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6 Characterization for Coal Combustion Residue Management M any variables affect the behavior and potential impacts of CCR place- ment in a mine setting, including chemical and physical properties of the CCRs (see Chapter 2), the hydrogeologic and biogeochemical set- ting at the mine site (see Chapter 3), and the proximity of sensitive receptors (see Chapter 4). The previous chapters have shown that these characteristics vary widely from site to site or from plant to plant. Therefore, decisions regarding CCR placement cannot be made based on broad generalizations but instead re- quire careful specific characterization of both the CCR material and the mine site in the context of CCR placement. Site characterization and CCR characterization are essential parts of CCR management and serve to guide engineering design, permitting decisions, reclamation management, and the development of effective monitoring programs (discussed further in Chapter 7). This chapter discusses the importance of site characterization and CCR characterization within a risk-in- formed framework for CCR management. The chapter also outlines the important categories of information that should be sought through a rigorous characteriza- tion program and summarizes the advantages and limitations of available meth- ods and modeling tools for use in the characterization process. RISK-INFORMED FRAMEWORK FOR CCR MANAGEMENT As described in Chapter 4, unmanaged disposal of CCRs can lead to con- taminant exposures, which can increase the risk of adverse impacts on public health and the environment. Viable management strategies are those that reduce CCR exposure and associated risks of adverse impacts to a level considered 127
128 MANAGING COAL COMBUSTION RESIDUES IN MINES unlikely or acceptable considering the associated benefits. At the same time, CCR management strategies should represent a reasoned application of financial resources balanced with the expectations of affected stakeholders. Overall, CCR management strategies should be informed by an evaluation of risk. Understanding the risks associated with CCR disposal at a mine site requires knowledge of · CCR characteristics (see Chapter 2 and "CCR Characterization" in this chapter); · The transport potential of CCR-derived contaminants in the mine envi- ronment (see Chapter 3 and "Site Characterization" in this chapter); · Toxicological properties of CCR constituents and an assessment of poten- tial human or ecological impacts, including knowledge of the location of poten- tial receptors and intended post-mining land uses (see Chapter 4 and "Site Char- acterization"); · The performance of various engineered CCR placement designs to miti- gate any CCR impacts to some standard of acceptability (see Chapter 7); and · Post-placement monitoring results to confirm predictions of contaminant transport and the performance of the CCR placement design (see Chapter 7). Thus, CCR characterization and site characterization are essential compo- nents of CCR management, providing the foundation for evaluating the safety of CCR placement at a particular site. CCR characterization can be used to estimate the rate and extent of contaminant leaching likely to be observed in the mine setting. Site characterization can help identify lower-risk placement sites based on such factors as their distance from the water table, the potential for downgradient attenuation of contaminants that are leached from the CCRs, and the hydraulic conductivity of surrounding geologic materials. Other characteris- tics that may influence the risk of CCR placement include the volume and method of CCR emplacement, the capacity of CCRs to neutralize any acid-generating materials at the placement site, and the distance of the CCRs from sensitive biota or drinking water wells. Just as characterization contributes to our understanding of risk, the evalua- tion of risk may, in turn, influence the amount of characterization needed. For example, a small volume of a relatively innocuous CCR with low leaching poten- tial may require less rigorous characterization of the site hydrogeology. Like- wise, a large volume of CCR to be emplaced in a higher-flow hydrogeological setting, close to ecological receptors or local residents that rely on groundwater as a drinking water source, may require more rigorous leaching tests and detailed characterization of the site hydrogeology and geochemistry. An assessment of potential risks may also influence the placement design by prompting further consideration of the engineering controls available to minimize the impact of CCR placement in the environment.
COAL COMBUSTION RESIDUE MANAGEMENT 129 Uncertainty is unavoidable in predictions of contaminant behavior and trans- port in the environment. Therefore, recognition of the many uncertainties regarding CCR placement in mines should also influence management decision making. Uncertainties may derive from simple measurement errors, which could occur with a poorly calibrated instrument, or from sampling errors, such as insufficient sam- pling to accurately assess the extent of subsurface fracturing. Uncertainties are also derived from errors in our conceptual understanding of complex systems, such as the transport of contaminants in complex mine settings or the stability of cement- itious ash over long time frames. Inaccuracies in mathematical simulation models also contribute to uncertainty. After all, simulation models are approximations of reality and are often predicted at a scale much coarser than the laboratory scale where our understanding of processes is usually the most reliable. Even if we could perfectly describe the processes governing CCR behavior at the centimeter scale, it is not a trivial matter to scale up these equations into a model that applies at the scale of meters or more (NRC, 2004). All of these sources of uncertainty add up to be rather significant, given the complex scenario of CCR placement in the mine environment. Several strategies can be used to cope with uncertainty in the CCR manage- ment decision-making process. Uncertainties about long-term performance could be reduced by the incorporation of redundant engineered liners and/or impermeable caps rather than more intensive characterization of the disposal site. Uncertainties about the potential for contaminant transport may be answered by more intensive characterization, perhaps including long-term column leaching experiments, inves- tigations of the extent of fracturing within natural geologic barrier materials, or research on the ability of geologic materials to naturally attenuate contaminant migration. Intensive subsurface monitoring could also be used to manage uncer- tainty and provide early warning of any problems, although site managers would have to be prepared to take additional steps to address unacceptable levels of contamination were they to occur. Characterization is, therefore, an important process by which managers can address uncertainties, and it contributes to the understanding of risk at potential CCR mine placement sites. The components of an effective characterization program are detailed in the following sections. SITE CHARACTERIZATION Site characterization is a dynamic process of developing and continually refining a site conceptual model, which captures relevant aspects of the site that affect the behavior and potential impacts of CCRs in the mine environment. According to the NRC (2001), a site conceptual model is "an evolving hypothesis identifying the important features, processes, and events controlling fluid flow and contaminant transport of consequence at a specific field site in the context of a recognized problem." It can also serve as a valuable tool to link potential
