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The method used depends on the type of mineral resource that is mined, its location at or beneath the surface, and whether the resource is worth enough money to justify extracting it. Each mining method also has varying degrees of impact on the surrounding landscape and environment.

Mercury contamination from historical gold mines represents a potential risk to human health and the environment. This fact sheet provides background information on the use of mercury in historical gold mining and processing operations in California, with emphasis on historical hydraulic mining areas. It also describes results of recent USGS projects that address the potential risks associated with mercury contamination.

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Miners used mercury (quicksilver) to recover gold throughout the western United States. Gold deposits were either hardrock (lode, gold-quartz veins) or placer (alluvial, unconsolidated gravels). Underground methods (adits and shafts) were used to mine hardrock gold deposits. Hydraulic, drift, or dredging methods were used to mine the placer gold deposits. Mercury was used to enhance gold recovery in all the various types of mining operations; historical records indicate that more mercury was used and lost at hydraulic mines than at other types of mines. On the basis of USGS studies and other recent work, a better understanding is emerging of mercury distribution, ongoing transport, transformation processes, and the extent of biological uptake in areas affected by historical gold mining. This information has been used extensively by federal, state, and local agencies responsible for resource management and public health in California.

Vast gravel deposits from ancestral rivers within the Sierra Nevada contained large quantities of placer gold, derived from the weathering of gold-quartz veins. Gold mining evolved from hydraulic mining of unconsolidated placer deposits in the early days of the Gold Rush, to underground mining of hardrock deposits, and finally to large-scale dredging of low-grade gravel deposits, which in many areas included the tailings from upstream hydraulic mines.

By the mid-1850s, in areas with sufficient surface water, hydraulic mining was the most cost-effective method to recover large amounts of gold. Monitors (or water cannons, fig. 1) were used to break down placer ores, and the resulting slurry was directed through sluices (fig. 2). As mining progressed into deeper gravels, tunnels were constructed to facilitate drainage and to remove debris from the bottom of hydraulic mine pits. The tunnels also provided a protected environment for sluices and a way to discharge processed sediments (placer tailings) to adjacent waterways. Gold particles were recovered by mechanical settling in troughs (riffles) within the sluices and by chemical reaction with liquid mercury to form gold-mercury amalgam. Loss of mercury during gold processing was estimated to be 10 to 30 percent per season (Bowie, 1905), resulting in highly contaminated sediments at mine sites, especially in sluices and drainage tunnels (fig. 3). From the 1850s to the 1880s, more than 1.5 billion cubic yards of gold-bearing placer gravels were processed by hydraulic mining in California's northern Sierra Nevada region. The resulting debris caused property damage and flooding downstream. In 1884, the Sawyer Decision prohibited discharge of hydraulic mining debris to rivers and streams in the Sierra Nevada region, but not in the Klamath-Trinity Mountains (fig. 4), where such mining continued until the 1950s.

Underground mining of placer deposits (drift mining) and of hardrock gold-quartz vein deposits produced most of California's gold from the mid-1880s to the 1930s. Another important source of gold from the late 1890s to the 1960s was gold-bearing sediment, which was mined using dredging methods. More than 3.6 billion cubic yards of gravel was mined in the foothills of the Sierra Nevada, where the dredging continued until 2003.

Most of the mercury used in gold recovery in California was obtained from mercury deposits in the Coast Range on the west side of California's Central Valley (fig. 4). Total mercury production in California between 1850 and 1981 was more than 220,000,000 lb (pounds) (Churchill, 2000); production peaked in the late 1870s (Bradley, 1918). Although most of this mercury was exported around the Pacific Rim or transported to Nevada and other western states, about 12 percent (26,000,000 lb) was used for gold recovery in California, mostly in the Sierra Nevada and Klamath-Trinity Mountains.

To enhance gold recovery from hydraulic mining, hundreds of pounds of liquid mercury (several 76-lb flasks) were added to riffles and troughs in a typical sluice. The high density of mercury allowed gold and gold-mercury amalgam to sink while sand and gravel passed over the mercury and through the sluice. Large volumes of turbulent water flowing through the sluice caused many of the finer gold and mercury particles to wash through and out of the sluice before they could settle in the mercury-laden riffles. A modification known as an undercurrent (fig. 5) reduced this loss. The finer grained particles were diverted to the undercurrent, where gold was amalgamated on mercury-lined copper plates. Most of the mercury remained on the copper plates; however, some was lost to the flowing slurry and was transported to downstream environments.

