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3
Behavior of Coal Combustion Residues in
the Environment
C ontaminants derived from CCRs have the potential to enter drinking
water supplies, surface water bodies, or biota at unacceptable concentra-
tions (discussed further in Chapter 4), thereby creating risks to human
health and the environment. The extent of contaminant release from CCR de-
pends on the volume and characteristics of the CCR emplaced and the disposal
environment. In the surrounding environment, hydrogeological conditions deter-
mine the potential for water to enter the CCR and transport contaminants away
from the disposal area. Additional biogeochemical processes control the rate and
distance of movement of contaminants from CCR disposal areas. This chapter
provides an overview of the hydrologic and biogeochemical processes control-
ling the release and transport of contaminants from CCR mine disposal sites to
locations where uptake may occur.
HYDROLOGICAL PROCESSES AFFECTING CCR BEHAVIOR
Recharge, unsaturated water flow, and saturated groundwater flow will all
affect the behavior of CCRs in the environment (see Sidebar 3.1). In a mine
setting, subsurface water flow will normally be the primary mechanism for trans-
porting CCR-derived contaminants from the disposal area to potential receptors
(e.g., aquatic life in streams supported by groundwater flow, local residents rely-
ing on groundwater as a drinking water source). Transport of CCR contaminants
through overland flow processes (Figure 3.1) is also possible in a mine setting,
especially where CCRs are used as capping material or as soil amendments;
59
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60 MANAGING COAL COMBUSTION RESIDUES IN MINES
SIDEBAR 3.1
Overview of Relevant Hydrologic Processes
A brief review of the water cycle provides perspective to understand the hydro-
logic processes affecting CCRs that are placed in the subsurface at mine sites.
Precipitation that falls on the land surface will either enter the soil through infiltra-
tion processes or flow over the land surface (overland flow) before eventually
reaching nearby streams (see Figure 3.1). Some of the water that enters the soil
will be lost through evaporation and plant transpiration (evapotranspiration), and
the remaining water will flow downward through the subsurface, eventually re-
charging the underlying aquifer.
Recharge rates vary from location to location and year to year, depending on
precipitation rates, evapotranspiration rates, topographic relief, and the ability of
the geologic materials to transmit water. Thus, recharge is difficult to quantify. In
humid, temperate climates, recharge can be 50 percent of precipitation, whereas
in dry, warm climates recharge can be as low as one percent or less of precipita-
tion (NRC, 1990).
Recharge water travels downward by gravity through the unsaturated zone,
where the pore space may be partly filled with air and partly filled with water, which
is held in the pores by the forces of surface tension (or capillary forces) (see Figure
3.2). A capillary fringe exists at the base of the unsaturated zone, where all pores
are saturated with water held by surface tension. Beneath the capillary fringe lies
the saturated zone, defined as the zone in which the pores are completely filled
with water at a pressure greater than atmospheric (Fetter, 1994). The boundary
between the saturated and unsaturated zones is called the water table. The water
level in a shallow well intersecting the saturated zone defines the height of the
water table. The elevation of the water table can fluctuate, rising into what was
previously the unsaturated zone or falling to create a thicker unsaturated zone.
Perched water tables may exist within the unsaturated zone in locations where
lenses of low-permeability material (e.g., clay layers) impede downward flow and
create a local saturated area.
Groundwater flow can occur in downward, upward, and lateral directions, de-
pending on the hydraulic properties of geologic materials and their relative orienta-
tion. Groundwater may travel long distances until it eventually discharges as a
spring or as seepage into a stream, lake, or ocean.
however, in most minefill scenarios, CCRs are covered by several feet of soil or
coal spoils, lessening the potential for overland transport of contaminants.
Water Flow in the Saturated Zone
Groundwater flow at CCR mine placement sites is controlled by the local
hydrogeology, which may be significantly altered by mining activities. Ground-
water flow in the saturated zone will depend on the thickness and orientation of
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BEHAVIOR OF COAL COMBUSTION RESIDUES 61
FIGURE 3.1 Near-surface hydrologic processes.
SOURCE: Modified after Drever, 1997.
individual geologic strata and the hydraulic conductivity of the geologic materi-
als (Figure 3.3). Hydraulic conductivity describes the capacity of a porous me-
dium to transmit water in response to an applied pressure. If the same pressure is
applied, saturated water flow is relatively rapid through porous media with high
values of hydraulic conductivity, such as sand and gravel, but much lower through
low-hydraulic-conductivity materials. Coal seams can occur as either thick or
thin beds that are typically layered between low-hydraulic-conductivity, fine-
grained shale or clay and higher-conductivity, coarse-grained silt or sandstone
sequences (Figure 3.4). The strata in coal-bearing areas may be flat-lying, moder-
ately undulating, or highly folded, leading to widely variable patterns of ground-
water flow. The strata in lignite and bituminous regions tend to be relatively
uniform and flat-lying or gently sloping. The coal seams are often more perme-
able than the interbedded sandstone and shale layers, and groundwater flow is
relatively more rapid through coal beds and fractured sandstones (Figure 3.4). In
anthracite deposits, the geologic materials are rigid, with low porosity and water
flow where the strata remain unfractured. However, the stresses placed on these
more brittle materials as the result of folding can lead to the development of
fractures, which facilitate preferential groundwater flow (NRC, 1990, 1996a).
