Formosa Mine is a 76-acre abandoned copper mine in Douglas County, Oregon. It operated from 1910 to 1937, reopened briefly from 1990 to 1993, and was added to the EPA's National Priorities List on September 19, 2007. The site was designated a State Orphan Site in 2000 because responsible parties were unable or unwilling to fund cleanup. Cleanup is currently underway and physical construction is not yet complete.
Every year the mine discharges millions of gallons of acid rock drainage and toxic metals into Middle Creek and South Fork Middle Creek. The four contaminants of concern identified in soil are arsenic, cadmium, copper, and zinc. The elevated metals and very low pH in the affected waterways make survival difficult for aquatic organisms and plants. Drinking or bathing directly in the creeks or other surface water downstream from the mine is a health hazard. A public health assessment determined that no public or private drinking water systems are affected. Human exposure is currently under control, but groundwater migration is not under control and contamination has not stabilized in the groundwater.
Cleanup is split into two operable units. Operable Unit 1 covers soil remediation. A Record of Decision issued April 18, 2016 selected excavation, onsite consolidation, and an engineered cap, along with revegetation, institutional controls, and ongoing monitoring. Remedial design was completed in November 2020 and restarted in October 2024, with estimated completion between June and August 2027. Soil remedial action began December 31, 2021, and is ongoing. This work is funded by the Bipartisan Infrastructure Law. In 2018, the Bureau of Land Management installed a hydraulic plug in the main adit to control water discharge into Middle Creek. Operable Unit 2 addresses surface water and groundwater. A remedial investigation and feasibility study began in July 2012 and is estimated to be complete between June and August 2028. A removal action occurred in April 2019, and another is planned for April through June 2028. A Record of Decision for groundwater is estimated for June through August 2028.
Community members can review site records at the Riddle Branch of the Douglas County Library at 637 First Avenue in Riddle. EPA has developed a Community Involvement Plan and published a 2022 cleanup update fact sheet. For questions, contact the Community Involvement Coordinator or the Remedial Project Manager.
