OSWER/OSRTI

Washington, DC 20460

NATIONAL PRIORITIES LIST (NPL)

[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.

***Proposed Site*** March 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, approximately 64 miles southeast of 

Hollister.

 Site History:

The site is 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 over 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 effect. It is highly probable that 

mercury is further transported down the watershed to the San Joaquin River, the Mendota Pool, 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 municipal 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.