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62 MANAGING COAL COMBUSTION RESIDUES IN MINES
Soil water z
zone
Unsaturated
zone
Capillary
fringe
Saturated
zone sat
FIGURE 3.2 The distribution of water in the unsaturated zone and the classification of
groundwaters according to Meinzer (1923). The figure shows the increasing volumetric mois-
ture content, , with depth, until it reaches saturation, sat, at the capillary fringe. The volumet-
ric moisture content is defined as the volume of water per bulk volume of soil sample.
SOURCE: George Hornberger, University of Virginia. Modified from Hornberger et al.,
1998.
There is a tendency for fractures to be most abundant near the surface and
then terminate at depth (Figure 3.5) (Callaghan et al., 1998). In this case, ground-
water flow might be directed primarily through the fractures near the surface but
through the pores of the rock matrix at greater depths. Groundwater velocities can
be quite high within an individual fracture. If the fractures are sufficiently wide,
groundwater flow volumes and velocities can be many times greater than in
unfractured materials (NRC, 1996a).
Removal of coal and reclamation of the mine site with coal spoils will alter the
pre-mining groundwater flow characteristics, often significantly. In some surface
mine settings, large volumes of rock are removed to gain access to the coal, and
during reclamation these materials are redeposited in the mine pit and surrounding
area. Water flow through coal spoils and similar materials can occur both through
discrete conduits or macropores that form between large pieces of spoil material
(pseudokarstic flow) and, more uniformly, through the finer spoil particles (matrix
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BEHAVIOR OF COAL COMBUSTION RESIDUES 63
FIGURE 3.3 Typical values for hydraulic conductivity of selected geologic materials.
SOURCE: Adapted from Heath, 1982, considering data from Hawkins, 1998; Harlow and
LeCain, 1993; VanVoast and Reiten, 1988; and Minns, 1993.
flow), leading to a wide range of hydraulic conductivities in coal spoils (Figure 3.3)
(Hawkins and Aljoe, 1991; Hawkins, 1998; Smith and Beckie, 2003).
Open pit lakes might also remain after large-scale surface mining operations.
Other mining methods, such as underground mining, may cause less disturbance
of surface materials, but large underground chambers are created during mining.
Mining often causes subsidence and increased fracturing in the surrounding strata
(Hornberger et al., 2004). Mine reclamation activities aim to restore surface
water flow paths and recreate similar recharge conditions, but, no effort is made
to restore the specific subsurface water flow paths (NRC, 1990). At most sites, a
new water flow field will be established that reflects the changes caused by
excavation and reclamation activities.
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64 MANAGING COAL COMBUSTION RESIDUES IN MINES
Hydraulic Conductivity (cm/sec)
10-7 10-6 10-5 10-4 10-3 Log
0
50
Explanation
(ft)
100 Coal Fire clay
Depth
Fracture Silty shale
Sandstone Shale
150
200
FIGURE 3.4 Range of hydraulic conductivity values with depth from a borehole in a
bituminous coal-bearing area of Kentucky. Hydraulic conductivities vary widely with
depth, and the highest values are generally found in the coal layers and fractured strata.
SOURCE: Modified from Wunsch, 1992. Courtesy of the University of Kentucky.
Water Flow in the Unsaturated Zone
Above the water table, water flow occurs in response to gravitational and
capillary forces and is therefore relatively complex. In homogeneous porous
media (e.g., well-sorted sand), unsaturated zone water will migrate predomi-
nantly downward to the water table as the result of gravitational forces. However,
depending on the soil moisture levels, the distance below the ground surface, and
the extent of evapotranspiration, unsaturated zone water may flow upward to-
ward the root zone. In porous media with layers or lenses of varying hydraulic
conductivity, lateral flow of water will also occur. Unsaturated flow through
coarse-grained coal spoils can occur in conduits, along the surfaces of large spoil
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BEHAVIOR OF COAL COMBUSTION RESIDUES 65
FIGURE 3.5 Conceptual model of the hydrogeologic flow system characteristic of cen-
tral Appalachian coal-bearing regions, showing the distribution of fractures with depth.
SOURCE: Harlow and LeCain, 1993.
fragments, or within the finer-grained matrix materials (Smith and Beckie, 2003).
Flow is strongly dependent on the orientation and the hydraulic conductivity of
the different spoil layers.
Hydraulic conductivity in the unsaturated zone is a function of moisture
content. In homogeneous porous media, the highest hydraulic conductivity in the
unsaturated zone occurs within the capillary fringe, where all pores are saturated
with water (Figure 3.2). As the water content of unsaturated geologic materials
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66 MANAGING COAL COMBUSTION RESIDUES IN MINES
decreases, large decreases in hydraulic conductivity--up to several orders of
magnitude--are observed. Because coarse-grained materials (e.g., gravel, some
coal spoils) have large pore spaces that drain more quickly than fine-grained
materials (e.g., silt, CCR), the hydraulic conductivity of coarse-grained materials
can be lower than that of fine-grained materials at the same moisture content
(Hillel, 1998). For example, at low to moderate moisture contents, unsaturated
water flow may be greater in fine-grained materials than in coarse-grained coal
spoil (Newman et al., 1997; Wilson et al., 2000).
