Geochemistry ~ Environmental Forensics ~ Mineralogy
 

Lead Isotopes as Age Sensitive, Genetic Markers in Hydrocarbons:

3. Leaded Gasoline, 1923-1990 (ALAS Model)

Anthropogenic Lead ArchaeoStratigraphy (ALAS Model) utilizes measured lead  isotope ratios of documented gasoline releases as preserved in a variety of matrices    (product, soils, water) to accurately estimate the year of the release.  ALAS is unique,   in that it is the only model for which historic data, i.e. U.S. lead production figures, are available that allow the nature of the calibration curve to be modeled and calculated for  time intervals where calibration samples are scarce (pre-1960).  Independent modeling of the data also allows one to evaluate the assumption that lead isotope ratios (e.g. ²⁰⁶Pb/²⁰⁷Pb) of gasoline additives, such as tetraethyllead, closely followed those of average annual U.S. industrial lead.  By integrating U.S. lead production figures with  lead isotope ratios of domestic, foreign, and recycled sources, ²⁰⁶Pb/²⁰⁷Pb ratios of average annual U.S. industrial have been calculated from the year lead additives were first introduced, 1923, through 1990, which, for practical purposes, marks the end of the leaded gasoline era.  The calculated ²⁰⁶Pb/²⁰⁷Pb ratios are in excellent agreement with those of ALAS Model calibration samples between 1923 and 1990 (R² = 0.945).  The results indicate that the production of alkylleads by firms such as Ethyl and DuPont  relied upon lead purchased from that available on the U.S. market rather than sources that were non-representative of that market.  In addition to the ALAS Model’s utility in estimating the year of gasoline releases, temporal fluctuations and trends observed in the model between 1923 and 1990 provide a series of chronostratigraphic markers, which can be related to variations in U.S. industrial lead sources and, in some instances,  historic events during the 20^th Century. 

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