130 MANAGING COAL COMBUSTION RESIDUES IN MINES sources to receptors through environmental fate and transport pathways and ex- posure routes (ASTM, 2003). The site conceptual model supports CCR manage- ment decisions, such as whether to place CCRs at a particular mine site. Concep- tual models are qualitative (for example, see Figure 6.1), but they provide the basis for numerical models, which translate the conceptual model into math- ematical equations that can be solved. A site conceptual model can only represent an approximation of the real world because of the complexity of the mine setting and the inherent scarcity of field data. Nevertheless, the conceptual model serves as the basis for identifying critical information gaps, so that additional characterization data can be gathered to evaluate risk. This additional characterization data is then used to further refine, or, if necessary, to completely revise the site conceptual model (Bredehoeft, 2005) to capture site-specific complexities in groundwater flow, CCR leaching, and contaminant transport. Although site characterization and CCR characterization are initially discussed separately in this chapter, CCR char- acterization information is an integral part of the site conceptual model because the total mass and leachability of contaminants in the CCRs affect the extent of natural (or engineered) isolation necessary to prevent downgradient ecological or human health impacts. The extent of pre-placement site characterization needed will depend on the aforementioned assessment of the risk of CCR mine placement as well as a consideration of the uncertainty in the site conceptual model. As uncertainty in the site characteristics and behavior of CCR increases, more effort should be placed on characterization. As discussed in Chapter 5, although the potential benefits of CCR mine placement are important to consider in CCR management decisions, these benefits do not reduce the need to characterize potential risks. Managers and regulators cannot make sound decisions about CCR placement unless both the benefits and the potential risks are well understood. Inadequate investment in site characterization up front may lead to an erroneous assessment of potential CCR impacts and improper placement or engineering design. The costs of adequate site characterization are likely to be far lower than the costs of remediating groundwater and surface-water contamination from a mine site with improperly sited CCRs. Information Needed for CCR Placement The SMCRA outlines general site characterization requirements to obtain a mining permit and to develop the reclamation and operation plan (30 CFR §779. 25, §780.22 (2004)) (see Sidebar 6.1). However, these site characteriza- tion requirements were intended to assess the potential impacts from coal min- ing and do not specifically consider the impacts of CCR placement. In most cases, additional site characterization data are needed to guide CCR placement, both to evaluate the potential for contaminant transport and to support the engi-
131 material Coal Shale Unconsolidated CCRs placement. with CCR with mixed site spoil mine a Mine at Sandstone flow water Coal showing table model, Water conceptual example Stream simple A 6.1 FIGURE
132 MANAGING COAL COMBUSTION RESIDUES IN MINES SIDEBAR 6.1 Site Characterization Under SMCRA The SMCRA requires mine operators to provide site characterization data be- fore the approval of a coal mining permit, although these requirements do not specifically consider CCR placement. These site characterization data inform the determination of probable hydrologic consequences prepared by the operator, the cumulative hydrologic impact assessment prepared by the regulatory agency, and the operator's surface and groundwater reclamation and monitoring plans (30 CFR §780.21(f-j)). 30 CFR §780.22 outlines geologic characterization requirements for the recla- mation and operation plan, including (a) . . . geologic information in sufficient detail to assist in determining: (1) The probable hydrologic consequences of the operation upon the quality and quantity of surface and ground water in the permit and adjacent areas . . . . [and] (2) All potentially acid- or toxic-forming strata . . . . (b) Geologic information shall include, at a minimum the following: (1) A description of the geology of the proposed permit and adjacent areas . . . and other parameters which influence the required reclamation and the occur- rence, availability, movement, quantity, and quality of potentially impacted surface and ground waters. (2) Analyses of samples collected from test borings . . . . The analyses shall result in the following: (i) Logs showing the lithologic characteristics including physical proper- ties and thickness of each stratum and location of ground water where occurring; (ii) Chemical analyses identifying those strata that may contain acid- or toxic-forming or alkalinity-producing materials and to determine their content . . . ; and (iii) Chemical analyses of the coal seam for acid- or toxic-forming mate- rials, including the total sulfur and pyritic sulfur . . . . neering plan for the placement and design of an effective groundwater moni- toring network (see Chapter 7). In cases where CCR placement is proposed in a new mine permit application, some areas of overlap will undoubtedly exist between the site characterization data needed for the mine permit and those needed as part of CCR management. However, in cases where large-volume CCR placement is proposed at a permitted and operating mine, existing site characterization data from the original mine permit should be evaluated care- fully before they are used to refine the site conceptual model. The purpose of this evaluation is to determine whether the mining process has altered certain site characteristics. For example, pumping and the formation of stress-relief
COAL COMBUSTION RESIDUE MANAGEMENT 133 In addition to geologic information, SMCRA and its implementing rules require the operator to provide detailed hydrologic information. For groundwater, the rules require information on "the location and ownership for the permit and adjacent areas of existing wells, springs, and other groundwater resources, seasonal qual- ity and quantity of groundwater, and usage. Water quality descriptions shall include, at a minimum, total dissolved solids or specific conductance corrected to 25°C, pH, total iron, and total manganese. Groundwater quantity descriptions shall include, at a minimum, approximate rates of discharge or usage and depth to the water in the coal seam, and each water-bearing stratum above and potentially impacted stratum below the coal seam" (30 CFR §780.21(b)(1)). For surface water, the operator must provide "[t]he name, location, ownership, and description of all surface-water bodies such as streams, lakes, and impound- ments, the location of any discharge into any surface-water body in the proposed permit and adjacent areas, and information on surface-water quality and quantity sufficient to demonstrate seasonal variation and water usage." As with groundwa- ter, "[w]ater quality descriptions shall include, at a minimum, baseline information on total suspended solids, total dissolved solids or specific conductance corrected to 25°C, pH, total iron, and total manganese. Baseline acidity and alkalinity infor- mation shall be provided if there is a potential for acid drainage from the proposed mining operation. Water quantity descriptions shall include, at a minimum, base- line information on seasonal flow rates" (30 CFR §780.21(b)(2)). The rules at 30 CFR §780.21 also require the operator to submit baseline cu- mulative impact area information and alternative water source information. Sup- plemental information may be required if the determination of the probable hydro- logic consequences indicates that adverse impacts may occur to the hydrologic balance or that toxic-forming material may result in the contamination of ground- water or surface-water supplies. "Such supplemental information may be based upon drilling, aquifer tests, hydrogeologic analysis of the water-bearing strata, flood flows, or analysis of other water quality or quantity characteristics" (30 CFR §780.21(b)(3)). SMCRA also allows the regulatory agency to require additional characteriza- tion data as necessary to meet the performance standards specified or to protect the hydrologic balance (30 CFR §780.22(c)). and/or subsidence-induced fracturing during mining may alter the permeabil- ity of the strata and the groundwater flow patterns. Because of the variability among mine sites, it is difficult to prescribe the precise site characterization data collection steps to follow prior to CCR place- ment. However, three broad categories of information are essential components of the site conceptual model: the hydrogeologic setting, the biogeochemical envi- ronment, and the proximity to sensitive receptors. These categories of site charac- terization information are discussed in detail below. The individual tools and methods available for collecting site characterization data have been described elsewhere (PADEP, 1998; ADTI, 2000) and are not discussed in detail here.