Gravel and cobbles that entered the sluice at high velocity caused the mercury to flour, or break into tiny particles. Flouring was aggravated by agitation, exposure of mercury to air, and other chemical reactions. Eventually, the entire bottom of the sluice became coated with mercury. Some mercury was lost from the sluice, either by leaking into underlying soils and bedrock or being transported downstream with the placer tailings. Minute particles of quicksilver could be found floating on surface water as far as 20 miles downstream of mining operations (Bowie, 1905). Some remobilized placer sediments, especially the coarser material, remain close to their source in ravines that drained the hydraulic mines.

Mercury use in sluices varied from 0.1 to 0.36 lb per square foot. A typical sluice had an area of several thousand square feet; several hundred lb of mercury were added during initial start-up, after which several additional 76-lb flasks were added weekly to monthly throughout the operating season (generally 6 to 8 months, depending on water availability). During the late 1800s, under the best operating conditions, sluices lost about 10 percent of the added mercury per year (Averill, 1946), but under average conditions, the annual loss was about 25 percent (Bowie, 1905). Assuming a 10- to 30-percent annual loss rate, a typical sluice likely lost several hundred pounds of mercury during the operating season (Hunerlach and others, 1999). From the 1860s through the early 1900s, hundreds of hydraulic placer-gold mines were operated in California, especially in the northern Sierra Nevada (fig. 6). The total amount of mercury lost to the environment from placer mining operations throughout California has been estimated at 10,000,000 lb, of which probably 80 to 90 percent was in the Sierra Nevada (Churchill, 2000).

Historical records indicate that about 3,000,000 lb of mercury were lost at hardrock mines, where gold ore was crushed using stamp mills (Churchill, 2000). Mercury was also used extensively at drift mines and in dredging operations. Mercury was used widely until the early 1960s in the dredging of auriferous sediment from alluvial flood-plain deposits. Today, mercury is recovered as a by-product from small-scale gold-dredging operations; also, mercury and gold are recovered as byproducts from some gravel-mining operations, especially in areas affected by historical gold mining. Understanding the present distribution and fate of the mercury used in historical gold mining operations is the subject of ongoing multi-disciplinary studies.

The USGS and cooperating agencies have identified several "hot spots" of mercury contamination and bioaccumulation by reconnaissance sampling of water, sediment, and biota at numerous hydraulic mine sites in the Bear-Yuba watersheds (Alpers and others, 2005). Subsequently, some mercury-contaminated mine sites have been remediated by other federal agencies, and remediation plans are being developed for other sites. Mercury contamination has also been investigated in dredge fields at lower Clear Creek (Ashley and others, 2002), the Trinity River, and the lower Yuba River (Hunerlach and others, 2004). These investigations show that total mercury concentrations in dredge tailings tend to be most elevated in the finest grained sediments. The State of California has listed several water bodies in the Bear-Yuba watersheds as impaired with regard to beneficial uses, starting a regulatory process that may include eventual mercury-load reduction through Total Maximum Daily Loads (TMDLs). The USGS is providing data and information to stakeholders through ongoing studies of mercury and methylmercury loads in the Bear River, mercury fluxes from reservoir sediments (Kuwabara and others, 2003), mercury methylation and demethylation processes in sediment, and mercury bioaccumulation in the food web of Camp Far West Reservoir.

Uranium is a naturally-occurring radioactive element that has been mined and used for its chemical properties for more than a thousand years. It is now primarily used as fuel for nuclear reactors that make electricity. Uranium can be recovered in two ways: by conventional mining of the rock (ore), or by using strong chemicals to dissolve uranium from the rock that is still in the ground and pumping it to the surface.

Uranium eventually decays to radiumradium A naturally occurring radioactive (unstable) element that forms when uranium or thorium decay. Elevated levels of radium can contaminate water supplies. Radium also poses a risk when it decays to form radon, a radioactive gas.. Radium decays to release a radioactive gas called radon. Open pit uranium milling and in situ mining sites do not pose a significant radon risk to the public or to miners; the radon disperses into the atmosphere. In the past, the waste rock produced by underground and open pit mining was piled up outside the mine. This practice has caused problems, including on Navajo lands where more than half of the small, abandoned uranium mines from the middle of the 20th century and their wastes remain. Wind can blow radioactive dust from the wastes into populated areas and the wastes can contaminate surface water used for drinking. Some sites also have considerable groundwater contamination. e24fc04721