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Pacific Southwest, Region 9: Superfund

Serving Arizona, California, Hawaii, Nevada, the Pacific Islands, and Tribal Nations

Iron King Mine And Humboldt Smelter

EPA #: AZ0000309013

State: Arizona(AZ)

County: Yavapai

City: Dewey-Humboldt

Congressional District: 01

Other Names:

Bulletin Board

Download the RI Report

EPA is planning a cleanup of contaminated soils in 15-30 residential yards. More information can be found in the following fact sheet: IKHS Fact Sheet 9.16.pdfIKHS Fact Sheet 9.16.pdf


Download the RI Report
Click the link above to access the Remedial Investigation Report electronically. You will be taken to a different web page. You can download individual components of the report from the "Document" list visible on the right side of the screen.

Description and History

NPL Listing History

NPL Status: Final

Proposed Date: 03/19/08

Final Date: 08/03/08

Deleted Date:

The former Iron King Mine and Humboldt Smelter lie within the Town of Dewey-Humboldt. The town, which covers an area of about 19 square miles at an elevation of about 4,600 feet, is located in the high desert of central Arizona, about 85 miles north of Phoenix. The Agua Fria River flows south through portions of the eastern section of the former Humboldt Smelter property.
There are two major contributors to contamination at the Iron King Mine / Humboldt Smelter (IKHS) site. (See figure at the bottom of this section.) First, the former Iron King Mine, where mining operations began in the late 1890s, consists of about 150 acres and is located west of Highway 69. Operators of the Iron King Mine extracted, milled, and concentrated solid rock ores for lead, zinc, copper, gold, and silver between about 1934 and 1970. Smaller mining facilities operated between 1906 and 1934.

Second, the former Humboldt Smelter, where mining and smelting operations began in the late 1870s, consists of about 180 acres and is located about 0.5 miles east of Highway 69. After small-scale operations in the late 1800s, the Humboldt Smelter purified copper from mine ores between 1906 and about 1937. Most production took place during World War I. Later, in the 1950s and 1960s, small processing operations attempted to recover metals from materials brought to the old smelter property.

The contamination that remains today at the Site is from these historical operations.

Where is the Waste Contamination On and Near the Mine and Smelter?

A 4-million cubic-yard tailings pile remains at the Iron King Mine property. Tailings are wastes left over after saleable metals are removed from mined ore during processing. The tailings at the IKHS Site contain high levels of arsenic and lead. These metals can be toxic to humans and wildlife and can be found in soils or sediments, or dissolved in water. The slopes of the tailings pile are too steep for long-term stability. In about 1964, part of the pile collapsed. Mine pile tailings flowed into the Chaparral Gulch, passed downstream, and mixed with tailings from the Humboldt Smelter. There are braids of mostly buried contaminated tailings in the Gulch downstream of 3rd Street. These contaminants move and mix with sediments from the mountains during heavy rains.

The Humboldt Smelter dumped tailings into a wide swale, or depression in the land, and into an expansive flood plain in the Lower Chaparral Gulch. Today, these tailings and tailings from the Iron King Mine have mixed in the flood plain. The tailings are held back by a 25-foot concrete dam downstream of the former smelter. The dam is wedged in a narrow canyon upstream of the Agua Fria River. The extensive tailings are contaminated with arsenic and lead.

The Chaparral Gulch is usually dry but can have very high water flow during peak rain events. Sediments are washed down from the mountains and into the canyon along with mine and smelter tailings. Cleanup of the tailings in the Gulch will present a significant engineering challenge because the entire Gulch continues to be subject to powerful erosion that cuts through and moves tailings, and carries in more sediment.

The former smelter operators also dumped slag, a molten waste material, over the side of a cliff on the smelter property overlying the Agua Fria River. The slag has solidified and has an appearance like hardened black lava. In addition, a fine, grayish material called aluminum dross was crushed above the Gulch on the former smelter property, most likely in the 1950s, with the intention of recovering saleable aluminum. The dross remains today and contains elevated levels of lead.

EPA Sampling in Residential Yards

Over decades, mine and smelter tailings and other forms of contamination also reached some residential yards. Tailings or particles may have blown in the wind, been used as fill material, or been left in areas that later became yards. If levels of arsenic and lead in residential soils are high enough, they can pose health risks to persons exposed to the soils. For this reason, EPA investigated where residential soils would potentially have higher levels of arsenic or lead because of the mine and smelter, and whether these elevated levels may pose a health risk to residents.