134 MANAGING COAL COMBUSTION RESIDUES IN MINES Hydrogeologic Setting To develop an accurate depiction of the hydrogeologic setting influencing the behavior of CCR placed in mines, information is needed about the geologic materials present at the site, climatological data, patterns of saturated and unsat- urated flow, and the local surface water flow system. The site characterization information needs identified below focus on those data that may be overlooked in the standard permitting process for coal mining but are essential to site character- ization for CCR placement. Meteorological Data. Local meteorological data, such as rates of precipitation, evapotranspiration, and groundwater recharge, provide information on water in- puts to the mine site and the relative importance of overland and groundwater flow. CCRs are often placed in the unsaturated zone; therefore, information on recharge rates is particularly important. Local recharge data are rarely available, but regional values should be used only with the understanding that there may be large uncertainty in these estimates (NRC, 1990). Variations in temperature throughout the year can provide useful information on freeze-thaw cycles that could affect the integrity of any protective caps. Geologic Materials. The geological materials in a mine site may offer a natural means for isolating CCRs, by limiting subsurface flow. To fully evaluate the potential for natural geologic isolation, the thicknesses, orientation, and hydrau- lic conductivity of the strata forming the sides and bottom of the CCR placement site should be determined. The characterization should examine the potential for fracture- or conduit-dominated flow in addition to flow through unfractured po- rous media, such as spoil materials. The presence of thick sequences of strata with hydraulic conductivities of 10-7 cm/sec or lower should reduce off-site groundwater flow, provided the orientation of these strata is optimal relative to the direction of groundwater flow. Depending on the depth to the water table, geologic units with higher conductivities may represent higher-risk placement sites. Engineering design considerations for sites lacking natural confining layers are discussed in Chapter 7. Soil properties may also have an impact on CCR management decisions at mine sites. Depending on its properties, on-site soil may be used for backfill, clay liners, protective soil cover, or low-permeability capping material. Hence, it may be valuable to collect data on the engineering properties of the soil at the site, such as in-place moisture density, Atterberg limits, grain size analysis and distribution, laboratory moisture-density relationships, and hydraulic conduc- tivity relationships. Subsurface Water Flow. To understand the potential for contaminant transport from CCR placement sites, three-dimensional flow processes should be included
COAL COMBUSTION RESIDUE MANAGEMENT 135 in the site conceptual model based on current theories of unsaturated and satu- rated flow in heterogeneous systems. The placement of CCR calls for thorough characterization of pre-mining groundwater flow and predictions of post-recla- mation flow through the entire mine area, including disturbed areas such as the mine spoil and the emplaced CCR. Site data to characterize groundwater flow would include seasonal fluctuations of the water table with respect to the CCR placement zone and hydraulic conductivities, rates, and directions of groundwa- ter flow in all aquifers potentially influenced by the CCR. Predictions of post- reclamation groundwater flow would require an understanding of the material properties of the spoil and CCR, including the hydraulic conductivities of the material upon emplacement, and an approximation of the placement geometry. As described in Chapter 3, water flow paths can change dramatically because of CCR emplacement. Groundwater flow through fractured rock, including coal, is difficult to quantify, and adequate site characterization information about frac- tures is costly to obtain, because prediction of flow requires knowledge of the number, size and thickness, and continuity of the fractures (NRC, 1990, 1996a; Domenico and Schwartz, 1998). Similarly, groundwater flow through heteroge- neous coal spoils, which may contain both matrix and conduit (or pseudokarstic) flow (Hawkins and Aljoe, 1991; Smith and Beckie, 2003), is difficult to quantify. Information on unsaturated flow characteristics is required to define the rate of contaminant migration into the groundwater zone, especially when CCRs are placed above but in close proximity to the water table. Prediction of water move- ment in the unsaturated zone requires information on values of hydraulic conduc- tivity for CCR, spoil, and other materials as a function of water content and wetting and drying histories. Information on surficial topography relative to hy- draulic conductivity variations may provide additional information about local infiltration at the land surface. Sufficient data should be collected to estimate travel times for contaminants to the habitats of sensitive receptors and to the nearest monitoring wells. A thorough groundwater flow characterization will also inform the design of an effective groundwater monitoring network that will intercept any contaminant plume from the CCR placement site. However, site managers should recognize the degree of uncertainty in groundwater flow data to determine the appropriate number of downgradient monitoring wells, with greater uncertainty warranting more monitoring wells. Groundwater monitoring is discussed in greater detail in Chapter 7. Surface Water Flow. Large amounts of surface water flow data are typically collected in the standard mine permit application. However, the addition of CCR placement at a mine site necessitates that there be a clear understanding of the interconnections between groundwater and surface-water flow under pre-mining, mining, and post-reclamation conditions. Due to concerns about flooding and
136 MANAGING COAL COMBUSTION RESIDUES IN MINES erosion at the CCR placement site, the configuration of the site with respect to the 100-year floodplain should also be verified. Biogeochemical Environment Flow characteristics alone are not sufficient to develop a site conceptual model of the behavior of CCRs in the mine setting because the biogeochemical conditions at the site have a significant influence on the mobility of contami- nants. Understanding the biogeochemical environment in the mine setting re- quires information on the groundwater chemistry, the mass and form of mineral phases present at the site, and the dominant microbially mediated geochemical reactions. With this information, geochemical models can be used to explore the likely reactions that may attenuate or enhance transport of contaminants in the subsurface. When this information is combined with knowledge of groundwater flow and transport properties and the leaching behavior of CCRs (discussed later in this chapter), the potential impacts of CCR placement on groundwater and surface-water quality can be estimated. Water Quality. A thorough assessment of pre-placement groundwater and surface-water quality in both disturbed and undisturbed areas of the mine is essential to evaluate possible environmental impacts of the CCR once it has been emplaced. Such characterization should assess the seasonal variability in water quality both upgradient and downgradient of the proposed placement site, as well as within the mine spoil. In addition to SMCRA-recommended water quality parameters (e.g., pH, alkalinity, iron, manganese, sulfate), background water samples should be analyzed for a complete suite of metals and metalloids poten- tially associated with CCRs (i.e., gold, aluminum, arsenic, boron, barium, beryl- lium, cadmium, chromium, cobolt, copper, mercury, molybdenum, nickel, lead, antimony, selenium, thallium, uranium, vanadium, zinc). Although this list ap- pears long, it is important that background groundwater and surface waters be analyzed for an extensive suite of metals and metalloids because the characteris- tics of all CCRs that will be placed in the mine cannot be known in advance. Even if the current CCR characteristics are known, the chemical characteristics of future CCRs may change with a new source fuel, different combustion technolo- gies, or additional pollution control technologies (see Chapter 2). A complete suite of major anions and cations should also be measured to support any geochemical modeling. As noted in Chapter 4, the EPA has encountered difficulties in evaluating the impacts of CCR placement at mine sites because of the inability to distinguish the effects of CCRs from preexisting mining-related activities (USEPA, 1999b). Im- proved pre-placement characterization of water quality across mine sites will help remedy this problem.
