EPA-600/R-08/077

 July 2008

Characterization of

Coal Combustion 

Residues from Electric 

Utilities Using Wet 

Scrubbers for Multi-Pollutant

Control
EPA/600/R-08/077 

July 2008 

Characterization of Coal Combustion Residues from 

Electric Utilities Using Wet Scrubbers for 

Multi-Pollutant Control 

F. Sanchez1

, D. Kosson1, 

R. Keeney2

, R. Delapp1, L. Turner3, and P. Kariher2 

1

Vanderbilt University 

Department of Civil and Environmental Engineering 

Nashville, TN 37235

2

ARCADIS 

4915 Prospectus Drive, Suite F 

Durham, NC 27713 

3

Turner Technology 

Nashville, TN 37235 

Category III / Applied Research 

Contract No. EP-C-04-023 

Work Assignment No. 4-26 

Project No. RN990234.0026 

U.S. Environmental Protection Agency 

Office of Research and Development 

National Risk Management Research Laboratory 

Air Pollution Prevention and Control Division 

Cincinnati, Ohio
Notice 

The U.S. Environmental Protection Agency (EPA) through its Office of Research and Development 

funded the research described here under Work Assignment No. 4-26 of Contract Number EP-C-04-023 

to Vanderbilt University. It has been subjected to the Agency’s peer and administrative review and has been 

cleared for publicationas an EPA document. Reference herein to any specific commercial product, process, 

or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its 

endorsement or recommendation. The views and opinions of authors expressed herein do not necessarily state 

or reflect those of EPA or any agency thereof. 

ii 
Foreword 

The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting the Nation's 

land, air, and water resources. Under a mandate of national environmental laws, the Agency strives to 

formulate and implement actions leading to a compatible balance between human activities and the ability 

of natural systems to support and nurture life. To meet this mandate, EPA's research program is 

providing data and technical support for solving environmental problems today and building a science 

knowledge base necessary to manage our ecological resources wisely, understand how pollutants affect 

our health, and prevent or reduce environmental risks in the future. 

The National Risk Management Research Laboratory (NRMRL) is the Agency's center for investigation 

of technological and management approaches for preventing and reducing risks from pollution that 

threaten human health and the environment. The focus of the Laboratory's research program is on 

methods and their cost-effectiveness for prevention and control of pollution to air, land, water, and 

subsurface resources; protection of water quality in public water systems; remediation of contaminated 

sites, sediments and ground water; prevention and control of indoor air pollution; and restoration of 

ecosystems. NRMRL collaborates with both public and private sector partners to foster technologies that 

reduce the cost of compliance and to anticipate emerging problems. NRMRL's research provides 

solutions to environmental problems by: developing and promoting technologies that protect and improve 

the environment; advancing scientific and engineering information to support regulatory and policy 

decisions; and providing the technical support and information transfer to ensure implementation of 

environmental regulations and strategies at the national, state, and community levels. 

This publication has been produced as part of the Laboratory's strategic long-term research plan. It is 

published and made available by EPA's Office of Research and Development to assist the user 

community and to link researchers with their clients. 

Sally Gutierrez, Director 

 National Risk Management Research Laboratory 

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Characterization of Coal Combustion Residues II 

ACKNOWLEDGMENTS 

Extensive input on the research program design was provided by G. Helms, U.S. EPA, Office of 

Solid Waste (Washington, D.C.). 

Laboratory testing described herein was carried out by ARCADIS with technical support from 

Vanderbilt University. R. Delapp was responsible for the chemical analyses carried out at 

Vanderbilt University. Technical assistance also was provided by A. Garrabrants. 

K. Ladwig and the Electric Power Research Institute (EPRI) are gratefully acknowledged for 

assistance in obtaining coal combustion residue samples and providing information from the 

EPRI database on coal combustion residues. 

S. Thorneloe is the U.S. EPA project officer for this research. 

Note: R. Keeney substantially participated in the work reported here and the preparation of this 

report but left employment of ARCADIS prior to the completion of this study and report. 

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Characterization of Coal Combustion Residues II 

TABLE OF CONTENTS 

Acknowledgments.......................................................................................................................... iv

Abstract........................................................................................................................................... v

Glossary of terms ........................................................................................................................... vi

Executive Summary....................................................................................................................... ix

Table of Contents.......................................................................................................................... xii

List of Tables ................................................................................................................................ xv

List of Figures.............................................................................................................................. xvi

1. Introduction................................................................................................................................. 1

1.1. Regulatory Context ...............................................................................................................6

1.1.1. Waste Management....................................................................................................... 6

1.1.2. Air Pollution Control .................................................................................................... 6

1.2. Configurations of U.S. Coal Fired Power Plants and Multi-pollutant Control 

Technologies................................................................................................................................7

1.2.1. Current Air Pollution Control Technologies................................................................. 8

1.2.2. Wet Scrubbers, NOx Controls and Multi-pollutant Controls ..................................... 11

1.3. Coal Combustion Residues.................................................................................................12

