|Natural Attenuation of the Lead Scavengers 1,2-Dibromoethane (EDB) and 1,2-Dichloroethane (1,2-DCA) at Motor Fuel Release Sites and Implications for Risk Management (EPA/600/R-08/107) September 2008
The lead scavengers Ethylene Dibromide (EDB) and 1,2-Dichloroethane (1,2-DCA) were added to leaded motor gasoline to prevent the buildup of deposits of lead oxide inside internal combustion engines. Recent studies demonstrate that lead scavengers may persist for long periods of time in certain ground water environments. Although lead and lead scavengers were phased out in conventional motor gasoline by the end of the 1980s, the lead scavengers from old releases may continue to contaminate ground water at many gasoline service station sites. In addition, aviation gasoline (Avgas) contains lead scavengers, and gasoline containing lead scavengers is still used for certain off-road applications such as automobile racing. There is a significant possibility that lead scavengers from releases of leaded gasoline pose an ongoing risk to ground water quality.
Domestic ground water wells and certain small public water supply wells that are in close proximity to sites where leaded gasoline may have been released should be of particular concern. These wells often produce ground water from shallow aquifers, which makes them more vulnerable to contamination than larger municipal water supply wells which usually produce water from deeper aquifers.
EPA has formed a team with the Association of State and Territorial Waste Management Officials to determine the scope and magnitude of the occurrence of lead scavengers at leaking UST sites. The team developed a three-phased approach to this problem: (1) developing an understanding of the magnitude of the potential problem by compiling existing background information, (2) assessing gaps in current knowledge, based on the findings of Phase 1, and implementing appropriate measures to fill the gaps, and (3) determining an appropriate response based on evaluation of the results of Phases 1 and 2.
Phase 1 culminated in development of a document entitled Lead Scavengers Compendium: Overview of Properties, Occurrence, and Remedial Technologies (U.S. EPA, 2006). Phase 2 consisted of collecting and analyzing ground water samples from 102 old gasoline release sites spread across the 19 states that chose to participate in the investigation. This report Natural Attenuation of the Lead Scavengers 1,2-Dibromoethane (EDB) and 1,2-Dichloroethane (1,2-DCA) at Motor Fuel Release Sites and Implications for Risk Management represents the culmination of Phase 2. It fills some of the data gaps on the expected distribution of lead scavengers at gasoline release sites, it discusses mechanisms for abiotic transformation and biodegradation of EDB and 1,2-DCA, and it provides new tools to recognize and use natural transformation and degradation of EDB and 1,2-DCA as part of a risk management strategy.
The survey found that significant concentrations of EDB continue to persist at many old leaded gasoline spill sites. Both EDB and 1,2-DCA were present at concentrations above their respective Maximum Concentration Level (MCL) at a significant number of sites; EDB was detected above its MCL of 0.05 µg/L at 42% of the sites sampled, and 1,2-DCA was detected above its MCL of 5.0 µg/L at 15% of the sites sampled. Benzene (with an MCL of 5.0 µg/L) was present at 100% of the sites sampled and was the primary risk driver at 75% of the sites where both benzene and EDB were present in ground water; EDB was the primary risk driver in the remaining 25% of sites.
The persistence of EDB at UST spill sites is consistent with its expected behavior in ground water. Simple physical weathering of EDB and 1,2-DCA from residual gasoline is a slow process that may require decades to centuries to reduce high concentrations of EDB or 1,2-DCA to their MCLs. At some sites, anaerobic biodegradation can provide substantial reductions in the concentrations of EDB and 1,2-DCA. At some sites, abiotic degradation caused by reaction with Iron(II) sulfide minerals in aquifer material can also produce substantial reduction in the concentration of EDB, particularly in ground water at neutral pH.
Although it is theoretically possible that anaerobic biodegradation or abiotic degradation will remove EDB at a particular site, it is frequently difficult to prove that degradation is occurring based on conventional monitoring data. Compound Specific Isotope Analysis (CSIA) can be useful to recognize biodegradation and abiotic transformation of EDB ground water. Degradation is recognized and documented by a change in the ratio of stable isotopes of carbon in the molecules of EDB that remain in the ground water after degradation. The change in the ratios can put a conservative boundary on the extent of degradation that has occurred in the ground water sampled by a particular well. This makes CSIA a useful tool to prove that degradation has happened at field scale at a particular site.
If the concentrations of EDB and 1,2-DCA in ground water in the source area of plumes do not attenuate, the hazard associated with these contaminants will persist indefinitely. MNA is most cost effective as a remedy when the concentrations of contaminants attenuate to their MCLs in a reasonable period of time. The concentrations of EDB and 1,2-DCA that would be expected in ground water in contact with unweathered leaded automobile gasoline are 1,900 and 3,700 µg/L respectively. To bring these initial concentrations to their MCL within 20 years, the first order rate of attenuation in concentration in the most contaminated well at a site should be 0.5 per year or greater for EDB and 0.33 per year or greater for 1,2-DCA. At certain sites, and under some circumstances, rates in excess of 0.5 per year for EDB or 0.33 per year for 1,2-DCA can be attained through anaerobic biodegradation or by abiotic reactions. To apply MNA at a specific site, rate constants for attenuation over time should be extracted from site-specific data and should be verified and validated by continued long-term monitoring.
Monitoring for concentrations of EDB in ground water can be a major cost of risk management at gasoline spill sites. The MCL for EDB is one hundred fold lower than the MCLs for Benzene or 1,2-DCA. Because the MCL for EDB is so low, not all analytical methods can detect EDB when it is present at its MCL. The EPA Method that is most commonly used to analyze for gasoline constituents in ground water (Method 8260B) has a detection limit for EDB of approximately 3.0 µg/L, which is sixty fold higher than the MCL. As a result, Method 8260B cannot be used to document that ground water is free of contamination from EDB. In contrast, EPA Method 8011 has a method detection limit for EDB of approximately 0.01 µg/L, which is sufficiently sensitive to measure EDB at its MCL.
Method 8260B would have only discovered 40% of the survey sites with concentrations of EDB above its MCL. At sites where benzene is the primary risk driver, Method 8260B would be appropriate to monitor the quality of ground water during active remediation. However, to determine if the site has reached the MCL for EDB, it is necessary to use Method 8011 or its equivalent.
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