Chronicling a Century of Lead Pollution in Mexico: Stable Lead Isotopic Composition Analyses of Dated Sediment Cores MARTIN F. SOTO-JIMENEZ,* ,†,§ SHARON A. HIBDON, † CHARLEY W. RANKIN, † JUGDEEP AGGARAWL, ‡ A. CAROLINA RUIZ-FERNANDEZ, § FEDERICO PAEZ-OSUNA, § AND A. RUSSELL FLEGAL † Department of Environmental Toxicology, WIGS, and Department of Earth Sciences, University of California at Santa Cruz, Santa Cruz, California 95064, and Institute of Marine Sciences and Limnology Institute, UNAM, Mazatla ´n, Mexico 82000. Analyses of lead isotopic compositions ( 204 Pb, 206 Pb, 207 Pb, and 208 Pb) of dated sediment cores from two coastal estuaries and two inland lakes chronicle the predominance of industrial lead emissions in Mexico over the past century. These isotopic ratios exhibit a shift in composition from the turn of the previous century (1900) that corresponds with measurable increases (from 2- to 10-fold) in lead concentrations in the cores above their baseline values (3-22 μg/g)sboth changes are consistent with the development of Mexican lead production for export and the manufacture of tetraethyl lead additives for Mexican gasolines. While subsequent changes in lead concentrations in the cores correspond with calculated emissions from the combustion of leaded gasoline in Mexico, isotopic compositions of the cores remain relatively constant throughout most of the 1900s (e.g., 206 Pb/ 207 Pb ) 1.200 ( 0.003; 208 Pb/ 207 Pb ) 2.463 ( 0.004). That isotopic constancy is attributed to the widespread pollution from lead production in Mexico and the dispersion of some of that lead used as an additive in Mexican gasolines. Introduction As in many other developing countries, Mexico experienced rapid economic and population growth over the past few decadessdepleting natural resources and increasing pollu- tion (1-3). Major factors in the country’s deterioration of environmental quality have been the smelting of lead ores and the use of some of that lead in the production of tetraethyl lead (TEL) additives for Mexican gasoline from the 1930s to the 1990s. That pollution has exposed urban populations in Mexico to atmospheric lead levels that cause measurable, adverse health effects (4-8). To partially quantify the extent of that pollution, we estimate atmospheric emissions from the combustion of leaded gasoline, alone, in Mexico over the past century totaled 541 000 metric tons (Figure 1). This estimate was derived from records (7, 9-15) of the consumption of gasoline in Mexico and the lead content of that gasoline. On the basis of studies of the dispersion of contaminant lead in soils and sediments published elsewhere (e.g., 16-18), it was assumed that most of that lead was deposited on Mexican sediments that now provide a record of those historical emissions. This assumption was corroborated by recent analyses of lead concentrations in dated sediment cores from estuarine and lacustrine sites in Mexico (18-19). Although there were differences in sediment accumulation rates and lead con- centrations among sites (due to differences in adjacent population sizes, industries, and weathering), cores exhibited similar systematic increases in lead fluxes above natural levels beginning around 1900 (Figure 2). Associated estimates of anthropogenic lead fluxes showed increments from low and constant lead values prior to 1900 (0.1-0.5 μg cm -2 year -1 ) to maximum values in the 1970s to 1980s (2-16 μg cm -2 year -1 ) followed by decreases toward 2000 (1-4 μg cm -2 year -1 ). Those temporal variations are comparable with our calculations of emissions of leaded gasoline in Mexico during the 1900s, detailed in subsequent sections. * Corresponding author phone: 01152 669 982 6631; fax: 01152 669 982 6631; e-mail: martin@ola.icmyl.unam.mx. † Environmental Toxicology, WIGS, University of California at Santa Cruz. ‡ Earth Sciences, University of California at Santa Cruz. § Institute of Marine Sciences and Limnology Institute. FIGURE 1. Lead content (g L -1 ) in Mexican gasoline over time and historic consumption of lead in gasoline (metric tons year -1 ) in Mexico (see the text for references). FIGURE 2. Historical anthropogenic lead flux (μg cm -2 year -1 ) estimates on 210 Pb-dated sediments from four representative environments in Mexico. Environ. Sci. Technol. 2006, 40, 764-770 764 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 3, 2006 10.1021/es048478g CCC: $33.50  2006 American Chemical Society Published on Web 12/20/2005