Gold Rush’s toxic legacy
That old fever thermometer in your medicine cabinet contains half a gram of mercury, enough to render all fish in a 25-acre lake unsafe to eat. In contrast, Gold Rush Era miners used over 26 million pounds of mercury in Nevada County alone, depositing as much as 8 million pounds into the watershed.
“History allows us to examine the consequences of what were once thought of as good ideas,” said archaeologist, author and photographer Hank Meals at a public conference on mercury held last month in Nevada City.
The “good idea” Meals referred to was the utilization of mercury, also known as quicksilver, to bond to small particles of gold that otherwise would have been washed away through hydraulic mining procedures.
“Quicksilver was added to amalgamate with fine gold,” he said. “It creates a clay-like substance that can be heated and the gold removed.”
Before the advent of hydraulic mining, rivers and streams throughout the Mother Lode were not the broad cobble-filled watercourses they are now. Much of the rocks and the sand deposited in streambeds are mine tailings, a byproduct of unbelievably erosive hydraulic mining practices. These mine tailings can be 80 to 100 feet deep.
Hydraulic mining removed more than 1.5 billion cubic yards of gravel from the 1850s until it was prohibited in 1884, though mercury continued to be used in dredging flood plain deposits until the 1960s. Dredgers and gold panners still recover copious amounts of mercury from rivers and streams.
Vast mercury deposits remain buried in streambed sediments. Mercury is dislodged as winter storms and human activities disturb watercourses. This mercury inexorably makes its way to the Sacramento-San Joaquin Delta and on to the San Francisco Bay.
Passing through areas of low water flow, mercury particles interact with warm water and bacteria-laden sediment, and are chemically converted into highly toxic methylmercury, which is readily absorbed by fish, birds, animals and humans.
The level of methylmercury becomes concentrated in tissue as larger fish eat smaller ones, a process known as bioaccumulation. Predatory fish such as bass and catfish can contain mercury levels harmful to humans and wildlife.
“The bigger the fish, the more mercury,” said Dr. Charlie Alpers, a research chemist with the United States Geological Survey (USGS).
“The bioaccumulation processes can increase mercury concentrations in fish 10,000 to 100,000 times,” said Dr. Susan Klasing, staff toxicologist with the Office of Environmental Health Hazard Assessment, California Environmental Protection Agency (Cal-EPA). “Fish consumption is the major source of human exposure. Almost all fish contain detectable levels.”
The human gastrointestinal tract absorbs up to 95 percent of the mercury in contaminated fish. As mercury builds up in the body, it can damage the immune, renal, cardiovascular and reproductive systems. As a neurotoxin, it can cause nerve damage resulting in loss of coordination, numbness, slurred speech, blurred vision, hearing loss, tremors and mental disturbances. High levels can lead to “blindness, seizures, and the inability to move,” said Klasing.
At highest risk are children and pregnant women. Decreases in IQ and developmental delays can be triggered by exposure in childhood or pre-natally, since methylmercury crosses the placental barrier.
Governmental agencies have posted numerous public health advisories for fish consumption due to elevated mercury levels in the Sacramento River Basin and in the San Francisco Bay.
Funded by a grant from the State Water Resources Control Board and hosted by the South Yuba River Citizens League and the Nevada County Resource Conservation District, last month’s mercury conference offered a cross-pollination of ideas from scientists, environmentalists and the public. Representatives of various federal, state and local agencies presented research data and strategies for mitigating mercury contamination in Sierra watersheds.
Though mercury was used at hardrock (or underground) mines, placer (or surface) mining is responsible for most of the mercury lost environmentally in gravel beds and watersheds.
Hydraulic mining used water cannons, or monitors, to break down gold-bearing gravel deposits, literally washing whole hillsides away, removing soil, sand and gravel down to bedrock at a rate of 30,000 gallons a minute in a 9-inch stream. The largest operation was at Malakoff Diggins, where over 41 million cubic yards of gravel were removed, leaving a pit over a mile long, half a mile wide and 600 feet deep.
The resulting slurry was channeled through quicksilver-charged drainage tunnels and sluices. The USGS estimates that during initial start-up, miners used 800 pounds of mercury in a sluice, a wooden box built over a watercourse that used water and gravity to separate gold from rock debris. Sediments were discharged in streams and rivers, carrying escaped gold and mercury particles downstream.
During the six- to eight-month placer mining season, hundreds of pounds of mercury were added weekly or monthly. At each sluice, several hundred pounds a year were lost in the watershed. For every three or four ounces of gold recovered, a pound of mercury may have been lost.
Sediments at sluice box sites and tunnels contain high concentrations of mercury; thousands of pounds may remain at each site. Mine tunnels pose the added risk of highly toxic mercury vapor.
In 1999, the USGS, Bureau of Land Management and the United States Forest Service began studying the Bear River and South Yuba River watersheds, attempting to locate mercury “hot spots” that might be remediated to reduce health risks to people and the environment.
“Decaying wood from sluices, stagnant water and sulfide compounds create the perfect environment for bacteria to methylate any mercury in sediments present,” said Steven Eubanks, Tahoe National Forest Supervisor. “People look at these sites, hoping to find leftover gold and get exposed to hazardous methylmercury.”
Miners built dams to capture debris, but dams captured mercury as well. “Not all these dams are stable,” Eubanks said.
Large reservoirs constructed in the Sierra Nevada between 1948 and 1968 also trap mercury-laden sediment, slowing its downstream transportation to the Delta and the San Francisco Bay.
“Over 150,000 people fish the Bay to feed 10,000 children and 50,000 women of childbearing age,” said Dr. Khalil Abu-Saba of the San Francisco Bay Regional Water Quality Control Board. Posing the same question scientists asked 20 years ago about lead, he wonders, “Are these kids going to do less well in school?”
EPA estimates indicate nearly 10 percent of California’s landmass would have to be dredged to recover all mercury lost in mining operations. A recent cleanup of a single sluice box and tunnel at Dutch Flat cost $1.4 million.
The enormity of the problem has given rise to creative solutions. Since bacteria that causes the formation of toxic methylmercury thrive in low oxygen conditions, Abu-Saba proposes increasing the oxygen content in water. Aerating reservoirs will decrease the amount of mercury that fish can absorb.
“If you can’t stop the load, start thinking about reducing methylation,” he said.
Alpers and the USGS propose another option: slowing bacterial growth by reducing the high sulfate content of water draining from contaminated mine sites. That’s easier and less expensive than removing mercury and prevents acid mine drainage. Methylation is prevented by depriving bacteria of its growing medium.
Scientists hope these and other strategies will mitigate damage caused by the continuing legacy of the “good ideas” of the Gold Rush.