U.S. EPA National Priorities List (NPL) Sites (Federal Superfund)

Iron King Mine and Humboldt Smelter

Community Involvement Activities Public Health Impact Site Hydrogeology
Contacts Public Meeting Calendar Site Map
Contaminants Public Notice Calendar Site Status Update
Information Repository Site History

Tailing swale piles

The Iron King Mine and Humboldt Smelter (site) are two separate facilities in the Town of Dewey-Humboldt. The Iron King Mine area occupies approximately 153 acres west of Highway 69, includes properties along the Iron King Mine Road and it is bordered by Chaparral Gulch to the north and Galena Gulch to the south. The Iron King Mine is comprised of the Iron King Mine proper area, the operations area and the former fertilizer plant. The 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 smelter stack. The investigation also includes soils in nearby residential areas, the waterways of Chaparral Gulch, Galena Gulch, the Agua Fria River and adjoining drainage channels and outfalls, shallow and deep groundwater. 

Site Status Update:

The EPA set up a temporary office in Dewey-Humboldt at the start of 2014 to perform extensive field work associated with the RI Addendum. The field work consisted of: sampling 368 additional residential yards and the collection of over 4,100 soil samples for arsenic and lead analysis; the collection of background samples from surrounding areas of the site; the collection of over 35 soil samples from residential yards to assess the bioavailability of arsenic in soil; the installation of six new groundwater monitoring wells and four piezometers; groundwater sampling of new and existing monitoring wells; and surface water sampling from the Agua Fria River. Three deep soil borings were advanced and converted to monitoring wells in the tailings pile and 268 borings were drilled, logged and sampled in the tailings floodplain, smelter tailings swale, upper and middle Chaparral Gulch, and areas adjacent to the smelter site and dross material. Borings were advanced and soil samples collected in the areas of the dam and slag piles to assess the content of metals and stability of soils.

Community Involvement Activities:

Fact sheets have been sent out to the community, which provide site updates. Public meetings have been held and EPA provided information on site status to the city council when requested.   

A Community Involvement Plan (CIP) was developed for this site.  The CIP identifies issues and concerns, describes ways that EPA will provide information to the community, receive community feedback, and outlines site timeframes and milestones.  This document is located at the site repositories.

Site History:

Humboldt Smelter and the
Iron King Mine at Humboldt, AZ

Late 1800s - 1969: The Humboldt Smelter operated from the late 1800s until 1969. The original smelter burned down in 1904 and was rebuilt in 1906. Ore from the Iron King Mine may have been processed at the Humboldt Smelter during this time period.

The Iron King Mine operated from the late 1800s until the early 1960s and was a periodically active mine for gold, silver, copper, lead and zinc.  The mine was expanded in 1936 to remove metals from underground. In 1938, a 140-ton mill was erected to crush ore and expanded to 225-ton capacity. In 1940, a cyanide processing plant was added to the site to treat the mill tailings and enhance precious metal recovery.

1974: Ironite Products Company (now North American Industries) purchased 85 acres of the former Iron King Mine comprised mainly of the tailings area and included an administrative building, a warehouse and process facilities. Ironite Products Company mixed mine tailings with sulfuric acid, urea and water to produce a fertilizer marketed as Ironite.

2002: A Preliminary Assessment/Site Inspection (PA/SI) report was completed for the Iron King Mine and tailings by the ADEQ Site Assessment Unit. Surface and subsurface soil samples were collected at 21 locations including the large tailings pile, ponds, Ironite plant area, former assay laboratory, former mill site, former fertilizer plant and nearby school. The conclusions were that heavy metals, including antimony, arsenic, cadmium, lead and mercury met observed release criteria and exceeded EPA and state benchmarks as documented sources of contamination and have been released to sediments in the Chaparral Gulch. Further study and evaluation was recommended.

Kuhles Capital acquired the Iron King Operations property including the laboratory and Glory Hole. Kuhles received a solid waste permit to operate a waste reduction and recycling facility at the former Glory Hole.

2003: Greenfields Enterprises purchased the Humboldt Smelter property.  Previous tenants included an aluminum recycling facility, wooden shed manufacturer, sawmill and Humboldt Precast. No operations are currently active at the smelter property. Ironite Products Company entered into the ADEQ voluntary remediation plan (VRP) as a result of the 2002 PA/SI which found that additional investigation of the site was needed.

