U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3152

February 2006

Arsenic in Coal

Printed on recycled paper

Sources of Arsenic

Arsenic is a persistent toxin that 

occurs naturally in trace amounts in 

rocks, sediments, and coal. Small 

amounts of arsenic may be present in 

ground water or, less commonly, surface 

waters, especially where there is a nearby 

source of arsenic. The most widespread 

natural source of arsenic is pyrite, a common mineral composed of iron and sulfur, 

which can contain a small amount of 

arsenic in its structure in place of sulfur.

Arsenic occurs in some ground-water 

aquifers due to chemical oxidation of 

pyrite or to reduction (the opposite of 

oxidation) of iron oxide minerals in the 

aquifer. Other sources of arsenic include 

past industrial activities, application of 

arsenic as a pesticide, and drainage from 

abandoned mine lands that contain pyrite. 

Prolonged consumption of drinking water 

from wells that greatly exceed arsenic 

health standards is the most serious arsenic-related health hazard in the United 

States and throughout the world.

Arsenic in bituminous coal occurs 

primarily in pyrite and, to a lesser extent, 

in organic portions of the coal. A small 

fraction of this arsenic is emitted during 

coal combustion. This Fact Sheet provides information on the arsenic content 

of U.S. coals, how arsenic occurs, and its 

behavior during mining, coal preparation, 

and coal combustion and in postcombustion beneficial uses.

Arsenic in U.S. Coal

All coals contain some arsenic. The 

U.S. Geological Survey (USGS) maintains 

an extensive database of over 7,000 analyses of U.S. coals. Data from this compilation indicate that the average arsenic concentration for U.S. coal is about 24 parts 

per million (ppm; Bragg and others, 1998; 

fig. 1). There is considerable variation by 

coal basin, ranging from an average low 

of 1.4 ppm in the Raton Mesa Basin of 

Colorado and New Mexico to a high of 71 

ppm in the southern (Alabama-Tennessee) 

portion of the Appalachian Basin (table 1).

Many Eastern U.S. coals are subjected to 

a cleaning process prior to use in coal-burning power stations. Coal cleaning is primarily intended to reduce the sulfur content 

of these coals, but in many cases arsenic 

contents are reduced as well. As a result, for 

coals that are cleaned, the arsenic concentration is, on average, lower than equivalent 

in-ground arsenic contents.

Table 1. Comparison of average arsenic content in U.S. coals by basin and calculated average powerplant input loadings of arsenic.

[Data from Bragg and others, 1998. ppm, parts per million; Btu, British thermal unit; lb, pound]

Arsenic Calorific value Arsenic input loadings

(ppm) (Btu/lb) (in 103 lbs per 1012 Btu)

Coal basin Median Mean Number of 

samples

Median Mean Number of 

samples

Median Mean Number of 

samples

Appalachian, Northern 16 28 1,607 12,570 12,440 1,500 1.3 2.3 1,500

Appalachian, Central 7.8 22 1,742 13,360 13,210 1,643 0.6 1.7 1,643

Appalachian, Southern 29 71 974 12,850 12,760 968 2.2 5.8 968

Eastern Interior 10 19 289 11,510 11,450 255 0.92 1.7 255

Fort Union 4.2 8.5 280 6,340 6,410 257 0.7 1.4 257

Green River 1.2 4.8 391 9,950 9,560 264 0.13 0.44 264

Gulf Coast 2.2 3.2 141 6,440 6,470 110 0.34 0.54 110

Pennsylvania Anthracite 3.2 8.1 51 12,860 12,530 39 0.25 0.79 39

Powder River 1.6 4.2 602 8,050 8,080 486 0.2 0.5 486

Raton Mesa 0.99 1.4 40 12,500 12,300 34 0.073 0.1 34

San Juan River 0.92 2.5 185 9,380 9,640 169 0.095 0.26 169

Uinta 0.7 1.5 249 11,290 10,820 222 0.074 0.14 222

Western Interior 14 21 286 11,320 11,420 261 1.2 1.9 261

Wind River 2.4 7 41 9,630 9,570 41 0.25 0.75 41

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Figure 1. Histogram showing arsenic distribution in U.S. coal samples (data from Bragg 

