Trace Elements in Coal Ash 

Overview

According to the U.S. Energy Information

Association (EIA), approximately 37 percent of 

the electricity currently produced in the United 

States is generated using coal (U.S. Energy 

Information Administration, 2013). Coal ash

(also known as coal combustion products) 

includes bottom ash, fly ash, and flue gas 

desulfurization products (fig. 1), which are

generated in amounts averaging 130 million

tons per year. In 2013, 14.5 million tons

of bottom ash, 53.4 million tons of fly

ash, and 35.1 million tons of flue gas

desulfurization products were produced 

(American Coal Ash Association, 2013).

The EIA predicts that coal use and coal ash 

generation in the United States will remain at 

current levels over the next few decades (U.S. 

Energy Information Administration, 2013).

Coal fly ash consists of fine particles, which

contain a mixture of minerals such as clays, 

quartz, iron oxides, aluminosilicate glass 

formed by melting of mineral matter at the high 

temperatures of combustion, and unburned 

carbon remaining after the combustion process. 

Major chemical constituents of coal fly ash 

typically include silicon (Si), aluminum (Al), 

and iron (Fe), listed in order of decreasing 

abundance when expressed as oxides (elements 

in combination with oxygen), with lesser 

amounts of oxides of calcium (Ca), magnesium 

(Mg), potassium (K), sulfur (S), titanium (Ti), 

and phosphorus (P) whose proportions tend 

to be more variable. Coal ash also contains 

minor amounts of trace elements, including 

chromium (Cr), nickel (Ni), zinc (Zn), 

arsenic (As), selenium (Se), cadmium (Cd), 

antimony (Sb), mercury (Hg), and lead (Pb). In 

addition, uranium (U) is commonly present at 

concentrations ranging from 10 to 30 ppm, which 

is near the upper limit of concentrations found in 

naturally formed rocks such as granite and black 

shale (Zielinski and others, 2007).

In the United States, coal ash is currently 

disposed of in ash impoundments or landfills 

(fig. 2). Storage or disposal of large volumes of 

coal ash in suitably engineered and monitored 

impoundments or landfills is costly and may be 

limited by near-site storage capacities. Long-term 

storage of coal ash can be problematic because 

water infiltration (from rain or snow) combined 

U.S. Department of the Interior

U.S. Geological Survey

Coal is first 

combusted in one 

or more furnaces, 

and part of the 

coal ash present 

(bottom ash) falls 

to the bottom of 

the furnace. 

The combustion flue

gas exits the furnace

carrying suspended 

fly ash. Emission 

controls such as 

electrostatic 

precipitators then 

remove the fly ash. 

Following this, flue gas 

desulfurization systems 

(scrubbers), if present, 

remove sulfur dioxide and 

other unwanted vapors from

the flue gas, and capture 

them as solid flue gas 

desulfurization products.

The remaining flue 

gas and a very 

small fraction of the 

particulates not 

captured by these 

emission control 

devices are emitted 

from the stack.

Coal pile 

Furnace 

Electrostatic

precipitator

Scrubber 

Stack 

Flue gas

Flue gas

Flue gas

Bottom ash Fly ash Flue gas

desulfurization products 

Coal ash 

Beneficial reuse Storage

Figure 1. The main processes involved in coal combustion and generation of coal ash 

(bottom ash, fly ash, and flue gas desulfurization products) at a coal-fired powerplant. 

The coal ash generated as a result of coal combustion is collected and stored or reused 

for beneficial purposes.

Aquifer 

. . . . . . . .

. . . . . . . .

. .

Ash

impoundment 

Precipitation

Aquifer NOT TO SCALE

Soil, 

lakes,

rivers Liner

Soil

Figure 2. Potential pathways by which coal ash and its elemental 

constituents can escape from ash impoundments. Precipitation (rain and 

snow) can lead to water infiltration through the ash into groundwater 

aquifers, soil, lakes, and rivers. The liner helps prevent migration of 

constituents from the impoundment to its surroundings.

Fact Sheet 2015–3037

May 2015
LA

ID

OR

NV

CA

NM

TX

UT

AZ

NE

MT ND

WY

CO

OK

KS

SD

AR

MO

GA

FL

IA

AL

TN

MS

NC

WI MI

MN

IL IN

KY

SC

OH

WV VA MD

DE

ME

NY

PA NJ

RI

WA

NH

MA

VT

CT

Appalachian

Basin

Powder River Basin

2

with leaky storage sites may result in the transport of coal 

ash and its constituent elements into the local environment. If 

ash impoundments fail, there is potential for widespread and 

prolonged impacts such as impairment of ecosystem functions 

and the loss of plant and animal life and habitat. Recent 

examples of ash releases include the Tennessee Valley Authority 

Kingston, Tennessee, coal ash spill in December 2008, which 

released approximately 5.4 million cubic yards of ash into the 

Emory and Clinch Rivers in Tennessee (U.S. Environmental 

Protection Agency, 2012a), and the Eden, North Carolina, spill in 

February 2014, which released up to 39,000 tons of coal ash into 

the Dan River in North Carolina (U.S. Environmental Protection 

Agency, 2015a). Additionally, a number of other ash impoundments across the United States have been identified by the U.S. 