2003 - 2005: Aquatec operated a permitted septage treatment facility on the former fertilizer plant site, previously operated by Ironite Products Company and located within the boundary of the former Iron King Mine.

2004: A PA/SI report was completed at the Humboldt Smelter and other areas by ADEQ. Samples were collected at the ore pile, evaporation pond, assay lab discharge pipe, large ash pile, gray ash, large tailings pile, retention basis, school yard and two private residences. Samples were analyzed for semi-volatile organic compounds (SVOCs), metals and cyanide. No SVOCs were detected, but elevated levels of arsenic and other metals were present.

2005: The Superfund Technical Assessment and Response Team (START) performed a removal assessment of 16 residential properties and one horse pasture along the Chaparral Gulch at the request of ADEQ. The assessment objective was to determine the level of arsenic and lead in soils due to erosion from tailings. There were 153 surface locations and 17 subsurface locations that were evaluated. Sample results were compared to the previously established background arsenic of 30.72 milligrams per kilogram (mg/kg) and lead background level of 20.05 mg/kg.

2006: North American Industries performed removal action of tailings impacted soil from four residential properties in the vicinity of the Ironite property and Chaparral Gulch under an Administrative Settlement Agreement and Order on Consent with EPA effective in May. Work was started in July and completed in May 2007.

An expanded site inspection report was completed on October 31st, for the Iron King Mine and Humboldt Smelter area by the ADEQ site assessment unit. Samples were collected at 12 drinking water wells including the main and backup wells for the Humboldt Water System as well as 10 private domestic wells. Arsenic concentrations were found to be above the EPA maximum contaminant level of 10 parts per billion (ppb) in a majority of the samples collected.

View of the slag pile and smelter

2008: On September 3rd, the EPA listed the site on the National Priorities List (NPL). A community site kick-off meeting was held in Dewey-Humboldt on August 20th, and the first remedial investigation (RI) sampling round begun. The primary RI objectives included assessing the nature and extent of contamination and to gather sufficient information so that EPA could select a remedy that eliminates, reduces, or controls risks to human health.  Sampling included soil, sediment, groundwater, surface water and air.

2009: Additional RI sampling was conducted during April.  Technical documents completed in 2009 included a reuse assessment, a cultural resource and historical buildings survey, a biological evaluation, and a riparian evaluation/ jurisdictional determination of the lower Chaparral Gulch.  A data validation report for sample analyses results was also completed. 

2010: By March, the RI was completed. EPA identified five areas of interest (AOI):

  • Iron King Mine – Includes the Iron King Mine proper area, Iron King Mine operations area, former fertilizer plant area, salvage yard, and ancillary associated properties.
  • Humboldt Smelter – Includes several abandoned buildings, a smelter stack, a tailings pile, a smelter ash pile, and a slag pile.
  • Waterways – Includes the Chaparral Gulch, Galena Gulch, Agua Fria River, and adjoining drainage channels and outfalls.
  • In-Town Soil – Includes residential, background, and ancillary properties
  • Groundwater – Includes shallow alluvium and deep bedrock ground water
Remedial alternatives evaluation and a closure cover alternatives evaluation for the Iron King Mine main tailings pile were also completed.

2011: EPA completed interim removal, control and improvement actions.  Three primary objectives were completed between September and November:

  • Eleven residential yards were stripped of soil, received up to two feet of clean soil backfill, and re-stored to their original condition. These residential yards contained elevated levels of arsenic and lead. The removed soil was placed on the Iron King Mine’s Main tailings pile. Hydro-seed was added on top of the soils to encourage vegetation growth and to act as a temporary dust suppressant.
  • The small tailings pile, located adjacent to the Chaparral Gulch, was relocated to a designated area on top of the Iron King Main tailings pile.  Over 20,000 cubic yards were moved.  Upon placement of the small tailings pile in the storage area, Gorilla Snot®, a temporary soil sealant was applied to the relocated material to act as a temporary dust suppressant. The former small tailings pile location area was restored to a natural drainage pathway. 
  • A soil sealant was applied to the ash piles at the Humboldt Smelter.

The final report documenting the removal action is available in the information repository, as well as on EPA web site.