EPA has now sampled or screened 580 residential yards in Dewey-Humboldt. Sampling results show that the great majority of yards sampled do not have levels of arsenic or lead that would pose a health risk to residents. However, EPA found a small number of yards with soils containing elevated levels of lead or arsenic and is now planning and executing cleanup actions for these yards as a first priority.


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Contaminants and Risks

Contaminated Media
  • Groundwater
  • Surface Water
  • Air
  • Soil and Sludges
  • Environmentally Sensitive Area

The contaminants of concern at this site are arsenic and lead. These are metals that can attach to soil, sediment, or mine waste particles and in certain conditions, dissolve into water.

The most prevalent contaminated arsenic and lead source at the site is mine tailings. The tailings are located at the former Mine Tailings Pile, in the Chaparral Gulch, in the former Smelter Tailings Swale, in the flood plain above the dam, and in a few scattered places in the Gulch below the dam. The tailings from the mine and some areas at the former smelter contain up to 350 times the background concentrations of lead and over fifty times the background concentrations of arsenic. Background concentrations are the levels of metals that are normally found in an environment and can be naturally occurring or can be caused by other contamination sources, such as lead paint or gasoline. Exposure to lead and arsenic at these concentrations in the tailings could present a health risk if a person is in contact with them over a long period of time.

Mine tailings can move in the environment in several ways. They can wash into storm drainages and be washed downstream, either alone or after mixing into sediments being carried along by stormwater. They can then result in toxicity to humans, plants, or wildlife at downstream locations where they are deposited. Tailings can also be blown by the wind from locations where there is no cover or crust over tailings piles and deposits. They can then mix into soils in locations downwind.

In addition, when the Humboldt Smelter was operating 80 to more than 100 years ago, it generated emissions of particles and vapors from its smoke stacks. This is called stack emissions. This occurred when the furnaces used to melt the ores were vented to the atmosphere. There were no regulatory air controls at the time when the smelter was in operation, and the stack emissions would have contained metals. Part of EPA’s investigation has focused on possible residual soil contamination related to smelter stack emissions.

The primary areas of contamination at this site are:

    The Main Tailings Pile at the Former Mine Site;

    Areas with contaminated soils immediately north, south, and west of the Iron King Mine Main Tailings Pile;

    Tailings deposited, or mixed into sediments and soils, in the Upper and Middle Chaparral Gulch east of Highway 69 and above and below the 3rd Street overcrossing;

    Tailings remaining in the Smelter Tailings Swale, a former tailings impoundment used by the Smelter, which lies above Chaparral Gulch but below the level of the rest of the smelter and town;

    Dry and wet tailings and contaminated sediments lying in the great tailings flood plain below the smelter; including water-deposited tailings and contaminated sediments from both the mine and the smelter, and held back by a 25-foot high dam;

    The original smelter property, which exhibits contaminated soils and piles of aluminum dross material imported to the property after the original smelter closed;

    Solidified slag material hanging over the Agua Fria River adjacent to the original smelter pyrometallurgical operations area.

    The locations of these source areas are shown on the map found above on this webpage.

Human Health Risk Assessment and Ecological Risk Assessment

EPA has now completed a comprehensive human health risk assessment to evaluate the potential harm to residents posed by site contaminants. EPA has fully sampled or screened 580 residential properties. Sampling results show that the great majority of yards sampled do not have levels of arsenic or lead that would pose a health risk to residents. However, EPA found a small number of yards with soils containing elevated levels of lead or arsenic. EPA is now planning cleanup actions for these yards as a matter of first priority.

When EPA does a human health risk assessment, very health-protective assumptions are made and all site-related contaminants are considered in terms of the toxicity of the contaminants, how much people could be exposed to and for how long, and how bioavailable the contaminants are for the most sensitive individuals. The results of the human health risk assessment are provided in Section 9 of the Remedial Investigation Report.

EPA conducted an ecological risk assessment to evaluate whether potential exposure to contaminants could harm plants or animals. The assessment found that contamination poses risks to many plant and animal groups (for example: mammals, birds, reptiles, and aquatic organisms). The results of the ecological risk assessment are provided in Section 10 of the Remedial Investigation Report.

Groundwater and Drinking Water

EPA has collected data from many monitoring wells installed in the aquifers (layers under the ground) from which public and private drinking water is supplied. The data indicate that the site is not presently causing arsenic or lead contamination in groundwater or drinking water.

