Past leaded gasoline emissions as a nonpoint source tracer in riparian systems: A study of river inputs to San Francisco Bay

Variations in the isotopic composition of lead in 1995-1998 river waters fl owing into San Francisco Bay trace the washout of lead deposited in the dra inage basin from leaded gasoline combustion. At the confluence of the Sacra mento and San Joaquin rivers where they enter the Bay, the isotopic composi tions of lead in the waters define a linear trend away from the measured hi storical compositions of leaded gas in California. The river waters are shi fted away from leaded gasoline values and toward an isotopic composition si milar to Sierra Nevadan inputs which became the predominant source of sedim entation in San Francisco Bay following the onset of hydraulic gold mining in 1853. Using lead isotopic compositions of hydraulic mine sediments and a verage leaded gasoline as mixing end members, we calculate that more than 5 0% of the lead in the present river water originated from leaded gasoline c ombustion. The strong adsorption of lead (log K-d > 7.4) to particulates ap pears to limit the flushing of gasoline lead from the drainage basin, and t he removal of that lead from the system may have reached an asymptotic limi t. Consequently, gasoline lead isotopes should prove to be a useful nonpoin t source tracer of the environmental distribution of particle-reactive anth ropogenic metals in freshwater systems.