OSWER/OSRTI 

Washington, DC 20460 

NATIONAL PRIORITIES LIST (NPL) 

***Final Site*** September 2011 

NEW IDRIA MERCURY MINE Idria, California 

San Benito County 

 Site Location: 

The site is an abandoned mercury mine located in the town of Idria in San Benito County, 64 miles southeast of Hollister. 

 Site History: 

The site was the second most productive mercury mine in North America, operating from approximately 1854 to 1972. 

 Site Contamination/Contaminants: 

Extensive waste rock and calcine tailings piles cover more than 40 acres of the site. Untreated acid mine drainage (AMD) 

currently discharges from an adit at the site at a rate between 10 and 50 gallons per minute. The AMD flows directly over 

waste rock and tailings piles into San Carlos Creek. Mercury from the mine wastes at the site is transported via San Carlos 

Creek to Silver Creek and into Panoche Creek. Mercury levels in San Carlos Creek downstream from the site are above 

background and exceed the criterion continuous concentration (CCC) for mercury in surface water according to EPA’s 

National Recommended Water Quality Criteria. The CCC is national guidance and is an estimate of the highest 

concentration of mercury in surface water to which an aquatic community can be exposed indefinitely without resulting in 

an unacceptable impact. It is likely that mercury is further transported down the watershed to the Mendota Pool, the San 

Joaquin River, and ultimately to the San Francisco Bay. 

 Potential Impacts on Surrounding Community/Environment: 

Over 20 miles of wetlands, located along San Carlos and Silver Creeks, are documented to be impacted with mercury. The 

aquatic and wetland habitats associated with these creeks are of regional ecological significance. This wetland area is only 

one of a few flowing east to the San Joaquin Valley from the 200-mile-long Diablo Range. There are federally endangered 

species in the vicinity of the site and wetlands areas. 

 Response Activities (to date): 

The Regional Water Quality Control Board (RWQCB) prescribed waste discharge requirements for the New Idria Mine for 

the discharge of mill waste, mine water and sewage effluent to San Carlos Creek in 1970. In 1975 and in 1988, the 

RWQCB found the mine to be in violation of the requirements. RWQCB was further involved in the site in 2003, and 

conducted an inspection, analyzing eight water samples, confirming the impact the site has on San Carlos Creek. In the mid 

1980s, Futures Foundation, a drug rehabilitation program, purchased the property and began dumping waste on the surface 

of the tailings piles. The California Integrated Waste Management Board and California Department of Toxic Substances 

Control, working in conjunction with EPA and the San Benito County Health and Human Services Agency, were able to 

have the waste materials removed from the site in February 2003. 

 Need for NPL Listing: 

A thorough investigation and long-term remedial action is necessary to address the site contamination, which poses a 

significant risk to endangered and threatened species and a unique wetland environment. Other federal and state cleanup 

programs were evaluated, but are not viable at this time. EPA received a letter of support for placing the site on the NPL 

from the state. 

[The description of the site (release) is based on information available at the time the site was evaluated with the HRS. The description may change 

as additional information is gathered on the sources and extent of contamination.] 

For more information about the hazardous substances identified in this narrative summary, including general information regarding the effects of exposure to these 

substances on human health, please see the Agency for Toxic Substances and Disease Registry (ATSDR) ToxFAQs. ATSDR ToxFAQs can be found on the Internet 

at http://www.atsdr.cdc.gov/toxfaq.html or by telephone at 1-888-42-ATSDR or 1-888-422-8737.