1.4. Residue Management Practices ..........................................................................................13

1.4.1. Beneficial Use............................................................................................................. 13

1.4.2. Land Disposal ............................................................................................................. 14

1.5. Leaching Protocol ...............................................................................................................16

2. Materials and Methods.............................................................................................................. 20

2.1. CCR Materials for Evaluation ............................................................................................20

2.1.1. Facilities Using Inhibited or Natural Oxidation of Scrubber Residues (Producing 

Scrubber Sludge or Fixated Scrubber Sludge)...................................................................... 24

2.1.1.1. Facility A (Natural Oxidation and SNCR)........................................................... 24

2.1.1.2. Facility B (Natural Oxidation and SCR).............................................................. 24

2.1.1.3. Facility K (Natural Oxidation and SCR) ............................................................. 25

2.1.1.4. Facility M (Inhibited Oxidation and SCR) .......................................................... 25

2.1.2. Facilities Using Forced Oxidation of Scrubber Residues (Producing FGD Gypsum) 26

2.1.2.1. Facility N (Forced Oxidation).............................................................................. 26

2.1.2.2. Facility O (Forced Oxidation and SCR) .............................................................. 26

2.1.2.3. Facility P (Forced Oxidation and SCR and SNCR)............................................. 26

2.1.2.4. Facility Q (Forced Oxidation and SCR) .............................................................. 27

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Characterization of Coal Cumbustion Residues II 

2.2. Leaching Assessment Protocols..........................................................................................27

2.2.1. Alkalinity, Solubility and Release as a Function of pH (SR002.1) ............................ 27

2.2.2. Solubility and Release as a Function of LS Ratio (SR003.1)..................................... 28

2.3. Analytical Methods.............................................................................................................28

2.3.1. Surface Area and Pore Size Distribution .................................................................... 28

2.3.2. pH and Conductivity................................................................................................... 28

2.3.3. Moisture Content ........................................................................................................ 28

2.3.4. Carbon Content Organic Carbon/Elemental Carbon Analyzer................................... 28

2.3.5. Dissolved Inorganic Carbon (DIC) and Dissolved Organic Carbon (DOC) .............. 29

2.3.6. Mercury (CVAA, Method 3052, and Method 7473) .................................................. 29

2.3.7. Other Metals (ICP-MS, Method 3052, and Method 6020)......................................... 30

2.3.7.1. ICP-MS Analysis ................................................................................................. 30

2.3.8. X-Ray Fluorescence (XRF) ........................................................................................ 32

2.3.9. MDL and ML for Analytical Results......................................................................... 32

2.4. Quality Assurance ASSESSMENT ....................................................................................33

2.4.1. Homogenization of Individual CCR Samples and Aliquots for Analyses.................. 33

2.4.2. Leaching Test Methods and Analytical QA/QC......................................................... 33

2.4.3. Improving QA/QC efficiency ..................................................................................... 34

2.5. Interpretation and Presentation of Laboratory Leaching Data............................................35

2.5.1. Interpretation of Mechanisms Controlling Constituent Leaching .............................. 36

2.6. Field pH probability distribution ........................................................................................38

2.7. Estimated leachate concentration as a function of pH........................................................41

3. Results and Discussion ............................................................................................................. 43

3.1. Total elemental content by digestion ..................................................................................43

3.2. total elemental content by xrf .............................................................................................51

3.3. Laboratory Leaching Test Results ......................................................................................57

3.3.1. Typical Characteristic Leaching Behavior as a Function of pH and LS..................... 58

3.3.2. Comparisons of the Ranges of Constituent Concentrations from Laboratory Testing 

with Measurements of Field Samples and the EPA Risk Report Database.......................... 77

4. Summary of Results, Conclusions and Recommendations....................................................... 87

5. References................................................................................................................................. 94

Appendices.................................................................................................................................... 97

A. Quality Assurance Project Plan ..................................................................................... A-1 

B. Total Elemental Content by Digestion.............................................................................B-1 

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Characterization of Coal Combustion Residues II 

C. Elemental Total Content (by XRF), Carbon, Loss on Ignition and 

Specific Surface Area ......................................................................................................C-1 

D. pH Titration and Constituent Leaching as a Function of pH (SR002 test results) ......... D-1 

E. pH and Constituent Leaching as a Function of LS (SR003 test results)..........................E-1 

F. Curve Fits.........................................................................................................................F-1 

G. Additional Facility Information ..................................................................................... G-1 

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Characterization of Coal Cumbustion Residues II 

LIST OF TABLES 

Table 1. Summary of facility configurations and sample identification contained in this report. 

