Abstract: Evaluating the Time and Source of Hydrocarbon Additions to Soils Using Lead Isotopes and Historical Changes in Industrial Lead Sources

Richard W. Hurst

Isotopic analyses of anthropogenic Pb in well-dated, southern California coastal sediments have been integrated with historical changes in ore Pb sources to produce calibration curves (206Pb/207Pb vs. time) that allow us to model the time anthropogenic Pb was added to a soil horizon. The major, historical sources of anthropogenic Pb in southern California are fossil fuels (e.g., gasoline). Hence, Pb model ages (LABILE model; Los Angeles Borderland Industrial Lead) provide time constraints on Pb deposition from fossil fuel combustion via airborne deposition, runoff, and/or sewage outfall in this region. The correlation between the LABILE model age and known times of anthropogenic Pb additions at 17 specific sites is good (r = 0.978); the accuracy of the method ranges from one to five y ars in the post-1960 time interval. Factors influencing accuracy include analytical uncertainties in Pb isotopic measurements (<= 0.1%), the scatter in isotopic ratios of anthropogenic Pb (circa 0.2%), and the uncertainty in the sediment age used to calibrate the method (0-1.5 yr). At one site, three statistically distinguishable events were identified; they correlate with residential development (1968), airborne vehicular Pb deposition (1983), and site remediation (1991). Gasoline incursions at two test sites have been dated accurately (± 1 yr). The limitations of the LABILE model (geographic, age, types of hydrocarbons, and industry to which it applies) are now under investigation.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994