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Mercury is a rare, dense metal, slightly more common than gold in the earth's crust. It has unusual properties that have made it valuable in metallurgy, electrical systems and chemical processes. It is a liquid at ordinary temperatures and evaporates when exposed to the atmosphere. These unusual physical characteristics, combined with mercury's common use from the beginning of the industrial revolution, have contributed to its widespread dispersion through the atmosphere to land and water around the globe by both wet and dry precipitation. The US EPA estimates that mercury vapor residence time in the atmosphere exceeds one year.

Gold pan with more than 30 grams of mercury from 1 kilogram of mercury-contaminated sediments collected in a drainage tunnel.
Gold pan with more than 30 grams of mercury from 1 kilogram of mercury-contaminated sediments collected in a drainage tunnel.

Mercury has been recognized as a serious environmental contaminant for many years. As a result, industrial uses have declined significantly over recent decades as effective substitutes have been developed. The US EPA estimates that in the United States the single largest remaining source of mercury discharges into the environment is coal combustion.

Mercury concentrations in coal are generally less than one part per million (ppm), but, in the United States alone in 1995, the large tonnage of coal combustion introduced an estimated 50 tons of mercury into the atmosphere. Mercury released to the environment from oil refined in the United States is approximately 5% of that which may be derived from coal combustion. The total amount of mercury emitted to the atmosphere from coal and oil combustion in North America was about 70 million tons in 2005. A comparable estimate for global mercury emissions from coal and oil combustion is 890 tons, of which 295 tons were emitted in China (Pirrone and others, 2010).

Environmental mercury contamination concerns in California are focused less on atmospheric sources, and more on aquatic sources for several natural and historic reasons.

Mercury's discovery in California predates the discovery of gold by several years. The first mines were located in New Almaden, about 10 miles south of present-day San Jose in the Santa Cruz Mountains. The site is now the Almaden Quicksilver County Park, Santa Clara County. The California Coast Ranges went on to be among the most productive mercury districts in the world, with major production centers from New Idria in the south to Clear Lake in the north.

In the Coast Ranges, mercury has been concentrated extensively in natural hydrothermal systems, including active thermal springs that continue to discharge into streams and lakes, and in fossil (inactive) systems that were the sites of commercial mercury mining. The hydrothermal activity contributes to high natural background levels of mercury in parts of the Coast Ranges. The discovery of commercial mercury ore bodies led to the development and operation of numerous mines from the 1840s to the early 1960s, from which more than 220,000,000 pounds of elemental mercury were produced. There were few controls on the dispersion of mercury from these operations, leading to significant increases in environmental mercury concentrations in affected soil, sediment, plants, fish, and other animals. Health advisories on fish consumption because of elevated mercury concentrations are widespread in the Coast Ranges, where more than a dozen separate water bodies are affected, including San Francisco Bay, Lake Berryessa, and Clear Lake.

The 1848 discovery of gold in the Sierra Nevada created a ready market for mercury produced by the mines in California's Coast Ranges. Mercury forms a relatively insoluble amalgam with gold, and miners used this property to increase gold recovery. Millions of pounds of mercury were used, especially in hydraulic placer mining operations that displaced and processed more than 1.5 billion cubic yards of gold-bearing sediments in the Sierra Nevada. Gold- bearing sediments were washed through sluice boxes over mercury that was loosely held in riffles and troughs. Coarse gold was trapped primarily by gravity separation, while the recovery of fine-grained gold was achieved largely with mercury. An estimated 10 to 30 percent of the mercury was lost to the environment in this process and transported into streams and reservoirs along with the discharged sediments (tailings or "slickens") from the hydraulic mining operations.

Mercury transport schematic
Tunnel sluice with mercury-contaminated sediments

In many gold-mining areas where mercury was used, it is still relatively easy to find quantities of liquid elemental mercury in sediments and stream channels. Of even greater environmental concern is the presence of methylmercury, an organic form of mercury that is a potent neurotoxin and is especially detrimental to developing fetuses and children. Methylmercury accumulates and biomagnifies in the food chain, reaching highest concentrations in predatory fish such as bass and other species which are prized by anglers. Numerous water bodies in California have fish-consumption advisories because of mercury contamination from historical mining. Several of these advisories are based on data collected by the USGS, including those in Trinity County, and in the Bear, Yuba, and American River watersheds in the Sierra Nevada. For information on these advisories, see the web site of the California Office of Environmental Health Hazard Assessment.

Mercury from hydraulic mining has been transported with sediments downstream into the San Francisco Bay/Sacramento-San Joaquin Delta estuary, where it has contributed to elevated mercury concentrations in fish, resulting in additional consumption advisories and regulatory action through the Total Maximum Daily Load (TMDL) process. For information on the TMDL process see the web sites of the California Regional Water Quality Control Boards:

San Francisco Bay Mercury TMDL

Sacramento - San Joaquin Delta Methylmercury TMDL

Ongoing studies by USGS are focused on characterizing and quantifying sources of mercury and methylmercury to the Bay-Delta, including continuing runoff from mercury mining areas in the Coast Ranges and from gold-mining areas in the Sierra Nevada, and from resuspension and diffusive transport from mercury-contaminated sediments already in the rivers and reservoirs in the Bay-Delta watershed.

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Page Last Modified: Thursday, 23-Feb-2012 14:33:35 EST