Historical gold and silver mining between 1859 and the early 1900s left an estimated 14 million pounds of mercury scattered across the Carson River watershed in western Nevada. About 236 mills processed ore using mercury, spreading contamination across soil, sediments, fish, and wildlife in five counties and more than 130 river miles. EPA added the site to the National Priorities List in 1990, and active cleanup began in 1996.
The main contaminants are mercury, arsenic, and lead. All three appear in soil at the old mill sites and tailings. Mercury is also widespread in river sediments and has worked its way into fish and wildlife tissue. Children six years old and younger face risk from direct contact with contaminated soil. Mercury can cause permanent nervous system damage and serious disabilities in developing fetuses and children. Lead reduces learning, memory, and attention in children and can damage kidneys. Arsenic causes gastrointestinal and skin problems, including cancer, and early childhood exposure harms cognitive development. Human exposure is not currently under control.
Cleanup is organized into operable units. Operable Unit 1 covers the old mill sites and tailings. EPA issued a cleanup decision for that unit in 1995, selecting excavation, containment, institutional controls, and other methods. An update to that decision in 2013 changed institutional controls and cleanup standards. A second remedial action phase for this unit began in September 2023. Operable Unit 2 covers the Carson River and its floodplain. EPA issued a Proposed Plan for that area in September 2021 and anticipates studies on the river and floodplain starting in September 2025. No final cleanup decision has been issued for Operable Unit 2 yet. EPA and the Nevada Division of Environmental Protection work with developers reviewing soil before new construction, having evaluated more than 70 development proposals since the early 2000s. The most recent five-year review occurred in September 2023, with the next estimated for late 2028.
Nevada recommends no consumption of fish from the Carson River between Dayton and Lahontan Dam, or from Lahontan Valley waters, Big and Little Washoe Lakes, or tribal fishing areas in the river and floodplain. Catch-and-release fishing and swimming are considered safe. Residents planning to disturb more than three cubic yards of soil should contact Nevada staff before starting work.
Community members can get involved through Citizen Advisory Boards in Silver City, Virginia City, Dayton, Silver Springs, and Fallon. EPA maintains government-to-government coordination with the Fallon Paiute Shoshone and Washoe Tribe of Nevada and California. An interactive StoryMap provides a site overview. Site records are available at four Nevada library branches in Fallon, Dayton, Carson City, and Silver Springs. EPA and Nevada project managers are available to answer questions directly.
