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Introduction
Guidelines
Irrigation
Refining
Mining
References
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MINING
Selenium Sources
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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).
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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)
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. |
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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. |
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. |
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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