Mercury

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.

Elemental mercury from the bottom of Peoples Creek

Mercury concentrations in coal are generally less than one part per million (ppm), but, in the United States alone, the large tonnage of coal consumed introduced an estimated 50 tons of mercury into the atmosphere in 1995.

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 13 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, transported into streams and reservoirs along with the discharged sediments (tailings or ³slickens²) from the hydraulic mining operations.

Tunnel sluice with mercury-contaminated sediments

Mercury from hydraulic mining has been transported with sediments downstream into the San Francisco Bay/Sacramento-San Joaquin Delta estuary, where it has likely contributed to elevated mercury concentrations in fish, resulting in consumption advisories. Ongoing studies will attempt to determine the present sources of mercury and methylmercury to the Bay-Delta, including continuing runoff from mercury mining areas in the Coast Ranges, from gold-mining areas in the Sierra Nevada, and from resuspension and diffusive transport from mercury-contaminated sediments already in the river and Bay-Delta systems.

Links to: Bear-Yuba, Dutch Flat, and Trinity River Watersheds