- ©2002. AAPG/DEG
The anthropogenic lead archaeostratigraphy (ALAS) model) uses 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 historical 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 on lead purchased from that available on the U.S. market rather than sources that were nonrepresentative 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 20th century.
Richard W. Hurst received his Ph.D. from the University of California, Los Angeles in 1975. He is a Professor of Biogeochemistry at California State University, Los Angeles and has been a consultant in environmental forensic geochemistry since 1978; he has served as an adjunct professor at the University of Maryland, Eastern Shore and is currently serving as a coadvisor to undergraduates in environmental geochemistry at the Massachusetts Institute of Technology. His research interests include the integration of isotope geochemistry with hydrogeological and statistical data to resolve problems associated with environmental remediation and petroleum exploration.