Two abandoned mercury mines in San Luis Obispo County operated from 1868 to 1970 and left behind soil and water contaminated with mercury, thallium, iron, cobalt, and arsenic. The site sits about 7 miles upstream from Lake Nacimiento, and contaminated runoff from Las Tablas Creek reaches the lake. EPA added the site to the National Priorities List (NPL), the federal roster of priority Superfund sites, in April 2006. The site is divided into four operable units: the Mine Proper, Las Tablas Creek Watershed, Lake Nacimiento, and a sitewide area.
Mercury is the primary concern. It can harm the nervous system, and levels in onsite water exceed state and federal safe drinking water standards. Mercury binds to lake sediments and builds up in fish tissue. The California Office of Environmental Health Hazard Assessment has issued a fish consumption advisory for Lake Nacimiento, recommending that the public, especially children and pregnant women, limit eating certain fish species from the lake. The lake water itself is safe for swimming and meets drinking water standards. People should not enter the mine site itself because of collapsing tunnels, unstable waste piles, and contaminated soil, water, and dust.
EPA reached a legal settlement with responsible parties in 2002. Since then, multiple emergency removal actions have been completed. Between 2000 and 2011, crews removed 120,000 cubic yards of contaminated material from the drainage channel, stabilized mine waste and slopes, removed the mercury processing building, and capped materials to prevent immediate threats to human health. The most recent removal work at the Mine Proper took place from September 2021 to March 2022. Contaminated materials remain temporarily capped on site pending long-term cleanup. EPA completed a Feasibility Study in 2014 to evaluate cleanup options, but results were inconclusive due to intense storms. A follow-up two-year study of water treatment technologies was also inconclusive because of drought. EPA continues collecting water, sediment, and fish tissue samples from Lake Nacimiento. According to EPA performance measures, human exposure is not currently under control, meaning unsafe contamination levels have been detected and people could reasonably be exposed. No final remedy has been selected and no remedial construction is complete.
The site received funding through the Bipartisan Infrastructure Law, which directed $3.5 billion to NPL Superfund sites. EPA developed a Community Involvement Plan based on interviews with local residents. A Community Advisory Group (CAG) was formed to give the public a forum to raise concerns about cleanup decisions. CAG meetings began in 2008, though they have not taken place in recent years. More community activities are planned as the investigation moves forward. Residents can contact EPA's Community Involvement Coordinator or Remedial Project Manager directly with questions.
