streger, S.H., Vainberg, S., Philp, R.P., Kuder, T. (2018) Carbon Isotope Fractionation of 1,2-Dibromoethane by Biological and Abiotic Processes, Environmental Science & Technology. 52 (6), 3440-3448. DOI: 10.1021/acs.est.7b05224