New isotopic evidence for chronic lead contamination in the San Francisco Bay estuary system: Implications for the persistence of past industrial lead emissions in the biosphere

Measurements of lead isotope compositions in unfiltered San Francisco Bay w aters from 1989 to 1998 have brought new insights into the cycling of anthr opogenic lead in estuaries. Isotopic compositions of lead in the shallow (< 2 m) southern reach were essentially invariant (similar to 90% derived from 1960s-1970s leaded gasoline) during the study period because of limited hy draulic flushing and the remobilization of lead from bottom sediments. In c ontrast, in the northern reach freshwater flushing from the San Joaquin and Sacramento rivers produced seasonal and decadal variations in lead isotope compositions. The seasonal shifts are attributed to advection of soils con taining late 1980s gasoline lead into the bay during winter rains. Mass bal ance calculations indicate that only a small fraction (1-10%) of this leade d gasoline fallout from the late 1980s has been washed out of the San Joaqu in and Sacramento rivers' drainage basin by 1995. Superimposed on this seas onal cycling was a long-term systematic shift in the component of gasoline lead expressed in the river systems, with a small (similar to 5-10%) decrea se in the amount of 1960s-1970s gasoline lead in river and North Bay waters . The retention of gasoline lead in the river systems draining into the bay as well as San Francisco Bay sediments indicates that historic gasoline de posits may remain in the combined riparian/estuarine system for decades. Su ch a persistence is in contraindication to recent reports of rapid (annual) decreases in lead contamination in other environments, and the link betwee n climate and contaminant transport suggests local or global climate change will have an impact on contaminant distribution and fate.