Environmental Protection Agency (EPA) as having the potential 

for significant hazardous impacts in the event of an ash spill 

(U.S. Environmental Protection Agency, 2014a). 

An alternative to ash storage and disposal is ash utilization, 

primarily as an additive to concrete or other modern construction 

materials, and secondarily as structural and embankment fill. The 

American Coal Ash Association estimates that approximately 

43 percent of the coal ash currently produced is beneficially 

reused (American Coal Ash Association, 2013). 

Trace Elements in Coal Ash 

Many trace elements such as chromium, arsenic, and 

mercury can be toxic to humans and ecosystem functioning at 

relatively low concentrations, and for these reasons, state and 

federal institutions monitor and regulate the concentrations of 

many trace elements in rivers and lakes, which provide sources 

of drinking water to towns and cities (U.S. Environmental 

Protection Agency, 2014b).

Trace elements are present in coal ash because the parent 

coal from which the ash is derived also contains trace elements 

as a result of natural formation processes. During combustion, 

most trace elements in the parent coal are retained in the residual 

coal ash and are concentrated in the smaller volume of the ash 

compared to the original coal. Trace element concentrations 

in coal ash are therefore higher compared to the parent coal 

(Carlson and Adriano, 1993; Meij, 1994; Jankowski and others, 

2006). Trace elements in coal ash are typically present at partper-million (ppm) to part-per-billion (ppb) levels, and vary due 

to differences in the composition of the parent coal, combustion 

processes, and ash collection methods at powerplants. 

Due to the trace element content in coal ash, there is interest 

in assessing (1) the properties and behavior of coal ash and 

its constituent trace elements in various environments, (2) the 

potential for human exposure, and (3) the potential for extracting 

elements of interest for reuse in various industries. For example, 

a recent study of runoff water from coal ash-amended concrete 

demonstrated that trace element dissolution from the concrete to 

the water was low (Cheng and others, 2008). Another concern 

is the emission of the radioactive element radon (Rn) from coal 

ash-amended construction materials. Studies have shown that 

the emission of radon from construction materials depends on 

many factors, such as the composition of the material and the 

manufacturing process, and is generally much lower than radon 

emissions from common rocks and soils (Kovler and others, 

2005; Zielinski and others, 2007).

Other studies have examined the impact of coal ash on 

natural ecosystems. A study of the Tennessee Valley Authority 

Kingston coal ash spill in 2008 found that river-water trace 

element concentrations were not elevated in areas impacted by 

the ash spill due to dilution (Ruhl and others, 2009) but river 

sediments contained elevated levels of arsenic and mercury 

(Ruhl and others, 2010; Deonarine and others, 2013). A further 

study by Ruhl and others (2012) examined the impact of 

discharges from coal-fired powerplants and coal ash treatment 

ponds into North Carolina lakes and found that arsenic, nickel, 

antimony, and selenium levels in water and sediments were 

elevated compared to EPA water quality standards (Ruhl and 

others, 2012). 

USGS Coal Ash Research

The U.S. Geological Survey (USGS) has conducted studies 

in which bulk chemical and mineralogical analyses, electron 

microscope observations, laboratory leaching experiments, and 

various micro-analytical techniques were utilized to (1) examine 

the properties of trace elements in coal fly ash, and (2) investigate the behavior of these trace element constituents under 

different environmental conditions.

Trace Elements in Coal Fly Ash: Speciation and Occurrence 

Coal fly ash samples were examined to determine which 

trace elements were present, their concentrations, and chemical 

speciation. The chemical speciation of a trace element refers 

to its chemical and physical form in the coal ash, for example, 

whether it occurs as a solid, liquid, or gaseous phase, its oxidation state, and chemical bonds with other elements. Chemical 

speciation is an important factor which influences the properties 

of trace elements and their behavior in various environments.

Coal fly ash samples (designated here as fly ash samples A, 

B, and C) were collected from three U.S. coal-fired powerplants. 

One plant produces coal fly ash (fly ash C) from low-sulfur coal 

mined in the Powder River Basin of Wyoming and Montana (see 

map below), whereas the other two plants produce coal fly ash 

(fly ashes A and B) from coal mined in the Appalachian Basin 

having high and relatively low sulfur contents, respectively. 

