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Linking Selenium Sources to Ecosystems: Local and Global Perspectives

Mining

Selenium Sources

image of globe indcating petrolium basins and phosphate deposits

The distribution of phosphate deposits (o) is overlain onto that of productive petroleum (oil and gas) basins (+) to generate a global plot of organic-carbon enriched sedimentary basins. The map indicates that ancient organic-rich depositional marine basins, unrestricted by age, are linked to the contemporary global distribution of Se source rocks. Thus, the map presents a base on which to predict environments that may be affected by Se loading. Given the geographic patterns, Se emerges as a contaminant within specific regions of the globe that may limit phosphate mining, oil refining, and drainage of agricultural lands because of potential ecological risks to vulnerable food webs (Click here to see more detail).

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. 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. 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)

Photograph of bird

Deformed American coot (Fulica americana) embryo (A) from a nest in the vicinity of a southeast Idaho phosphate mine tailings reservoir. The deformity exhibited here—curly toe—is similar to that induced by Se in chickens (B, deformed; C, normal). The scale bar is 10 mm in each image. This coot egg was artificially incubated and analyzed using a fluorescence-based micro-digestion.

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. Other detected, but inconclusively documented sheep-die-offs have ocurred and undetected die-offs of wildlife are highly probable. 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. 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/Valley Fills

Our emphasis in determining Se sources is 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 coals are included as a subset of petroleum source rocks on our global conceptual model.

An area of expanded mountaintop coal mining encompasses 12 million acres, extending over portions of West Virginia, Kentucky, Virginia, and Tennessee. Most of the major rivers and tributaries east of the Mississippi River originate in the mountains of the Appalachians. Coal is a recognized source of Se both through Se enriched particulates from the burning of fossil fuel and fly-ash disposal in aquatic environments.

Shales associated with coals that are displaced at the time of mining and concentrated at fill sites also are a source of Se to areas downstream of valley fill construction. Proposed control measures to neutralize coal mine drainage with alkaline addition may exacerbate the mobility of selenium and hence its loading to the environment. Proposed remedial sedimentation ponds and associated wetlands, would likely become high Se risk environments for bird and fish exposures to Se because of favorable conditions to biomagnification in food webs. Based on Se concentrations in streams and sedimentation ponds receiving discharges from valley fills, adverse ecological effects from selenium are likely to occur.



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Page Last Modified: Wednesday, 09-Jun-2010 13:46:56 EDT