Implications of CCR for Subsurface Flow
As noted in Chapter 2, emplacement of CCR at mine sites can occur above or
below the water table. The physical properties of CCRs can differ greatly from
the physical properties of coal spoils and surrounding geologic materials (Table
3.1; Figure 3.3). As a result, large-volume CCR disposal can substantially alter
groundwater flow paths. CCRs can be disposed as large monofills, as layers of
CCR interbedded with coal spoils, or as blended mixtures of CCR and coal spoils
(Figure 2.9). Considerations of potential saturated and unsaturated water flow in
and around these CCR emplacement zones have implications for mine disposal of
CCRs.
CCR Impacts on Saturated Flow
In the saturated zone, given the same pressure conditions, groundwater flow
will be greatest in high-hydraulic conductivity materials. Where monofills of
fine-grained CCR are placed within coarse-grained coal spoils, the water will
have a tendency to flow around the CCR monofill because the lower hydraulic
conductivity of the CCR will impede flow. Where CCR fills an entire surface
mine pit, the impacts on groundwater flow will depend on the hydraulic conduc-
tivity of the surrounding geologic materials as well as the extent of compaction
during emplacement. If the surrounding materials are relatively intact strata with
a lower hydraulic conductivity than the CCR, groundwater will flow through the
CCR. Alternatively, if the hydraulic conductivity of the surrounding geologic
materials is higher than that of the CCR, water will tend to flow around the CCR.
When coal spoils and CCR are placed as interbedded layers during the mine
reclamation process (Figure 2.9), the contrasts in hydraulic conductivity will fur-
ther alter the groundwater flow. Under these conditions, the impacts on groundwa-
ter flow will depend on the orientation of the groundwater flow direction relative to
the orientation of the layers of CCR and spoil. If the groundwater flow direction is
parallel to the CCR layers, water will flow preferentially through the coarse spoil
layers, with only minor flow through the CCR. If the groundwater flow direction is
perpendicular to the CCR layers, the fine-grained CCR will impede the flow and
reduce groundwater velocities through the emplacement zone.
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BEHAVIOR OF COAL COMBUSTION RESIDUES 67
TABLE 3.1 Hydraulic Conductivity at Saturation (Ks)
and Particle Diameter (d) for Some Soils and Typical
CCRs After Placement
Ks (cm/sec) d (mm)
Claya 10-8 to 10-6 <0.002
Silta 10-6 to 10-4 0.002-0.05
Sanda 10-3 to 10-1 0.05-2
Gravela 1.0 to 10-3 >2
Fly ash: 0.006 0.130(g)
Unstabilized, compacted 4 × 10-5(b )
Stabilized with lime 10-7(b,c )
Bottom ash 10-3 to 10-2(d ) 0.2 10(h)
Boiler slag 10-3 to 10-2(d ) 0.6 3(h)
FGD residue: 0.02 0.04*(i)
Dewatered unstabilized FGD 10-5 to 10-4(e )
Stabilized or fixated FGD 10-7 to 10-6(f )
*Mean diameter.
Note that hydraulic conductivities of CCRs may vary significantly
based on the degree of compaction methods. Particle size diameter
data for CCRs reflect the mean grain sizes at the 10th and 90th weight
percentiles, unless otherwise noted.
SOURCES: Hillel, 1998; Ghosh and Subbarao,1998; Koury et al., 2004;
a b c
dMajizadeh et al., 1979; Prusinski et al., 1995; Smith, 1985; Morenoa et
e f g
al., 2005; Moulton, 1973; Tishmack, 1996.
h i
As described in Chapter 2, some CCRs have cementitious properties, while
others can become cementitious with the addition of lime or some other base.
Table 3.1 shows the notable reduction in hydraulic conductivity that can occur
when CCRs are "stabilized" with the addition of lime. It should be noted that
some uncertainty remains regarding the long-term stability of cementitious ash
and whether these low hydraulic conductivities can be maintained in the environ-
ment over time (McCarthy et al., 1997; Weinberg and Hemmings, 1997).
CCR Impacts on Unsaturated Flow
Predictions of unsaturated flow are complex, even without the addition of
CCRs, and research on unsaturated flow through CCRs is extremely limited.
Nevertheless, some observations of the potential impacts of CCRs on unsaturated
flow at mine sites are provided here based on relevant studies of unsaturated flow
through layered fine- and coarse-grained materials and through waste rock piles
at coal and metal mine sites.
The impacts of CCRs on unsaturated flow will depend on a number of
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68 MANAGING COAL COMBUSTION RESIDUES IN MINES
factors, including the degree of contrast in hydraulic properties between the CCR
and the surrounding spoil or geologic strata, the moisture content, and the geom-
etry of CCR emplacement (Hillel, 1998; Smith and Beckie, 2003). As discussed
previously, research on unsaturated flow suggests that at times of low infiltration,
water in the unsaturated zone may flow preferentially through fine-grained CCR
layers rather than through the coarser-grained spoil materials. During periods of
high infiltration rates, research suggests that flow might be dominantly through
the coarse-grained spoils (Bussičre et al., 2003; Smith and Beckie, 2003). Thus,
large uncertainties remain regarding flow in the unsaturated zone in complex
mine settings, especially those with great contrasts in hydraulic conductivity.
When CCRs are placed close to the water table, a thick capillary fringe could
form within the materials. Studies of groundwater flow through mine tailings
with similar particle size distributions and hydraulic conductivities as fly ash,
noted a thick capillary fringe, ranging from tens of centimeters up to six meters in
thickness (Blowes and Gillham, 1988; Al and Blowes, 1996a,b). Under such
conditions, the addition of only a small amount of water, such as a minor precipi-
tation event, can lead to a pronounced rise in the water table and increased
potential for contaminant transport to surface water bodies.
