Captain Jack Mill sits about 1.5 miles south of Ward in Boulder County, Colorado. Historic gold and silver mining left behind contaminated soil, sediment, and surface water. EPA added the site to its National Priorities List (NPL) in September 2003, and cleanup has involved EPA, the Colorado Department of Public Health and Environment (CDPHE), and responsible parties. The site has not yet reached construction completion and remains on the NPL.
EPA identified eight metals as contaminants of concern: antimony, arsenic, cadmium, copper, lead, manganese, thallium, and zinc. Most of these metals appear in soil, sediment, leachate, groundwater, and fish tissue. All contamination falls within one operable unit covering surface waste and the underground Big Five Adit area.
Surface cleanup wrapped up in November 2012. Workers consolidated contaminated mine waste into two capped cells, one at the former mill area and one at the Big Five Waste Rock Pile. Vegetated soil covers and water diversion structures now prevent direct contact with contaminated material and reduce metal leaching into surface water. The vegetation held up against major rain events after establishing by August 2013. Underground, a long-term remedy targets acid mine drainage from the Big Five tunnel. An in-tunnel treatment system and engineered bulkhead were installed in early summer 2016, but in October 2018 acidic water with elevated heavy metals from the tunnel caused a fish kill in upper Left Hand Creek about five miles downstream. EPA and CDPHE responded with a temporary treatment system, first using sodium hydroxide, then switching to a lime-based system in December 2018. That temporary system continues while the in-tunnel system is modified. Infrastructure Investment and Jobs Act funding has supported this ongoing work.
Current assessments show human exposure is under control, meaning no unacceptable exposure pathways exist right now. Groundwater migration is also under control. However, the Left Hand Water District draws drinking water from Left Hand Creek, and the site poses a potential future contamination risk to that source. Additional remedial design and construction work is estimated between March and May 2027, with further action potentially continuing into 2028. The most recent five-year review was completed in September 2022, and the next is estimated for fall 2027.
Community members with questions can contact the EPA Community Involvement Coordinator or the Remedial Project Manager. For state-related questions, contact CDPHE.
