OSWER/OSRTI Washington, DC 20460



# N ATIONAL 

# P RIORITIES 

# L IST 

# (NPL)

[The description of the site (release) is based on informa tion 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 ca n be found on the Internet

at http://www.atsdr.cd c.gov/toxfaq.html or by telephone at 1

- 888

- 42

- ATSDR or 1

- 888

- 422

- 8737. 

### ***Proposed Site *** March 2011


### NEW IDRIA MERCURY MI NE Idria, C alifornia San Benito County

 **Site Location:**

The s ite is an abandoned mercury mine located in the town of Idria in San Benito County, approximately 64 miles southeast of

Hollister

.



###  **Site History:**

The s ite 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 s ite. Untreated acid mine drainage (AMD) currently discharges from an adit at the s ite 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 s ite is transported via San Carlos Creek to Silver Creek and into Panoche Creek. Mercury levels in San Carlos Creek downstream from the s ite are above background and exceed the

c riterion c ontinuous c oncentration (CCC) for mercury in surface water accordin g 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 t o 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 wetlan ds, 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 f ederally endangered species in the vicinity of the s ite and wetlands areas.

 **Response Activities (to date):**

The Regional Water Quality Control Board ( RWQCB ) prescribed waste discharge re quirements 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 i n 2003, and conducted an inspection, analyzing eight water samples, confirming the impact the s ite 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, w orking in conjunction with EPA and the San Benito County Health and Human Services Agency, were able to have municipal waste ma terials removed from the site in February 2003.



###  **Need for NPL Listing:**
A thorough investigation and long

- term remedial action is necessary to address the s ite contamination, which poses a significant risk to endangered and threatened species and a uniqu e wetland environment. Other federal and state cleanup programs were evaluated, but are not viable at this time.