BIOGEOCHEMICAL PROCESSES AFFECTING CCR BEHAVIOR
As groundwater comes in contact with CCR in the mine environment, the
material will be impacted by an array of geochemical and biological processes.
Dissolution and desorption processes can release constituents into water from the
CCR through an initial set of rapid reactions, which will be followed by slower
reactions over months or years. Once these constituents enter the groundwater,
they may be transported away from the CCR. Some contaminants will be trans-
ported conservatively, moving with the flow of water because they are unaffected
by adsorption to aquifer materials. However, other contaminants may be attenu-
ated by adsorption or precipitation reactions or transformed by microbially medi-
ated biological reactions.
The biogeochemical environment in the coal mine setting can vary widely
between sites and within a single site. Oxidation-reduction conditions at a mine
site are generally oxic, but suboxic conditions may occur at depth. The ground-
water pH may be near neutral at some coal mine sites, particularly western mines,
and highly acidic at others due to sulfide mineral oxidation reactions that cause
acid mine drainage (AMD) (Sidebar 3.2). A large range of pH and oxidation-
reduction conditions may develop within a single site as the result of variability
in the amount of acid-generating materials and the availability of acid-consuming
materials (Cravotta, 1994). When CCRs are emplaced at a site, there is potential
for the pore water pH to rise to very high values (pH > 9) due to the substantial
alkalinity in many CCRs.
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BEHAVIOR OF COAL COMBUSTION RESIDUES 69
SIDEBAR 3.2
Acid Mine Drainage
Coal mine drainage waters can vary widely in composition, from highly acidic to
alkaline. Acid mine drainage (AMD) is a common problem at coal mines in the
eastern United States and is formed by the oxidation of sulfide minerals (e.g.,
pyrite, FeS2), which exist in coal spoils and surrounding geological materials. Acid
mine drainage contains elevated concentrations of acid, iron, manganese, alumi-
num, and associated trace elements, such as zinc, nickel, and arsenic, which can
be transported to surrounding waters (Williamson and Rimstidt, 1994; Blowes et
al., 2003a). The following reactions characterize the various steps in the genera-
tion of acidity by pyrite oxidation (Stumm and Morgan, 1996):
FeS2(s) + /2 O2 + H2O = Fe2+ + 2 SO4
7 2- + 2 H+ (3.1)
Fe2+ + /4 O2 + H+ = Fe3+ + /2 H2O
1 1 (3.2)
FeS2(s) + 14 Fe3+ + 8 H2O = 15 Fe2+ + 2 SO4 2- + 16 H+ (3.3)
Fe3+ + 3 H2O = Fe(OH)3(s) + 3 H+ (3.4)
Oxygen entering pyrite-rich coal spoils is usually consumed through sulfide and
iron oxidation reactions catalyzed by bacteria (e.g., Thiobacillus ferrooxidans) (Sing-
er and Stumm, 1970; Nordstrom and Southam, 1997; Nordstrom and Alpers, 1999).
Acid mine drainage can be neutralized by reactions with carbonates (e.g., limestone)
or aluminosilicate minerals (Campbell et al., 2001; Skousen et al., 2002; Blowes et
al., 2003b; Jambor, 2003; Weber et al., 2004). The rate and extent of acid production
will depend on a number of factors, including the amount of pyrite present, the
amount of neutralizing minerals, the rate of oxygen influx, the pH, and the microbial
community. It may take several decades to many centuries for all available sulfide
minerals to oxidize and for minerals contributing to acid-neutralization reactions to
be consumed (Banwart and Malmström, 2001; Blowes et al., 2003b).
Leaching Behavior of CCR
Trace elements can be tightly bound within the CCR minerals, or they can
occur as leachable coatings on grain surfaces (see Chapter 2). Water chemistry--
primarily pH--influences the solubility of CCR-derived constituents. Many met-
als and metallic compounds found in CCRs exhibit the highest solubilities at very
low and very high pH, with lower solubilities at near neutral pH (Figure 3.6).
Under acidic (low-pH) conditions, elevated dissolved concentrations of many
constituents can be expected due to the high mineral solubility (Pankow, 1991;
Stumm and Morgan, 1996). Under alkaline (high-pH) conditions, the formation
of soluble hydroxide and carbonate complexes leads to increased dissolution of
many metals (Pankow, 1991). There are other elements--in particular, oxyanion-
forming elements such as arsenic, selenium, and molybdenum--that remain
soluble under near-neutral pHs.
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70 MANAGING COAL COMBUSTION RESIDUES IN MINES
10000
1000
100
10
1
mg/L
0.1
0.01
0.001
0.0001
4 5 6 7 8 9 10 11 12 13 14
pH
FIGURE 3.6 Calculated solubilities of selected metallic elements with pH based on
systems containing metal hydroxide and water, without other complexing agents present.
SOURCE: Scheetz et al., 2004. Courtesy of Pennsylvania Department of Environmental
Protection.
