The New Idria Mercury Mine operated from 1854 to the early 1970s and produced over 38 million pounds of mercury, making it the second most productive mercury mine in North America. The 40-acre site in San Benito County, California was added to the EPA's National Priorities List (NPL) in September 2011. It carries significant contamination from decades of mining activity and has not yet reached the remedy-selection stage of cleanup.
The main concern is mercury, which has been detected at elevated levels in San Carlos, Silver, and Panoche creeks, reaching at least 20 miles downstream. Testing in 2010 also found aluminum, arsenic, copper, iron, nickel, selenium, and zinc in San Carlos Creek. Flooded mine levels produce acidic mine drainage (AMD) that flows out of the Level 10 adit and carries mercury and metals into those waterways. The site also holds 0.5 to 2 million tons of waste rock and calcine tailings piles. People can be exposed by ingesting or touching contaminants in surface water, sediments, and groundwater.
The EPA has completed two removal actions to limit the spread of contamination. Work from October 2011 to February 2012 rerouted AMD so it no longer ran directly through tailings piles, built a settling pool to allow metals to drop out before water reached San Carlos Creek, and installed surface water diversions to reduce erosion. A second removal action ran from December 2015 to February 2016 to prepare the site for additional rainfall. A combined Remedial Investigation and Feasibility Study (RI/FS) began in May 2012 and moved to EPA oversight in February 2016. A settlement with Buckhorn Inc. was finalized in December 2018, and a Consent Decree was completed in February 2023, establishing the framework for continued investigation.
The site is still in the investigation phase. No final cleanup remedy has been selected, physical construction has not begun, and the site has not been deleted from the NPL. The EPA has stated there is not yet enough data to determine whether human exposure is under control. Additional investigations will define the full extent of contamination.
Community members with questions can contact the EPA's Community Involvement Coordinator or the Remedial Project Manager. Site documents are available at the Superfund Records Center in San Francisco at 75 Hawthorne Street, Room 3110.