and others, 1998). Arsenic (in parts per million, 

ppm) is expressed on a remnant moisture 

whole coal basis. Forty samples having arsenic values exceeding 300 ppm are excluded 

to make the sample distribution easier to view 

but are included in statistics given in table 1.
Mode of Occurrence

The mode of occurrence of arsenic is 

important in determining its relative mobility during mining, combustion, and storage 

of coal for electric power generation; disposal or use of coal combustion byproducts; 

and leaching of coal beds in contact with 

ground water. Iron sulfides such as pyrite 

and marcasite are common inorganic constituents of coal, composing anywhere from 

a negligible amount to about 5 percent by 

weight. Other than iron and sulfur, arsenic 

is generally the most abundant element in 

pyrite and marcasite, but its concentration is 

highly variable (fig. 2).

In bituminous coals, pyrite generally 

accounts for the single largest fraction of 

arsenic, with lesser fractions in the organic 

portion of the coal and in other inorganic 

forms (fig. 3). Lower rank coals (lignite 

and subbituminous) generally have a larger 

proportion of their arsenic in the organic 

portion. Some coals have unusually high 

levels of arsenic, independent of coal rank. 

This enrichment is thought to result from 

past geologic interaction of coal beds with 

metal-enriched fluids, similar to the process 

by which metallic ore deposits are formed 

(Goldhaber and others, 2003).

Arsenic in Waste from Coal 

Mining, Processing, and Storage

Mining operations in the United States 

generally must meet water-quality standards for discharge of mine water. In some 

abandoned mines or undisturbed pyrite-rich 

strata, however, decomposition of pyrite 

in coal and rock can lead to dispersal of 

arsenic to the environment. For example, in 

the Warrior Coal Field of Alabama, stream 

sediments in areas that have been mined 

have higher arsenic concentrations than 

stream sediments outside the coal-mining 

areas (Goldhaber and others, 2001).

With exposure to moisture and atmospheric oxygen, arsenic in pyrite is rapidly 

converted to an oxidized form known as 

arsenate that is readily leached by precipitation and ground water (Huggins and others, 

2002). The presence of arsenic in pyrite is 

thought to destabilize it, possibly hastening its decomposition to iron oxyhydroxide 

minerals. Because pyrite in coal oxidizes 

rapidly, coal storage facilities and waste 

materials from coal washing operations are 

potential sources of arsenic mobilization.

Arsenic in Coal Combustion 

Products

USGS studies indicate that, during 

coal combustion in modern coal-fired 

utilities, 90–100 percent of arsenic is 

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Figure 2. Elemental maps and microprobe analysis points showing distribution of arsenic (As) in pyrite in two Alabama coal samples. Maps are falsecolor images using a “thermal” intensity scale. A, Pyrite from a bituminous coal sampled in Cullman County, Alabama. Arsenic concentration at the 

points shown ranges from less than the detection limit (0.01 weight percent) to 2.6 weight percent. Image by Allan Kolker, USGS. B, Pyrite collected 

from a fracture in bituminous coal from Walker County, Alabama. Image shows zoning of arsenic content ranging up to nearly 4.5 weight percent. 

Image by Rob Koeppen, USGS; from Goldhaber and others (2003).

Figure 3. Selective leaching results for 11 U.S. 

bituminous coal samples. Plot shows that arsenic leached by nitric acid (yellow sub-bars) is 

the dominant form in these samples. Nitric acid 

is known to dissolve pyrite. The fraction of arsenic not leached (100 percent minus total bar 

height) represents organic-associated arsenic 

not removed by the leaching process. Arsenic 

present in all other forms is shown as “other.” 