Fly ash C contained lower concentrations of most measured 

trace elements compared to fly ashes A and B (fig. 3). Also, the 

two Appalachian Basin fly ashes had different trace element 

compositions, especially for arsenic. 
0.1 

1 

10 

100 

1,000 

Arsenic

(As)

Cadmium

(Cd)

Cerium

(Ce)

Cobalt

(Co)

Chromium

(Cr)

Copper

(Cu)

Lithium

(Li)

Molybdenum

(Mo)

Nickel

(Ni)

Antimony

(Sb)

Selenium

(Se)

Uranium

(U)

Vanadium

(V)

Concentration, in parts per million 

Trace element 

A B C 

Average for U.S. coals 

EXPLANATION

Coal fly ash sample

Figure 3. Concentrations of selected trace elements in three coal fly ash samples from the Appalachian Basin (A and B) and the 

Powder River Basin of Wyoming (C), compared to average trace element concentrations in U.S. coals (Bragg and others, 1998).

Figure 4. Electron microscopy images of fly ash. A, Two components of fly ash: glassy spheres and fine-grained surface coating. 

B, Higher resolution image of the surface coating, showing regions lacking crystalline form. Images were obtained at the Virginia 

Polytechnic Institute and State University Nanoscale Characterization and Fabrication Laboratory. 1 micrometer (μm) equals 

1/1,000 of a millimeter; 1 nanometer (nm) equals 1/1,000,000 of a millimeter.

3

1 μm 10 nm

Surface coating

Glassy spheres

A B

Electron microscopy techniques were used to examine fly 

ash structure and elemental composition. Transmission electron 

microscopy images of fly ash A show that there are two components of fly ash: glassy spheres and surface coating (fig. 4A). A 

higher resolution image of the fly ash surface coating (fig. 4B) 

indicated that it consists of fine nanometer-sized structures 

which lack crystalline form and consist largely of carbon. Electron microscopy coupled with compositional mapping indicated 

that trace elements such as vanadium (V), zinc, molybdenum 

(Mo), thallium (Tl), and lead are associated with the surface 

coating of fly ash A. 
Figure 5. A, Arsenic (As) elemental map of fly ash sample A showing relative concentrations; the relative highest concentrations 

of arsenic are shown in red and lowest concentrations are shown in blue. B, X-ray absorption spectrum of the region of analysis 

circled in A, and fit to spectrum with materials of known arsenic species, which demonstrates that two chemical forms of 

arsenic, As3+ and As5+, are present. Synchrotron analyses were performed at beam line 2–3 at the Stanford Synchrotron Radiation 

Lightsource in Menlo Park, California.

11,850 11,870 11,890 11,910 11,930 11,950 11,970 11,990 

Energy, in electron volts (eV)

Normalized absorption

Results of spectrum analysis from circled region in A 

EXPLANATION

Fit to spectrum with materials of known arsenic

 species: 11% As3+ and 89% As5+

Arsenic peak 

 10 μm 

Region of 

spectrum

analysis 

A

B

4

High-energy X-rays from a synchrotron radiation source 

were also utilized to study the chemical speciation of trace 

elements present in the fly ash samples. These techniques 

identified regions enriched in elements such as arsenic (fig. 5A) 

and its oxidation state. Using this approach, the X-ray absorption 

spectrum of fly ash A indicated that most of the arsenic (89%) 

was present as the more oxidized As 5+, whereas only a small 

fraction (11%) was present as As 3+ (fig. 5B). These two forms of 

arsenic are the most common in aquatic environmental systems 

such as rivers and lakes and also exhibit different properties. 

For example, As 3+ is considered to be more toxic than As 5+

(Smedley and Kinniburgh, 2001). 

Similarly, chromium in fly ash A was present as two different chemical forms: Cr 3+ and a very low percentage of Cr 6+

(spectrum not shown). Cr 6+ compounds readily dissolve in water 

while Cr 3+ compounds do not (U.S. Environmental Protection 

Agency, 2013a). Both chromium species are found in aquatic 

environments. Cr 3+ is a nutritionally essential element in humans 

while Cr 6+ is a carcinogen (U.S. Environmental Protection 

Agency, 2013b). 

Behavior of Coal Fly Ash and Its Trace Element Constituents 

Under Different Environmental Conditions

A set of laboratory experiments was conducted to investigate how different redox conditions (oxic and anoxic) affect the 

dissolution or leaching of trace elements from coal fly ash into 

simulated freshwater. Both oxygen-rich (oxic) and oxygen-poor 

(anoxic) regions can be found in natural ecosystems. Oxic 

regions are typically found in the surface waters of rivers and 

lakes while anoxic regions are commonly found in bottom 

waters and sediments. Redox condition is an important factor 

in environmental systems because it influences the chemical 

speciation and behavior of trace elements. 
Figure 6. Results of dissolution experiments with fly ashes A and B

in simulated oxic and anoxic freshwater at pH 7. A, For arsenic

experiments, arsenic dissolved from fly ashes A and B under both

oxic and anoxic conditions, and increased over the course of the

28-day (672-hour) experiment. B, For chromium experiments: for

fly ash A, under oxic conditions, dissolved chromium concentrations were as high as 80 ppb, while little or no dissolved chromium 

was measured under anoxic conditions; for fly ash B, no dissolved chromium was detected under oxic or anoxic conditions during 

the course of the experiment. Total arsenic and chromium concentrations in 6A and 6B include all dissolved arsenic and chromium 

chemical forms. 

