Pacific Southwest, Region 9: Superfund
Serving Arizona, California, Hawaii, Nevada, the Pacific Islands, and Tribal Nations
Cyprus Tohono Mine
EPA #: AZD094524097
City: Casa Grande
Congressional District: 0
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Description and History
NPL Listing History
NPL Status: Not on the NPL
Cyprus Tohono Corporation (CTC) operated a copper mine and processing facility (the Cyprus Tohono Mine Site) on tribal lands leased from the Tohono O'odham Indian Nation (Nation). The Cyprus Tohono Mine Site (Site), previously known as the Casa Grande Mine and the Lakeshore Mine, is located on Highway 15 approximately 32 miles south of Casa Grande, Arizona. Development of the mine Site began in the 1880s, when low-grade oxide ore was mined from surface outcrops. During the 1950s and 60s, Trans-AZ operated a small open pit copper oxide mine. In the late 1960s, Trans-AZ and the El Paso Natural Gas Company enlarged the open pit for removal of 350,000 tons of ore. Mining continued into the 1980s, with several changes of operators. CTC began operating the Site in 1987, after leasing it from the Nation. It expanded the open pit mining activities and heap leaching operations. Since January 2005, the CTC continued with the extraction/electrowinning operations for processing heap solutions. In 2009, the mine was moved into "care and maintenance" status under the jurisdiction of the Bureau of Land Management (BLM).
In 1992, there was a release of 1.4 million gallons of process waste water to surface washes on the mine site due to a break in a line running to the evaporation ponds.
In 2000, EPA required a Supplemental Environmental Project, including geo-chemical and hydro-geological characterization and a reclamation plan. Groundwater investigations discovered elevated sulfate and uranium concentrations in the groundwater beneath the evaporation ponds and tailings impoundments at the mine site.
A number of removal and remedial investigation activities have taken place at the Site. These include the construction of a solution containment system in 2000, the implementation of a wildlife management program beginning in 2001, the installation of domestic groundwater wells for the village of North Komelik in 2002, completion of a temporary water treatment plant for pit lake waters in 2004, and the movement of the contaminated soils from the evaporation ponds to a lined cell near the heap leach piles in 2006-2007.
The remedial investigation (RI) phase began in 2009, the same year that EPA identified the Site as a Superfund Alternative Site (SAS). The RI Report was submitted by the CTC in late 2012. EPA provided the CTC "conditional approval" of the RI Report in 2016.
Contaminants and Risks
There is a groundwater plume with uranium, sulfate and perchlorate that extends approximately one-half mile from the site.
Who is Involved
EPA, in consultation with the Nation, oversees the remedial investigation/feasibility study work conducted by the CTC, the potentially responsible party.
In 2002, the CTC provided bottled water to the village of North Komelik on the Nation to temporarily replace the drinking water supply for the residents. Several wells were discovered to be contaminated with uranium at concentrations above background that exceeded health based guidelines. In 2003, the CTC replaced the North Komelik drinking water supply wells with two new wells to provide safe drinking water. The wells were located 6 miles south of North Komelik and extracted water from an uncontaminated aquifer. The bottled water program was terminated once the new supply wells were in place. Subsequently, in 2013, the Nation's Water Resources Department provided a connection to the Greater Santa Rosa regional water system for the village of North Komelik. This system draws drinking water from groundwater wells upgradient from the Site that do not have elevated levels of naturally-occurring arsenic, fluoride or sulfate. This system is in compliance with EPA's primary drinking water standards.
Investigations began at the Site in the early 2000s. The investigative activities were conducted by the CTC, in coordination with the BLM and the Nation, in response to EPA's involvement at the Site. During the period of 2001 to 2008, the investigative work included: surface geophysical studies; installation of 55 new groundwater monitor wells; borehole geophysics; depth-specific groundwater quality sampling; hydraulic testing; aquifer testing; and routine water quality and water level monitoring.
In 2005, EPA completed an Environmental Evaluation/Cost Analysis (EE/CA) and held a public meeting to obtain comments on an option analysis for addressing the source areas at the mine site by a Removal Action. In 2006-2007, the CTC conducted a Non-Time Critical Removal Action for removal of contaminated material from the former evaporation tailing ponds, the former mill tailings impoundment,and the vat leach tailings embankment and placed them in a high density polyethylene (HDPE) lined repository located southeast of the open pit near the heap leach piles, on the mine site. The former evaporation ponds area was reclaimed with an earthen cover and re-vegetated. CTC conducts quarterly inspections of the re-vegetated areas. The estimated cost for this Removal Action was $19 million.
During the period of 2009-2012, as part of EPA's Remedial Investigation (RI) process, the CTC continued to conduct field studies, and prepared plans and reports for the mine site. This work included the installation of an additional 10 groundwater monitor wells, updated sampling and monitoring plans, and the preparation of a 4 volume Remedial Investigation (RI) Report.. The RI Report included the evaluation and findings of groundwater data gathered over a 10 year period, as well as a baseline risk assessment for both human health and for ecological resources. The RI Report was submitted to EPA for review in December 2012. In 2013, CTC informed EPA that it was evaluating whether to resume mining activities at the Site. EPA provided the CTC "conditional approval" of the RI Report in early 2016. An updated groundwater model was completed in 2016, and the Feasibility Study process is underway in 2017.
In 2004, CTC constructed a water treatment plan to treat the acidic Pit Lake water on a temporary basis, that was discharged from the heap leach piles into the open pit mine area.
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.
Cyprus Tohono Corporation (CTC)
Documents and Reports
Public Information Repositories
The public information repositories for the site are at the following locations:
EPA Site Manager
Mail Code SFD
75 Hawthorne Street
San Francisco, CA 94105
EPA Community Involvement Coordinator
Mail Code SFD
75 Hawthorne Street
San Francisco, CA 94105
EPA Public Information Center
After Hours (Emergency Response)