Facilities are identified by code letter only (e.g., “Facility A”) to preserve the confidentiality 

of the CCR source. All fly ash samples are ASTM Class F. .................................................... 5 

(EPA, 2005). CCR samples evaluated in this report are from configurations indicated by 

shaded (light gray) rows. Current capacity reflects date of data collection for EPA report 

Table 4. Beneficial uses of CCRs (ACAA, 2007). Total production of CCRs during 2006 was 

Table 6. Method detection limits (MDLs) and minimum level of quantification (ML) for ICPTable 7. Comparison of summary statistics for field pH data and pH probability distributions 

Table 2. General Characteristics of Coals Burned in U. S. Power Plants (EPA, 2005). ............... 7 

Table 3. Projected Coal-Fired Capacity by APC Configuration as per data collection in 1999 

(EPA, 2005). ........................................................................................................................... 10 

124,795,124 short tons............................................................................................................ 15 

Table 5. MDL and ML of analysis of DIC and DOC. ................................................................. 29 

MS analysis on liquid samples................................................................................................ 31

used in Report 1 and this report. ............................................................................................. 41 

Table 8. Fly Ash. Summary of results. ....................................................................................... 90 

Table 9. FGD Gypsum. Summary of results............................................................................... 91 

Table 10. Scrubber Sludge. Summary of results......................................................................... 92 

Table 11. Fixated Scrubber Sludge. Summary of results............................................................ 93 

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Characterization of Coal Combustion Residues II 

LIST OF FIGURES 

Figure 1. Flow diagram describing processing and nomenclature of FGD scrubber residues and 

samples included in this study. ................................................................................................. 2

Figure 5. Fly ash (FA) comparisons (CFA, AFA, DFA, etc refer to sample identification codes; 

Figure 6. Gypsum (Gyp-U, Gyp-W) comparisons (NAU, NAW, OAU, etc. are sample

Figure 7. Scrubber sludge (ScS) comparisons (CGD, AGD, DGD, etc. are sample identification 

Figure 8. Fixated scrubber sludge (FSS) and fixated scrubber sludge with lime (FSSL) 

Figure 9. Coefficient of variation (C.V.) from XRF elemental analysis of 10 subsamples of 

Figure 2. Multi-pollutant control systems in coal fired power plants.......................................... 12

Figure 3. Life-cycle evaluation of coal combustion residues (EPA, 2002)................................. 13

Figure 4. Uses of CCRs based on 2006 Industry Statistics (ACAA, 2007)................................. 16

see Table 1). Shorthand is used for when SCR is in use (“on”) or not in use (“off”)............ 20

identification codes; see Table 1). .......................................................................................... 21

codes; see Table 1). Shorthand is used for when SCR is in use (“on”) or not in (“off”)....... 22

comparisons (DCC, BCC, KCC, etc. are sample identification codes; see Table 1).............. 23

FSSL sample MAD after mixing by coning and quartering................................................... 33

Figure 10. An example of extract concentrations as a function of pH from SR002.1................. 38

Figure 11. Probability distributions for field pH used in Report 1 (LogLogistic) and this report 

(BetaGeneral). Summary statistics for the field data and the probability distribution used in 

this report (BetaGeneral) are provided to the right of the graph............................................. 40

Figure 12. Example of regression fit and corresponding coefficients for a 5th order polynomial 

equation used to represent solubility and release as a function of pH (antimony for fly ash 

from Facility B with SCR bypassed (DFA)). ......................................................................... 42

Figure 13 and Figure 14. Mercury and Aluminum. Comparison of total elemental content by 

digestion.................................................................................................................................. 45

Figure 15 and Figure 16. Antimony and Arsenic. Comparison of total elemental content by 

Figure 17 and Figure 18. Barium and Cadmium. Comparison of total elemental content by 

Figure 19 and Figure 20. Chromium and Cobalt. Comparison of total elemental content by 

Figure 21 and Figure 22. Lead and Molybdenum. Comparison of total elemental content by 

Figure 23 and Figure 24. Selenium and Thallium. Comparison of total elemental content by 

digestion.................................................................................................................................. 46

digestion.................................................................................................................................. 47

digestion.................................................................................................................................. 48

digestion.................................................................................................................................. 49

digestion.................................................................................................................................. 50

Figure 25. Fly Ash - Total content by XRF................................................................................. 52

Figure 26. Gypsum – Total content by XRF................................................................................ 53

Figure 27. Scrubber Sludge – Total content by XRF................................................................... 54

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Characterization of Coal Cumbustion Residues II 

Figure 28. Fixated Scrubber Sludge – Total content by XRF...................................................... 55

Figure 30. Mercury - Examples of characteristic leaching behavior as a function of pH (SR002.1 

Figure 47 and Figure 48. Mercury and Aluminum. Comparison of maximum, minimum and 

natural pH concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤

Figure 49 and Figure 50. Antimony and Arsenic. Comparison of maximum, minimum and 

natural pH concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤

Figure 51 and Figure 52. Barium and Boron. Comparison of maximum, minimum and natural 

pH concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤ pH≤ 12.4.

