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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
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
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).
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
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.
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
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
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.
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
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 (Bussière 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.
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.
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
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.
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.
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
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.
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
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
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
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-
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-
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.