Selenium Investigations in Southeast Idaho
In 1996, isolated livestock deaths associated with excessive selenium uptake in the vicinity of historic phosphate mines in southeastern Idaho prompted concerns regarding potential human health and ecological effects from past mining operations. In response to these concerns, the primary mine operators in the region formed the Idaho Mining Association (IMA) Selenium Committee, an ad hoc organization to jointly and voluntarily investigate and address any mining-related environmental and public health issues associated with past operations. Similarly, an Interagency/Phosphate Industry Selenium Working Group (SeWG) consisting of voluntary participants from federal, state, and tribal agencies, as well as other stakeholder groups, was established to collaborate on these efforts.
From 1997 to 2000, numerous phased investigations were conducted by the IMA Selenium Committee's contractor, agency investigators, and regional academic scholars and scientists. Due to the similarities in operations and mine design, and to provide some level of cost effectiveness, the investigations were directed at a limited number of constituents and conducted on a representative area-wide basis.
Investigative Efforts to Date
Through the voluntary efforts of the IMA Selenium Committee and SeWG participants, investigators were able to confirm the release of selenium and other related metals in localized areas. The primary source of observed releases appears to be infiltration of runoff through external waste rock piles at the historic sites. These piles were designed to accumulate and store the overburden and waste rock generated from the phosphate mining process. The piles partially consist of middle waste shales, a naturally occuring, mineral-enriched layer of waste rock that can release dissolved metals to water when highly fractured, weathered and oxidized. The metal-laden waters emanating from the piles from localized seeps and springs are transported to adjacent stream segments and wetlands where they are deposited in sediments, absorbed as plant uptake or accumulated in biological systems. Vegetation on and adjacent to the waste rock piles also accumulate root zone-available dissolved metals.
Because of the described transport mechanisms, the highest metal concentrations are observed during the spring runoff when the piles are initially flushed. Concentrations generally decrease in flowing streams through the summer months. Conversely, isolated surface water bodies such as drainage basins and ponds tend to concentrate through the summer months due to evaporation. Other media, including vegetation and sediments, can retain high concentrations of accumulated metals throughout the year.
Existing data indicate that these effects are currently focused around approximately 75 square miles of active and historic mine lease areas within the approximate 2,500-square-mile resource area. Future activities will be directed at defining the nature and extent of contamination at the individual mine sites.
DEQ Tasks to Date
In August of 2000, DEQ formally took the agency lead for the selenium area-wide investigation through voluntary agreements with the companies and interagency participants. DEQ hired a contractor for technical assistance and developed a scope of work and project schedule for completing the area-wide investigation targeted at ultimately developing regional risk-management guidance. DEQ also established an interagency technical group to coordinate their activities with other jurisdictional and administrative agencies, and a Selenium Area-Wide Advisory Committee (SeAWAC) to continue to solicit input from mining company representatives, project stakeholders, and other participants in the former SeWG. DEQ's initial technical tasks were to:
- review all existing data, reports, and preliminary risk evaluations for data quality and usability
- develop Conceptual Site Models (CSMs) for Human Health and Ecological Risk Assessments to determine critical data needs for baseline risk assessment requirements
- perform a comprehensive data gaps analysis to plan for data collection efforts
During the 2001 field season, additional surface water, soil, sediment, vegetation and biota sampling events were conducted to fill the remaining area-wide risk assessment data gaps. Sampling activities included expanding the list of target parameters to a full range of potential mining-related metals to ensure the final list of contaminants of potential concern was appropriately screened.
These data were used to perform an area-wide risk assessment that was published in December 2002 after conducting a formal public comment period. The risk assessment generally concluded that regional human health and population-level ecological risks were unlikely due to the limited amount of area impacted by previous releases. However, subpopulation risks to aquatic, riparian, and terrestrial ecological receptors were evident in localized areas based on significant exceedances of toxicological benchmarks and threshold reference values.