Selenium at the Gay Mine

Area-Wide Selenium Investigations in SE Idaho

In the mid-1990s, a naturally-occurring trace mineral called selenium, found in the “center waste shale” of phosphate mines (the rock layer found between layers of phosphate ore), was found to have leached into vegetation and waterways near mines in Caribou County, Idaho, at measurable concentrations. The selenium was determined to be the cause of injured livestock that had grazed on this vegetation. In July 2000, the US Forest Service (USFS), the US EPA, the Bureau of Land Management (BLM), the Bureau of Indian Affairs (BIA), the US Fish and Wildlife Service (USFWS), the Shoshone-Bannock Tribes (Tribes), and the Idaho Department of Environmental Quality (IDEQ) entered into a Memorandum of Understanding (MOU). In May 2001, five companies that were current or former operators of Southeast Idaho phosphate mines (Agrium/NuWest, FMC, Monsanto, J.R. Simplot Co., Rhodia) entered into a Consent Order/ Administrative Order on Consent (CO/AOC) with IDEQ to conduct a Selenium Area-Wide Investigation at these mine sites. IDEQ took the lead in conducting the Area-Wide Investigation under the CO/AOC, including a human health and ecological risk assessment.

IDEQ reviewed the existing data and prepared a draft data gap analysis. Based on the data gap memo, the companies prepared an area-wide work plan for additional surface water and soil sampling, which IDEQ approved. Additional field sampling, including sampling at the Gay Mine, occurred during the spring and summer 2001. IDEQ issued a final area-wide investigation report in 2002.

IDEQ utilized the area-wide investigation report to prepare a human health and ecological risk assessment and risk management plan. In April 2002, IDEQ released its preliminary area-wide risk assessment for public review. IDEQ made the following conclusions:

  1. Overall there are no significant human health risks or ecological risks (at the population-level) from selenium in the SE Idaho Resource Area.
  2. IDEQ did not recommend any area-wide actions, but did recommend development of site-specific best management practices for current and future mining, and remediation of “hot-spots” based on the site-specific investigations that would be conducted at the individual mine sites.

As part of the Selenium Area-Wide Investigation, numerous media and locations were sampled for selenium and other trace metals at the Gay Mine. Selenium concentrations exceeding Safe Drinking Water Act Maximum Contaminant Levels (SDWA MCL) were detected in five “pit lakes” (artificial lakes formed when mining ceased and the excavated area was allowed to fill up with water). Selenium levels in samples collected in 1997-1998 from the natural streams/creeks at the Gay Mine did not exceed Clean Water Act Water Quality Standards (CWA WQS) for surface water. Sampling of soil and vegetation at overburden backfill/dumps and mill shale piles found above-background concentrations of selenium and other trace metals; however, there are no established regulatory criteria for selenium and/or trace metals in vegetation.

FMC and Simplot Recommendations for Addressing Gay Mine Pit Lakes Contamination

Based on the available data, FMC and Simplot commissioned the nationally-recognized environmental engineering firm Brown and Caldwell to evaluate alternatives to mitigate the contamination in the pit lakes. The alternatives, which included institutional controls (fencing to minimize access) and partial pit backfill to eliminate the pit lakes, were discussed at a meeting between the companies, BLM, BIA, and the Tribes on August 3, 2000. The companies’ objective at that meeting was to restart discussions on proceeding with final reclamation, discuss the results of the Area-Wide and Gay Mine-specific selenium investigations conducted from 1997-2000, and obtain the Agencies’ approval to partially backfill and eliminate the pit lakes with identified elevated selenium concentrations. The Agencies and the Tribes did not approve or disapprove or otherwise act on the companies’ proposal.

IDEQ Final Area-Wide Human Health and Ecological Risk Assessment Report

IDEQ issued the final Area-Wide human health and ecological risk assessment report in 2003. The IDEQ report concluded that “regional human health and population-level ecological risks are unlikely to occur in the overall Resource Area.” In 2004, IDEQ issued the final Area-Wide Risk Management Plan that identified four objectives:

  1. Protect surface water resources
  2. Protect wildlife, habitat and ecological resources
  3. Maintain and protect multiple beneficial uses
  4. Protect groundwater resources

During April 2004, Simplot and FMC conducted site-specific surface water sampling at Gay Mine in anticipation of a site-specific investigation consistent with the IDEQ area-wide risk management plan “model” Statement of Work (SOW). The sample results were consistent with past Gay Mine sampling. No streams exceeded the federal Clean Water Act Water Quality Standards for Selenium (Se), Cadmium (Cd), Chromium (Cr), Nickel (Ni), Vanadium (V) or Zinc (Zn).

The Bureau of Community and Environmental Health (BCEH) of the Idaho Department of Health and Welfare, under a cooperative agreement with the federal Agency for Toxic Substances and Disease Registry (ATSDR), issued a “Public Health Assessment for the Southeast Idaho Phosphate Mining Resource Area” (PHA) on February 24, 2006. Conclusion #1 of the PHA states that “BCEH classifies the Southeast Idaho Phosphate Mining Resource Area as a no apparent public health hazard according to ATSDR’s interim public hazard categories.” The PHA superseded a prior health consultation that suggested an advisory for consuming trout from East Mill Creek (ATSDR HC May 2003).