A Superfund Site.
The Lava Cap Mine site occupies approximately 33 acres in a semi-rural residential area of the Sierra Nevada foothills in western Nevada County, California. The site is approximately 5 miles southeast of Nevada City and 6 miles east of Grass Valley at an elevation of about 2700 feet. The site includes the mining area where ore was processed to recover gold, and areas where tailings which originated at the mine have been washed downstream and deposited over time. The downstream areas of the site include Lost Lake, a private lake surrounded by homes, located approximately 1-1/4 miles downstream of the Lava Cap mine site.
In 1994, an estimated 1,776 people lived within one mile of the site, and 24,091 lived within four miles of the site. The immediate watershed basin ecosystem contains two California Species-of-Special-Interest: foothill yellow-legged frog and western pond turtle, in addition to more common species of reptiles, amphibians, fish, birds, and mammals.
Gold and silver mining activities were initiated at Lava Cap Mine in 1861. From 1861 to 1918, processing of the ore and disposal of the waste rock, overburden, and tailings occurred off-site at the Banner Mine, which is located approximately 1.5 miles north of the Lava Cap Mine.
The Lava Cap Mine was inactive from 1918 to 1934, at which time mining activities were resumed and a flotation plant was built to process the ore at the site. The gold and silver concentrates from the flotation plant were shipped to two smelters, one in California and the other in Washington. In 1940, a cyanide plant was built to recover the concentrates on site. However, this operation proved to be relatively ineffective. From 1941 to 1943, the cyanide plant only handled the middlings and tailings from the flotation plant. The middlings and tailings were ground to a very fine size (i.e., able to pass through a 400-mesh screen), then vat leached with cyanide to remove the residual gold and silver. Slurries from the flotation and cyanide processes were deposited in a ravine on the site. Where the ravine steepened and narrowed, a log dam approximately 60 feet high was built to hold the tailings in place. The waste rock and overburden were also deposited in two piles located at the site between the mineshaft and the tailings pond. In 1943, Lava Cap Mine was closed due to World War II. An attempt was made to re-open the mine in the mid-1980s. However, community opposition resulted in the defeat of a proposed re-zoning of the property which would have allowed mining activities to resume at the site.
In 1979, complaints from local residents initiated an action from California’s Central Valley Regional Water Quality Control Board (RWQCB) that led to issuance of a Cleanup and Abatement Order (CAO). The CAO called for the property owners at that time to take measures to limit tailings discharges to Little Clipper Creek, to divert surface water runoff from the mine and mill waste fill deposits, and to obtain an evaluation of the dam. This Order resulted in the construction of three small settling basins below the dam and some flow diversions around the tailings. No improvements were made to the dam.
During a major winter storm in January 1997, the upper half of the log dam collapsed, releasing over 10,000 cubic yards of tailings into Little Clipper Creek. In early 1997 staff from the California Department of Fish and Game and the Nevada County Department of Environmental Health inspected the site. Extensive deposits of tailings were observed in and on the shoreline of Little Clipper Creek, at the confluence of Little Clipper and Clipper Creeks, and in and on the shoreline of Lost Lake. The tailings were also observed in wetland areas contiguous with these water bodies, in some cases completely covering the vegetation.
Several times during 1997, DTSC has conducted sampling at the mine site and off-site at Lava Cap Mine to determine the locations and concentrations of arsenic contamination. Following the dam collapse in January 1997, the current property owner constructed a drainage ditch upstream of the mill tailings which partially diverted surface away around the tailings.
The EPA formally listed the Lava Cap Mine site on the National Priorities List (NPL) in February 1999, allowing Superfund funding to be spent on investigation and cleanup of the site. In 2004, as a result of initial studies of the site, EPA divided site into four project areas, or Operable Units.
Source: US EPA Region 9, Superfund.