However, groundwater is naturally high for arsenic in sporadic locations in the Dewey-Humboldt area. This is due to veins of rock in the local geology that contain arsenic in certain locations. Many private home water supply wells have very low levels of arsenic, while others have elevated arsenic. The arsenic level in any given private supply well depends on how close the well screen is to an underground rock vein containing high arsenic. Private household wells supplying arsenic above safe drinking water standards are more prevalent on the eastern side of the Agua Fria River.

While the data indicate that elevated arsenic in groundwater is not presently related to the site, residents’ private well water may nonetheless have naturally elevated levels of arsenic. Arsenic in drinking water is known to increase the risk of many health effects. EPA therefore strongly recommends that residents have their private well water tested and take action to install in-home filtration systems to remove arsenic if their water exceeds 10 micrograms per liter of arsenic.

The public water supply in Dewey-Humboldt is provided by purveyor Humboldt Water. For several years, the purveyor was under an enforcement action by the Arizona Department of Environmental Quality (ADEQ) due to violations of the Clean Water Act. As of 2016, Humboldt Water has installed new equipment, and ADEQ has informed the public that the public drinking water again complies with drinking water standards.

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Who is Involved

The Iron King Mine / Humboldt Smelter Site is a federal Superfund site and the U.S. EPA is the lead agency for investigation and cleanup activities related to the site.
Other independent agencies and entities with knowledge and/or involvement on the Superfund project for this site include:

    The Arizona Department of Environmental Quality

    The Town of Dewey-Humboldt

    The U.S. Agency for Toxic Substances and Disease Registry

    The Arizona Department of Health Services

    The University of Arizona Superfund Research Program

EPA has issued a Technical Assistance Grant (TAG) to the Community Coalition of Dewey Humboldt, with Chief Officer Rose Eitemiller. The funds under this grant are used for a community group to hire a technical consultant to assist them in understanding information provided by EPA about the site in its presentations, investigations, assessments, and reports.

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Investigation and Cleanup Activities

Remedial Investigation and Risk Assessment

Under the Superfund process, a remedial investigation (RI) is performed to evaluate the nature and extent of the contamination at a site. The RI explores what, where, and how much contamination is present, how and where it may have moved or be moving, and what risks it poses. The RI defines the problem that subsequent phases of the process develop and select actions to address.

As of September 2016, EPA has issued the comprehensive Remedial Investigation for the IKHS Site. This 3-volume document includes the human health risk assessment and ecological risk assessment. A link to an electronic version of this document can be found at the top of this webpage. The document can be found in hard copy at the site’s information repository at the Dewey-Humboldt Town Library.

Cleanup in Residential Areas: Residential Soils Removal Action

While sampling during the investigation has shown that the great majority of residential yards do not pose a risk to residents, EPA is now planning to implement an expedited cleanup action to address risks posed by contaminated soils at about 15-30 residential yards with elevated levels of lead and/or arsenic.

Cleanup in Non-Residential Areas: Feasibility Study

Now that the investigation into contamination has been completed and is documented in the RI, EPA is starting the feasibility study (FS) in the non-residential areas of the site. This includes the Main Tailings Pile and properties surrounding it, all affected segments of the Chaparral Gulch, the Humboldt Smelter Property (including but not limited to the dross piles, operations areas, and slag), the Smelter Tailings Swale, the great tailings flood plain, the concrete dam, the confluence area of Chaparral Gulch and the Agua Fria River, and other contaminated areas.

The purpose of the FS is to develop, evaluate, and compare cleanup alternatives. The FS is the basis for public comment and input prior to EPA’s selection of a cleanup alternative.

Investigation History Leading Up To the Remedial Investigation

Formal concerns related to the accumulated contamination at the former Iron King Mine and Humboldt Smelter properties began as a result of EPA and Arizona Department of Environmental Quality (ADEQ) involvement in the various air quality, stormwater, landfill operation, and wastewater treatment permits held by facility operators. Complaints were documented and permit compliance violations reported. EPA and/or ADEQ performed the first CERCLA investigations (preliminary assessment/site inspections) at the properties in 2002-2004. A removal assessment was completed by EPA in 2005, and an expanded site inspection was completed by EPA in 2006.

The principal findings of these investigations were that hazardous materials, primarily metals, had been released at the former Iron King Mine and former Humboldt Smelter properties (or adjacent properties), and the contaminant sources. The findings of the initial investigations formed the basis for listing of the combined Iron King Mine / Humboldt Smelter Superfund site on EPA’s National Priorities List (NPL) on September 3, 2008.

