Pacific Southwest, Region 9: Superfund
Serving Arizona, California, Hawaii, Nevada, the Pacific Islands, and Tribal Nations
New Idria Mercury Mine
EPA #: CA0001900463
County: San Benito
City: New Idria
Congressional District: 17
A removal action was completed November 2015 to prepare the site for an expected rainy winter.
Investigation activities are ongoing.
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Description and History
NPL Listing History
NPL Status: Final
Proposed Date: 03/10/11
Final Date: 09/16/11
The New Idria Mercury Mine (NIMM) site is on 8,000 acres of private land in the Diablo Mountain Range and incorporates the abandoned town of Idria in San Benito County, California. The mine is located in the New Idria Mining District, which includes over a dozen smaller mercury mines. The NIMM operated from 1854 to the early 1970s, was the second most productive mercury mine in North America, and produced over 38 million pounds of mercury.The abandoned town of Idria was comprised of dozens of dilapidated buildings until a fire in July 2010 destroyed nearly half of the historic structures in the northern portion of the town.
The New Idria mining claim was declared in 1854 by prospectors and investors. In 1857, the first brick furnace to roast cinnabar ore was built at the Site. The mine operations expanded to include San Carlos and Molina mines and miles of tunnels, shafts and drifts were used to access the cinnabar. In the 1920s, the overburden was stripped down to form pits in order to access cinnabar. The mining continued nearly uninterrupted with a few idle and low productivity periods due to low mercury values and land owner disputes. The mine operated until the early 1970s. Several furnaces were built over the years including four large furnaces still located at the site adjacent to the Level 10 adit. Mercury was extracted from the cinnabar ore by crushing the ore and roasting it to release elemental mercury vapor which was cooled and condensed for bottling. The roasting process is called calcination and the roasted ore is known as calcines. Typically, calcines still contain some soluble mercury.
Extensive waste rock and calcine tailings piles (0.5 to 2 million tons) cover over 40 acres at the Site. A large furnace and process area and other mine working features remain at the Site. Over 30 miles of tunnels and 20 levels (levels are inversely synonymous to floors of building) were constructed as part of mining operations at the Site. The Level 10 adit was used as the main haulage level and is connected to other levels by shafts and raises. The extensive mine levels have flooded with water which reacts with the high iron and sulfur content of the bedrock to form an acidic solution, typically known as acid mine drainage (AMD), which drains from the Level 10 adit.
Although regulated by the Regional Water Quality Control Board (RWQCB) starting in the late 1960s, surface water discharges of AMD from the mine remained uncontrolled after the mine shut down in the early 1970s. The EPA, U.S. Geological Survey (USGS), RWQCB, and academic institutions have conducted investigations at the NIMM site and surrounding area to assess mercury contamination in sources at the Site and in creeks downstream of the mine.
Contaminants and Risks
- Surface Water
- Soil and Sludges
- Environmentally Sensitive Area
There are a few residences and vacation homes, but no schools, day-care centers or regularly occupied workplaces on the NIMM site. In addition, there are no drinking water sources that are impacted by the Site.
Surface water from the Site drains to San Carlos Creek, which flows northward to Silver Creek and continues north to Panoche Creek. Panoche Creek flows to the Mendota Pool and San Joaquin River during periods of heavy precipitation and flood events. The Mendota Pool and San Joaquin River are recreational fisheries and are located approximately 45 river miles downstream from the Site. The San Joaquin River flows to the San Francisco Bay, which is a commercial fishery. The San Joaquin River Restoration Project is a state- and federally-funded effort to restore and maintain fish populations in “good condition,” including naturally reproducing and self-sustaining populations of salmon and other fish. Sensitive habitats and wetlands are found along the surface water pathway between the Site and San Joaquin River.
Several metals were detected at concentrations significantly above background concentrations in AMD and calcine tailings piles. Some of these same hazardous substances were also detected at concentrations significantly above background levels in San Carlos Creek. Mercury was detected at concentrations significantly above background levels in the San Carlos, Silver, and Panoche creeks as far as 20 miles downstream from the Site.
The 2010 Expanded Site Inspection (ESI) sampling results documented releases of aluminum, arsenic, copper, iron, mercury, nickel, selenium and zinc in the San Carlos Creek, and of mercury in the entire length of Silver Creek into Panoche Creek. The extent of mercury contamination in the Panoche Creek is undetermined.
Over 20 miles of wetlands along the San Carlos and Silver creeks are documented as impacted by mercury. The San Carlos and Silver creeks are characterized by extensive wetlands in a predominantly arid, upland environment. As such, the aquatic and wetland habitats, and dependent animal species, are of regional ecological significance. This is one of only a few large watersheds in the 200 mile-long Diablo Range that are characterized as having intermittent or perennial flow conditions for water flowing east to the San Joaquin Valley. Habitats for six threatened and seven endangered species are present in the vicinity of the NIMM and downstream of the Site.
San Carlos Creek downstream of the Site is listed on the Federal Clean Water Act 303(d) list of impaired water bodies for mercury impacts from the mine. Investigations have indicated the presence of mercury in surface water, stream sediments, and wetlands downstream of the mine.
Who is Involved
This site is being addressed through Federal and State actions. Negotiations with Potentially Responsible Parties to conduct the investigation work are ongoing.
EPA conducted a removal action to reduce the release of contaminants from the Site. These actions included: re-routing the AMD so that it does not run directly through the tailings piles; construction of a settling pool for the AMD to allow some of the metals to settle out prior to its discharge into San Carlos Creek; and construction of surface water diversions to control erosion. These actions were completed November 2011. Additional investigations will be conducted to define the full extent of contamination.
Potentially Responsible Parties
Potentially responsible parties (PRPs) refers to companies that are potentially responsible for generating, transporting, or disposing of the hazardous waste found at the site.
Documents and Reports
Public Information Repositories
The public information repositories for the site are at the following locations:
San Benito County Integrated Waste Management District Office
3220 Southside Road
Hollister, CA 95023
EPA Site Manager
Mail Code SFD
75 Hawthorne Street
San Francisco, CA 94105
EPA Community Involvement Coordinator
Mail Code SFD
75 Hawthorne Street
San Francisco, CA 94105
EPA Public Information Center
After Hours (Emergency Response)