Modified from Palmer and others, 1998.

A B
captured in coal combustion byproducts 

(Brownfield and others, 1999). Coal 

combustion byproducts such as fly ash 

and bottom ash likely contain multiple 

forms of arsenic. Some researchers have 

concluded that arsenic in coal ash is 

incorporated into iron-rich glass and crystalline silicate minerals that constitute the 

bulk of the solid ash. A more widely held 

view is that arsenic in flue gases combines with calcium oxide in the hot gases 

to form calcium arsenate (Ca3(AsO4)

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that is concentrated on the surfaces of 

coal ash particles.

Coal combustion byproducts are 

widely used in a variety of commercial 

applications. These include cement and 

concrete, aggregates, structural fill, and 

even reclamation of abandoned coal 

mines. Such uses for coal ash are permitted by its designation as nonhazardous 

waste under the Resource Conservation 

and Recovery Act. Standardized tests, 

including the U.S. Environmental Protection Agency’s (USEPA) Toxicity Characteristic Leaching Procedure, show that 

harmful substances such as arsenic are 

not readily leached from these materials 

under simulated environmental conditions. Additional information on coal 

combustion products is given in Kalyoncu and Olson (2001).

Technological Impact

Coal-fired utilities have been required 

to reduce emissions of nitrous oxides 

(NOx) to meet air quality standards. 

To comply with these new emissions 

regulations, many existing coal-fired 

powerplants and new installations are 

equipped with selective catalytic reduction (SCR) technology. SCR works by 

injecting ammonia into the flue gas 

stream. The ammonia passes over a 

catalyst that reduces the nitrous oxides 

(NOx) in the flue gas to elemental 

nitrogen and water. Although SCR is 

very effective in controlling NOx emissions, the effectiveness of the catalyst is 

greatly reduced if there are elevated concentrations of arsenic and selenium present in the flue gas stream. As a result, 

the amounts of these two elements in 

utility feed coal are a potential concern 

to utilities (Rigby and others, 2000) and 

may be monitored closely.

Figure 4. Interior view of residence in southwestern Guizhou Province, China, where arsenic-rich 

coal and coal briquettes are used to dry crops (chili peppers) that are later consumed, resulting in 

arsenic toxicity. Note the lack of ventilation in this dwelling. Photograph by Harvey Belkin, USGS.

Health Impact

In the United States, human exposure to arsenic primarily occurs in 

rural areas, through consumption of 

water from wells that greatly exceed 

the arsenic drinking water standard (10 

micrograms per liter) specified by the 

USEPA. In the vast majority of cases, 

this exposure is unrelated to coal or 

emissions from coal-burning powerplants. More direct exposure to arsenic 

from coal occurs where coal is used for 

domestic purposes. In the United States, 

such exposure is very limited. Domestic 

coal use is far more common in countries such as China, India, and South 

Africa. Health effects from arsenic have 

been documented where extraordinarily 

arsenic-rich coals were used to dry foods 

that were later consumed (Finkelman 

and others, 2002; fig. 4).
Summary

Arsenic in bituminous coals is present primarily within the mineral pyrite. 

Pyrite occurs in small amounts in many 

rock types and sediments and coal. Pyrite 

and its weathering products are potential 

sources of arsenic in ground water and, 

more locally, in surface water. During 

coal combustion in modern coal-fired 

utilities, most of the arsenic is captured 

in coal combustion byproducts, which are 

considered nonhazardous waste, on the 

basis of acceptable performance in standardized leaching tests. Arsenic exposure 

in the United States primarily results 

from consumption of drinking water from 

private wells that exceed drinking water 

standards. Documented health effects 

from arsenic in coal are known elsewhere 

in the world in cases where unusually 

arsenic-rich coal is used domestically.