Following listing of the Site on the NPL in 2008, EPA performed the following four phases of investigations between 2008 and 2015:

Investigation Name
Performed by
Phase 12008 to 2009 Initial RI Performed by EPA’s contractor EA Engineering, Science, and Technology, Inc.
Phase 22010 and 2012 Supplemental RI Performed by EPA’s contractor EA Engineering, Science, and Technology, Inc.
Phase 32012 to 2013 Background and Surface Soil SamplingPerformed by EPA Region 9
Phase 42013 to 2015 Data Gap RIPerformed by EPA’s Environmental Response Team and their contractor Lockheed Martin Scientific, Engineering, Response and Analytical Services
The RI Report discussed above provides a synthesis of the previous and current investigations and provides additional analysis, evaluation, and risk assessment conducted in 2015 and 2016.

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Cleanup Results to Date

As discussed in the previous section, EPA is currently planning a removal (cleanup) action for residential soils.

In addition, two previous initial removal (cleanup) actions were performed at residential and municipal properties in response to elevated concentrations of arsenic and lead in surface soils. These actions were performed before the overall residential investigation was complete. The first removal action was performed in 2006-2007 by a property owner/operator, Ironite Products Company, and addressed about six residential parcels. Soils were removed and replaced with clean soil. The second removal action was performed by EPA in 2011-2012 and addressed about 13 parcels. Most of these parcels were located along Sweet Pea Lane near a former rail spur that led into the former Humboldt Smelter. Soils were removed and replaced with clean soil.

Also in the 2011 removal action, a small tailings pile just north of the Main Tailings Pile at the Iron King Mine Property was removed and safely stockpiled on the Main Tailings Pile. This small tailings pile lay within the Chapparal Gulch west of Highway 69, and contaminated tailings were washing into the Gulch from that location.

These removal actions are discussed in detail in Section 3 of the RI Report.

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Potentially Responsible Parties

Potentially responsible parties (PRPs) refers to companies that are potentially responsible for generating, transporting, or disposing of the hazardous waste found at the site.

Potentially responsible parties (PRPs) refers to entities that are potentially liable under the Superfund law to pay for, and/or perform investigation and cleanup of, site contamination. PRPs may include current owners and operators at the site and parties who were owners or operators during a time of release of hazardous substances. Parties responsible for generating, transporting, or disposing of the hazardous substances may also be PRPs. As part of the Superfund process, EPA identifies those parties that are "potentially responsible parties" who fall into one or more of the above categories of PRPs, and then may negotiate with the PRPs to perform cleanup activities under EPA oversight and/or help pay EPA's response costs. EPA has conducted a search for parties associated with the IKHS site and on November 19, 2009 issued a General Notice Letter to 14 parties advising them of their status as PRPs under CERCLA.

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Documents and Reports

Hide details for Administrative RecordsAdministrative Records
Hide details for Community InvolvementCommunity Involvement
10/01/09Community Involvement Plan - Iron King Mine – Humboldt Smelter Site
04/27/10Iron King Mine - Humboldt Smelter Superfund Site - Presentation
08/31/11Public Meeting Presentation
09/01/11Frequently Asked Questions
Hide details for Fact SheetsFact Sheets
01/01/06U.S. EPA Completes Removal Assessment
04/01/08Iron King Mine-Himboldt Smelter Proposed for Superfund List
08/01/08EPA Begins Site Investigation
01/30/09EPA Conducts Ambient Air Sampling
05/01/09Chaparral Gulch Show Elevated Arsenic
10/02/09Investigation Report Due This Winter
02/01/10Safe Gardening in Dewey-Humboldt
02/16/10Federal, State, and Local Environmental Resources
04/13/10EPA Completes Remedial Investigation - Additional Residential Sampling Offered
11/04/10Cleanup Documents Available for Review
04/12/11Residential Soil Sampling Completed Ahead of Upcoming Yard Cleanup
08/12/11Residential Yard Cleanup To Start this Fall
04/02/12EPA Completes Interim Cleanup
09/05/12Site Update
11/22/13Upcoming EPA Field Work and December 7 Meeting Announcement
10/30/14Iron King Mine/ Humboldt Smelter Update flyer
04/30/15Sampling Results for Residential Yards in Dewey-Humboldt
09/27/16EPA Issues Remedial Investigation and Risk Assessment Reports; Next Steps in Cleanup of Residential Yards
Hide details for ImagesImages
07/01/10Iron King Mine – Humboldt Smelter Superfund Site (Frequently Asked Questions)
Hide details for MapsMaps
05/01/08Investigation Areas - Iron King Mine - HS
01/20/09Site Map
Hide details for Technical DocumentsTechnical Documents
08/01/08Data Presentation Air- Ambient Air Data Summary Table
01/01/09Data Summary Air - Ambient Air Data Summary Table - December 2008/January 2009
02/04/09A Cultural Resource and Historic Building Survey for a Remedial Investigation/Feasibility Study at the Iron King Mine-Humboldt Smelter Superfund Site
03/01/09TEOM - Particulate Monitoring Graph
07/01/09Reusing Superfund Sites - Summary Overview
03/01/10Remedial Investigation Report
03/01/10Remedial Investigation Report (Appedix A)
06/01/10Reuse Assessment for Iron King Mine - Humboldt Smelter Superfund Site
10/11/10Data Evaluation Summary Report Addendum II
10/13/10Remedial Alternatives Evaluation Technical Memorandum
10/20/10Cover Alternatives Evaluation for Iron King Main Tailings Pile
07/01/12Iron King Mine Removal Report

