‘Hot spot’ cools
May 21, 2007
Men in hard hats scrambled 25 feet up a ladder to the opening of an abandoned mine tunnel found near Greenhorn Creek. The dark 200-foot chasm known as the Boston Mine leached mercury into the environment for more than a century after operations were abandoned there in the 1880s, when hydraulic mining became outlawed.
The mine is free of mercury now, thanks to a team of mine experts, engineers and geologists who spent several months cleaning up the hot spot using 75-year-old gold-extracting technology.
The team, along with a local film maker who documented the project, recently flew to Washington D.C. to accept the U.S. Department of the Interior’s 2006 Environmental Achievement Award.
The Boston Mine is one of 12 mercury hot spots out of 126 abandoned mines identified in a regional study of federal lands.
“This is the first time we know of that this technology has been used successfully at an abandoned hydraulic mine site,” said David Lawler, coordinator of the Abandoned Mine Lands Program for the Bureau of Land Management.
“We applied an existing technology to fix a modern problem,” Lawler said.
Toxic mines widespread
California’s unique geology provides the richest gold and mercury deposits in the world. Because of this, there are 49,000 abandoned mines in California alone, and Nevada County is considered the largest hydraulic mining center in the world, said Lawler.
Miners used mercury in their sluice boxes because of the element’s ability to amalgamate with gold.
“Any place gold was mined, they used mercury,” Lawler said.
It is estimated that 10 to 20 percent of the mercury used in gold extraction during placer and hardrock mining during the mid-to-late 1800s was lost to the environment, Lawler said.
As much as 10 million pounds of mercury resides in the headwaters and lower reaches of the Bear and Yuba rivers, Lawler estimates. Mercury is an element that doesn’t decompose and poses a risk to fish and other organisms up the food stream, especially the developing nervous systems of infants and children.
In the Greenhorn Creek watershed – where thousands of miners once lived and worked in the Red Dog and You Bet mining districts – dozens of abandoned mine tunnels remain on private property.
“Question is, who is going to pay for it on private lands?” asked Lawler as he and Tetra Tech geologist Greg Reller maneuvered a rented sport utility car over bumpy, narrow back roads.
In the underbrush on the road’s edge, gaping mouths of mine tunnels continue to spill water just as they were designed to 150 years ago.
“There’s hardly a 100-acre area that hasn’t been mined,” Lawler said.
As the car passes through the hydraulic-scarred Greenhorn Creek, the evidence of mining is apparent in the miles of gravel deposits and terraces lining the banks. Up to 400 pounds of mercury can be found in the gravel of Greenhorn Creek, 40 times that of the Yuba River, Lawler said.
“Who is going to clean up Greenhorn Creek? That’s where the majority of the mercury load is.”
Because of the mercury load, Greenhorn’s fishery is nonexistent and fishery advisories have been posted at Rollins Lake.
Before the cleanup, puddles of silver mercury could be found in and around the tunnel after a heavy rain. Water skeeters, banana slugs and other living creatures had high bio-accumulations of the element. Every time someone entered the tunnel, they stirred up the sediment and increased the amount of mercury discharge at the site.
The tunnel was 3 feet deep with sediment and mercury puddles when the crews from Bureau of Land Management, Tetra Tech and Cherokee Development began mucking inside the tunnel.
A double-drum “slusher” scooped the viscous gunk to the front of the tunnel, where a backhoe loader transferred the material to an on-site processing plant. The material was then run through a series of graduated screens before entering a centrifugal bowl, where heavy minerals collected onto the sides.
“At the end of the day, we had a panful of mercury,” Lawler said.
Once the mercury was removed, the tunnel floor was capped with cement. The mercury was sent to an authorized mercury recycling company in Pennsylvania for re-use in the medical equipment industry.
With the success of the Boston Mine cleanup, work can begin on larger sites such as the Pond Hydraulic Mine and sluice tunnel in Foresthill and the nearby Poore Mine, Lawler said.
The pilot project cost the federal government $550,000 to recover two and a half pounds of mercury.
“Cleaning these things obviously isn’t inexpensive, but technically it’s totally doable,” said Tim Callaway, contractor for Cherokee Development.
Local Filmmaker Craig Rohrsen, who lives near the Boston Mine, has entered his film of the project in next year’s South Yuba River Citizens League Environmental Film Festival. The 18-minute film will be available for checkout soon from SYRCL’s library at its Nevada City office, Rohrsen said.
To contact Staff Writer Laura Brown, e-mail firstname.lastname@example.org or call 477-4231.