More than 150 years ago California's Sierra Nevada foothills echoed with the jubilant cry "There's gold in them thar hills!" Today, those words have morphed into a more subdued but equally urgent warning: there's mercury in the San Francisco Bay, and it isn't going anywhere soon.

Mercury contamination from mining operations in the Sierra Nevada a century and a half ago continues to pollute San Francisco Bay. (Photo Roy Kaltschmidt)

The lingering problem dates back to the Gold Rush, when miners used mercury by the wagonload to extract gold flecks from sediment. This toxic legacy is still finding its way into tributaries that feed the San Francisco Bay. How long it remains in the Bay once it arrives has been difficult to pin down, but an innovative new study indicates that it takes as long as 50 years for the Bay's mercury concentrations to respond to changes in input.

"The good news is the San Francisco Bay is getting cleaner; the bad news is it will take a long time to flush mercury out of the system," says Tom McKone, a staff scientist in Berkeley Lab's Environmental Energy Technologies Division and an Adjunct Professor at UC Berkeley's School of Public Health. He collaborated with former Berkeley Lab scientist Matthew MacLeod and Donald Mackay of Canada's Trent University on the study, which appears in a recent issue of the journal Environmental Science & Technology.

"Our work also demonstrates that we have to live with the messes we make for a long time. It's been 150 years since the Gold Rush, and we're still paying for it," says Mckone.

For the past several years, McKone and his colleagues have developed ways to model the ebb and flow of harmful chemicals such as PCBs (polychlorinated biphenyls) and pesticides across large regions of North America. They track a chemical's movement and accumulation by taking into account the chemical's ability to reach equilibrium with a region's soil, water, air, and vegetation. These chemical mass balance models allow the scientists to determine the extent to which a harmful chemical is absorbed into the environment, and how much is free to ride the winds and currents to other regions.

The team's San Francisco Bay research began as part of a UC Berkeley Superfund Basic Research Program project to develop methods for reconstructing historical exposures to metals in estuaries. They turned their attention to mercury after meeting with scientists from the San Francisco Estuary Institute, which, along with several regulatory agencies, is working to understand and resolve this longstanding problem.

Mercury, however, is especially challenging to model because it readily converts between three different species, each of which reacts with the environment in a different way. There's elemental mercury, which is the volatile liquid metal used by the 49'ers, who called it liquid silver. There's salt-like divalent mercury. And there's organic or methyl mercury, which is a potent neurotoxin that is known to be detrimental to developing fetuses and young children. It is passed from prey to predator along the food chain, which is why the California Office of Environmental Health Hazard Assessment issues fish consumption advisories for the San Francisco Bay.

"You can pick any part of the environment — water, air, and soil — and find all three mercury species in some kind of chemical balance," says McKone.

	Tom McKone and his colleagues devised a mass balance model to account for how mercury reacts with air, soil, vegetation, water, and sediment. (Photo Roy Kaltschmidt)

Modeling these permutations in the San Francisco Bay area is even more difficult because the region represents the largest estuary on the West Coast, encompassing roughly 1,600 square miles of central California and draining almost one-half the land area of California. Complicating matters, the Bay is composed of a north and south estuary, with a huge flux of water moving through each system. Every year, an estimated 2,450 million kilograms of sediment enter the Bay, some of which is deposited in the Bay and some of which exits the Golden Gate.

To track the behavior of this constantly changing toxin in a large and dynamic body of water, the team started with a mass balance model that accounts for how the three mercury species react with the region's air, soil, vegetation, water, and sediment. Based on previous research, they also assumed that mercury exits the San Francisco Bay via the Golden Gate at a rate that is slower than its conversion rate between the three different species. This allowed them to assume a constant ratio between the three species in each type of environmental medium (air, soil, etc.).

They applied this model to the entire Bay Area, extending as far east as the Carquinez Strait, where the Sacramento and San Joaquin rivers empty into the Bay. The resulting mass balance calculations revealed that continental and global background levels of mercury are largely responsible for the Bay Area's airborne mercury concentrations. But they found a much more localized source of mercury in the waters of the San Francisco Bay: contaminated sediments from long-ago mining activities. In addition, they determined that it takes decades for the Bay's mercury concentrations to respond to changes.

"If we alter the amount of mercury that enters the Bay, it doesn't reach a new equilibrium until about 50 years later," says McKone. "We are learning that it takes a long time to clean up the Bay. Unfortunately, the upstream input of mercury is very large, and will remain large for a long time."

Mining devastated California's environment in the 1850s; its toxic effects persist.

That's because California is littered with thousands of defunct mercury mines that date back to between 1850 and 1900, when miners used mercury-lined sluices to capture tiny grains of gold from sediment. Over the years, tailings from the mercury mines that supplied this process leached into the watershed, and eventually into the San Francisco Bay. These mines have been largely cleaned up and no longer pose a threat to the environment, but the mercury they released is still cycling through the sediment carried by the Sacramento and San Joaquin rivers.

"Gold mining was really hard on California's environment, and the harm remains apparent even now," says McKone. "But we are using the mistakes of the past to learn more about the life cycles of persistent chemicals in the Bay, and to learn how to protect the Bay for the future."

Additional information

• More about the Berkeley Trent (BETR) model for studying the transport of contaminants