2012: The ATSDR and ADHS offered blood lead screening events to the Dewey-Humboldt community on April 19th and May 21st. The screening was conducted in response to concerns expressed by parents about potential toxins related to the Iron King and Humboldt Smelter Superfund site. Children younger than 6 and pregnant women were especially encouraged to get the screenings.  No children under the age of six included in the recent Dewey-Humboldt community testing exceeded the 5 micrograms per deciliter (ug/dl) recommended blood lead level. The Centers for Disease Control & Prevention (CDC) recently adopted a new reference value for children under six years of age, which decreased to 5 micrograms per deciliter (µg/dl) from 10 µg/dl.

Iron King Mine
Standing on top of the orange tailings
pile looking towards Dewey-Humboldt

Additional data was collected to evaluate the stability of the Iron King Mine Main tailings pile. Preliminary results indicated the tailings pile volume as approximately 3.2 million cubic yards (instead of the original estimate of 6.4 million cubic yards). The results and analysis will be included in the RI  addendum.

EPA began evaluating naturally occurring metals in the area.  In-situ x-ray fluorescence (XRF) level results were measured and shallow soil samples were collected throughout the Dewey-Humboldt area.  The assessment area extended approximately 20 square miles (5 miles east to west and 4 miles north to south). The XRF and soil data analyses results will assist in determining contaminant (metals) background concentrations at the site.  Once the soil data has completed validation, results will be provided to the residents whose properties were sampled. The sampling event analytical results will be part of the RI addendum.

EPA installed four additional groundwater wells at the Iron King Mine tailings pile and two wells down-gradient of the mine. These groundwater wells will provide more information on the groundwater arsenic concentrations at, and down-gradient, from the Iron King Mine main tailings pile. The sampling event analytical results will be part of the RI addendum.

2013: Field activities for the RI addendum included emergency response action sampling for future remediation of several residential properties and extensive sampling for background study and dispersion of contaminants from the smelter.  A site-wide sampling plan to address all areas of tailing deposits, smelter materials and gulch sediments was implemented with preliminary field investigations. The installation and sampling of new groundwater wells are planned to determine the conditions and overall stability of the main tailings pile.


The current contaminants of concern at the site include arsenic, lead, and other metals that may have contaminated soil, sediments, surface water and groundwater in concentrations significantly above background levels. The full extent of off-site soil contamination and possible groundwater contamination has not been fully assessed. Runoff from the mine tailings along the Chaparral Gulch may be entering the Agua Fria River. Contaminants of concern may change as new data become available.

Public Health Impact:

The Arizona Department of Health Services finalized a health consultation in 2009. A health assessment is to follow shortly after completion of the RI and FS. Groundwater and soil were evaluated for exposure to arsenic and lead. Two wells were identified to pose a public health hazard due to high arsenic levels. These included one deep irrigation well and one shallow domestic well.

Site Hydrogeology:

Groundwater is encountered within the alluvium at depths between 30 and 50 feet below ground surface (bgs) under unconfined conditions and generally the flow follows the local topography.  Shallow groundwater is thought to flow east from the mine and smelter towards 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 1000 ft bgs. The regional groundwater flow direction is not fully understood and will be evaluated during the RI.


Name Phone/Fax E-mail
Sara Benovic, ADEQ Project Manager (602) 771-4248*/(602) 771-4138 fax [email protected]
Wendy Flood, ADEQ Community Involvement Coordinator (602) 771-4410*/(602) 771-4236 fax [email protected]
Jeff Dhont, EPA Project Manager (415) 972-3020**/ (415) 947-3526 fax [email protected]

*In Arizona, but outside the Phoenix area, call toll-free at (800) 234-5677.
**Call EPA’s toll-free message line at (800) 231-3075.

Information Repository:

The local repository is at the Dewey-Humboldt Town Library, 2735 N. Corral Street, Dewey-Humboldt, AZ 86329, (928) 632-5049.

Interested parties can review site information here on this page and at the ADEQ Record Center located at 1110 W. Washington Street, Phoenix, Arizona. Please contact (602) 771-4380 or (800) 234-5677 ext. 6027714380 for hours of operation and to schedule an appointment.

The complete official site file can be reviewed at the EPA Region IX, Records Center, Mail Stop SFD-7C, 95 Hawthorne Street, Room 403, San Francisco, CA 94105, (415) 536-2000.

Site Map

Site Update

Arsenic and Lead Sampling Results Fact Sheet

U.S. EPA Site Description