The Bear River's Gold Rush Legacy
One hundred and twenty two years after the last hydraulic gold mining operation in the Sierra Foothills closed down, the Bear River is still a Gold Rush-era river in recovery from mercury contamination and will remain so for hundreds of years to come.
Continued high levels of mercury in present day river sediments indicate that the bulk of the estimated 2.5 million pounds of the heavy metal that were lost in the Bear River Watershed during 32 years of hydraulic mining are still there, trapped in the 1.5 billion cubic yards of sediment that were stripped from hillsides by high pressure water cannons the miners called monitors.
When added to the riffles and troughs of large sluices, liquid mercury captured gold particles falling out of the tons of gravel and soil being washed through in the swift current. Mercury was inevitably lost into the river and accumulating sediments. In the post-hydraulic mining era, extensive use of mercury in dredging an estimated 3.6 billion cubic yards of flood plain deposits for gold kept contamination levels rising into the early 1960s.
Scientists who are studying the problem agree that the Bear River Watershed was the most heavily mined in the Sierra Nevada. One of them, Charles Alpers, is a U.S. Geological Survey research chemist and project chief of several projects and studies related to mercury in the environment and its relationship to historical mining sites.
All that glitters is not gold. Mercury may shine in the bottom of your , as millions of pounds remain in the river gravels as a legacy of the mining era. More dangerous is methyl mercury, a natural breakdown of mercury which moves up the food chain and can be found at dangerous levels in fish tissue. Charles Alpers of the US Geological Survey is investigating mercury methylation in the Bear River watershed.
Alpers says recent data on the environmental fate of those vast accumulations of contaminated mining sediments doesn't support the optimistic theory held in the early 1900s that they would wash out in 100 years or so.
"It's actually going to take a lot longer than that," he says. "The finer sediment does gets washed out, but the coarse stuff gets left behind and the cobbles end up armoring the stream and you need bigger and bigger floods to move stuff. In some places, Greenhorn Creek for instance, the deposits are still 60-to-100 feet deep. It will be hundreds of years before Greenhorn returns to pre-mining conditions."
And however long it takes for mining sediments to be flushed out of the river, mercury in the water will be the river's enduring legacy from the quest for gold. That in turn will perpetuate the bioaccumulation of mercury in the river's fish populations, with concentrations increasing by an average of five times in each succeeding stage of the food chain.
Alpers says it is more accurate to speak of methyl mercury, the organic form of mercury that is produced by microorganisms and which is a neurotoxin for humans, as the pollutant found in fish.
"Methyl mercury is the critical form for getting mercury into the food web and the primary risk to people and animals in the ecosystem is from eating fish," he says. "Bass species (spotted, large and small mouth) and catfish are the highest level of fish predators in the Bear River. That's where the human health risk is. There's less risk to human life and ecosystems at lower parts of food web, and from swimming and actual contact with the water."
Testing done in 1999 confirmed the risk from eating fish, resulting in an advisory against eating bass caught in Camp Far West Reservoir for women and children, and a general advisory not to eat more than one sport fish meal a week from the Bear River.
How higher dietary levels of mercury may be affecting other wildlife populations isn't well understood, Alpers says. "There's been relatively little work done on the effects of mercury on wildlife. Nobody has tested ospreys, for instance, and we can't catch and test river otters."
He says that the levels of mercury found throughout the watershed are by no means equal or uniform, and that frequently occurring spikes trigger increased water sampling to try and locate the source.
"Levels coming out of Rollins Lake are pretty low, but then they spike around the Dog Bar area," he says, "and we don't know why it happens. There's another marked increase below Camp Far West all the way to Highway 65 near Wheatland. USGS is proposing to take samples to try and pinpoint where the source is, and then restore habitat in that same reach."
Often those spikes can be traced to abandoned tunnels that were originally opened by drift (gravel) miners. Alpers says estimates of the numbers of such tunnels in the Sierra Nevada run into the hundreds, and that maybe only as many as 20 in the entire Bear River Watershed have been found.
"In Greenhorn Drainage alone we've mapped 12-15 tunnels and that's not all of them," he says. "We know of several big ones draining other diggings, including two big ones draining the Malakoff Diggins. Every hydraulic pit had at least one major tunnel, along with several smaller ones."
The rediscovery of these old tunnels is seldom a cause for general rejoicing. Alpers thinks the recent ground collapse that killed a man in Alta was likely an old drift mine tunnel. The rush of would-be miners following the reappearance of the Polar Star tunnel near Dutch Flat in 1999 actually started on the Web.
USGS got wind of it after a man from the Midwest posted a story on the Internet about pulling some gold out of an old tunnel during a summer vacation. That he also took 40 pounds of mercury out of the tunnel was what really caught the agency's attention. The excellent directions he gave to find the tunnel were so accurate that by the time Alpers and representatives from the Regional Water Quality Control Board arrived late on the scene "the place had a high beer can index," Alpers says.
"One guy was stripping old sluice boards to use making planter boxes. We tested the water coming in and going out and where the numbers were already 20 times higher than the standards for mercury, they shot up to 30 times higher in disturbed water samples. The EPA did an emergency response and spent about $1.4 million to clean up the attractive nuisance aspects of the tunnel, but nobody's paying for ongoing monitoring."
For more information:
- Mercury Contamination from Historical Gold Mining in Califorinia (USGS Fact Sheet)
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