COAL COMBUSTION RESIDUE MANAGEMENT 137 Geological Materials. Coal mine spoils contain sulfide minerals (e.g., pyrite) that can have a significant impact on local water chemistry and, therefore, on the behavior of CCRs in the mine setting. Where oxygen is plentiful, as in the unsaturated zone, sulfide minerals will oxidize, releasing acid, sulfate, and trace metals (see Sidebar 3.2). Two primary forms of information can be collected to help assess the potential for AMD: (1) physical and mineralogical data and (2) acid-base accounting analyses. Information on the mineralogy of geologic solids in the coal spoils can be used to estimate both the rates of sulfide mineral oxidation (and thus acid genera- tion) and the rates of acid neutralization through reactions with other minerals, such as calcite. For example, framboidal pyrite is much more reactive than pyrite crystals with other morphology. Similarly, calcite is more reactive than other carbonate minerals in coal spoils, leading to higher neutralization reaction rates (Blowes et al., 2003b). Data on the physical properties of geologic materials, including moisture content, grain-size distribution, and the overall shape of the spoils, are necessary to quantify the rates of in-situ sulfide oxidation (Blowes et al., 2003a,b). Acid-base accounting quantifies the potential for acid generation and acid neutralization in a geologic solid based on laboratory tests. This approach is frequently used to calculate the blending ratio of CCRs to spoil in order to neutralize acidity generated at the site and thereby help remediate the effects of AMD (see Chapter 3). Acid-base accounting methods fall into two categories: static testing methods (e.g., Sobek et al., 1978; Kania, 1998) and kinetic testing methods (e.g., Hornberger and Brady, 1998; ASTM, 2001). The static method commonly involves batch titrations with strong acids and/or bases to determine the potential for acid generation and acid neutralization reactions to occur. Ki- netic methods are designed to emulate field conditions and may involve exposing a sample to alternating wetting and drying conditions to promote oxidation reac- tions before the drainage waters are analyzed for pH, alkalinity, and concentra- tions of sulfate and dissolved metals. Qualitative comparison of these acid-base accounting methods to field be- havior suggests that the predictions of acid generation are generally good. Never- theless, there can be notable variability in how acid-base accounting tests are performed in the laboratory (e.g., Sobek method, modified Sobek), and those who interpret the data should understand what analytical method was used and how the results of that method relate to field behavior. There are a substantial percentage of cases where the acid neutralization potential has been overesti- mated, especially with static tests. For example, many sources of bases may not be available for dissolution due to the formation of coatings that prevent contact of the acid with the base. For cementitious CCRs in particular, the base may not be accessible for acid neutralization. When the results of acid-base accounting are inaccurate, the addition of CCRs may not be sufficient to neutralize acidic conditions or they may neutralize the acid over the short term, only to see the
138 MANAGING COAL COMBUSTION RESIDUES IN MINES return of AMD after the source of alkalinity is exhausted. Careful attention should be given to acid-base accounting, considering the potential uncertainties in the analyses and site conditions, when CCRs are placed at mine sites. Because of the potential for error in acid-base accounting, kinetic tests have been increasingly utilized, and efforts have been made to more accurately quantify the spatial variability at mine sites in acid-base accounting calculations (Perry, 1998). Also, safety factors have been employed when determining the amount of base (e.g., carbonate minerals, lime, alkalinity in CCRs) needed to neutralize acid-generat- ing material in the reclamation process (Hornberger and Brady, 1998; Perry, 1998). The above tests are focused primarily on predicting the potential for acid generation at mine sites and to a lesser extent the potential for the release of toxic elements. The tests are not focused on assessing attenuation reactions other than those resulting as a direct consequence of acid neutralization. There is potential for other reactions to contribute to the attenuation of toxic elements, particularly where groundwater flow paths are long and geological conditions are such that they promote contaminant attenuation reactions (e.g., clays, reactive organic matter). This is an area of active research and a uniform approach to predicting these attenuation reactions has not been developed. Proximity to Sensitive Receptors Management decisions regarding CCR disposal at mine sites require an un- derstanding of the risks to local receptors. Therefore, the locations of sensitive receptors should be identified as part of the site characterization process. These receptors may include neighbors who rely on groundwater for their drinking water supply, local communities that may be affected by CCR placement opera- tions, and aquatic and terrestrial biota or grazing livestock inhabiting areas that could potentially be impacted by CCR-derived contaminants. Prediction Methods for Site Characterization A large portion of the site characterization information discussed can be gathered from field data collection and laboratory analyses. Nevertheless, predic- tive tools can also be useful to assess the potential impact of CCR placement on water flow and water quality. The advantages and limitations of using computer modeling tools for predicting subsurface water flow and contaminant transport in site characterization of CCR mine placement sites are discussed below. Methods for Predicting Water Flow Methods for predicting water flow in homogeneous porous media have been well developed. There are a number of computer models available for prediction
COAL COMBUSTION RESIDUE MANAGEMENT 139 of both unsaturated and saturated groundwater flow based on site-specific data input (see Sidebar 6.2). Below the water table, flow calculations are usually conducted in two or three dimensions. Above the water table, calculations for predicting water flow are more computationally extensive, and therefore are often performed in only one or two dimensions. Porous media groundwater flow models can be successfully applied to some coal spoils, particularly those with finer-grained materials without large conduits and voids. Most commonly applied groundwater flow models, however, are not suitable for application in aquifers with fracture-dominated flow. In fact, EPA stated that it was unable to estimate risks from minefills because its models were "not able to account for conditions such as fractured flow that are typical of the hydrogeology associated with mining operations" (65 FR 32214). Several models are available to simulate flow in fractured rock (see Sidebar 6.2) but the application of such models to mining projects has been limited due to the complexity of the models and the difficulty of obtaining the required input data. NRC (1996a) provides an overview of approaches for modeling flow in fractured media. For model applications, uncertainty in groundwater flow calculations is of- ten related to limited or poor-quality model input data, including inadequate representation of hydraulic conductivity variations, lack of data on water re- charge rates, and lack of information on the rate of water flowing into and out of the model domain. These parameters are often estimated or extrapolated over large areas, resulting in model calculations of limited value. Unsaturated flow models require additional data on local precipitation rates, evaporation and evapo- transpiration rates, and values of hydraulic conductivity as a function of wetting and drying history, which are rarely available at the local scale. Therefore, at most mine sites, model calculations to estimate water flow are generally limited to saturated flow through materials that can be treated as porous media. Methods for Predicting Contaminant Transport Prediction of contaminant release requires integration of physical transport processes and biogeochemical reactions, including oxidation reactions, neutral- ization reactions, and attenuation reactions. Because the number of processes that require integration is large, computer models can be valuable tools for exploring the effect of various CCR placement options on contaminant release over time. Existing Models and Research Needs. A number of modeling tools have been developed to predict contaminant transport (see Sidebar 6.2), and there is poten- tial for applying these tools to predict the rate of contaminant release and trans- port at CCR mine placement sites. In these models, the degree of system com- plexity ranges widely. The most complete models include equations describing the physical transport of oxygen in the unsaturated zone, the flow of water under unsaturated and saturated conditions, and reactions both in the solution phase
