Managing Coal Combustion Residues in Mines (2006) / Chapter Skim
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6 Characterization for CCR Management
6 Characterization for CCR Management
Pages 127-154
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From page 127... ...
Therefore, decisions regarding CCR placement cannot be made based on broad generalizations but instead require 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)
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From page 128... ...
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)
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From page 129... ...
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.
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From page 130... ...
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.
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From page 131... ...
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
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From page 132... ...
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 carefully 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.
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From page 133... ...
Because of the variability among mine sites, it is difficult to prescribe the precise site characterization data collection steps to follow prior to CCR placement. However, three broad categories of information are essential components of the site conceptual model: the hydrogeologic setting, the biogeochemical environment, and the proximity to sensitive receptors.
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From page 134... ...
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.
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From page 135... ...
Groundwater monitoring is discussed in greater detail in Chapter 7. Surface Water Flow.
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From page 136... ...
. Improved pre-placement characterization of water quality across mine sites will help remedy this problem.
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From page 137... ...
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)
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From page 138... ...
Nevertheless, predictive 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.
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From page 139... ...
, and there is potential for applying these tools to predict the rate of contaminant release and transport at CCR mine placement sites. In these models, the degree of system complexity ranges widely.
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From page 140... ...
. For the majority of coal mine sites, these data are not available, limiting the application of reactive solute transport models.
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From page 141... ...
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.
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From page 142... ...
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.
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From page 143... ...
Characterization of CCRs is an essential component in the development of a site conceptual model that will help site managers and regulators make management 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.
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From page 144... ...
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)
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From page 145... ...
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.
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From page 146... ...
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)
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From page 147... ...
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)
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From page 148... ...
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)
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From page 149... ...
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.
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From page 150... ...
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.
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From page 151... ...
Such postaudit-type studies provide opportunities to explore many geochemical processes that occur at CCR placement sites over time, such as acid generation and neutralization, oxidation-reduction, precipitation and dissolution, adsorption and desorption, 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.
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From page 152... ...
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.
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From page 153... ...
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 modeling tools, to predict contaminant transport potential and assess the potential impacts of CCR disposal at a mine site.
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From page 154... ...
improving the quality of model input data, (2) focusing first on understanding conservative contaminant transport, (3)
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