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