140 MANAGING COAL COMBUSTION RESIDUES IN MINES SIDEBAR 6. 2 Modeling Tools for Site Characterization Numerical models are valuable tools for assessing the potential for contami- nant transport from CCR mine disposal sites. Numerical flow and reactive trans- port models can be divided into three broad categories: water flow models, geochemical models, and models that integrate flow and geochemistry. Water flow models can be further divided into those that predict unsaturated water flow, satu- rated water flow, and surface water flow and those that integrate flow between two or more of these domains. Static geochemical models typically are equilibrium- based and include acid-base, oxidation-reduction, aqueous complexation, precip- itation-dissolution, ion exchange, adsorption-desorption, and gas transfer reac- tions. Prediction of contaminant transport requires integration of water flow and geochemical processes. This integration can range from a simple mixing cell ap- proach to fully coupled multicomponent reactive transport formulations with the possible inclusion of equations to account for slow reaction kinetics. A number of computer models are available for integrating water flow, oxygen transport, and a broad range of geochemical reactions to predict the movement of contaminants in the subsurface (e.g., Lichtner et al., 1996; Wunderly et al., 1996; Brown et al., 1998, 2001; Stollenwerk, 1998; Kent et al., 2000; Mayer et al., 2002; Nordstrom, 2003). These models have been applied to a number of mine sites representing a range of climatic settings (e.g., Wunderly et al., 1996; Bain et al., 2000, 2001; Banwart and Malmström, 2001; Mayer et al., 2002, 2003; Garvie et al., 2003; Nordstrom, 2003; Jurjovec et al., 2004; Molson et al., 2005). For exam- ple, reactive solute transport models have been applied to assess the longevity of solute release from coal mines (Gerke et al., 1998; Banwart and Malmström, 2001) and to evaluate closure scenarios (Garvie et al., 2003). In general, the quality of the predictions is related to the quality and amount of data used for the calculations (e.g., complexation) and between the solution and the solid phase (e.g., adsorp- tion, cation exchange, dissolution). Reaction kinetics can be included to represent variations in mineral dissolution and precipitation rates, reaction catalysis by microorganisms, and other reaction-limiting processes. The limitations in applying these models are primarily the acquisition of suffi- cient data and the commitment of sufficient resources to model development and testing for the specific application of CCR placement in coal mine sites. Successful application of these models requires detailed information on the geologic media through which the solutes are transported (e.g., moisture content as a function of wetting and drying history, grain size, solid-phase mineralogy, composition of mineral coatings, surface area, presence of organic matter). For the majority of coal mine sites, these data are not available, limiting the application of reactive solute transport models. Application of these models to CCR sites would require addi- tional data (e.g., moisture content relationships for CCR as a function of wetting
COAL COMBUSTION RESIDUE MANAGEMENT 141 and the complexity of processes included. These models have not been applied to coal mine sites containing CCRs. Model Categories:a Saturated Water Flow Through Porous Media (not suitable for fractured media): e.g., MODFLOW Unsaturated Water Flow Through Porous Media; e.g., SEEPW, HYDRAS, HYDRAS2D Saturated Water Flow Through Fractured Media: e.g., FRAC3DVS, FRACTRAN, NETFLO, SWIFT-98, TRAFRAP-WT Unsaturated Water Flow Through Fractured Media: e.g., FRAC3DVS Surface Water Flow: e.g., HEC-RAS Geochemical Equilibrium Models: e.g., MINTEQA2, PHREEQC, GEOCHEMIST'S WORKBENCH Sulfide Oxidation Models: e.g., PYROX, SULFIDOX Reactive Solute Transport Models: e.g., PHREEQC, MINTRAN, FLOTRANS, HYDROGEOCHEM Reactive Solute Transport Models Incorporating Sulfide Oxidation Reactions: MINTOX, MIN3P, MULTIFLOW aModels listed are examples and include those that fall into the public domain, proprietary codes, and those developed for research applications. and drying history, reactions controlling the interactions between CCR leachate and coal spoil materials) and some research contributing to model development to answer questions such as: What geochemical reactions control CCR leaching? What is the influence of CCR leachate on microbial activity? There have been only limited applications of these reactive transport models to coal spoil sites and no application at coal sites with CCR placement. Therefore, research is needed to apply the models to real field sites and to evaluate whether the transport and reaction processes included in the model adequately describe the processes taking place at CCR mine disposal sites, including those that occur over protracted time scales. Additional research may be needed to modify the models accordingly. Therefore, research is necessary before current reactive trans- port models can be used to make meaningful long-term predictions of the poten- tial environmental impacts from CCR disposal in mines. Even after these models are tested and become widely available, their use may still be limited except at
142 MANAGING COAL COMBUSTION RESIDUES IN MINES the largest CCR disposal sites due to the input data requirements and the need for skilled specialists to run the models. Interim Suggestions for Using Predictive Modeling Tools. Simpler models exist today that can be used to increase understanding of contaminant transport at CCR sites. The application of such models in concert with a robust field sampling campaign can allow for reasonable estimates (although still with significant un- certainty) of potential CCR-related contaminant movement. Steps can be taken to improve modeling efforts that will lead to enhanced predictions of contaminant transport from mine sites and improved placement of groundwater monitoring wells. These steps include (1) improving the quality of model input data, (2) focusing first on understanding conservative contaminant transport, (3) incorpo- rating unsaturated zone flux, and (4) conducting a "post-audit" to evaluate the success of the modeling. Improving the quality of model input data is one of the best ways to increase the usefulness of groundwater flow and contaminant transport modeling at CCR mine disposal sites. For example, saturated flow models are routinely used to assess groundwater flow directions based on very limited data--often water-level and hydraulic conductivity data from only a few wells. Increasing the amount of input data and including data on the hydraulic conductivity of CCRs should significantly improve groundwater flow predictions. Numerical models can also be used to examine the sensitivity of simulated outcomes to uncertainty in the input data and can potentially identify the most critical site characterization data needs to improve predictions of contaminant transport. A well-developed site conceptual model that is supported by site characterization data should form the basis of the numerical model. For example, understanding the role of fracture- driven flow is essential to determine whether a porous media model can be used to make meaningful predictions. Predicting contaminant transport rates and di- rections in fractured media requires extensive additional site characterization and a more involved modeling effort, and even with such an intense effort, the simu- lations are likely to contain sizable uncertainties. Site managers could derive significant benefit from numerical modeling, even if it focuses only on conservative transport of contaminants. These more simplistic models can be used to estimate the directions of flow and the time of travel, which is essential information for siting monitoring wells that can detect potential contamination within an early time frame. Such models can also explore the effects of various CCR emplacement scenarios on groundwater flow. Depending on the site characteristics and the CCR characteristics, significant leaching may occur at sites where CCR is placed above the water table. There- fore, models should incorporate, at a minimum, a contaminant flux term from the unsaturated zone, considering estimated recharge rates for the site. Collection of subsurface flow and water quality data close to the CCR em- placement area after disposal provides valuable information for testing model
