Specific Petroleum Compounds and Dating Releases
Most people are very familiar with the BTEX test by EPA method 8021. BTEX is the acronym for benzene, toluene, ethylbenzene and xylene. This test is performed using a gas chromatograph with a sensitive detector. Additional "add on" compounds to this test include methyl tertiary butyl ether (MTBE) and naphthalene. All of these compounds share one thing in common, they are found primarily in gasoline, and as such, serve as useful indicators of contamination from gasoline sources.
A common misconception that we often see is that naphthalene is a good indicator of diesel fuel contamination. This is not the case! Naphthalene is a volatile, aromatic compound that is an intentional component of gasoline. Diesel fuel is largely composed of semi-volatile, aliphatic compounds. It is true that there are a variety of volatile, aromatic compounds in diesel fuel such as naphthalene. These however exist in lower levels than found in gasoline and other lighter fuels. Consequently, naphthalene is not a good indicator of diesel fuel contamination.
In addition to the expected gasoline constituents, various additives have been used in gasoline. Lead in the form of tetraethyl lead was first added to gasoline in the 1920s to reduce engine "knock". In the early 1960s tetramethyl lead was substituted for tetraethyl. In 1984 the allowable level of lead utilized in gasoline was reduced from 4.2 to 0.1 grams per gallon. Ethylene dibromide and 1,2-dichloroethane were also added to older gasoline formulations as "lead scavengers" in an effort to reduce the amount of lead accumulating and eventually fouling engines.
In 1980 the EPA approved methyl tertiary butyl ether (MTBE) as a gasoline oxygenate. Oxygenates such as MTBE were required by the EPA as a mechanism for reducing carbon monoxide content and increasing oxygen content. The concerns for MTBEs role as an air and water pollutant are rising. MTBE has been found in high concentrations in the air surrounding cities with severe inversion problems.
The above mentioned and other compounds can provide a means of quantifying the date of a gasoline release. Additionally, the varying solubility and volatility cause gasoline compounds to degrade at different rates. This provides another useful mechanism for evaluating how recent the release occurred. Conveniently the order of degradation is in the same order as the acronym BTEX. Benzene tends to degrade faster than toluene, and toluene degrades faster than ethylbenzene and so on. MTBE is more volatile than benzene and naphthalene is less volatile than xylene. Gaseous hydrocarbons such as s-pentane, isobutane and n-butane degrade very quickly when released to soil. Samples containing these compounds are almost certainly from a recent release.
Fuel weathering can be affected by a number of factors including soil type, depth of release, and temperature. Great caution must be exercised in making judgments about the date of the release based on the analytical information. A thorough evaluation must be made of all available information.
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Speciation Of Metals: ChromiumSampling Volatiles In Soil by 5035 MTBE - The Next Challenge Utilizing TPH Methods Effectively TPH Method Overlap Specific Petroleum Compound Testing TPH 1664 vs. 413.1 and 418.1 TCLP vs. SPLP Extraction Procedures Analyzing for Metals by ICP/MS ICP/MS Qualitative All Elements Scan