TRACING OF POLLUTION SOURCES IN URBAN DUST AND SOILS FROM HERMOSILLO CITY (NORTHWESTERN MEXICO) USING PB ISOTOPES

Anthropogenic activities have undoubtedly a negative impact on the environment as a consequence of the rapid urbanization, industrialization, agriculture, and mining. Few decades ago, population growth and associated activities were not a concern to the sustainable urban development. Studies have shown the hazardous potential of high metal concentration in urban areas and how this can become a serious problem to human health. Therefore, the constrainment of pollution is of key importance to identify sources. Geologic materials are important sources of airborne particulate matter but the contribution of contaminated soil to concentrations of Pb in airborne dust has not been widely documented. Hermosillo has experienced a noticeable population and industrial growth for the past three decades. Recent studies show high metal concentration in dust deposited on roofs of elementary schools and airborne particulate matter. In this study we analyzed the Pb isotope ratios and concentration of airborne particulate matter and surrounding rocks. We also analyzed the Pb isotope ratios of dust samples deposited on elementary school roofs. The data show clearly three groups: a more radiogenic Pb group defined by the rocks (geogenic end-member), a less radiogenic Pb group defined by the airborne particulate matter (anthropogenic end-member), and an intermediate group (dust on roofs). Spatial distribution of Pb concentration and isotope composition of airborne and sedimented dust show a re-suspension/sedimentation dominating process controlled by erosion, traffic and topography. Pb-isotope data of leaded gasoline are imprinted in sedimented dust on roofs. Considering that leaded-gasoline has not been in use in Mexico for the past two decades, the isotope data show not only a Pb-legacy in soil but also a re-suspension process affecting the air column below 3 meters height. The combination of the ²⁰⁷Pb/²⁰⁶Pb data of the surrounding rocks and dust identify three well-defined zones with remarkable anthropogenic influence that corresponds to the oldest urbanized sectors and heavily traffic zones. The use of the Pb isotope systematic is clearly a powerful tool in the determination of contaminating sources using the urban dust below 3 meter height, and during the risk assessment and identification of exposure paths to humans.