COAL COMBUSTION RESIDUE MANAGEMENT 143 predictions. A number of soluble constituents in CCRs, such as borate, are good indicators of conservative transport, and these constituents can be used to assess whether subsurface flow calculations are reasonable. Valuable lessons can be learned through these post-audits to improve the simulation at the site and to provide guidance for future modeling at CCR placement sites (Konikow, 1986). Such a review could also confirm that monitoring wells were placed in meaning- ful locations. CCR CHARACTERIZATION CCRs vary greatly in chemical and mineralogical composition. Trace ele- ments can be tightly bound within glasses and residual minerals in CCRs, or they can occur as easily leachable coatings on grain surfaces. Some, but not all, CCRs contain large quantities of alkaline materials (see Chapter 2). To understand the potential risks involved in placing significant volumes of a particular CCR in the mine setting, careful CCR characterization is needed. Characterization of CCRs is an essential component in the development of a site conceptual model that will help site managers and regulators make manage- ment decisions regarding CCR placement at mine sites. Characterization of CCRs prior to mine placement may involve analyses of bulk chemical and physical properties and trace element leaching potential. The results of these characteriza- tion tests should be used in conjunction with an assessment of the mine hydro- geology and biogeochemistry to provide an evaluation of the potential for benefi- cial and/or deleterious impacts from CCR placement at a mine site. Many states rely on CCR characterization tests--primarily leaching tests-- to determine whether CCRs are suitable for mine placement (see Sidebar 6.3 and Table 6.1). Currently, characterization methods required to determine the safety of CCR placement are relatively simple tests compared to those that have been developed for research purposes. This section describes common and alternative CCR characterization methods, including their limitations and advantages. Bulk Chemical and Physical CCR Analyses Leaching of contaminants from CCRs is dependent on a range of physical and chemical properties of the CCR, including bulk properties and microscale properties. Characterization of bulk properties provides information that can be used to assess the stability of CCRs in the mine setting. Bulk physical properties include the grain size distribution and surface area available for reaction, permeability upon compaction, and whether or not the CCR grains are cemented together. In general, fine-grained materials will have a higher surface area, increasing the opportunity for reaction. Highly cemented materials will limit ingress of water into the CCR, thereby limiting reaction. Laboratory testing can be conducted to assess the cementitious properties of
144 MANAGING COAL COMBUSTION RESIDUES IN MINES SIDEBAR 6.3 Uses of Leaching Tests at the State Level States use a range of methods to characterize CCRs and thereby evaluate the appropriateness of CCR mine placement. Most states require the Toxicity Charac- teristic Leaching Procedure (TCLP), although some states use alternate leaching tests such as the Synthetic Precipitation Leaching Procedure (SPLP) or the Amer- ican Society for Testing and Materials (ASTM) D-3987 (see Table 6.2) (USEPA, 2002c), and other states may use multiple tests. Likewise, states differ in the stan- dards they use to evaluate the results of leaching tests and to classify the wastes. Generally, the leaching test data are evaluated against either (1) Resource Conser- vation and Recovery Act (RCRA) standards for "characteristic" hazardous wastes based on the concentrations of toxic metals in the leachate or (2) an end-use water quality standard (e.g., drinking water maximum contaminant levels [MCLs]) multi- plied by a factor that takes into account dilution and attenuation processes likely to occur in the environment. For example, Ohio uses a leaching test limit of 30 times the drinking water standards for specific metals, while Illinois uses class I ground- water standards as its leaching test limits (Table 6.1). It should be noted that these standards for classifying or categorizing materials do not consider site-specific conditions and are applied across an entire state. Table 6.1 presents some exam- ples of the variability of leaching test limits across several states and includes the RCRA toxicity limits, MCLs, and secondary MCLs for the purpose of comparison. CCRs and whether any additions to CCR such as lime can promote greater cementation. Bulk chemical properties include the total metal content of the CCRs, the residual sulfide content, and the content of alkali or acid in the solid. The bulk chemical composition of CCRs provides an indication of the maximum concen- trations of major and trace elements that can be leached from them. Materials with unacceptably high total concentrations of highly toxic elements such as mercury can be segregated and handled with greater caution. Analyses of bulk metal content also provide a basis for comparing the results of specific leach tests to assess the relative fraction of metals that can be leached under different condi- tions that might occur at the mine site. Analyses of residual sulfide content and form will influence the potential acid that might be generated as the CCR weath- ers. Similarly, determinations of acid neutralization potential are necessary to evaluate whether particular CCRs can moderate the AMD generated at the mine site. Acid-base accounting tests (discussed previously in this chapter) are applied routinely at mine sites and can be applied in a similar manner to assess the neutralization potential of CCR materials relative to the acid generation at coal sites (e.g., Kania, 1998; Perry, 1998).
COAL COMBUSTION RESIDUE MANAGEMENT 145 Leaching Tests Leaching tests assess the potential release of trace elements from CCRs and are commonly applied approaches to CCR characterization (see Sidebar 6.3). Nevertheless, the effectiveness of these testing procedures for predicting CCR behavior in the mine environment has not been thoroughly evaluated. Numerous leach tests are available commercially or in the research community. Kim (2002b) provides a summary of more than 100 leaching protocols, including single-point static batch tests, multipoint serial or sequential batch experiments, and column leaching methods. The concentrations of trace elements that leach from CCRs depends on the initial composition of the CCRs, the composition of the leaching solution, and the rates of water flow. The advantages and limitations of these leaching approaches are discussed below. Single-Point Batch Leaching Tests The simplest leaching tests are static batch methods in which a CCR sample is placed in a set volume of leaching solution and the mixture is agitated for a fixed time. A leachate sample is then collected and analyzed, providing water chemistry data for this single sampling point. Ideally, these tests would represent post-placement conditions; and a wide variety of single-point batch leaching tests with different leaching solutions, contact times, and solid-to-solution ratios are available. Leaching solutions can range from solutions of very low pH (e.g., < 2), as observed in high-sulfur coal fields, to highly basic solutions, similar to those associated with alkaline ash. Some examples of commonly used single-point leach tests are presented in Table 6.2. The Toxicity Characteristic Leaching Protocol (TCLP), developed by the EPA (USEPA, 1994), is the most widely used leaching procedure to evaluate leaching of CCRs for placement in mines. The TCLP was originally developed to provide a standardized method for assessing the potential for leaching of contaminants from solid wastes disposed in a municipal solid waste (MSW) landfill and acetic acid solution was selected as the leaching solution to simulate the composition of pore waters present in MSW landfills during early stages of operation. The TCLP is specifically required in the CCR permitting process by 8 out of 23 states examined by EPA (USEPA, 2002c) and many commercial laboratories have the capability to perform this test. Other states require different single-point leaching tests. For example, New Mexico requires an 18-hour distilled water test (American Society for Testing and Materials [ASTM]-3987), while Pennsylvania requires the Syn- thetic Precipitation Leachate Procedure (SPLP; see Table 6.2). The reliance on single-point batch leaching procedures, such as the TCLP, for prediction of CCR stability in mine settings has been widely criticized be- cause (1) the composition of the initial leaching solution may not be representa- tive of the range of leaching conditions encountered in the field; (2) the character-
146 MANAGING COAL COMBUSTION RESIDUES IN MINES TABLE 6.1 CCR Leaching Test Methods and Standards for Evaluating Test Results Across Several Maximum Acceptable CCR Leachate Concentrations (mg/L) West Virginia Ohio Pennsylvainia Illinois Test TCLP TCLP SPLP ASTM Method: D-3987-85 Al 0005.0 Sb 001 0000.15 0.006 As 005 00.30 0001.25 0.05 Ba 100 60 0050 2 Be 000.007 0.004 B 0031.50 2 Cd 001 00.15 0000.13 0.005 Co 1 Cr 005 03 0002.5 0.1 Cu 0032.5 0.65 Fe 0007.5 5 Pb 005 00.45 0001 25 0.0075 Mn 0001.25 0.15 Hg 000.2 00.06 0000.05 0.002 Mo 0004.38 Ni 070 0002.5 0.1 Se 001 01.5 0001.00 0.05 Ag 005 0.05 Tl 007 0.002 Zn 0,125 5 SO4 2,500 Cl 2,500 NOTE: MCL = maximum contaminant level; RCRA = Resource Conservation and Recovery Act aLead and copper are regulated by a treatment technique. If more than 10% of tap water samples exceed the action level, water sys- tems must take additional steps. istics of the final leaching solutions are not usually controlled or even monitored in the tests and may differ markedly from those of the initial leaching solution; and (3) secondary precipitates may form due to solubility limitations in the batch experiments (USEPA SAB, 1991, 1999). The TCLP has also been criticized for its short extraction time, which might overlook the potential for slow release of constituents, and for the lack of field validation of the test (USEPA SAB, 1991). As a consequence of these limitations, leaching may either be under- or overesti- mated by single-point tests leading to inappropriate decisions regarding place- ment of CCRs in mines (Vories, 2002). To address these concerns, alternative