Figure 53 and Figure 54. Cadmium and Chromium. Comparison of maximum, minimum and 

natural pH concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤

Figure 55 and Figure 56. Cobalt and Lead. Comparison of maximum, minimum and natural pH 

Figure 57 and Figure 58. Molybdenum and Selenium. Comparison of maximum, minimum and 

natural pH concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤

Figure 29. Fixated Scrubber Sludge with Lime – Total content by XRF.................................... 56

results)..................................................................................................................................... 61

Figure 31. Aluminum. Examples of characteristic leaching behavior as a function of pH. ....... 62

Figure 32. Antimony. Examples of characteristic leaching behavior as a function of pH. ........ 63

Figure 33. Arsenic. Examples of characteristic leaching behavior as a function of pH............. 64

Figure 34. Barium. Examples of characteristic leaching behavior as a function of pH. ............ 65

Figure 35. Boron. Examples of characteristic leaching behavior as a function of pH. .............. 66

Figure 36. Cadmium. Examples of characteristic leaching behavior as a function of pH.......... 67

Figure 37. Chromium. Examples of characteristic leaching behavior as a function of pH. ....... 68

Figure 38. Chromium. Examples of characteristic leaching behavior as a function of pH. ....... 69

Figure 39. Cobalt. Examples of characteristic leaching behavior as a function of pH............... 70

Figure 40. Lead. Examples of characteristic leaching behavior as a function of pH. ................ 71

Figure 41. Molybdenum. Examples of characteristic leaching behavior as a function of pH.... 72

Figure 42. Molybdenum. Examples of characteristic leaching behavior as a function of pH.... 73

Figure 43. Selenium. Examples of characteristic leaching behavior as a function of pH. ......... 74

Figure 44. Selenium. Examples of characteristic leaching behavior as a function of pH. ......... 75

Figure 45. Thallium. Examples of characteristic leaching behavior as a function of pH........... 76

Figure 46. Natural pH (pH in distilled water at LS=10) observed in SR02 extracts. .................. 79

pH≤ 12.4.. ............................................................................................................................... 80

pH≤ 12.4.. ............................................................................................................................... 81

................................................................................................................................................. 82

pH≤ 12.4.. ............................................................................................................................... 83

concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤ pH≤ 12.4. . 84

pH≤ 12.4.. ............................................................................................................................... 85

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Characterization of Coal Combustion Residues II 

Figure 59. Thallium. Comparison of maximum, minimum and natural pH concentrations 

observed in SR02 and SR03 extracts over the pH domain 5.4≤ pH≤ 12.4............................. 86

xi
Characterization of Coal Cumbustion Residues II 

ABSTRACT 

This report evaluates leaching characteristics of air pollution control residues from coal-fired 

power plants that use acid gas scrubbers, which may also reduce air emissions of mercury and 

other pollutants. Leaching of mercury and other constituents of potential concern (COPCs) 

during land disposal of coal combustion residues (CCRs)1

 is evaluated in this report. The data 

presented in this report will be used in a future report to evaluate the fate of mercury and other 

COPCs from the management of CCRs resulting from the use of multi-pollutant control 

technologies. This research is part of an on-going effort by U.S. Environmental Protection 

Agency (EPA) to use a holistic approach to account for the fate of mercury and other metals in 

coal throughout the life-cycle stages of CCR management including disposal and beneficial use. 

The specific objectives of the research reported here are to: 

1. Evaluate the potential for leaching to groundwater of mercury and other COPCs (i.e., 

aluminum, antimony, arsenic, barium, boron, cadmium, chromium, cobalt, lead, 

molybdenum, selenium, and thallium) removed from the flue gas of coal-fired power plants 

by facilities that use wet scrubbers as part of a multi-pollutant control strategy to reduce air 

emissions. 

2. Provide the foundation for assessing the impact of enhanced mercury and multi-pollutant 

control technology on leaching of mercury and other COPCs from CCR management 

including storage, beneficial use, and disposal; and, 

3. Perform these assessments using the most appropriate evaluation methods currently 

available. The laboratory leach testing followed the approach developed by Kosson, et al. 

(2002), which considers the effects of varying environmental conditions on waste constituent 

leaching. 

Air pollution control residues (fly ash, gypsum, and scrubber sludge samples) were obtained 

from coal combustion electric utility facilities using wet scrubbers. A range of facility 

configurations was selected representing differences in air pollution control technology 

configurations and coal rank. Each of the residues sampled has been analyzed for selected 

physical properties, and for total content and leaching characteristics of selected COPCs. Results 

of laboratory leaching tests were used to develop estimates of constituent release under field 

management scenarios. Laboratory leaching test results also were compared to field observations 

of leaching. 

This report includes results for 23 CCRs (5 fly ashes, 6 gypsum samples, 5 scrubber sludges, 7 

fixated scrubber sludges) sampled from eight facilities. Each CCR sampled was evaluated in the 

laboratory for leaching as a function of pH and liquid-to-solid ratio. Results are presented for 

mercury, aluminum, antimony, arsenic, barium, boron, cadmium, chromium, cobalt, lead, 

molybdenum, selenium and thallium. 

