THE ALAS MODEL: THE ONGOING SAGA OF LEAD FORENSIC GEOCHEMISTRY IN URBAN/RURAL ENVIRONMENTS
The ALAS Model was developed using measured 206Pb/207Pb ratios of well documented leaded gasoline and leaded gasoline impacted soil (1923 - 1990). Anthropogenic 206Pb/207Pb ratios exhibit a systematic increase after ~ 1965 due to systematic increases in the use of radiogenic Mississippi Valley Type (MVT) lead ores in the production of alkyllead gasoline additives. Releases of leaded gasoline can be quite accurately dated between ~ 1962 and ~ 1985 given ALAS Model 206Pb/207Pb ratios increase rapidly.
Case study 1: Lead paint was suspected of being the primary, if not sole source of lead in older homes in a northern U.S. city. Although there is evidence of paint contamination, analyses of house dust lead isotope ratios clearly indicate that gasoline lead contamination is present, and in many cases, may be the predominant lead source in the houses, the result of windblown soil particulates with adsorbed legacy lead from gasoline.
Case study 2: This is a re-evaluation of anthropogenic lead sources in fluvial sediment from the Pettaquamscutt River Basin, Rhode Island (Lima et al., 2005). The authors stipulate that coal-derived lead is the primary lead source, however, they fail to account for differences in coal and gasoline lead concentrations. When concentration is considered, a systematic shift is observed in the source of anthropogenic lead in the sediment with coal dominating from 1930 to 1950; gasoline lead is the dominant source from ~ 1950 to the early 1980s. The trend shifts towards coal as leaded gasoline is phased out during the 1980s.
It is imperative that we recognize the ongoing impact of lead, particularly in urban soils and from windblown dust that have been heavily impacted by anthropogenic lead from a variety of sources. However, lead from gasoline combustion continues to be a major hazard that should not be overlooked in forensic investigations involving lead pollution of both urban and rural environments.