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Mercury: A California Gold Rush Legacy

Throughout the Sacramento River Basin there are numerous lakes, rivers, and streams currently on the Clean Water Act Section 303(d) list of impaired water bodies for mercury. Mercury enters waterways when soils erode, atmospheric dust falls to the ground, and mineral springs discharge. Another significant source is cinnabar ore (mercury sulfide) that was mined in the Inner Coast Ranges for elemental mercury (quicksilver). This liquid form of mercury was transported from the Coast Ranges to the Sierra Nevada for gold recovery where several million pounds of mercury were lost to the environment during the gold rush.

Today, contaminated mine sites, riverbanks, and debris basins are a legacy source of mercury, and the scale is daunting, with more than 47,000 abandoned mines in California. Liquid mercury is encountered today by large- and small-scale dredging operations in the rivers and streams once used to wash away unwanted waste from historical mine operations. In urban areas, municipal stormwater and sewage also contain mercury from various consumer products.

Why Mercury Is a Problem

Methylmercury is a potent neurotoxin, and thousands of people eat fish and shellfish from mercury-contaminated waterways in the Sacramento River Basin. Methylmercury is readily absorbed from water and food, and therefore concentrations multiply greatly between water and top predators of aquatic food chains. The cumulative result of this bioaccumulation is more than million-fold increases in concentrations of methylmercury in predatory fish such as bass and fish-eating wildlife such as terns and eagles.

State regulators recently listed 100 water bodies throughout the Central Valley as impaired by mercury. Mercury contamination affects aquatic life and wildlife habitat; sport, subsistence, and commercial fishing; and rare and endangered species habitat. State health managers have posted dozens of fish consumption advisories.

Addressing the Mercury Problem

Government and land management agencies are tasked with cleaning up mercurycontaminated areas on public lands. Meanwhile, there are few incentives for voluntary private lands cleanup, and regulations regarding assessment and cleanup target levels are not always consistent or understandable. Reducing methylmercury levels in fish will require local, waterway-specific solutions because each waterway has its own unique set of mercury sources, land ownership patterns, and available resources. Both sources and the process of converting elemental mercury into methylmercury must be addressed. It takes less than half of a measuring cup of methylmercury to contaminate 3 million fish to levels above consumption advisories. That amount is a small fraction of the 1,500 pounds (which would fill 25 two-liter bottles) of all the mercury carried by the Sacramento River annually, which in turn is a minuscule portion of the 3 million tons (which would fill 40,000 truckloads) of sediment delivered by the river contaminated by that mercury.

Transformation of various kinds of mercury into methylmercury is a crucial step in the bioaccumulation process. An unfortunate irony is that more productive ecosystems such as wetlands and flooded agriculture tend to generate more methylmercury. A national survey by the USGS of 21 large watersheds across the U.S. demonstrated that methylmercury levels in fish were determined largely by methylmercury concentration in water, and that the percentage of wetlands in these watersheds was an important factor influencing methylmercury concentrations in water. The CALFED Program provided more than $30 million for scientific research into the status of mercury in the Delta and the mechanisms that control the formation of methylmercury. Now actions are needed to implement control measures. Proponents of projects—to rehabilitate land, restore riparian habitat, and manage sediment and water—need tools and incentives to encourage environmentally sound removal techniques that minimize the transport of mercury and its transformation to methylmercury. Regulators need to measure current conditions and to monitor over time the effectiveness of their policies on project outcomes. Health officials need to convey accurate, relevant public messages.

The above article is reprinted A Roadmap to Watershed Management, in the chapter Sacramento River Basin.