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GeoInsighter Spring 2000 Newsletter
Volume 5 Number 1

Gasoline Age Dating Using Lead Isotope Analysis

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Age dating of gasoline releases is a major concern at sites where more than one party is a potential source or where the likelihood of insurance recovery hinges on the date of a release. A typical practice is to take samples of the gasoline from monitoring wells and send them to a laboratory for "fingerprinting" analysis. Fingerprinting involves running the samples through a gas chromatograph and comparing the resulting chromatograms to those obtained from "fresh" and "weathered" gasoline samples. Another approach is to use "time-of-travel" arguments. In general, this involves using the estimated velocity of ground water flow, together with the distance traveled by the more soluble gasoline components, to calculate a likely date of release. Due to the uncertainty inherent in these methods, they are often unable to narrow the release date down to the level required to resolve the issue at hand.

For cases that require a more precise and defensible estimate of the release date, the use of lead isotope analysis may be the answer. This method involves analysis of lead isotopes that are present in both leaded and unleaded gasolines. (Unleaded gasolines contain parts-per-billion levels of lead.) First, some background information: Isotopes of an element (such as lead) are atoms that have a different number of neutrons and, therefore, a different mass. Lead has four naturally-occurring isotopes. By comparing the amount of each lead isotope present in a sample, lead isotope ratios can be calculated. 

The basis of the lead isotope age dating method is this: Research has shown that lead isotope ratios in gasoline varied with the time of gasoline production. In particular, the ratio of Lead-206 to Lead-207 in gasoline showed a gradual increase between 1965 to approximately 1990. It has been theorized that at least some of this increase is due to the gradually increased use of Mississippi Valley lead ores by the United States lead industry during this time. By obtaining a gasoline sample (or contaminated soil sample) and subjecting it to lead isotopic analysis, an estimate of the age of the gasoline can be obtained. For gasolines produced between approximately 1965 and 1980, the estimated age is reportedly accurate to within one year. For gasolines produced prior to 1965, resolution is limited to classification as "pre-1965." The age of gasolines produced between 1980 and 1990 can be estimated to within approximately 1.5 to 2 years.

What happens when several releases may have occurred at a site? This method has the potential to be very helpful in these cases. By doing careful, repeated (and expensive) extractions of contaminated soil samples, information on the history of releases can be obtained. The most easily extracted lead tends to represent the most recent releases, while the more difficult-to-extract lead tends to represent older releases. Isotope ratios can be calculated for each extract, potentially leading to estimated ages for multiple releases.

Due to the analytical effort involved, the lead isotope method is expensive compared to fingerprinting methods. However, the increased costs may be justified when a more precise estimate of the release date is needed.

Richard E. Doherty
redoherty@geoinc.com

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