Conda/Woodall Mountain Mine Site
Southeast 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 eight 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 U.S. Department of the Interior, Bureau of Land Management (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). If the BRA determines that any areas of the mine pose an unacceptable risk, then potential cleanup alternatives will be identified and evaluated for those areas in a Feasibility Study (FS).
Progress was made in 2015 toward completing the RI. The draft final RI report was submitted by Simplot and reviewed by the agencies. The draft site-specific human health risk assessment, ecological risk assessment, and livestock risk assessment were also submitted and reviewed in 2015. The RI report and all three site-specific risk assessments should be finalized in 2016. In addition, a supplemental ground water investigation for the western side of the mine was completed in 2015. Results from this investigation will be incorporated into the final RI report as well. The final reports will be posted on this webpage.
Data collection continued for a third year on the five-year Plant Uptake Field Scale Pilot Study, which evaluates the uptake of selenium into vegetation with various soil cover types and into vegetation planted directly on overburden materials. Information 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.
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, the ODA was 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, re-grading the ODA, placing a clean soil cover over the ODA, and seeding it with shallow-rooted, low selenium accumulating grasses. In addition, newly constructed ditches divert clean water around the ODA into two infiltration basins and four stormwater/sediment control basins. Two basins manage contaminated water from toe seeps. Four wells monitor ground water quality downgradient of the ODA. A post removal action site control plan was finalized in 2015 and includes requirements for long-term effectiveness monitoring and 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 RI/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.