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
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Contact: Tim Stephens
stephens@cats.ucsc.edu
831-459-4352
University of California - Santa Cruz
10-Sep-2000