Use of Oxygenates to Date a Gasoline Release

Background

Oxygenates include a number of chemical compounds (containing functions with oxygen) that have been used as gasoline additives to improve gasoline characteristics and combustion in the United States (U.S.) and worldwide over the years. Apart from ethanol that was mixed with gasoline in the Midwest U.S. prior to World War II, the other oxygenates were more recently added to gasoline. Thus, from about 1980, with the phase-out of lead additives, a peak in the use of oxygenates as alternatives to organic lead in gasoline occurred. Apart from ethanol, the most common oxygenate compounds used include: methyl tertiary butyl ether (MTBE), tertiary amyl methyl ether (TAME), tertiary butyl alcohol (TBA), ethyl tertiary butyl ether (ETBE), di-isopropyl ether (DIPE), and tertiary amyl ethyl ether (TAEE). The relatively recent introduction of most oxygenates as gasoline additives, as well as variation in the types or combination of oxygenates used and the different chronological changes in the total amount of added oxygenates following legislative changes (in order to meet a certain amount of total oxygen needed for burning gasoline), are the basis for the forensic use of oxygenates to determine the age and possibly the source of gasoline spills. This editorial points out some major changes in the total amount and composition of oxygenates in the past several decades that can be used as starting point to age-date relatively recent (post-1980s) gasoline releases in environment. Along with such general information, for an accurate age dating process one should account for the (bio)degradation rates of different oxygenates as well as specific environmental conditions that determine the time required for the release of oxygenates from an initial nonaqueous phase liquid (NAPL) spill and thus the starting point of most degradation processes. Several hints in this respect are also given in addition to few useful references. This editorial is intended to be useful by providing general guidance on how to interpret oxygenate data and estimate gasoline age.

Historical Changes in Oxygenate Composition and Total Percentage Used (Table)

The most well-known and used oxygenate, MTBE (more than 30% of gasoline sold in the U.S. contained oxygenates, of which more than 80% was MTBE), has been blended in gasoline starting in 1979 when ARCO obtained a federal waiver for its use in the amount of 7 vol%. MTBE started to be used mostly after 1980 due to its advantages compared with gasoline-grade TBA (GTBA) or other oxygenates (as it has significantly lower water solubility and does not absorb water to break phase). MTBE total initial amount of 7 vol% increased rapidly. By 1981, 11 vol% MTBE was added to gasoline, which increased to 15 vol% in 1988 through an Environmental Protection Agency (EPA) waiver to Sun Oil Company (Gibbs, 1990), and even up to 17 vol% in 1992 due to Clean Air Act requirements. A widespread national use of MTBE could be noticed after 1987. In 1990, the EPA suggested 15 vol% MTBE (equivalent to 2.7 wt% oxygen) for winter gasoline use. This amount (15 vol%) was lowered to 11 vol% (equivalent to 2 wt% oxygen) in the reformulated gasoline in 1995 in order to prevent excessive atmospheric formation of nitrogen oxide. Due to the increased awareness of environmental problems, starting from 2000, different states initiated bans on MTBE usage.

Although the most used oxygenate, MTBE was not the first introduced after ethanol. Thus, in 1969, the refinery-synthesized alcohol TBA was introduced by ARCO and started being marketed in 1979 as 7 vol% blend with gasoline (known as “gasoline-grade TBA (GTBA)).” An increase in TBA from 7 to 10 vol% was noted around 1981 (Kaplan, 2003).

Other oxygenates, ethers such as ETBE or TAME, were introduced as gasoline additives in the early 1990s. TAME was first manufactured by Exxon Company (Bayton, TX) in 1987, and later by other companies (e.g., Chevron in 1995). It has been used in California and nationwide in combination (in various proportions) with MTBE (generally at a lower concentration than MTBE). In California, it first appeared in gasoline in winter of 1993 (NIPER, 2000) and reached a maximum usage in 1996.

ETBE was not widely used (with use recorded in four locations—the maximum use in Louisville, KY, in 1995 and minor use in Baltimore, MD, in 1996 while no use was recorded for 1997). ETBE was recorded to be used in mixture with MTBE and ethanol in the following proportions: 17.14% ETBE + 57.71% MTBE + 24% ethanol.

Along with MTBE and other oxygenates, ethanol continued to be widely used in gasoline. Thus, according to an EPA waiver agreement in 1978, ethanol was used in gasoline in concentrations not to exceed 10 vol% (equivalent to 3.7 wt% oxygen). Of note is that MTBE and ethanol were not intentionally blended together in gasoline.

Useful Hints in Forensically Interpreting Oxygenate Data

•	TAME and TBA are also impurities (< 1%) in the manufacture of MTBE, thus a detection at low concentrations along with MTBE does not necessarily imply the use of a combination of MTBE and TAME or TBA, but rather the use of only MTBE presenting TAME or/and TBA impurities.
•	MTBE detection is in general an indication of post-1980 gasoline releases; however, MTBE is not contained in all post-1980 gasoline.
•	When using MTBE for age-dating gasoline, one should consider potential false-positives in laboratory testing when EPA Method 8020/8021 is used because various methyl-pentanes co-elute with MTBE (Morrison, 2000).
•	Any potential contribution from non-point sources must be evaluated and identified.
•	Cross-contamination from one fuel to another should be considered, especially in pipelines or tanker trucks. Thus, MTBE has been detected in the presence of jet fuel, diesel fuel, heating oil, aviation gas, and waste oil.
•	A useful summary of U.S. gasoline compositions comparing 1990–1992 conventional gas range properties with winter/summer reformulated gasoline (RFG) I range and annual average for 1995–1996 nationwide is given by Kaplan (2003). The referred publication also contains a summary of gasoline compositions and physical properties comparing 2000 California reformulated gasoline ranges and annual averages with winter/summer conventional gasoline and RFG II properties for gasoline nationwide.

Biodegradation Rates

The detection of MTBE or other oxygenates in a spill sample indicates a spill after 1980. The presence of other oxygenates in the spill that were introduced later (such as TAME) may indicate an even more recent spill (after 1990). The proportions of oxygenates found in the sample may also give an indication of the age and possible source of the spill as long as the detected concentrations are interpreted correctly. In order to do this, a forensic scientist should have a clear understanding of the specific environmental conditions around the spill, as well as of the literature data regarding the biodegradation and degradation rates for different oxygenates. Corroborating such information, a more accurate final concentration of oxygenates in the spill can be determined that will allow narrowing the period when the spill occurred. For example, in the case of ethanol, an average decay rate is equal to 0.014 day⁻¹ (Malcolm Pirnie, Inc., 1998) with a half-life of about 50 days. Thus, if ethanol is detected in a spill sample, it indicates a very recent spill. Assuming a known initial concentration of ethanol in the spill, the exact age of spill may be calculated. Complications occur in the case of MTBE, which is in general resistant to biodegradation (although this may occur under both aerobic or anaerobic conditions), is dependent on the presence of other gasoline compounds, and is strongly influenced by different environmental conditions. TBA is the primary metabolite of MTBE biodegradation; however, it may also be present as impurity in the original MTBE or be added to MTBE up to 5 vol%. Evidence in the past decade shows that MTBE can be intrinsically attenuated by in situ microbial activity with a half time extending from 1 month to 1.7 years. Both MTBE and TBA are rapidly biodegraded under high redox conditions, whereas benzene, toluene, ethylbenzene, and xylenes, as well as other gasoline hydrocarbons retard MTBE and TBA metabolism. Of note is also that rates of TBA degradation seem to be more rapid than those for MTBE.