The Iron King Mine and Humboldt Smelter site sits in Dewey-Humboldt, Arizona, where mining and smelting operations ran for most of the 20th century. Mining left behind roughly four million cubic yards of tailings high in arsenic and lead. Smelting operations produced dross, slag, and contaminated soils. Storms have carried these wastes into nearby drainages and the Agua Fria River. The site was added to the National Priorities List in September 2008 and is being addressed through a single operable unit covering soil, groundwater, surface water, and solid waste.
The EPA has identified 34 contaminants of concern. The main ones are arsenic and lead, but cadmium, copper, zinc, mercury, and a range of other heavy metals are also present. Chlorinated dioxins and furans appear in soil and solid waste as well. Long-term exposure to arsenic and lead at high levels can cause serious health effects. Most residential yards investigated did not pose unacceptable health risk, but 50 properties had contamination high enough to require action. EPA cleaned up surface soils at those properties to at least 12 inches deep between 2006 and 2017. At some properties, deeper soils still exceed cleanup levels and are marked with orange warning markers so residents know not to dig below them.
EPA selected a final cleanup remedy on October 20, 2023. The plan calls for consolidating more than 8 million tons of contaminated material from non-residential areas into two permanent, covered waste repositories, one at the former mine and one at the former smelter. The remedy also includes slope stabilization, an engineered cap, containment measures, institutional controls to restrict future land use, and long-term monitoring of groundwater, soil, surface water, and solid waste. Interim protective measures completed since 2019 include dust control at the smelter, warning signs, and additional fencing. Active remedial construction started in September 2024 and is expected to run through mid-2028. EPA is also continuing a civil investigation to identify parties potentially responsible for cleanup costs.
Community members are encouraged to stay away from both properties because of physical hazards including an unstable brick stack, collapsing buildings, sharp cliffs near the Agua Fria River, and waste materials with high levels of toxic metals. Field work for the cleanup design is scheduled from March through August 2026 and will use heavy equipment in residential and nonresidential areas, though no road closures are planned.
EPA shares updates through fact sheets, public meetings with the Dewey-Humboldt Town Council, the town newsletter, and a dedicated town webpage (https://experience.arcgis.com/experience/c37b8129bf304fa0ad034d3ebf2a4626). Documents are available at the Dewey-Humboldt Town Library in Humboldt, Arizona, and at the Superfund Records Center in San Francisco, California.
