Chronic lead contamination in San Francisco Bay shows long-term persistence of lead from past use of leaded gasoline

SANTA CRUZ, CA--The use of lead as a gasoline additive was phased out years ago in California, but an enormous reservoir of lead-contaminated soils and river sediments remains in the Central Valley and will continue to contaminate the waters of San Francisco Bay for decades to come, according to a study published in the September 12 issue of the Proceedings of the National Academy of Sciences.

The findings by researchers at the University of California, Santa Cruz, are based on an analysis of lead in water samples collected over a ten-year period in San Francisco Bay and the mouths of the Sacramento and San Joaquin Rivers. Although there is no evidence that lead pollution in San Francisco Bay threatens the health of humans or marine organisms, the study has implications for the persistence of a wide range of potentially harmful contaminants, said Russell Flegal, professor and chair of environmental toxicology at UCSC.

"We can use this as a model for other contaminants, and it shows that many contaminants simply don't go away once you stop polluting the environment," Flegal said. "We're seeing lead contamination from the 1960s still coming into the bay, and our calculations indicate it will be another 50 to 100 years before all the lead from gasoline emissions in the Central Valley is washed into the bay."

Even after lead stops entering the bay, lead-contaminated sediments are likely to remain there indefinitely, he added. These findings contrast with those of other researchers who have reported significant reductions of lead contamination in other types of environments. Since the phaseout of leaded gasoline, lead concentrations have fallen in urban air, ocean surface water, polar ice and snow, and even human blood. But the UCSC researchers now show that contaminated rivers and estuaries can retain pollutants like lead for a very long time.

Douglas Steding, a graduate student in Flegal's lab, is the first author of the study. The coauthors are Fl

Contact: Tim Stephens
University of California - Santa Cruz

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