Mercury Contamination from Historic Gold Mining in California Notes
Mercury Contamination from Historic Gold Mining in California
Historic Gold Mining and Mercury Use
Miners in the western United States used mercury to recover gold from placer and hardrock mines. Hydraulic mining, which involved using monitors (water cannons) to break down placer ores, resulted in significant mercury loss, estimated at 10 to 30 percent per season.
Hydraulic Mining
Around 1852, hydraulic mining evolved, utilizing water to strip soil and gravel, directing the resulting slurries through sluices for gold recovery. In 1884, the Sawyer Decision restricted discharge of mining debris in the Sierra Nevada region. Dredging of gold-bearing sediments continued into the early 1960s.
Mercury Mining
Most mercury used in gold recovery came from the Coast Ranges. Total mercury production in California between 1850 and 1981 was over lb, with about 12 percent used for gold recovery in California.
Mercury Use in Hydraulic Mining
In sluices, mercury was added to enhance gold recovery. Modifications like undercurrents were developed to capture fine-grained sediment. Mercury loss occurred through flouring, leakage, and downstream transport. Annual mercury loss from a typical sluice was likely several hundred pounds. The total mercury lost to the environment from hydraulic placer-gold mines may have been 3–8 million lb.
Transport and Fate of Mercury
The distribution and fate of mercury used in historic gold mining remains largely unknown and is the focus of ongoing studies by the USGS.
The Bear–Yuba Project
The American, Bear, Yuba, and Feather River watersheds have been affected by hydraulic mining. USGS scientists have demonstrated a positive correlation of mercury bioaccumulation with intensity of hydraulic gravel mined in the Sierra Nevada. Sampling of water, sediment, and biota is ongoing to identify mercury "hot spots."
Mercury Contamination: Key Issues
Risks to Human Health:
Consumption of contaminated fish
Improper handling of contaminated sediments
Inhalation of mercury vapors
Challenges for Land Management:
Public access to contaminated areas
Physically hazardous sites
Environmental consequences of resource development
Environmental Fate of Mercury:
“Hot spots” at mine sites
Contaminated sediments
Transport to downstream areas
Bioaccumulation and biomagnification in food chain
Fish Consumption Advisories for Mercury
Methylmercury , a potent neurotoxin, is especially harmful to fetuses and young children. Fish consumption advisories for mercury are in place in several California waterbodies. Mercury methylation, the transformation from elemental mercury to methylmercury, is influenced by environmental factors. Methylmercury concentrations increase up the food chain through biomagnification.