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Community Involvement

Public Meetings: EPA recognizes that communities have the right to be involved in the decisions that affect their lives. EPA’s experience has been that when the public is involved in EPA’s work, the cleanup process results in a better outcome and a more robust remedy.

At the IKHS site, EPA’s Community Involvement Program helps citizens participate throughout the cleanup process, including the investigation phase and the remedy selection phase. Among the goals of EPA’s Community Involvement Program are to:

    1. Provide opportunities for the public to become actively involved;

    2. Meet the community’s information needs;

    3. Incorporate issues and concerns into cleanup decisions; and

    4. Give feedback to the public on how their issues and concerns were addressed.

EPA and ADEQ have a number of ways for the community to become more knowledgeable about and involved with the IKHS site.

Community Involvement Plan
The Iron King Mine / Humboldt Smelter Superfund Site Community Involvement Plan provides a detailed explanation of how EPA and ADEQ will encourage public participation in the cleanup decision-making process. It is a flexible plan that organizes both EPA’s and ADEQ’s efforts but can change based on new community needs. A copy of the Community Involvement Plan is located at the Dewey-Humboldt Town Library and is also available in the documents section of this website.

Fact Sheets
EPA periodically issues fact sheets to all mailing addresses in proximity to the site. These fact sheets provide information on a wide variety of topics including updates on the latest data and findings, announcements of upcoming public meetings, issuance of key reports, attainment of project milestones, opportunities for public input, public advisories, and other Site information. Several fact sheets are available on this website and are linked below.

Public Meetings
Since the Site investigation and listing on the National Priorities List, EPA has held numerous meetings for the public. These meetings may include a presentation followed by input and discussion from the community or may follow an “open house” format with exhibits and staff available to answer questions. EPA plans to continue to announce and hold public meetings at appropriate points in the Superfund process.

EPA has also given presentations at Town Council meetings. EPA plans to continue to provide such briefings to the Town Council, and to coordinate with and keep the Town Manager aware of developments regarding the project in the future.

Town Newsletter
EPA periodically provides brief site updates and announcements through the Dewey-Humboldt Town Newsletter, which is available to all residents.

Community Technical Assistance Grant
EPA awarded a Technical Assistance Grant to the Community Coalition of Dewey-Humboldt, a local non-profit group. The purpose of the Technical Assistance Grant is to promote public participation by providing financial assistance to a community group for independent technical assistance. After the independent technical advisor reviews Site-related documents, the advisor can help the community formulate its questions and concerns and communicate them effectively to EPA.

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Public Information Repositories

The public information repositories for the site are at the following locations:

Dewey-Humboldt Town Library
2735 N. Corral Street
Dewey-Humboldt, AZ
(928) 632-5049

The most complete collection of documents is the official EPA site file, maintained at the following location:

Superfund Records Center

Mail Stop SFD-7C

95 Hawthorne Street, Room 403

San Francisco, CA 94105

(415) 820-4700

Enter main lobby of 75 Hawthorne street, go to 4th floor of South Wing Annex.

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EPA Site Manager
Jeff Dhont
US EPA Region 9
Mail Code SFD
75 Hawthorne Street
San Francisco, CA 94105
EPA Community Involvement Coordinator
Heather Parker
US EPA Region 9
Mail Code SFD
75 Hawthorne Street
San Francisco, CA 94105
EPA Public Information Center
State Contact
Karin Harker
1110 West Washington St.
Phoenix, AZ 85007
PRP Contact
Community Contact
Rose Eitemiller
Other Contacts
After Hours (Emergency Response)
(800) 424-8802

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