Laboratory research has examined the potential for fly-ash leaching under a
broad range of pH. Kim et al. (2003) conducted a series of 30- to 90-day column
leaching experiments to evaluate the leaching of 32 fly ash samples by several
different leaching fluids, including deionized water, simulated acid mine drain-
age (pH 1.2), and an alkaline solution (pH 11.1) representative of pore fluids that
might develop in alkaline fly ash. Analyses of the effluent showed that the great-
est extents of leaching occurred with the acidic leaching solutions for many of the
cations analyzed, including aluminum, cobalt, chromium, copper, manganese,
nickel, and zinc (see Figure 3.7), due to the enhanced dissolution of the ash
particles. In contrast, the leaching of arsenic, antimony, and selenium, was great-
est for alkaline solutions. The committee was unable to find any research on the
effects of various oxidation-reduction conditions on CCR leaching, although
suboxic conditions may occur when CCRs are placed beneath the water table.
Limited research has been done to understand the field leaching behavior of
CCRs. However, one major research study was recently completed and collected
field leachate samples at 37 CCR landfill and surface impoundment disposal sites
(Ladwig et al., 2006). In this study, leachate samples were collected from leachate
wells, lysimeters, drive points, core samples, sluice lines, and from ponds at the
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BEHAVIOR OF COAL COMBUSTION RESIDUES 71
1
0.1
0.01
0.001
L/T
M
0.0001
0.00001 HOH
H+
OH-
0.000001
As Ba Be Cd Co Cr Cu Ni Pb Sb Se Zn
Element
FIGURE 3.7 Box plot showing the relative solubility (ML/T: total mass leached divided
by total initial mass in the fly ash) for trace elements leached from 32 fly ash samples
using acidic (H+), neutral (HOH), and alkaline (OH-) leaching solutions in laboratory
column experiments. The box represents the 10th and 90th percentiles; the solid line
within the box represents the median; and the whiskers (or error bars) represent the 5th
and 95th percent confidence intervals.
SOURCE: Kim et al., 2003. Courtesy of the American Chemical Society.
ash/water interface. The field data show fairly wide ranges of trace element
concentrations in the leachate at these sites, with some species (e.g., chromium,
cobolt, selenium) showing variability up to four orders of magnitude between the
maximum and minimum concentrations detected (see Figure 3.8).
CCR Interactions with Acid-Generating Coal Spoil
Coal spoil when exposed to water and oxygen can generate AMD (Sidebar
3.2). Many CCRs, however, are alkaline and may be capable of neutralizing the
acidity, depending on the manner of emplacement (Daniels et al., 2002). As
discussed in Chapter 2, mine placement of alkaline CCRs has been used explic-
itly for treating AMD, and AMD reduction is often considered an added benefit
in large-volume CCR mine disposal operations. This section discusses research
on the interactions between acid-generating coal spoils and alkaline CCRs, high-
lighting the implications for CCR placement design in the mine reclamation
process.
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72 MANAGING COAL COMBUSTION RESIDUES IN MINES
10,000
1,000
100
)L/
(ug 10
ation
1
Concentr 0.1
0.01
0.001
0.0001
Hg Pb Ag Tl Be Cr Co U Cd Sb Cu Zn Ni Se As V
FIGURE 3.8 Data showing field leachate concentrations from 37 CCR sites. Data were
collected from fly ash, bottom ash, and flue gas desulfurization ash placed in landfills and
surface impoundments. Boxes represent the range of data within the 25th and 75th percen-
tiles (or the inter-quartile range, IQR), and whiskers (or error bars) reflect the minimum
and maximum non-outlier concentrations detected. Outliers are considered to be values
that are greater than the 75th percentile + 1.5*IQR or less than the 25th percentile -
1.5*IQR and are shown as diamonds.
SOURCE: Data from Ken Ladwig, Electric Power Research Institute.
Stewart et al. (1997, 2001) evaluated leaching from different blends of fly
ash and acid-producing coal refuse1 using a series of multi-year unsaturated
column experiments. Ash-free coal refuse columns showed a rapid decline in
leachate pH values from 8.0 to less than 2.0 and substantial increases in concen-
trations of dissolved metals (iron, manganese, aluminum, copper, and zinc). In
contrast, columns with the highest proportions of alkaline fly ash (20 percent and
33 percent by weight) showed no evidence of AMD, maintaining a relatively
constant pH (above pH 7) throughout the course of the experiment (Figure 3.9).
Low concentrations of metals leached from these ash-amended columns, although
high concentrations of boron and sulfate were detected. In columns with lower
proportions (5-10 percent) of fly ash and in columns blended with low-alkalinity
1The coal refuse used in these studies was primarily waste rock material mined with coal and
subsequently removed at the coal preparation plant.
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BEHAVIOR OF COAL COMBUSTION RESIDUES 73
14
FIGURE 3.9 Mean pH in column leachate over a three-year column experiment to
examine the impacts of different blending ratios of coal refuse and two types of fly ash
(Clinch River fly ash [CRF] and WestVaco fly ash [WVF]). The CRF is moderately
alkaline, whereas the WVF is a lower-alkalinity CCR. Error bars represent one standard
deviation above and below the mean.
SOURCE: Lee Daniels, Virginia Polytechnic Institute and State University. Modified
from Stewart et al., 2001. Courtesy of Virginia Polytechnic Institute and State University.
fly ash (up to 20 percent), the pH eventually declined to low values during the
course of the experiment. This decline was attributed to insufficient alkalinity
addition. Once the pH declined, concentrations of metals increased substantially
in the leachate.
