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Mercury contamination at historic gold mining sites represents a potential risk to human health and the environment. Elemental mercury (quicksilver) was used extensively for the recovery of gold at both placer and hardrock mines throughout the western United States. In placer mine operations, loss of mercury during gold recovery was reported to be as high as 30 percent. In the Dutch Flat mining district located in the Sierra Nevada region of California, placer mines processed more than 100,000,000 cubic yards of gold-bearing gravel. The placer ore was washed through mercury-charged ground sluices and drainage tunnels from 1857 to about 1900, during which time many thousands of pounds of mercury were released into the environment. Mine waters sampled in 1998 had total unfiltered mercury concentrations ranging from 40 ng/L (nanograms per liter) to 10,400 ng/L, concentrations of unfiltered methyl mercury ranged from 0.01 ng/L to 1.12 ng/L. Mercury concentrations in sluice-box sediments ranged from 600 μg/g (micrograms per gram) to 26,000 μg/g, which is in excess of applicable hazardous waste criteria (20 μg/g). These concentrations indicate that hundreds to thousands of pounds of mercury may remain at sites affected by hydraulic placergold mining. Elevated mercury concentrations have been detected previously in fish and invertebrate tissues downstream of the placer mines. Extensive transport of remobilized placer sediments in the Bear River and other Sierra Nevada watersheds has been well documented. Previous studies in the northwestern
Sierra Nevada have shown that the highest average levels of mercury bioaccumulation occur in the Bear and South Fork Yuba River watersheds; this study has demonstrated a positive correlation of mercury bioaccumulation with intensity of hydraulic gravel mining.