COAL COMBUSTION RESIDUE MANAGEMENT 147 States Compared to RCRA Toxicity Limits, MCLs, and Secondary Drinking Water Standards RCRA Toxicity Drinking Water Secondary Drinking Limits (mg/L) MCLs (mg/L) Water Standards (mg/L) 000.05 to 0. 2 0.006 005.0 0.01 100 2 0.004 Under review 001.0 0.0050 005.0 0.1 1.3a 001.0 000.3 005.0 0.015a 000.05 000.2 0.002 001.0 0.05 005.0 000.1 0 002 005 250 250 SOURCES: Ziemkiewicz and Skousen, 2000; USEPA, 2004a; IL Title 35 Subtitle F Chapter 1 Section 620.420; 40 CFR §261.24. leaching procedures are being examined in hopes of finding a test that more accurately represents the potential for leaching hazardous substances from CCRs. Serial, Sequential, and Multipoint Batch Leaching Tests To improve predictions of long-term leaching and address concerns about potential solubility limitations and inappropriate leaching solutions, serial and sequential batch leaching procedures have been developed (Kim, 2002b). For example, a serial batch procedure has been developed to simulate leaching ex-
148 (hr) 1440 to Time 18 24 18 18 48 Up Solid Liquid/ Ratio 20 20 20 20 10 20 pH Initial Leachant 5.0 4.2 2.88 5.0 (g) Size 0 Sample 100 100 100 50 70 acid Tests buffer sulfuric groundwater acid acid, oracetate Leaching citrate acid acetic nitric synthetic Batch O7 O, O, O, 2 2 2 2 Leachant H H H Acetic Sodium H Static of WET) (EPTOX) Extraction D3987) Procedure Procedure (CA Procedure Single-Point Test of Test Shake (ASTM for Leaching Leaching Leaching Toxicity Water Extraction Examples 2004. Method with 6.2 Procedure Test Precipitation Waste Characteristic Groundwater Kim, Waste TABLE Method Standard Solid Extraction Synthetic (SPLP) Toxicity (TCLP) California Synthetic (SGLP) SOURCE:
COAL COMBUSTION RESIDUE MANAGEMENT 149 pected to be encountered in weathered coal spoils and workings. This mine water leaching procedure uses simulated AMD or actual mine water as the leaching solution and replenishes this solution until a pH of 3 is reached in the leachate (Ziemkewicz et al., 2003). Other serial batch methods include ASTM D5284 and EPA's multiple extraction procedure. Test methods have also been developed to evaluate leaching that could po- tentially occur under a range of geochemical conditions that might be encoun- tered at a site. For example, sequential batch experiments have been designed that utilize multiple leaching solutions of different compositions in a prescribed se- quence (e.g., Tessier et al., 1979; Palmer et al., 1999). Similarly, multipoint procedures have been developed to evaluate leaching under a range of geochemi- cal conditions or solid-to-solution ratios (e.g., Kosson et al., 2002). The advan- tage of these procedures is that the results obtained are theoretically more repre- sentative of the wide range in composition of leaching fluids that can be observed in the field, and test results can be obtained in a relatively short time (days to months). A multipoint leaching procedure that varies solid-to-solution ratios and leach- ing times and covers a large range in leachate composition is described by Kosson et al. (2002). This proposed leaching framework also includes a combination of batch and column leaching methods. By combining laboratory leaching data, field data, and advanced geochemical modeling within the proposed leaching framework, a conceptual model can be developed that can be applied to evaluate leaching under the range of geochemical conditions expected to be encountered in the field. Limitations of this approach include higher skills required to perform the test and subsequent model calculations, therefore leading to higher costs. The above methods range greatly in cost and the final selection of a method may require a trade-off between characterizing a few samples in great detail or a larger number of samples in less detail. It should also be noted that the above methods do not specifically address the potential for leaching under the suboxic conditions that often prevail beneath the water table in coal spoils. Performing leaching tests under conditions that mimic suboxic field conditions requires high technical skills and specific equipment, which would add further to testing costs. Column Leaching Tests Column leaching tests evaluate contaminant leaching under continuous flow conditions. The advantage of column leaching tests is that reaction products are flushed from the column and are not allowed to build up to concentrations that may artificially lead to precipitation. Column experiments using fine-grained CCRs require slow flow rates and operation for long periods of time, on the order of months or more. Thus, column leaching experiments are more costly than batch tests. Chapter 3 discusses two column leaching studies (Stewart et al., 2001; Kim
150 MANAGING COAL COMBUSTION RESIDUES IN MINES et al., 2003) that demonstrated the potential for fly ash to exhibit notably different leaching behavior under acidic, neutral, or alkaline pH. These studies suggest that careful CCR characterization using representative leaching solutions is required to predict adequately the potential for leaching at mine sites. Field-Leaching Tests A number of CCR characterization tests can be carried out at the field scale; these range from relatively small test plots to much larger instrumented pilot test areas. Test plots have been used to assess the leaching of elements from co- blended CCR and coal spoils (e.g., Stewart and Daniels, 1995). In these types of studies, CCRs are emplaced and leaching is monitored by collecting water samples in pan lysimeters. These test plot characterization approaches are more likely to approximate the behavior of CCRs in real-world settings, although one limitation of these tests is that they are usually carried out very close to the ground surface where contact with atmospheric oxygen is highest. Results ob- tained from the tests, therefore, may not be representative of leaching conditions that occur in deeper zones where suboxic conditions might prevail. Recent research has compared laboratory leaching tests with field behavior at CCR disposal sites (Ladwig et al., 2006). Field leachate from sluiced ash was compared to the results from two laboratory leaching protocols--the SPLP and a synthetic sluicing procedure. The agreement between the field data and the leach- ing test protocols was variable. The laboratory results generally ranged about one order of magnitude both above and below the field-collected data, depending on the trace element of interest, although some trace element concentrations showed variations of more than two orders of magnitude between the laboratory and field leaching data (Ladwig et al., 2006). These results suggest that improvements in laboratory leaching protocols are necessary if they are to be considered represen- tative of CCR behavior in the field. Research Needs Currently there is a lack of detailed assessment of the applicability of labora- tory test methods to predict field behavior of CCRs emplaced in mines. There has been little evaluation of whether leaching results obtained using small-scale labo- ratory batch and column tests correlate well with results obtained in field test cells and from field leaching monitoring at full-scale emplacement sites. Re- search is needed to continually improve and field-validate leaching tests that can be used to better predict the mobilization of CCR-derived constituents in mine settings. Two approaches can be taken to fill this knowledge gap. The first is to establish a carefully planned research program designed specifically to fully characterize the leaching characteristics of CCRs at the laboratory scale and