These findings suggest that the addition of fly ash to coal spoils in a suffi-
cient quantity can prevent AMD formation. However, less is known about the
ability of CCRs to prevent AMD over extended time frames. For example, it is
not known how the presence of Fe3 and other oxidized metals in the CCRs may
+
enhance pyrite oxidation in the surrounding spoils (see Sidebar 3.2). Stewart et
al. (2001) speculated that the high pH of the CCR suppresses the microbially
mitigated oxidation of pyrite while also limiting the movement of oxygen to the
sulfide minerals, so that the acid generated through slower abiotic sulfide oxida-
tion reactions can be effectively neutralized by the CCR. However, if there is an
insufficient addition of alkalinity, low-pH conditions will eventually be gener-
ated, perhaps after several years, potentially leaching metals and other elements
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74 MANAGING COAL COMBUSTION RESIDUES IN MINES
from the ash at high concentrations. Stewart et al. (2001) recommend that if coal
ash is to be used in reclamation activities, close attention should be paid to balanc-
ing the acid-generating potential of coal refuse with the alkalinity of the ash. Many
sources of alkalinity in the aquifer material may not be available for reaction
because of the formation of surface coatings or due to dissolution kinetics. There-
fore, some practitioners recommend increasing the alkalinity by some safety factor
to prevent the unanticipated return of acidic conditions (Daniels et al., 1996).
If the CCR is thoroughly mixed with coal spoils, the alkalinity of the CCR
will contribute to acid neutralization reactions close to where acid generation
occurs. Daniels et al. (2002) examined various CCR and coal refuse mixing
strategies to determine their effectiveness in reducing acidity. However, none of
the CCR placement strategies tested, including layering the CCRs within the coal
refuse and partially blending the CCRs with refuse before layering, proved as
effective at preventing acid generation as the bulk-blending approach of the
previous column experiments. Thus, understanding the mobility of CCR con-
stituents in mines with the potential to generate AMD requires information on
acid-base accounting (see Chapter 6) and the manner of CCR placement relative
to acid-generating materials. Much less is known about the effectiveness of CCRs
for treating AMD under suboxic conditions.
Mobility of CCR Constituents in Mine Environment
The degree to which CCR-derived constituents are mobile in the mine envi-
ronment depends on both aqueous speciation and reactions with surrounding
geologic materials. Trace elements released from CCRs can form neutral, posi-
tively, or negatively charged species in one or more valence states in solution
(Table 3.2). The speciation of elements is dependent on pH, oxidation-reduction
potential in the mine setting, and the concentrations of other species in solution
that might contribute to the formation of soluble complexes.
The mobility of these CCR-derived species varies widely in the mine envi-
ronment. Some species do not interact strongly with the surrounding geologic
materials (e.g., coal spoils, shale, clay) over the entire range of pH and oxidation-
reduction potential likely to be encountered at a coal mine site. Other species will
be mobile under a limited range of pH and oxidation-reduction potential; still
others will have low mobility under all conditions. Only limited information is
available on attenuation reactions influencing the fate of CCR elements of con-
cern at coal mine sites where large-volume CCR disposal has occurred. However,
insights can be gained through other studies on the transport of metals and metal-
lic compounds under geochemical conditions that develop in mine settings or
other types of sites, since many of the constituents of interest are the same as
those found at CCR disposal sites (Table 3.3). Examination of these data provides
information about the potential mobility of CCR-derived elements under near-
neutral conditions at coal mine sites.
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BEHAVIOR OF COAL COMBUSTION RESIDUES 75
TABLE 3.2 List of Selected Elements Observed to Leach from CCR,
Including Common Hydrolysis Species
Element Important Species Between pH 2 and 12
Ag Ag (II): Ag2 +
Ag(I): Ag+
Al Al3 , Al(OH)2 , Al(OH)2 , Al(OH)4
+ + + -
As As(III): H3AsO3 , H2AsO3
0 -
As(V): H2AsO4 , HAsO4
2-
B H3BO3 , H2BO3 , HBO3
0 - 2-, BO3 3-
Ba Ba2 +
Be Be2 , BeO2
+ 2-
Cd Cd2 , CdO2
+ 2-
Co Co(III): Co3+
Co(II): Co2 , HCoO2
+ -
Cr Cr(VI): HCrO4 , CrO4
- 2-
Cr(III): Cr3 ,Cr(OH)2 , Cr(OH)2
+ + +1, Cr(OH)3 , Cr(OH)4
0 -1
Cu Cu2 , CuO2
+ 2-
Fe Fe(III): Fe3 , Fe(OH)2 , Fe(OH)2 , Fe(OH)3 , Fe(OH)4 , Fe2(OH)2
+ + + 0 - 4+
Fe(II): Fe2 , Fe(OH)+, Fe(OH)3 , Fe(OH)2
+ - 0
Hg Hg(II): Hg2 , HgOH+, Hg(OH)2 , Hg(OH)3
+ 0 -
Hg(0): Hg0
Mn Mn2 , MnOH+, Mn(OH)3
+ -
Mo Mo(VI): MoO4 2-, HMoO4 2-, H2MoO4 , MoO2
0 2+
Mo(V): MoO2 +
Mo(III): Mo3 +
Ni Ni2 , NiOH+, HNiO2
+ -
Pb Pb2 , PbOH+, Pb(OH)2 , Pb(OH)3
+ 0 -
S S(VI): HSO4 , SO4
- 2-
S(0): S0
S(II): H2S0, HS-
Sb SbO+, SbO2 -
Se Se(VI): HSeO4 - 2-
,SeO4
Se(IV): H2SeO3, HSeO3 -
,SeO3 2-
Se(0): Se0
Se(II): H2Se, HSe-
Tl Tl(III) :Tl(OH)2 , Tl(OH)2
+ +
Tl(I) : Tl+
U U(VI): UO2 2+, UO2OH+, (UO2)3(OH)5 +
U(V): UO2 +
U(IV): U4 , UOH3 , U(OH)2
+ + 2+, U(OH)3 , U(OH)4 , U(OH)5
