Pb and Sr Isotopic Compositions of Soils in Urbanized Industrial Settings, East Los Angeles, California, U.S.A: From Geogenic to Anthropogenic Sources

Preliminary lead and strontium isotopic compositions of soil profiles collected from highly urbanized and industrialized sites were measured to develop a survey of possible sources of lead. Potential lead and strontium sources range from geologic (e.g., bedrock and bedrock-derived soils) to anthropogenic (e.g., vehicular traffic and legacy of burning fossil fuels, heavy metal smelters, manufacturing of paint, plastics, lubricants, residential development, agricultural amendments, atmospheric deposition, etc.). Twenty-one soil profiles were sampled (6 samples/profile site to a depth of 60 cm). Lead concentrations and isotopic compositions (thermal ionization mass spectrometry and inductively coupled mass spectrometry) were measured for bulk soils, acid-leachates, and residues of bulk leaching. The soils include rock-forming silicates (e.g., feldspars, quartz, mafic minerals) and secondary minerals and amorphous materials (e.g., iron-oxyhydroxides, clays, organics). The lead isotopic compositions of acid-leachates of the soils represent lead adsorbed to mineral surfaces or held in soluble minerals (for example, iron- and manganese-hydroxides, and carbonate); soil residues likely reflect feldspar and other rock-forming minerals in bedrock-derived soils. The soils plot as a broad band in ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb space extending from radiogenic values (typical of rock-derived lead (bedrock and bedrock-derived soils) to lower ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb values that likely indicate anthropogenic/industrial types of lead (for example, heavy metal aerosols, leaded paint, agricultural amendments). Lead isotopic values of the Los Angeles soils form a broad band  intermediate between values of lead ores from the highly radiogenic U.S. Mississippi Valley type deposits and less radiogenic lead deposits of China.  Labile lead (leach fractions from soils) shows a moderate range in values of ²⁰⁶Pb/²⁰⁷Pb ~1.1671 to 1.1928, and ²⁰⁸Pb/²⁰⁷Pb ~2.4319 to 2.4498. Residues overlap the leach compositions and extend to more radiogenic values (rock-derived lead). The lowest values of ²⁰⁶Pb/²⁰⁷Pb (~1.1754) and ²⁰⁸Pb/²⁰⁷Pb (~2.4375) and highest lead contents (~4000 ppm) were measured in soils near where lead was smelted. Higher values of ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb and lower values of lead (~25 ppm) are found in other bulk soils and may represent lead derived from bedrock. The variations in lead isotope values in the soil profiles are consistent with contributions from distinct sources, which include both natural and anthropogenic lead.