Department of
Environmental Quality

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 the 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 state and 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 mined 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.

Investigation and Remediation Efforts

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 clean-up alternatives will be identified and evaluated for those areas in a Feasibility Study (FS).

The RI for the entire mine site is currently underway. Numerous samples of surface water, ground water, soil, sediment, vegetation, aquatic and terrestrial biota have been collected and analyzed. Other characterization information such as water level measurements in wells, geophysical information, and surface water flow measurements have been collected. A complete summary of all available data and conclusions based on it will be presented in the future in a RI Report.

The CO/AOC also called for the development of a Community Involvement Plan to determine community concerns, interests, and involvement in the CERCLA decision-making process. The plan identifies outreach activities the agencies will use to promote public participation in the clean-up process. The goals of the plan are to:

• Inform the public of current and planned site activities
• Maintain open, two-way communication with the public, ensuring questions are answered and concerns are addressed as they occur
• Provide the public with information and opportunities to comment on the information
• Promote public participation in the decision making process

Public comment was solicited on the plan. It is anticipated that the plan will be updated as project developments require.

Cleanup of Pedro Creek Overburden Disposal Area

The Conda-Woodall Mountain Mine site includes several Overburden Disposal Areas (ODAs) where waste rock excavated from the mining pits was dumped. The Pedro Creek ODA is one of the most significant ODAs in terms of potential impacts to the human health and the environment. Elevated concentrations of selenium are found in surface soils, surface water, alluvial ground water, sediments, and vegetation. In addition, the Pedro Creek ODA is unstable and subject to erosion from rain and snowmelt, which has resulted in the release of selenium and other contaminants to ground water and surface water downgradient.

Actual or threatened releases of hazardous substances from the Pedro Creek ODA are present and imminent and substantial endangerment to public health, welfare, or the environment exists. As a result, DEQ and EPA have decided to clean up the ODA as a Non Time-Critical Removal Action under CERCLA. An Engineering Evaluation/Cost Analysis (EE/CA) was finalized in October 2010. The EE/CA outlined the clean-up objectives for the Pedro Creek ODA and evaluated six potential cleanup alternatives in terms of effectiveness, implementability, and costs. The EE/CA also identified the clean-up alternative preferred by EPA and DEQ. A public comment period on the potential clean-up alternatives was held in January-February 2011 (Fact Sheet). Public input was considered and, in July 2011, DEQ, EPA, and BLM selected an alternative that calls for:

• Consolidating and regrading of the overburden/waste rock on the steep side slopes
• Regrading the ODA to eliminate flat spots and pooling areas
• Placing a 12-18 inch thick cover over the regraded areas
• Revegetating with non-selenium-accumulator plant species
• Installing diversion ditches and other erosion and sedimentation controls to reduce infiltration through the waste rock
• Developing and implementing a long-term performance monitoring plan

The removal action will be conducted and funded by the J. R. Simplot Company under DEQ and EPA oversight with BLM participating as a cooperating agency to EPA. Action is slated to begin in 2011 and estimated to require one to two years to complete. A detailed description of the cleanup plan, the rationale for the selected alternative, and a summary of public comments and responses are available in the Pedro Creek ODA Action Memorandum.