+ 0 -
V V(V): H2VO4 , HVO4
- 2-, VO4 3-
V(IV): VO2 +
Zn Zn2 , ZnOH+, ZnO2
+ 2-
xx
Under near-neutral pH conditions, constituents such as sulfate, magnesium,
ferrous iron (Fe2 ), zinc, nickel, arsenic, selenium, and boron often migrate
+
readily, especially in the suboxic conditions that exist in many coal spoils. In
contrast, the concentrations and mobility of some other constituents, such as
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76 MANAGING COAL COMBUSTION RESIDUES IN MINES
aluminum, lead, and cadmium, are expected to be limited due to adsorption to
solids within the aquifer or because of the formation of secondary precipitates
(e.g., carbonates, sulfates). The transport of these sparingly soluble constituents,
however, may be enhanced in coarse-grained or fractured media by colloids
(particles that generally range in size from 1 nm to 10 µm) (McCarthy and
Zachara, 1989; Russel et al., 1989; Kretzschmar et al., 1999). Over time, geo-
chemical conditions at a site (e.g., pH, redox conditions) can change as the more
reactive or soluble minerals dissolve and are flushed from the CCR, thereby
affecting the transport potential of trace elements from the CCR.
As groundwater moves away from the CCR disposal area, this water has the
potential to discharge contaminants to surface water bodies, where additional
geochemical processes can occur that may affect their mobility and bioavailability.
Abundant information is available on the transport and bioavailability of contami-
nants in surface waters downstream from coal mine sites without CCR. In contrast,
virtually no information is available for sites with CCR placement. Coal mines that
generate acid mine drainage (Sidebar 3.2) can contribute large quantities of iron to
streams adjacent to mine sites. Oxidation of the iron results in the precipitation of
ferric (oxy)hydroxide solids, which can scavenge some trace elements of concern,
lowering their concentrations in the stream. However, at many sites this process is
inefficient, and trace elements can migrate long distances from the mine in surface
water, at unacceptable concentrations.
POTENTIAL FOR CONTAMINANT TRANSPORT FROM
COAL COMBUSTION RESIDUES IN COAL MINES
Contaminants entering groundwater can be transported away from the CCR
source area potentially resulting in the degradation of drinking water supplies or
of surface-water quality. The degree of degradation of downgradient water qual-
ity will depend on the concentration and volume of contaminated water entering
the flow system and the ability of the aquifer or receiving water body to dilute or
attenuate the contamination. The concentration and volume of contaminated wa-
ter, in turn, depend on the leachable mass of toxic constituents in the CCR, the
emplacement design, and the local hydrogeologic setting. The leachable mass of
toxic constituents is a function both of the leachability of the constituents of
concern and the total mass of CCR materials disposed at a site.
For example, if CCRs are placed in the unsaturated zone at a site where
unsaturated water movement is slow, there might be potential for dilution of the
contaminants to acceptable concentrations if groundwater velocities in the satu-
rated zone are relatively high. This situation would most likely occur when the
areal extent of CCR emplacement and the total leachable contaminant mass are
relatively small. Similarly, if CCRs are placed in low-hydraulic-conductivity
geologic materials so that the volume of groundwater discharging to a surface
water body is small and if the contaminant concentrations are also low, there
OCR for page 77
77
al.
Non- et
of 2000);
(1991)
LeBlanc
(1995,
Studies (2000)
al. (2002)
al.
(1998); et al.
et (2000) (1998)
Wittbrodt (2004)
Case et
al. al. al.
Kent
on et et and et
References McCreadie Stollenwerk (1991); Davis Brown Heikkinen Palmer Curtis
Based
of
Mo,
Conditions presence
Cr(VI),
in -
3
Constituents Cu, Zn
B, Se, Zn HCO
Mobile As As, Ni, Mn, Ni Cr(VI) U(VI)--mobility increased high
Groundwater
and aquifer
Media types
aquifer
gravel
Near-Neutral material aquifer aquifer
aquifer
and
Under Environmental Tailings underlying Sand Alluvial Aquifer Mixed Alluvial
sites
Creek
Contaminants Ontario MA Pinal
Finland CO
AZ
Cod, site,
Lake, contaminated
Mobile
Location Red Cape Mine Basin, Western U.S. Naturita,
of
Sites
mine
injection uranium
Examples gold
from
3.3 from tracer wastes
wastes
impoundment
mine wastes waters tailings
TABLE CCR-Contaminated Source Porewater tailings Controlled Gold Mine Industrial Pore mill xx
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78 MANAGING COAL COMBUSTION RESIDUES IN MINES
might be sufficient dilution in the surface water body to reduce concentrations of
contaminants to acceptable levels. If the leachate contaminant concentrations
from the CCRs are low and the distances to sensitive water bodies are long, the
contaminant removal capacity of the aquifer solids may be sufficient to reduce
the concentrations to acceptable levels. There are, however, other scenarios where
the outcome of CCR mine disposal may not be positive. Large contaminant
plumes could form where leaching rates are moderate to high, where there is
substantial water flow through the CCRs (in either the saturated or the unsatur-
ated zone), and where the CCR emplacement zone covers a sizable aereal extent.