COAL COMBUSTION RESIDUE MANAGEMENT 151 compare the results to field-leaching test plots and to highly instrumented full- scale field sites. This characterization research should utilize fully coupled reac- tive solute transport models that incorporate the major physical and biogeochemi- cal processes controlling leaching behavior. This approach is currently being taken at metal mine sites to help develop meaningful laboratory tests. The major limitation of this approach is the long time required to obtain results. The second approach for addressing these knowledge gaps is to conduct several post-CCR-placement studies at coal mine sites, such as the work under way at the Universal Mine site in Indiana (Murarka et al., 2002). Detailed moni- toring systems could be installed to evaluate groundwater and leachate water quality. If sufficient site characterization data are collected, advanced numerical models can be used to integrate these data. The results could then be compared with leaching tests performed at the time of emplacement and with more detailed tests performed on original archived samples of CCR, if available. Such post- audit-type studies provide opportunities to explore many geochemical processes that occur at CCR placement sites over time, such as acid generation and neutral- ization, oxidation-reduction, precipitation and dissolution, adsorption and des- orption, and the potential for biological catalysis of these reactions. Interim Suggestions for CCR Characterization To contribute to the evaluation of the risk of CCR mine placement, CCRs should be characterized prior to significant mine placement and with each new source of CCRs. CCR characterization should continue periodically throughout the mine placement process to assess any changes in CCR composition and behavior. Current characterization practice relies heavily on laboratory leaching tests, in particular the TCLP, to evaluate the potential hazards of CCR placement in mines. These tests do not use leaching solutions that are representative of the large range of geochemical conditions likely to be encountered in mines, and they may greatly underestimate the actual leaching that will occur. It is recommended that leaching procedures be continually improved to encompass the range of pH and oxidation-reduction conditions that might be encountered in pore-water close to the CCR placement area over an extended time (many decades to centuries). Leaching tests should also assess slower dissolution reactions. Until some recently proposed leaching protocols are evaluated more thor- oughly, some simple improvements to currently applied leaching protocols can be made. As a first step, a wider range of leaching conditions should be applied in static leach tests. These leaching conditions should include low-pH leaching solutions to represent the aggressive leaching that may occur in the most reactive areas of the unsaturated zone. The composition of the leaching solution should be monitored both before and after leaching is complete to ensure that the final leaching solution is representative of expected conditions at the mine site. Leach-
152 MANAGING COAL COMBUSTION RESIDUES IN MINES ing tests should be conducted over longer periods (e.g., several weeks) and a few solid-to-solution ratios should be evaluated to assess whether precipitation con- trols are limiting leaching characteristics. Samples that do not pass a predeter- mined criterion should be rejected for mine placement. Samples that do pass the criterion may still have to be evaluated in greater detail, depending on the poten- tial risks of CCR placement determined from site characterization, including column leaching tests and longer-term evaluations of leaching as CCR materials age. INTEGRATION OF CCR AND SITE CHARACTERIZATION DATA Current site characterization is usually conducted independently of CCR characterization. In practice, site characterization and CCR characterization should be carried out in an integrated fashion to provide the information needed to develop a site conceptual model that adequately informs CCR management decision making in a way that is protective of the environment. For example, site characterization data are needed to inform the design of relevant leaching tests, by providing the range of geochemical conditions that might be encountered over long periods of time (decades to centuries) at the mine site. Likewise, an under- standing of the total mass and leachability of the contaminants in CCRs to be disposed at a mine site is needed to evaluate the potential for attenuation through reaction with geological materials. Given the complex hydrology and geochemis- try of mine sites, the site conceptual model should be reevaluated as additional site data are obtained (at least annually during active placement). SUMMARY To ensure effective CCR management at mines, thorough CCR characteriza- tion and hydrogeologic and biogeochemical characterizations of the mine site are needed. Characterization is an essential part of the CCR management process and serves to guide engineering design, risk-informed permitting decisions, reclama- tion management, and the development of effective monitoring programs. Char- acterization is also one means by which managers can address uncertainties. The components of an effective characterization program have been detailed in this chapter. Site characterization is a dynamic process of developing and continually refining a site conceptual model that captures the relevant aspects affecting the behavior of CCRs in the mine environment. Current site characterization require- ments typically are focused on assessing potential impacts from coal mining and do not specifically consider the impacts of CCR placement. The committee recommends comprehensive site characterization specific to CCR placement at all mine sites prior to substantial placement of CCRs. The mine site hydrogeology and biogeochemical environment should be defined in both undis-
COAL COMBUSTION RESIDUE MANAGEMENT 153 turbed areas and preexisting disturbed areas, and the site's proximity to sensitive receptors should be determined. Due to the variability among mine sites, it is difficult to prescribe the precise site characterization data collection steps to follow prior to CCR placement. However, specific categories of site characteriza- tion information relevant to CCR placement are detailed in this chapter. To contribute to the evaluation of risk of placing CCRs at mine sites, the committee recommends characterization of CCRs prior to significant mine placement and with each new source of CCRs. CCR characterization should continue periodically throughout the mine placement process to assess any changes in CCR composition and behavior. Characterization of CCR materials prior to mine placement may involve analyses of bulk chemical and physical properties and trace element leaching potential. Leaching tests are commonly applied to assess the potential release of trace elements from CCRs, and this chapter has discussed advantages and limitations of general classes of leaching protocols. The limitations of single-point batch tests are well recognized (e.g., solubility limitations, inappropriate leaching solu- tions), although these tests remain in widespread use and have a major role in the regulation of CCR mine placement in many states. The committee concludes that information on the applicability of laboratory leaching test methods to predict CCR leaching behavior in the field is lacking. Therefore, the committee recom- mends additional research to continually improve and field-validate leach- ing tests to better predict the mobilization of constituents from CCRs in mine settings. Specifically, post-placement field studies could be conducted that would allow the comparison of leaching test results against detailed water quality monitoring. Some alternative leaching tests are being developed to address these concerns, but until these proposed leaching protocols are evaluated more thor- oughly, the committee recommends some simple improvements to currently ap- plied leaching protocols. In particular, the CCR characterization methods used should provide contaminant leaching information for the range of geochemical conditions that will occur at the CCR placement site and in the surrounding area, both during and after placement. Those samples that do not pass a pre-determined criterion should be rejected for mine placement, although those samples that do pass may still need to be evaluated in greater detail, depending on the potential risks of CCR placement determined from the site characterization. Site characterization and CCR characterization data should be thoroughly integrated into a site conceptual model, perhaps supplemented by numerical mod- eling tools, to predict contaminant transport potential and assess the potential impacts of CCR disposal at a mine site. Computer models are valuable tools for integrating physical transport processes and biogeochemical reactions. However, the committee recommends additional research to apply existing reactive transport models to real field sites and to evaluate whether the transport and reaction processes included in the model adequately describe the processes taking place at CCR mine disposal sites, including those processes that occur
154 MANAGING COAL COMBUSTION RESIDUES IN MINES over protracted time scales. In the interim, several steps are identified that can improve modeling efforts that will lead to enhanced predictions of contaminant transport from mine sites, including (1) improving the quality of model input data, (2) focusing first on understanding conservative contaminant transport, (3) incorporating unsaturated zone flux, and (4) conducting a post-audit to evaluate the success of the modeling against monitoring data.