1

 Coal combustion residues (CCRs) refer collectively to fly ash, scrubber residues and other air pollution 

control solid residues generated during the combustion of coal collected through the associated air 

pollution control system. Resultant CCRs may be managed as separate or combined residue streams, 

depending on individual facility configuration. 

xii
Characterization of Coal Combustion Residues II 

GLOSSARY OF TERMS 

ACI Activated Carbon Injection 

Al Aluminum 

APC Air Pollution Control 

APPCD Air Pollution Prevention and Control Division 

As Arsenic 

ASTM American Society for Testing and Materials 

B Boron 

Ba Barium 

BET Brunauer, Emmett and Teller (method for quantification of surface area) 

BML Below Method Limit 

CAIR Clean Air Interstate Rule 

CAMR Clean Air Mercury Rule 

Cd Cadmium 

CCRs Coal Combustion Residues 

CCV Continuing Calibration Verification 

Co Cobalt 

COPCs Constituents of Potential Concern 

Cr Chromium 

C.V. Coefficient of Variation 

CVAA Cold Vapor Atomic Adsorption 

DIC Dissolved Inorganic Carbon 

DOC Dissolved Organic Carbon 

DOE United States Department of Energy 

DI Deionized (i.e., deionized water) 

DRC Dynamic Reaction Chamber 

dw dry weight basis 

DWEL Drinking Water Equivalent Level 

EPA United States Environmental Protection Agency 

EPRI Electric Power Research Institute 

ESP Electrostatic Precipitator 

ESP-CS Cold-side Electrostatic Precipitator 

ESP-HS Hot-side Electrostatic Precipitator 

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Characterization of Coal Cumbustion Residues II 

FF Fabric Filter (bag house) 

FGD Flue Gas Desulfurization 

FID Flame Ionization Detector 

FO Forced Oxidation 

FSS Fixated Scrubber Sludge 

FSSL Fixated Scrubber Sludge with Lime 

Gyp-U Unwashed Gypsum 

Gyp-W Washed Gypsum 

Hg Mercury 

HHV Higher Heating Value 

Ho Holmium 

ICP-MS Inductively Coupled Plasma-Mass Spectrometry 

ICV Initial Calibration Verification 

In Indium 

IO Inhibited Oxidation 

I.Ox. Inhibited Oxidation (this abbreviation used in some figures to improve 

clarity) 

LOI Loss On Ignition 

LS Liquid-to-Solid Ratio (LS ratio) 

M Molar 

Max Maximum 

MCL Maximum Contaminant Level (for drinking water) 

MDL Method Detection Limit 

Mg Lime Magnesium Enriched Lime (often also referred to as “mag-lime”) 

Min Minimum 

ML Minimum Level of Quantification 

Mo Molybdenum 

NETL National Energy Technology Laboratory (DOE) 

NIOSH National Institute of Occupational Safety and Health 

NO Natural Oxidation 

NOx Nitrogen Oxides 

NSPS New Source Performance Standards 

OC/EC Organic Carbon/Elemental Carbon 

ORD Office of Research and Development (EPA) 

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Characterization of Coal Combustion Residues II 

OSWER Office of Solid Waste and Emergency Response 

PAC Powdered Activated Carbon 

Pb Lead 

PM Particulate Matter 

PRB Sub-bituminous coal mined in Wyoming’s Powder River Basin 

PS Particulate Scrubber 

QA/QC Quality Assurance/Quality Control 

RCRA Resource Conservation and Recovery Act 

RFA Reference Fly Ash 

SAB U.S. EPA Science Advisory Board 

SCA Specific Collection Area 

Sb Antimony 

ScS Scrubber Sludge 

SCR Selective Catalytic Reduction 

SNCR Selective Non-Catalytic Reduction 

SDA Spray Dryer Absorber 

Se Selenium 

SO2 Sulfur Dioxide 

SPLP Synthetic Precipitation Leaching Procedure 

SRM Standard Reference Material 

S/S Stabilization/Solidification 

SWDA Solid Waste Disposal Act 

TC Toxicity Characteristic 

TCLP Toxicity Characteristic Leaching Procedure 

Tl Thallium 

XRF X-Ray Fluorescence 

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Characterization of Coal Cumbustion Residues II 

EXECUTIVE SUMMARY 

This report evaluates changes that may occur to coal combustion residues (CCRs)2 in response to 

changes in air pollution control technology at coal-fired power plants, which will reduce 

emissions from the flue gas stack by transferring pollutants to fly ash and other air pollution 

control residues. Congress has directed EPA to document that the Clean Air Act regulations do 

not have the net effect of trading one environmental burden for another. The Air Pollution 

Prevention and Control Division (APPCD) of EPA’s Office of Research and Development 

(ORD) is conducting research to evaluate potential leaching and cross media transfers of 

mercury and other constituents of potential concern (COPCs) resulting from land disposal or 

beneficial use of CCRs. The focus of this report is to present data that will be used to evaluate 

the fate of mercury and other metals from the use of wet scrubbers at coal-fired power plants. 

