Field evidence from underground storage tank (UST) sites where leaded gasoline leaked indicates the lead scavengers 1,2-dibromoethane (ethylene dibromide, or EDB) and 1,2-dichloroethane (1,2-DCA) may be present in groundwater at levels that pose unacceptable risk. These compounds are seldom tested for at UST sites. Although dehalogenation of EDB and 1,2-DCA is known to occur, the effect of fuel hydrocarbons on their biodegradability under anaerobic conditions is poorly understood. Microcosms (2 L glass bottles) were prepared with soil and groundwater from a UST site in Clemson, South Carolina, using samples collected from the source (containing residual fuel) and less contaminated downgradient areas. Anaerobic biodegradation of EDB occurred in microcosms simulating natural attenuation, but was more extensive and predictable in treatments biostimulated with lactate. In the downgradient biostimulated microcosms, EDB decreased below its maximum contaminant level (MCL) (0.05 µg/L) at a first order rate of 9.4 ± 0.2 yr-1. The pathway for EDB dehalogenation proceeded mainly by dihaloelimination to ethene in the source microcosms, while sequential hydrogenolysis to bromoethane and ethane was predominant in the downgradient treatments. Biodegradation of EDB in the source microcosms was confirmed by carbon specific isotope analysis, with a 13C enrichment factor of -5.6. The highest levels of EDB removal occurred in microcosms that produced the highest amounts of methane. Extensive biodegradation of benzene, ethylbenzene, toluene and ortho-xylene was also observed in the source and downgradient area microcosms. In contrast, biodegradation of 1,2-DCA proceeded at a considerably slower rate than EDB, with no response to lactate additions. The slower biodegradation rates for 1,2-DCA agree with field observations and indicate that even if EDB is removed to below its MCL, 1,2-DCA may persist.
Separate experiments were carried out to assess the potential inhibitory interactions between 1,2-DCA and EDB, which might explain the observed persistence of these compounds where leaded gasoline was released. Preliminary experiments were conducted to determine if an enrichment culture that chlororespires PCE and TCE developed at Clemson University was also capable of respiring EDB and 1,2-DCA. The culture was found to have the ability to rapidly dehalorespire EDB and 1,2-DCA, currently the only mixed culture known to do so. However, when the culture was fed both compounds simultaneously, it degraded EDB at the expense of 1,2-DCA in all cases. When the culture was enriched on EDB, activity on 1,2-DCA was completely inhibited, even after EDB was gone. No amount of 1,2-DCA inhibited the rate of EDB degradation down to part-per-trillion levels. Any previous exposure to EDB precluded the culture's ability to consume 1,2-DCA. Remarkably, when the culture was enriched on 1,2-DCA and subsequently exposed to both EDB and 1,2-DCA, EDB was consumed first. EDB clearly inhibited 1,2-DCA biodegradation, and the degree of 1,2-DCA inhibition was roughly proportional to the concentration of EDB. This clear pattern of 1,2-DCA inhibition by EDB may contribute to its observed persistence in laboratory and field studies and merits further evaluation.
Currently, decision makers have little information to guide remedial choices at UST sites contaminated with leaded gasoline additives. An analytical model was used to simulate the effects of partial source removal and plume remediation on EDB and 1,2-DCA plumes at contaminated UST sites. The risk posed by EDB, 1,2-DCA, and comingled gasoline hydrocarbons varies throughout the plume over time. Dissolution from the light nonaqueous phase liquid (LNAPL) determines the concentration of each contaminant near the source, but biological decay in the plume has a greater influence as distance downgradient from the source increases. For this reason, compounds that exceed regulatory standards near the source may not in downgradient plume zones. At UST sites, partial removal of a residual LNAPL source mass may serve as a stand alone remedial technique if dissolved concentrations in the source zone are within a couple orders of magnitude of the applicable government or remedial standards. This may be the case with 1,2-DCA; however EDB is likely to be found at concentrations that are orders of magnitude higher than its low MCL of 0.05 µg/L. For sites with significant EDB contamination, even when plume remediation is combined with source depletion, significant timeframes may be required to mitigate the impact of this compound. Benzene and MTBE are commonly the focus of remedial efforts at UST sites, but simulations presented here suggest that EDB, and to a lesser extent 1,2-DCA could be the critical contaminants to consider in the remediation design process at many sites.