Superfund Site | Iron King Mine and Humboldt Smelter

Environmental Protection Agency (EPA) #: AZ0000309013

Superfund National Priority List (NPL) Status: Placed on Aug. 3, 2008


In the town of Dewey-Humboldt, the site includes two separate and former facilities: the Iron King Mine and Humboldt Smelter.

Bordered by Chaparral Gulch to the north and Galena Gulch to the south, the Iron King Mine encompasses approximately 153 acres west of Highway 69. The operations area and fertilizer plant are currently operated by North American Industries. 

The former Humboldt Smelter area occupies approximately 182 acres along Chaparral Gulch including property at the east end of Main Street in the town of Dewey-Humboldt around the former smelter stack. The investigation includes soils in nearby residential areas, waterways of Chaparral Gulch, Galena Gulch, the Agua Fria River and adjoining drainage channels and outfalls, shallow and deep groundwater.

Contaminants of Concern

Arsenic, lead and other metals may have contaminated soil, sediments, air, surface water and/or groundwater in concentrations significantly above background levels. Primary sources of contamination include the Iron King Mine tailing pile and impoundment/ponds; the Humboldt Smelter slag (a molten waste material), aluminum dross, tailing piles and impoundment/pond; and lower Chaparral Gulch. Runoff from the mine tailings along the Chaparral Gulch may be entering the Agua Fria River. 

Public Health Impact

The Arizona Department of Health Services finalized a health consultation in 2009. According to public health and ecological risk assessments conducted by the EPA, contamination at the site could pose risks if no cleanup actions are taken. It is recommended that residents limit or avoid contact with soils and any water in the contaminant of concern areas mentioned above. Residents that use private wells in the area are encouraged to test their wells for arsenic.

Site Hydrogeology

Groundwater is encountered within the alluvium at depths between 30 and 50 feet below ground surface under unconfined conditions and generally the flow follows the local topography. Shallow groundwater is thought to flow east from the mine and smelter toward the Aqua Fria River and along the Chaparral Gulch. Deep groundwater under confined conditions moves within the fracture system of the metamorphic bedrock. Bedrock wells have been drilled to depths ranging from 200 to 1,000 feet below ground surface. The site is located within the Prescott Ground Water Active Management Area (AMA). The AMA contains two sub-basins with regional groundwater flow directions to the north and/or southeast.

Action Taken

In 2008, the EPA initiated a remedial investigation (RI) with the primary objectives of determining the nature and extent of contamination and gathering information to select a remedy that eliminates, reduces or control risks to public health. The EPA collected soil, sediment, surface water, groundwater and air samples from the Iron King Mine, Humboldt Smelter, residential and commercial areas, and waterways (Chaparral Gulch, Galena Gulch and Agua Fria River). The results from the RI indicate a need for cleanup actions to protect public health and the environment.

Between 2011 and 2012, the EPA completed a removal action that included cleanup of a subset of residential yards, removal of the Iron King Mine small tailing pile adjacent to the Chaparral Gulch and application of a soil sealant to the Humboldt Smelter dross piles. The EPA also evaluated background levels of arsenic and lead in the area and continues to evaluate groundwater at the site.

Between 2013 and 2014, the EPA continued investigations on and near the mine and smelter, and within residential yards to better understand contamination at the source areas. The investigation will determine whether the soil in residential yards have been impacted by the former mine and smelter activities.


The EPA has completed the RI and Risk Assessment Report and beginning the feasibility study (FS). The FS will focus on developing cleanup options that will reduce the health risks posed by the site. As a first priority the EPA is planning and executing cleanup actions for a small number of residential yards.