Leaching tests are being conducted on the residues to determine the potential transfer of 

pollutants from the residues to water resources or other environmental systems (e.g., soils, 

sediments). 

The specific objectives of the research reported here are to: 

1. Evaluate the potential for leaching to groundwater of mercury and other COPCs (i.e., 

aluminum, antimony, arsenic, barium, boron, cadmium, chromium, cobalt, lead, 

molybdenum, selenium, and thallium) removed from the flue gas of coal-fired power 

plants by facilities that use wet scrubbers as part of a multi-pollutant control strategy to 

reduce air emissions. 

2. Provide the foundation for assessing the impact of enhanced mercury and multi-pollutant 

control technology on leaching of mercury and other COPCs from CCR management 

including storage, beneficial use, and disposal; and, 

3. Perform these assessments using the most appropriate evaluation methods currently 

available. The laboratory leach testing followed the approach developed by Kosson, et al. 

(2002), which considers the effects of varying environmental conditions on waste 

constituent leaching. 

Air pollution control residues were obtained from coal combustion electric utility facilities using 

wet scrubbers. A range of facility configurations was selected representing differences in air 

pollution control technology configurations and coal rank. Each of the residues sampled has been 

analyzed for selected physical properties, and for total content and leaching characteristics. 

Results of laboratory leaching tests were used to develop estimates of constituent release under 

field management scenarios. Laboratory leaching test results also were compared to field 

observations of leaching. 

2

 Coal combustion residues (CCRs) refer collectively to fly ash and other air pollution control solid 

residues generated during the combustion of coal collected through the associated air pollution control 

system. Resultant CCRs may be managed as separate or combined residue streams, depending on 

individual facility configuration. 

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Characterization of Coal Combustion Residues II 

This report includes results for 23 CCRs (5 fly ashes, 6 gypsum samples, 5 scrubber sludges, 7 

fixated scrubber sludges) sampled from eight facilities.3

 The samples are considered to be 

representative of likely facility configurations indicative of 84 and 74 percent, respectively, of 

the current and future facility configuration types with acid gas scrubbers based on generating 

capacity; however, only a limited number of facilities representing each configuration type have 

been sampled. A range of coal ranks typically combusted is also represented. Each CCR sampled 

was evaluated in the laboratory for leaching as a function of pH and liquid-to-solid ratio. Results 

are presented for mercury, aluminum, antimony, arsenic, barium, boron, cadmium, chromium, 

cobalt, lead, molybdenum, selenium and thallium. 

The selected testing approach was chosen for use because it evaluates leaching over a range of 

values for two key variables [pH and liquid-to-solid ratio (LS)] that both vary in the environment 

and affect the rate of constituent release from waste. The range of values used in the laboratory 

testing encompasses the range of values expected to be found in the environment for these 

parameters. Because the effect of these variables on leaching is evaluated in the laboratory, 

prediction of leaching from the waste in the field is expected to be done with much greater 

reliability. 

In addition, results from laboratory leaching evaluation were compared to field leachate 

concentrations from CCR management facilities available from a U.S. EPA database and an 

Electric Power Research Institute (EPRI) database to determine if laboratory testing results 

reasonably represented field observations. 

Summary of Conclusions 

The data presented in this report will be used in a future report that provides a probabilistic 

assessment of plausible management practices to evaluate the fate of mercury and other COPCs. 

Leach results contained in this report are compared to health-based values to identify where there 

may be potential concerns. The intended use of these results is to suggest that for values less than 

MCLs or DWELs there is unlikely a potential for environmental concern. The thresholds used 

here for preliminary screening do not account for additional dilution and attenuation processes 

that may occur under field management scenarios4

. Therefore the results are considered 

environmentally conservative and actual release rates would be less. For values greater than 

MCLs or DWELs, additional research is needed to determine potential release rates. 

Based on the results of testing and evaluations in this study, the following conclusions are drawn: 

1. For each CCR type, the following constituents exceeded either the maximum 

contaminant level (MCL) or drinking water equivalent level (DWEL) in at least one 

laboratory leaching test condition over the range of pH and LS ratios considered, and 

therefore potentially may present unacceptable environmental risks under some 

management scenarios. These cases warrant more detailed evaluation, including 

consideration of site-specific conditions. 

3

 Fly ash is collected by the particulate collection device, such as an electrostatic precipitator; gypsum is 

dewatered material collected from forced oxidation flue gas desulfurization; scrubber sludge is collected 

from natural or inhibited oxidation flue gas desulfurization; and fixated scrubber sludge is a mixture of 

scrubber sludge, fly ash and often with additional lime added. 