0 

20 

40 

60 

80 

100 

120 

140 

160 

0 100 200 300 400 500 600 700 

Concentrations, in parts per billion 

Time, in hours 

Sample A, oxic 

EXPLANATION

Sample B, oxic 

Sample A, anoxic 

Sample B, anoxic 

EPA Maximum Contaminant Level (MCL)

EPA Maximum Contaminant Level (MCL)

0 

20 

40 

60 

80 

100 

120 

0 100 200 300 400 500 600 700 

Time, in hours 

A. Total arsenic B. Total chromium

5

In these experiments, small volumes of oxic and anoxic 

freshwater without measurable arsenic and chromium were 

exposed to fly ashes A and B, and dissolved arsenic and chromium 

concentrations in the freshwater were measured over time (fig. 6). 

The results of the experiments showed that arsenic dissolved from 

fly ashes A and B into the freshwater under both oxic and anoxic 

conditions (fig. 6A). Dissolved arsenic concentrations increased 

over time, and maximum concentrations at the end of the 28-day 

(672-hour) experiment ranged from 30 to 150 ppb, and were above 

the EPA Maximum Contaminant Level (MCL) of 10 ppb for total 

arsenic in drinking water (U.S. Environmental Protection Agency, 

2014b). Arsenic is a trace element of concern because exposure can 

lead to skin damage, circulatory system problems, and an increased 

risk of cancer in humans (Eckley and Branfireun, 2008). 

In contrast to arsenic, chromium dissolved only from fly 

ash A in oxic freshwater, and little or no chromium dissolved 

from fly ash B in oxic or anoxic freshwater (fig. 6B). In these 

cases, chromium was most likely present as an insoluble species 

and did not dissolve under the conditions tested (Guertin and 

others, 2004). During the timeframe of the experiment, dissolved 

chromium concentrations in freshwater were below the EPA MCL 

of 100 ppb for total chromium in drinking water (U.S. Environmental Protection Agency, 2013b). Chromium is a trace element 

of concern because concentrations in drinking water above the 

MCL can be toxic (U.S. Environmental Protection Agency, 2013b). 

Overall, the data obtained from these experiments improve 

our understanding of the structure of fly ash particles and their 

trace element content, as well as the leaching behavior of trace 

elements in coal ash, such as arsenic and chromium, under 

environmental conditions typically found in rivers and lakes. 

Regulations Concerning Coal Ash Disposal and Reuse

The EPA has recently revised regulations pertaining to the 

disposal of coal ash in landfills and surface impoundments. Coal 

ash has been classified as a non-hazardous solid waste, under 

Subtitle D of the Resource Conservation and Recovery Act 

(U.S. Environmental Protection Agency, 2015b). The final rule 

on coal combustion residuals generated by electric utilities 

(U.S. Environmental Protection Agency, 2015c) requires that 

coal ash facilities implement: (1) minimum federal regulations 

for the disposal of coal ash in landfills and surface impoundments, including site location and structural design criteria; 

(2) routine structural integrity and groundwater assessments; 

(3) remediation of coal-ash-contaminated groundwater sites; 

and (4) closure of coal ash disposal facilities with the intention 

of reducing the risk of failure of coal ash storage facilities and 

protecting groundwater aquifers. The new regulations do not 

pertain to beneficial uses of coal ash as defined by the final rule.

Coal Ash Studies Going Forward

Long-term projected use of coal in electricity generation 

will require continuing efforts to address the safe storage, 

disposal, and reuse of large volumes of coal ash. In particular, 

more research is needed to determine the mobility of environmentally sensitive trace elements during coal ash handling, use, 

and accidental spills, and ascertain the resulting implications 

for human health and the environment. Objective, unbiased 

scientific research such as that performed by the USGS provides 

information needed for responsible policymaking and to better 

inform the general public. 
ISSN 2327-6916 (print) ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20153037

By Amrika Deonarine, Allan Kolker, and Michael Doughten

6

For more information, please contact:

Director, Eastern Energy Resources Science Center

U.S. Geological Survey

12201 Sunrise Valley Drive

Mail Stop 956 National Center

Reston, VA 20192

Phone: 703-648-6401

http://energy.usgs.gov/GeneralInfo/ScienceCenters/Eastern

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