Sep. 26, 2000 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 Flegal and Charles Dunlap, a research fellow in the Environmental Toxicology Department and an assistant professor at the American University of Armenia.
Three factors conspire to maintain elevated levels of lead in the waters of San Francisco Bay, Steding said. One is the large amount of lead that remains in the Central Valley watershed, where it is associated with soils that gradually wash into the Sacramento and San Joaquin Rivers and flow into the bay. A second factor is the large amount of lead-contaminated sediments within the bay, which continually release lead into the water. The third factor is the limited transport of contaminated sediments from the bay into the open ocean.
"The bay just doesn't clean itself efficiently," Steding said. "The only outflow is through the Golden Gate, so sediment flow out of the bay is limited."
Furthermore, that flow has been drastically reduced by diversions of water from streams around the bay for urban, industrial, and agricultural uses. Contaminated sediments in the southern reach of the bay seem likely to remain there indefinitely, the researchers found.
"There is very little movement of sediments out of the South Bay," Steding said. "It's akin to a stagnant lagoon, and only during periods of high water flow does it get any substantial flushing."
Steding was able to analyze water samples from the bay and determine not only how much of the lead in a given sample came from gasoline, but how much came from, for example, gasoline produced in the 1960s and 1970s. Lead's unique elemental properties made this possible, he said. Different forms of lead, called stable isotopes, occur naturally, and their proportions can indicate the origins of the lead in a sample.
"Lead has a unique isotopic variability that allows us to fingerprint the sources," Steding said.
The results showed that gasoline emissions from the 1960s and 1970s still account for most of the lead in water samples from the rivers and bay. Leaded gasoline consumption peaked in the mid-1970s. The researchers estimated that cars burning leaded gasoline emitted 33,000 tons of lead in the Sacramento and San Joaquin drainage basins in the 1960s and 1970s.
In the South Bay, 1960s and 1970s gasoline lead still accounts for 90 percent of the lead found in water samples. Moreover, this percentage remained unchanged throughout the study period, from 1989 to 1998. In the North Bay and the rivers, however, the proportion of lead from the 1960s and 1970s declined by 5 to 10 percent relative to lead from 1980s gasoline emissions.
The researchers found that only 1 to 10 percent of the total amount of lead deposited in the Central Valley during the drought years of 1986-92 had been washed into the bay by 1995. The study also indicated that lead from 1980s gasoline is more readily mobilized by surface runoff than lead from 1960s and 1970s gasoline, which is probably located deeper in California soils and in the sediments of the river beds.
According to Flegal, other heavy metals and some organic pollutants are likely to show similar patterns of persistence in the environment. These include contaminants, such as mercury, that are serious problems in San Francisco Bay, posing threats to humans and wildlife.
"We can't fingerprint these other contaminants, but we know they cycle like lead does in the environment," Flegal said.
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