References Cited

Bragg, L.J., Oman, J.K., Tewalt, S.J., 

Oman, C.L., Rega, N.H., Washington, P.M., and Finkelman, R.B., 1998, 

U.S. Geological Survey Coal Quality 

(COALQUAL) Database, version 2.0: 

U.S. Geological Survey Open-File 

Report 97–134, CD-ROM. (Available 

on the Web at http://pubs.usgs.gov/

of/1997/of97-134/.)

Brownfield, M.E., Affolter, R.H., 

Cathcart, J.D., O’Connor, J.T., and 

Brownfield, I.K., 1999, Characterization of feed coal and coal combustion 

products from power plants in Indiana 

and Kentucky: Proceedings of the 24th 

International Technical Conference 

on Coal Utilization and Fuel Systems, 

Clearwater, Fla., p. 989–1000.

Finkelman, R.B., Orem, William, Castranova, Vincent, Tatu, C.A., Belkin, 

H.E., Zheng, Baoshan, Lerch, H.E., 

Maharaj, S.V., and Bates, A.L., 2002, 

Health impacts of coal and coal 

use—Possible solutions: International 

Journal of Coal Geology, v. 50, p. 

425–443.

Goldhaber, M.B., Irwin, E., Atkins, 

B., Lee, L., Black, D.D., Zappia, H., 

Hatch, J.R., Pashin, J.C., Barwick, 

L.H., Cartwright, W.E., Sanzolone, R., 

Ruppert, L., Kolker, A., and Finkelman, R.B., 2001, Arsenic in stream 

sediments of northern Alabama: U.S. 

Geological Survey Miscellaneous Field 

Studies Map MF–2357, 1 sheet. (Available on the Web at http://pubs.usgs.gov/

mf/2001/mf-2357/.)

Goldhaber, M.B., Lee, R.C., Hatch, J.R., 

Pashin, J.C., and Treworgy, J., 2003, 

Role of large scale fluid-flow in subsurface arsenic enrichment, in Welch, 

A.H., and Stollenwerk, K.G., eds., 

Arsenic in ground water—Geochemistry and occurrence: Boston, Kluwer 

Academic Publishers, p. 127–164.

Huggins, F.E., Huffman, G.P., Kolker, 

Allan, Mroczkowski, S.J., Palmer, 

C.A., and Finkelman, R.B., 2002, 

Combined application of XAFS spectroscopy and sequential leaching for 

determination of arsenic speciation in 

coal: Energy and Fuels, v. 16, no. 5, p. 

1167–1172.

Kalyoncu, R.S., and Olson, D.W., 2001, 

Coal combustion products: U.S. Geological Survey Fact Sheet FS–076–01, 

4 p. (Available on the Web at

http://pubs.usgs.gov/fs/fs076-01.)

Palmer, C.A., Mroczkowski, S J., Finkelman, R.B., and Crowley, S.S., 1998, 

The use of sequential leaching to 

quantify the modes of occurrence of 

elements in coals: Proceedings of the 

Fifteenth Annual International Pittsburgh Coal Conference, 28 p., 

CD-ROM.

Rigby, K., Johnson, R., Neufort, R., 

Pajonk, G., Hums, E., Klatt, A., 

and Sigling, R., 2000, SCR catalyst 

design issues and operating experience—Coals with high arsenic concentrations and coals from the Powder 

River Basin: Proceedings of the 2000 

International Joint Power Generation 

Conference, Miami Beach, Fla., July 

23–26, 2000, 8 p.

By Allan Kolker, Curtis A. Palmer, Linda J. 

Bragg, and Joseph E. Bunnell

Contacts

Arsenic in coal and modes of occurrence

Allan Kolker (703) 648-6418 akolker@usgs.gov

Selective leaching studies

Curtis A. Palmer (703) 648-6185 cpalmer@usgs.gov

The coal quality database

Linda J. Bragg (703) 648-6451 lbragg@usgs.gov

Health effects from arsenic exposure

Joseph E. Bunnell (703) 648-6497 jbunnell@usgs.gov

U.S. Geological Survey

956 National Center

Reston, VA 20192