SEPA
Untied States
Environmental Protection
Agency
National Priorities List (NPL)
**
*Proposed Site
***
OSWER/OSRTI
Washington, DC 20460
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.
n. Site History:	
The site is the second most productive mercury mine in North America, operating from approximately 1854 to 1972.
1 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.
rtft 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.
B 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.
[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.]
F or 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.

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