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Geotimes
 Published by the American Geological Institute
November 2000
Newsmagazine of the Earth Sciences

News Notes

Environmental geochemistry
Lingering lead

Lead is persisting in the San Francisco Bay in contrast to decreased concentrations in other nonestuarine environments, say University of California, Santa Cruz (UCSC) scientists. Using lead isotopic compositions, they found that lead from 1960s-1970s gasoline still remains in the bay and at least 90 percent of late 1980s lead has yet to enter the bay. Information about lead’s unexpected persistence may also have implications for other contaminants like mercury, which have biological ramifications.

“Even though we phased out leaded gasoline in 1992, we are still affecting the environment,” says Doug Steding, an earth sciences Ph.D. student at UCSC. “We are seeing that, isotopically, there is no change in lead over the last 10 years in the South Bay. The degree of persistency is surprising,” Steding says.


Twice a year the University of California, Santa Cruz, takes out the 
Research Vessel David Johnston to help with sampling the bay's surface
water. Photo courtesy of Genine Scelfo.

Throughout the bay, lead currently suspended in the water column and sediment is mostly from 1960s-1970s leaded gasoline. More than 90 percent of late 1980s lead emissions is still in soils within the river basins of the Sacramento and San Joaquin rivers and are gradually being washed into the northern reach of the bay by winter rain. The hydraulic mine sediments from the rivers have slightly diluted the relative amount of 1960s-1970s gasoline lead in the northern reach.

“In the ocean, we can see declines in lead concentrations that we can’t see in the bay because of the nature of the geologic setting,” says Sam Luoma, a USGS hydrologist. Lead concentrations have decreased in the ocean, atmosphere, polar snow and ice, and rivers in North America since the phase-out of leaded gasoline, Luoma adds.

Steding, Charles Dunlap, a research fellow in the Environmental Toxicology Department, and Russell Flegal, a professor of Earth Sciences, Aquatic and Environmental Geochemistry, both of UCSC, published their results in the Sept. 12  Proceedings of the National Academy of Science. The 10-year-long study used samples taken from the San Francisco Bay and the mouths of the Sacramento and San Joaquin rivers.

Sediment was deposited in the bay until around 1950 when erosion became the dominant process, Luoma says. A large factor in this change was the construction of dams in the river basins that essentially starved the bay of sediment. Because the bay is shallow and wind and tidal currents strongly affect it, sediment contaminated with lead is constantly resuspended in the water column.

“Estuaries are usually thought of as sinks. But in this case, it is a source — the sediment within the estuary is a source for lead contaminants,” Steding says. The size of the bay’s mouth also confines the sediment flow from the bay to the ocean. This natural limitation, combined with human-induced changes in the hydrology of the area, make the bay unable to efficiently reduce contaminants as effectively as other environments. As a result, contaminants retained by hydrogeological conditions affect the terrestrial and aquatic communities in the bay.

Lead concentrations in the bay are currently not hazardous. However, other contaminants, like mercury, are exceeding suggested limits and are a danger to the bay’s wildlife. “This research really gives us a model for contaminants in the bay,” Steding says.

Mercury cycles through estuarine environments in a similar way to lead. But scientists do not know the exact residence time of mercury within the bay because, unlike lead, mercury does not have a natural variability in its radioactive decay.

Isotopic data allow scientists to fingerprint the nonpoint sources — such as gasoline from different decades in the case of lead isotopes. Isotopic compositions, therefore, are better indicators for how long contaminants stay within a system than are mass concentrations. By knowing the degree of persistency of lead, scientists may gain a better understanding of mercury’s potential impacts on the biological communities of the bay. Researchers can use this model for other contaminants, such as PCBs, chromium and nickel.

Julia Cole
Geotimes contributing writer



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