4

 Dilution and attenuation factors are specific to individual sites and management scenarios and may 

range from less than 10 to greater than 100. 

xvii
Characterization of Coal Cumbustion Residues II 

(i) Fly ash – antimony, arsenic, boron, cadmium, chromium, molybdenum, 

selenium and thallium. 

(ii) FGD gypsum – boron, cadmium, molybdenum, selenium and thallium. 

(iii) Scrubber sludge – mercury, antimony, arsenic, boron, chromium, lead, 

molybdenum, selenium and thallium. 

(iv) Fixated scrubber sludge – mercury, antimony, arsenic barium, boron, cadmium, 

chromium, lead, molybdenum, selenium and thallium. 

However, (i) typically, evaluation results from only a subset of samples of a given 

material type exceeded the indicated criteria, and (ii) never did the full range reported 

exceed the indicated threshold. 

2. Leaching of individual constituents may vary over several orders of magnitude, 

depending on the conditions of the management scenario. Thus, these results can be used 

to suggest design conditions that would reduce or minimize constituent release (e.g., pH, 

and other conditions). 

3. Leaching concentrations do not correlate with total content except for specific 

constituents in selected materials where the constituent (i) is weakly retained, and (ii) 

leaching concentrations have a low variability relative to pH. Thus, total content is not a 

good indicator of leaching. 

4. Results of this study suggest that it appears that Cr leachability is associated with the use 

of post-combustion NOx controls. This is based on a limited set of paired samples from 

the same facility operating with and with SCR or SNCR in use. This finding will be 

further evaluated as additional data are collected. 

5. The systematic leaching behavior of COPCs observed in the range of samples evaluated 

suggests that the geochemical mechanisms controlling leaching can be discerned and 

quantified using geochemical speciation modeling, which in turn, can serve as the basis 

for evaluating and improving design of CCR management scenarios. Development of 

generalized geochemical speciation models for the CCR materials evaluated in this study 

is recommended. 

The new information reported here provides an expanded basis for future assessments and may 

impact risk evaluations. Ranges of concentrations of some constituents in laboratory leaching 

test extracts and field data included in this study suggest different applicable concentration 

ranges for risk evaluation other than used in the recent risk assessment on coal combustion waste 

found in docket # EPA-HQ-RCRA-2006-0796 

(http://www.regulations.gov/fdmspublic/component/main). 

This is the second of a series of reports that will evaluate the potential for leaching of COPCs 

from CCRs from coal-fired power plants that use wet scrubbers. The first report focused on the 

use of sorbents for enhancing mercury capture at coal-fired power plants. (Sanchez et al., 2006) 

The third report will evaluate CCRs from facilities with different air pollution control 

configurations and coal ranks that were not previously covered in the first two reports. The 

fourth and final report will provide a probabilistic assessment of the leaching potential of 

mercury and other COPCs based on plausible management strategies. The data will be used to 

correlate leaching characteristics to coal rank, air pollution control configurations, and 

combustion facility characteristics. 

xviii
Characterization of Coal Cumbustion Residues II 

1. INTRODUCTION 

Changes are occurring to air pollution control technology at coal-fired power plants which will 

reduce emissions from the flue gas stack by transferring pollutants to fly ash and other air 

pollution control residues. Congress has directed EPA to document that the Clean Air Act 

regulations do not have the net effect of trading one environmental burden for another. The Air 

Pollution Prevention and Control Division (APPCD) of EPA’s Office of Research and 

Development (ORD) is conducting research to evaluate potential leaching and cross media 

transfers of Constituents of Potential Concern (COPCs) resulting from land disposal or beneficial 

use of coal combustion residues (CCRs). This report is part of a series of reports being prepared 

to document the fate of mercury and other metals found in coal that are being controlled at the 

power plant stack through implementation of multi-pollutant control technology. 

The focus of this report is to present an evaluation of air pollution control residues that may 

result from the use of SO2 scrubbers as mercury control technology at coal-fired power plants, 

and the potential for transfer of pollutants from the resulting residues to water resources or other 

environmental systems (e.g., soils, sediments). The residues studied for this report were 

unwashed and washed flue gas desulfurization (FGD) gypsum, scrubber sludge, fixated scrubber 

sludge, and fly ashes generated from power plants that have SO2 scrubbers. This report compares 

the impact of NOx control technology [selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR)] on characteristics of CCRs obtained from the same facilities during 

periods when the NOx control was and was not in use. 

The potential for leaching of mercury and other COPCs during land disposal or beneficial use of 

the CCRs is the more narrow focus of this assessment. This research is part of an on-going effort 

by EPA to use an integrated, comprehensive approach to account for the fate of mercury and 

other metals in coal throughout the life-cycle stages of CCR management (Thorneloe et al., 

2008; Sanchez et al., 2006). Related research and assessment on environmental fate of 

constituents during CCR management includes conducting thermal stability studies, leach 

testing, and probabilistic assessment modeling to determine fate of mercury and other metals that 

are in coal combustion residues resulting from implementation of multi-pollutant control 

technology (Kilgroe et al., 2001; EPA, 2002). 

