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 resulted in open pits and piles of waste rock overburden (materials covering the phosphate ore). Some of the waste rock 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. When precipitation moves through the waste rock, chemical processes cause release of selenium and other naturally-occurring metals. The infiltrating water can flush these contaminants downward to the water table where the contaminated shallow groundwater can then flow beyond the overburden disposal area boundary. Shallow groundwater can intercept streams down gradient of the overburden disposal area, transporting contaminants to the surface water.
Conda/Woodall Mountain Mine, looking southwest from Woodall Mountain. Visible in photo: old tailings pond (now dry), mid-limb mine pit, and several grass-covered overburden disposal areas.
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.
Remedial Investigation/Feasibility Study
In early 2008, DEQ, EPA, and the U.S. 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 and the BRA were completed in 2016
During 2018, work continued on the Feasibility Study (FS), identifying and screening potential cleanup alternatives for those areas of the mine that were determined to pose unacceptable risks. A portion of the FS work also focused on development of site-specific preliminary remediation cleanup goals (PRGs) that will be used to determine which sites require remedial actions. Once the agencies reach agreement on the PRGs and on a list of viable technologies, the second part of the FS will conduct a detailed analysis of those remedial alternatives, comparing them against each other to determine which would work better at Conda. A complete FS is will be submitted for review in 2020, and will be uploaded to this website when it is finalized.
Data collection for the five year Plant Uptake Field Scale Pilot Study was completed in 2018. A final report will be issued in 2019. 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 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 take up and accumulate selenium present in the soil.
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, re-grading 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. 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.
Shallow Groundwater Pilot Scale Treatability Study
During 2018, a below-ground permeable reactive barrier (PRB) was designed and constructed as a pilot scale treatability study at one of the ODA seeps in the headwaters of Pedro Creek. The PRB is designed to intercept and treat contaminated shallow groundwater that flows from the toe of the ODA. Physically, the PRB consists of a 22 ft deep trench roughly perpendicular to groundwater flow across the stream valley where contaminated groundwater from the ODA occurs. The trench was filled a prescribed mixture of chopped alfalfa hay, wood chips, and sand, collectively known as reactive media.
As contaminated shallow groundwater flows through the PRB, the selenium is chemically reduced and attaches to the reactive media, removing those contaminants from the groundwater. This technology has been used with success at other phosphate mines in southeast Idaho, but not at a site with groundwater having selenium concentrations as high as Conda. Therefore, in addition to the PRB trench, some former water management ponds between the trench and the ODA were also filled with the same reactive media, transforming the former ponds into treatment cells. Contaminated water seeps out of the ODA toe, through the treatment cells, and then through the PRB trench. It is hoped that this multi-stage in-ground treatment process will provide enough residence time to reduce the high concentrations of selenium to acceptable levels in the shallow groundwater.
To assess treatment performance, groundwater samples will be collected and analyzed from monitoring wells installed up-gradient, within, and down-gradient of the treatment cells and PRB. In addition, surface water samples will be collected and analyzed from Pedro Creek downstream of the treatability study units. Data collected from the first year of operation of the treatment cell and PRB system will be used to support detailed evaluation of groundwater cleanup alternatives in the final FS.
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.