At numerous mine sites, contaminant leaching from other materials placed in the
unsaturated zone has resulted in the development of large plumes of contami-
nated groundwater downgradient of the disposal area (Dubrovsky et al., 1984;
Moncur et al., 2005).
These general scenarios provide some guidance as to the types of mine
settings that may contribute to higher- or lower-risk CCR disposal. To fully
assess the potential for degradation of groundwater and surface-water quality, a
detailed analysis is required that takes into account the specific characteristics of
the CCR and the hydrogeology and geochemistry of the site, which are discussed
further in Chapter 6.
The time frame for contaminant transport depends on local rates of unsatur-
ated and saturated groundwater flow and potential attenuation reactions in the
surrounding environment, but it is worth noting that it may take many years
before groundwater contamination from CCR mine disposal reaches down-
gradient monitoring wells. Changing geochemical conditions (e.g., the depletion
of alkalinity from CCR) add further uncertainty regarding the potential for mo-
bilizing contaminants over extended time frames. Sizable uncertainty is associ-
ated with our current understanding of CCR behavior in the mine environment
because few, if any, studies have analyzed the long-term behavior of CCRs in
the mine setting. Long-term (>10 years) studies that encompass a range of
climatic and geologic settings are needed to accurately characterize CCR behav-
ior in mine sites so that the types of mine settings, CCRs, and placement tech-
niques most protective of human and ecological health can be identified. Addi-
tional research is also necessary to determine whether placement of CCR in
mines can ameliorate the adverse effects of AMD in surface waters, particularly
over protracted time scales.
SUMMARY
Successful prediction of CCR behavior in the mine environment requires a
thorough understanding of the complex physical and biogeochemical processes
that control the release and transport of CCR-derived constituents. This chapter
provides an overview of the hydrologic and biogeochemical processes control-
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BEHAVIOR OF COAL COMBUSTION RESIDUES 79
ling the release and transport of contaminants from CCR mine disposal sites to
locations where uptake may occur. In a mine setting, subsurface water flow will
be the primary mechanism for transporting CCR-derived contaminants from the
emplacement area to potential receptors. Subsurface flow at CCR mine place-
ment sites is controlled by the local hydrogeology, which may be significantly
altered by mining activities, and the addition of CCR further alters groundwater
flow paths. The manner and degree to which the pathways are altered will depend
on the manner of CCR emplacement and the location of the disposal site relative
to the water table. When CCRs are placed in close proximity to the water table, a
thick capillary fringe could form, which increases the potential for downgradient
contaminant transport.
As water comes in contact with CCR in the mine environment, the material
will be impacted by a broad array of geochemical and biological processes. The
mobility of CCR-derived constituents varies widely in the mine environment
depending on the pH, oxidation-reduction potential, and chemical composition of
the water encountered at a mine site. Low-pH water can mobilize metals and
nonmetallic constituents in the CCR. Depending on their acid-neutralizing poten-
tial and the methods of emplacement, CCRs may be effective in neutralizing
AMD and therefore reducing the overall transport of contaminants from the mine
site. However, several potentially toxic constituents in CCRs are mobile at neu-
tral or alkaline pHs. Thus, the committee concludes that acid neutralization will
not reduce the mobility of all contaminants of concern from the CCR.
Impacts on downgradient water quality from CCR disposal at mine sites will
depend on the concentration and volume of contaminated water entering the flow
system and the ability of the aquifer or receiving water body to dilute or attenuate
the contamination. The concentration and volume of contaminated water, in turn,
depend upon the leachable mass of toxic constituents in the CCR, the emplace-
ment design, and the local hydrogeologic setting. General scenarios are presented
to provide some guidance as to the types of mine settings that may contribute to
higher- or lower-risk CCR disposal. Specifically, one high-risk scenario occurs
where leaching rates are moderate to high, where there is substantial water flow
through the CCRs (either in the saturated or the unsaturated zone), and where the
CCR emplacement zone covers a sizable areal extent.
Abundant information exists regarding the transport of toxic metals and
metalloids in groundwater, which may assist our understanding of the behavior of
CCR-derived constituents in the mine setting. However, the committee concludes
that there remains a poor understanding of the conditions influencing the field
behavior of CCRs, such as pH, oxidation-reduction conditions, and hydraulic
conductivity, over extended time frames at CCR placement sites. Sizable uncer-
tainty exists in our current understanding of CCR behavior in the mine environ-
ment because few, if any, studies have analyzed the long-term behavior of CCRs
in the mine setting. The committee recommends additional research to exam-
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80 MANAGING COAL COMBUSTION RESIDUES IN MINES
ine the long-term (>10 years) environmental behavior of CCR at mine sites,
including differing climatic and geologic settings, so that the types of mine
settings, CCRs, and placement techniques most protective of human and
ecological health can be identified. This research should include studies to
determine under which conditions CCRs can effectively ameliorate the adverse
effects of AMD in surface waters, particularly over protracted time scales.
Representative terms from entire chapter:
ccr placement
