Protecting Public Health and the Environment.

Conda/Woodall Mountain Mine Site

Southeastern Idaho is a major phosphate-producing region, and phosphate mining has been an important industry in the area since the early 20th century. The process of mining phosphate ore from the earth generated open pits and piles of overburden materials (materials covering the phosphate ore). The overburden material is naturally elevated in selenium and other trace metals. When exposed to the elements, these overburden piles can release selenium and other trace metals to the environment.

The Conda/Woodall Mountain Mine site is located east of State Highway 34 about 8 miles northeast of Soda Springs in Caribou County. Mining for phosphate ore at the Conda area began about 1906 and continued until 1984. The site is located on private lands as well as public lands administered by the Bureau of Land Management (BLM). Mining operations took place primarily on property owned by the J.R. Simplot Company, which operated the mine from 1960 until phosphate mining ceased at the site in 1984. Overburden piles comprised of shales high in selenium and other trace metals are present on the site. Simplot is the potentially responsible party for cleanup of the mine.

Remedial Investigation/Feasibility Study

In early 2008, DEQ, EPA, and the US Department of the Interior, BLM entered into a Consent Order/Administrative Order on Consent (CO/AOC) with Simplot to investigate contamination at the Conda/Woodall Mountain Mine under state law and the federal Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). The CO/AOC requires Simplot to perform a Remedial Investigation (RI) to look for and assess contamination from past mining activities and then to evaluate any resulting threats to human health and the environment through development of a Baseline Risk Assessment (BRA).

The RI Report was finalized in late 2016. The human health, ecological, and livestock risk assessments were also completed in 2016. Work has begun on development of the Feasibility Study (FS) to identify and evaluate potential cleanup alternatives for those areas of the mine that were determined to pose unacceptable risks. Simplot submitted a draft of the first part of the FS to the agencies in December 2016. This preliminary document summarizes key findings of the RI and risk assessments, and identifies all available cleanup technologies to address unacceptable conditions and risks at the site. Once the agencies reach agreement on a list of viable technologies, the second part of the FS will conduct a detailed analysis of the alternatives, comparing them against each other to determine which would work better at Conda. The FS is expected to be completed in 2018 and will be posted on this web page when it is finalized.

Data collection continued for year four of the 5-year plant uptake field scale pilot study. The pilot study evaluates the uptake of selenium into vegetation with various soil cover types, as well as uptake into vegetation that are planted directly on overburden disposal area (ODA) materials. Information generated from the pilot study will be used in the FS to determine the appropriate cover thickness to help reduce risks to animals from eating plants that uptake selenium in the soil. 


Conda Field Sampling

FSPS Field Sampling - August 2013

Cleanup of Pedro Creek Overburden Disposal Area (ODA)

The Pedro Creek ODA Non Time-Critical Removal Action (NTCRA) was completed in 2015. The NTCRA is an early cleanup action that was conducted in 2013–2015 to address a steep and unstable Pedro Creek ODA located at the headwaters of Pedro Creek that served as a source of selenium and other contaminants that impacted Pedro Creek and the shallow groundwater. The NTCRA is a source control action that included excavating approximately 1.6 million cubic yards of overburden, regrading the ODA, placing a clean soil cover over the ODA, and seeding it with shallow-rooted, low-selenium-accumulating grasses. In addition, ditches were constructed to divert clean water around the ODA into two infiltration basins and four stormwater/sediment control basins. Two basins were built to manage contaminated water from toe seeps.  Four wells were installed to monitor groundwater quality downgradient of the ODA. The Post Removal Action Site Control Plan, finalized in 2015, includes requirements for long-term effectiveness monitoring and an operations and maintenance activities for the NTCRA.

This photo from May 2010 shows the Pedro Creek ODA before the NTCRA. The overburden was exposed at the ground surface and the very steep side slopes were unstable and prone to erosion and slumping.

In 2015, grasses growing on new soil cover on re-graded Pedro Creek ODA. Stormwater basins and run-on/run-off control ditch in foreground.

What's Next

Following completion of the FS, a proposed cleanup plan will be presented for public comment. After considering public comment on the plan, a record of decision, that selects the final cleanup actions (including any additional actions at the Pedro Creek ODA), will be issued.  

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Staff Contacts

Pocatello Region Mining Project Manager
Margie English
DEQ State Office
1410 N. Hilton
Boise, ID 83706
(208) 373-0306

More Information

Remedial Investigation/Feasibility Study

Pedro Creek ODA Non Time-Critical Removal Action

Related Pages

Selenium Investigations in Southeast Idaho