The study, published in the Proceedings of the National Academy of Sciences, is based on analysis of lead isotopes in water samples collected over a ten year period up to 1998, and found that 90% of the lead originated from gasoline produced in the 1960s and 1970s. The research also indicated that lead from 1980s gasoline is more easily mobilised by surface runoff than lead from 1960s and 1970s gasoline. Lead from the two earlier decades is probably located deeper in soils and in the sediments of river beds, say the researchers.
“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,” said Russell Flegal, Professor and Chair of Environmental Toxicology at UCSC. “We’re seeing lead contamination from the 1960’s 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.”
Though there is no evidence that the lead in the bay threatens human health or marine organisms, other heavy metals and some organic pollutants are likely to show similar patterns of persistence in the environment, says Flegal. These include contaminants, such as mercury, that are serious problems in San Francisco Bay, posing threats to humans and wildlife.
Even after the lead stops entering the bay, the lead-contaminated sediments are likely to remain there indefinitely, explained Flegal. According to the study’s primary author, Douglas Steding, a graduate student at UCSC, there are three reasons why the bay maintains such a high level of lead sediments:
- 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;
- the large amount of lead-contaminated sediments within the bay, which continually release lead into the water;
- the limited transport of contaminated sediments from the bay into the open ocean.
“The bay just doesn’t clean itself efficiently,” said Steding. “The only outflow is through the Golden Gate, so sediment flow out of the bay is limited.”
Flow has also been considerably reduced by diversions of water for urban, industrial and agricultural uses, leaving contaminated sediments in the southern reach of the bay to remain there indefinitely, say the researchers.
“There is very little movement of sediments out of the South Bay,” said Steding. “It’s akin to a stagnant lagoon, and only during periods of high water flow does it get any substantial flushing.”