Linking Selenium Sources to Ecosystems: Mining
Science Center Objects
Environmental sources of selenium (Se) such as from organic-enriched sedimentary deposits are geologic in nature and thus can occur on regional scales. A constructed map of the global distribution of Se source rocks informs potential areas of reconnaissance for modeling of Se risk including the phosphate deposits of southeastern Idaho and the coals of Appalachia.
Global Prediction of Selenium Sources
From the combined global distribution of phosphate deposits and petroleum-generating basins, it is possible to produce a world-wide map that shows the distribution of organic-carbon enriched sedimentary basins (Figure 1). Current anthropogenic activity, when combined with our forecasts, helps locate areas that may warrant investigations of Se dynamics during development or expansion. The United States has remained the world's largest producer of phosphate rock throughout most of the last century and into the 21st century. North Africa and the Middle East together produce a comparable amount. Major oil production is from the Middle East (6,870 million barrels per year) with Latin America, Central Eurasia, Asia and the Pacific, the United States, and Europe each contributing in the range of 2,500 million barrels per year (see Refining page). Areas of the Alaskan North Slope, North Africa, and Kazakhstan represent areas where both commodities are available or where industries possibly will expand.
Phosphoria Formation/Valley Fills (Southeast Idaho)
The Meade Peak Member of the Permian Phosphoria Formation extends throughout southeastern Idaho, and adjacent areas of Wyoming, Montana, and Utah. Over the last half of the 20th century, mining in Idaho provided approximately 4.5% of world demand for phosphate, used mainly in fertilizer. This tonnage represents approximately 15% of the estimated one billion tons accessible to surface mining within the Phosphoria Formation. The Phosphoria Formation also is estimated to have generated about 30 billion metric tons of oil.
Mining removes phosphate-rich beds and exposes organic carbon-rich waste rock to subaerial weathering. Waste rock is generated at a rate of 2.5 to 5 times that of mined ore. Individual dumps contain 6 to 70 million tons of waste-rock that is either contoured into hills, used as cross-valley fill, or used as back-fill in mine pits. Waste shale in comparison to ore, is more enriched in selenium (80 ppm Se v. 50 ppm Se). In terms of Se chemistry, when Se hosted by organic matter in source rocks is exposed to the oxic conditions of the atmosphere and surface and ground water, Se is oxidized from relatively insoluble selenide and elemental Se to soluble oxyanions, selenite and selenate. Organic Se also can exist in the dissolved phase.
Eight horses, approximately 250-300 sheep, and more than 250 tiger salamanders have died at seven mining sites because of acute dietary exposure to Se. Elk are being evaluated for public health risks and permits for grazing have been suspended for some mine-disturbed areas.
Selenium-contaminated impoundments appear to present greater risks to wildlife than Se contaminated streams and rivers. Avian egg samples were collected in spring when ephemeral vernal wetlands provide habitat and breeding birds are present. Coot eggs reached 80 ppm Se (dw), above the 10-ppm Se embryo viability threshold and the 65-ppm Se concentration above which 100% teratogenesis has been observed. Reproductive impairment was found at one impoundment in spite of the fact that egg collection was limited (Figure 2). The egg tissue contained 12 ppm, a value just above the threshold for substantive risk. Of the 27 coot eggs collected, nine embryos were assessable for presence or absence of overt deformities. One deformity in nine embryos is a factor of 75 above the background rate for overt deformities. This deformity is considered "mild" and, as such, is considered with the sets of ecological data (Se concentrations in water, sediment, plants, invertebrates, and fish) it represents additional evidence of risk to resident birds and those using this part of the Central Flyway.
Appalachian Mountaintop Coal Mining and Valley Fills
Our emphasis in determining selenium sources was on marine oil shales, with 31 of the 47 basins considered in the analysis of petroleum basins being of type II kerogen (marine oil shales). The other 13 basins are of type III kerogen and/or coal (continental deposits) and three are of type I Kerogen (mainly lacustrine deposits). Thus, mining of coal seams and their associated waste rock are primary geologic selenium sources that have the potential to affect aquatic ecosystems. Selenium release to the environment during coal burning for power generation can be direct during combustion or indirect from disposal of solid combustion waste (i.e., fly ash).
Large-scale land disturbance is associated with mountaintop coal mining and waste-rock management in the southern and central Appalachian Mountains. Tops of mountain ridges are sheared off as near-surface, thin-layered coals are mined, and adjacent valleys are filled with waste rock (valley fills) (Figures 3 and 4). The four Appalachian states of West Virginia, Kentucky, Virginia, and Tennessee are the most affected.
Valley fills provide a reservoir of reduced selenium (relatively insoluble selenide and elemental selenium in host rocks) that is oxidized to mobile selenate over time. Waste leachate selenium concentrations have been found to be related to the overall magnitude of the selenium reservoir available for release over time. Additionally, alkaline conditions (in surrounding strata and aquifers or introduced) can neutralize traditional acid mine drainage and in the process speed selenium mobility. All of these factors result in selenium being transported regionally within watershed systems (Figure 3: streams, reservoirs, ponds) and potentially bioaccumulating in aquatic food webs.
Studies in mining-affected watersheds in West Virginia (Figure 5) resulted in basin hydrologic schematics and food-web diagrams that document the progression of selenium trophic transfer across suspended particulate material, invertebrates and fish for each site. This type of analysis serves as the basis for developing a site-specific ecosystem-scale model to predict selenium exposure within the hydrologic conditions and food webs of southern West Virginia. The range of outcomes of these model runs: 1) accounts for critical sources of variability; 2) establishes an understanding of relevant and controlling variables; and 3) illustrates that environmentally safe dissolved selenium concentrations will differ among ecosystems depending on the ecological pathways and hydrological conditions in those systems.
Presser, T.S., 2013, Selenium in Ecosystems within the Mountaintop Coal Mining and Valley-Fill Region of Southern West Virginia-Assessment and Ecosystem-Scale Modeling, U.S. Geological Survey Professional Paper 1803, 86 p.
Presser, T.S., Piper, D.Z., Bird, K.J., Skorupa, J.P., Hamilton, S.J., Detwiler, S.J. and Huebner, M.A., 2004, The Phosphoria Formation: a model for forecasting global selenium sources to the environment, in J. Hein, ed., Life Cycle of the Phosphoria Formation: From Deposition to the Post-Mining Environment: Elsevier, New York, p. 299-319.
Presser, T.S., Hardy, M.A., Huebner, M.A., and Lamothe, P., 2004, Selenium loading through the Blackfoot River watershed: linking sources to ecosystems: in J. Hein, ed., Life Cycle of the Phosphoria Formation, From Deposition to the Post-Mining Environment: Elsevier, New York, p. 437-466.
Skorupa, J.P., Detwiler, S., and Brassfield, R., 2002, Reconnaissance Survey of Selenium in Water and Avian Eggs at Selected Sites Within the Phosphate Mining Region Near Soda Springs, Idaho, May-June, 1999: U.S. Fish and Wildlife Report, U.S. Fish and Wildlife Service, Sacramento, California, 95 p.
Piper, D.Z., Skorupa, J.P., Presser, T.S., Hardy, M.A., Hamilton, S.J., Huebner, M.A., and Gulbrandsen, R.A., 2000, The Phosphoria Formation at the Hot Springs Mine in southeast Idaho: a source of trace elements to ground water, surface water, and biota: U. S. Geological Survey Open-File Report 00-050, 73 p.