California Water Science Center
Mercury Contamination from Historic Gold Mining in CaliforniaBy Charles N. Alpers and Michael P. Hunerlach
Mercury contamination from historic gold mines represents a potential risk to human health and the environment. This fact sheet provides background information on the use of mercury in historic gold mining and processing operations in California, and describes a new USGS project that addresses the potential risks associated with mercury from these sources, with emphasis on historic hydraulic mining areas. Miners used mercury (quicksilver) to recover gold throughout the western United States at both placer (alluvial) and hardrock (lode) mines. The vast majority of mercury lost to the environment in California was from placer-gold mines, which used hydraulic, drift, and dredging methods. At hydraulic mines, placer ores were broken down with monitors (or water cannons, fig. 1) and the resulting slurry was directed through sluices and drainage tunnels, where gold particles combined with liquid mercury to form gold-mercury amalgam. Loss of mercury in this process was 10 to 30 percent per season (Bowie, 1905), resulting in highly contaminated sediments at mine sites (fig. 2). Elevated mercury concentrations in present-day mine waters and sediments indicate that hundreds to thousands of pounds of mercury remain at each of the many sites affected by hydraulic mining. High mercury levels in fish, amphibians, and invertebrates downstream of the hydraulic mines are a consequence of historic mercury use.
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 historic gold mining. This information will be useful to agencies responsible for prudent land and resource management and for protecting public health. Origins of Hydraulic Mining
Hydraulic mines operated on a large scale from the 1850s to the 1880s in California's northern Sierra Nevada region, where more than 1.5 billion cubic yards of gold-bearing placer gravels were worked. In 1884, the Sawyer Decision prohibited discharge of mining debris in the Sierra Nevada region, but not in the Klamath-Trinity Mountains (fig. 4), where hydraulic 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 early 1900s. Dredging of gold-bearing sediments in the Sierra Nevada foothills has been an important source of gold since the early 1900s. Mercury also was used extensively until the early 1960s in the dredging of flood plain deposits, where over 3.6 billion cubic yards were mined. Mercury is recovered today as a by-product from large- and small-scale dredging operations.
Mercury Mining Mercury Use in Hydraulic Mining Most of the mercury used in gold recovery in California was obtained from the Coast Ranges mercury belt on the west side of California's Central Valley (fig. 4). Historic mercury production peaked in the late 1870s (fig. 5). Gravel and cobbles that entered the sluices 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 escaped from the sluice through leakage into underlying soils and bedrock, and some was transported downstream with the placer tailings. Some remobilized placer sediments remain close to their source in ravines that drained the hydraulic mines. Minute particles of quicksilver were found floating on surface water as far as 20 miles downstream of mining operations (Bowie, 1905).
Averill (1946) estimated that, under the best operating conditions, 10 percent of the mercury used was lost and, under average conditions, the annual loss of mercury was up to 30 percent. Mercury use varied from 0.1 to 0.36 pounds per square foot of sluice. We estimate that a typical sluice had an area of 2,400 square feet and used up to 800 lb of mercury during initial start-up, after which several additional 76-lb flasks were added weekly to monthly throughout its operating season (generally 6 to 8 months, depending on water availability). Assuming a 10-30 percent loss, the annual loss of mercury from a typical sluice was likely several hundred pounds during the operating season. From the 1860s through the early 1900s, hundreds of hydraulic placer-gold mines operated in the Sierra Nevada. The total amount of mercury lost to the environment from these operations may have been 3-8 million lb or more, from estimates by Churchill (1999) that about 26,000,000 lb of mercury were used in California. Historic records indicate that about 3 million lb of mercury were used at hardrock mines in stamp mills, where ores were crushed. Mercury was also used extensively at drift mines and in dredging operations. The present distribution and fate of the mercury used in historic gold mining operations remains largely unknown, and is the focus of ongoing studies.
The Bear-Yuba Project
Fish Consumption Advisories for Mercury Methylmercury (CH3Hg+) is a potent neurotoxin that impairs the nervous system. Fetuses and young children are more sensitive to methylmercury exposure than adults. Methylmercury can cause many types of problems in children, including brain and nervous system damage, retardation of development, mental impairment, seizures, abnormal muscle tone, and problems in coordination. Therefore, the consumption guidelines in areas where CH3Hg+ is known to occur in fish at potentially harmful levels tend to be more restrictive for children as well as for pregnant women, nursing mothers, and women of childbearing age. In the United States, as of 1998, there were a total of 2,506 fish and wildlife consumption advisories for all substances, of which 1,931 (more than 75 percent) were for mercury. Forty states have issued advisories for mercury, and ten states have statewide advisories for mercury in all freshwater lakes and (or) rivers. In California, as of 1999, there were fish consumption advisories for mercury in 13 waterbodies, including the San Francisco Bay and Delta Region and several areas in the Coast Ranges affected by mercury mining (fig. 10; compare with fig. 4). Data on CH3Hg+ levels in fish are presently insufficient for public agencies to determine whether advisories are warranted for lakes and rivers in areas affected by historic gold mining, such as the Sierra Nevada foothills.
References Cited Averill, C.V., 1946, Placer mining for gold in California: California State Division of Mines and Geology Bulletin 135, 336 p. Bowie, A.J., 1905, A practical treatise on hydraulic mining in California: New York, Van Nostrand, 313 p. Bradley, E.M., 1918, Quicksilver resources of the state of California: California State Mining Bureau Bulletin 78, 389 p. California Office of Environmental Health Hazard Assessment, 1999, California Sport Fish Consumption Advisories, 1999: Sacramento, Calif., 9 p. Churchill, R., 1999, Insights into California mercury production and mercury availability for the gold mining industry from the historical record: Geological Society of America Abstracts with Programs, v. 31, no. 6, p. 45. Hunerlach, M.P., Rytuba, J.J., and Alpers, C.N., 1999, Mercury contamination from hydraulic placer-gold mining in the Dutch Flat mining district, California: U.S. Geological Survey Water-Resources Investigations Report 99-4018B, p. 179-189. (PDF) Long, K.R., DeYoung, J.H., Jr., and Ludington, S.D., 1998, Database of significant deposits of gold, silver, copper, lead, and zinc in the United States: U.S. Geological Survey Open-File Report 98-206A, 33 p. Slotton, D.G., Ayers, S.M., Reuter, J.E., and Goldman, C.R., 1997, Gold mining impacts on food chain mercury in northwestern Sierra Nevada streams (1997 revision), Appendix B in Larry Walker Associates, 1997, Sacramento River watershed mercury control planning project?report for the Sacramento Regional County Sanitation District, 74 p. Tunnel sluice with mercury-contaminated sediments
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