In the Gold Rush-era foothills community of Jackson, the worst-case scenario painted by the Environmental Protection Agency last year about a 100-year-old dam there held a chilling warning.
In a scant two minutes, a breach at the Eastwood Multiple Arch Dam would send a 15-foot-deep mudflow with high levels of arsenic and other toxic chemicals downhill into the center of the historic community. On the way down, the mudflow would pass the entrance to Jackson Junior High School and nearby residences.
The weathered 100-year-old concrete dam is built on the former Argonaut mine roughly a mile north of the town and is so unstable that a big storm or an earthquake could lead to a breach, according to the EPA and the Army Corps of Engineers. The result: millions of dollars of property damage and loss of life, with Jackson Creek and the city’s sewage system contaminated.
“It would put that sludge at the base of the hill and would inundate the shopping center at the bottom of the hill,” said Jackson Mayor Keith Sweet. “Then it would go down Main Street and fill the National Hotel.”
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The dam holds back 165,000 cubic yards of silty and wet soil filled with contaminated mine tailings, the polluted material left over from decades of ore mining.
Like many towns in the Gold Country, the environmental legacy of gold and silver mining activity is writ large across the landscape of this community of 4,651 residents. Many homes in the town have been built near where chemicals such as lead and mercury have been found in contaminated soil.
The EPA spent $3 million for emergency cleanup work at the Argonaut mine last year, working on 11 residential yards and a vacant lot, plus installation of a protective cover over steep soil slopes at Jackson Junior High School.
The agency has also put the Argonaut mine on a shortlist of eight sites across the U.S. for possible listing as a Superfund project, which would bring in more federal environmental money. The agency expects to rule on the Superfund designation by the end of September.
“Dam failure would be catastrophic for the town of Jackson,” said Jared Blumenfeld, Pacific Southwest regional director for the EPA. The agency estimates about 1 million cubic yards of tailings sit behind the dam and upstream of the mine.
The Argonaut mine is one of 46,000 legacy mines in the state with no current mine ownership or whose owners are incapable of paying for cleanup.
A top danger at many of the old mines is aging, badly maintained “mine dams” straining to hold back toxic sand and tailings. On a recent afternoon at the Agonaut mine dam, metal cables used as rebar poked out of the concrete structure where more concrete supporting the structure was once attached.
State officials contend dam failure is unlikely and would only be possible under a worse case scenario.
The most likely danger is heavy rainfall where water would pool behind the dam and place pressure on its crumbling walls.
Facing such a likelihood, the state is racing to retrofit the dam before it deteriorates further at an expected cost of $10 million to $11 million, said Charlie Ridenour, hazardous substances engineer with the state Department of Toxic Substances Control.
The state hopes to select a retrofit design for the dam by December with construction beginning next spring. Completion is expected by October 2017, Ridenour said.
The expectation of a rainy winter last year led the department to install a $1 million diversion system on an embankment above the dam that sends rainwater past the mine site into to a culvert under Argonaut Drive below.
“The diversion system worked beautifully and it even managed to deal with one storm which dumped 4 inches of rain,” Ridenour said.
Earthquakes are less of a threat to the dam, which sits near two ancient fault zones – the Melones and Bear Mountain fault zones – that have not been active for centuries, said Donald Drysdale, spokesman for the state’s Department of Conservation.
The area below the dam remains an arsenic hotspot, with soil samples collected by the EPA in 2013 showing arsenic readings above 1,000 parts per million. Normal background levels for arsenic in soil are in the 20 parts per million range.
In some cases, mercury levels at the mine site itself are 20 times acceptable levels, and lead levels are 40 times acceptable levels, Blumenfeld said.
An EPA site inspection report found that many homes were built directly over the 330-acre Argonaut mine property on waste rock and mine tailings covered with fill prior to construction. The affected communities include Argonaut Estates, Jackson View Estates, Sierra View Estates and Argonaut Heights II.
“These kinds of mines have had a pretty devastating impact on the state as a whole, but the Argonaut mine is different,” he said. “Unlike most mines, this one is in and under residential properties.”