CCRs include bottom ash, boiler slag, fly ash, scrubber residues and other miscellaneous solids 

generated during the combustion of coal. Air pollution control can concentrate or partition metals 

to fly ash and scrubber residues. The boiler slag and bottom ash are not of interest in this study 

because enhanced mercury emission controls are not expected to change their composition. Use 

of multi-pollutant controls minimizes air emissions of mercury and other metals by the transfer 

of the metals to the fly ash and other CCRs. This research will help determine the fate of 

mercury and other COPCs from the management of CCRs through either disposal or reuse. Fly 

ash may include unburned carbonaceous materials and inorganic materials in coal that do not 

burn, such as oxides of silicon, aluminum, iron, and calcium. Fly ash is light enough to be 

entrained in the flue gas stream and captured in the air pollution control equipment. 

The type and characteristics of FGD scrubber residue produced is primarily a function of (i) the 

scrubber sorbent used (i.e., limestone, lime, magnesium enriched lime referred to as Mg lime, or 

alkaline fly ash), (ii) the extent of oxidation during scrubbing (i.e., forced oxidation, natural 

oxidation, or inhibited oxidation), (iii) post-scrubber processing, including possibly dewatering 

or thickening, drying, water rinsing, or blending with other materials, and (iv) coal rank 

1
Characterization of Coal Combustion Residues II 

combusted. The presence and leaching characteristics of the constituents of potential concern in 

scrubber residues is a consequence of the coal combusted, process sequence employed, process 

conditions, process additives and use or disposal scenario. Figure 1 illustrates the processes used 

in the production of materials that were sampled for this study, sample nomenclature, and the 

typical management pathways for each material. FGD gypsum is defined here as the by-product 

of the SO2 wet scrubbing process when the scrubber residue is subjected to forced oxidation. In 

forced oxidation systems, nearly all of the by-product is calcium sulfate dihydrate (CaSO4•H2O). 

The resulting wet gypsum is partially dewatered and then either disposed in a landfill (unwashed 

gypsum; Gyp-U) or water rinsed (in some cases) and dried to produce washed gypsum (washed 

gypsum; Gyp-W) that then potentially can be used in wallboard manufacturing or agricultural 

applications. Scrubber sludge (ScS) is the by-product of the SO2 wet scrubbing process resulting 

from neutralization of acid gases at facilities that use either inhibited oxidation or natural 

oxidation of scrubber residue. In inhibited oxidation systems, nearly all of the by-product is 

calcium sulfite hemihydrates (CaSO3•1⁄2H2O). In natural oxidation systems, the by-product is a 

mixture of CaSO3•1⁄2H2O and CaSO4•H2O. Scrubber sludge typically will be either partially 

dewatered in a thickener and then disposed in a surface impoundment, or after thickening, further 

dewatered and mixed with fly ash to form fixated scrubber sludge (FSS). In most cases, 

additional lime is also blended with the scrubber sludge and fly ash to form fixated scrubber 

sludge with lime (FSSL). The blend of fly ash and scrubber sludge is typically between 0.5 to 1.5 

parts fly ash to 1 part scrubber sludge on a dry weight basis, with 0 or 2-4% additional lime 

added (FSS or FSSL, respectively). Fixated scrubber sludge typically is either disposed in a 

landfill or supplied to a beneficial use (e.g., fill in mining applications). This report evaluates the 

characteristics of fly ash, FGD gypsum, scrubber sludge, and fixated scrubber sludge (as 

produced with or without lime) from several coal combustion facilities. 

Absorber

(Forced Oxidation) Facilities N, O, P, Q Facilities A, B, K, M

Wet Gypsum

Absorber

(InhibitedOxidation

orNaturalOxidation)

Thickener Impoundment Dewatering (Scrubber Sludge; ScS)

Landfill(Agriculture?)

(Unwashed Gypsum; Gyp‐U) Drying

Rinsing Mixing of Scrubber Sludge

& Drying Fly Ash and Lime

(Fixated Scrubber Sludge) (Washed Gypsum; Gyp‐W) (FA+ScS; FSS)

(FA+ScS+lime; FSSL)

Wallboard Landfillor

Beneficial Use

Figure 1. Flow diagram describing processing and nomenclature of FGD scrubber residues and 

samples included in this study. 

2
Characterization of Coal Cumbustion Residues II 

When coal is burned in an electric utility boiler, the resulting high combustion temperatures 

vaporize the Hg in the coal to form gaseous elemental mercury (Hg0

). Subsequent cooling of the 

combustion gases and interaction of the gaseous Hg0 with other combustion products result in a 

portion of the Hg being converted to gaseous oxidized forms of mercury (Hg2+) and particlebound mercury. The specific chemical form — known as the speciation — has a strong impact 

on the capture of mercury and other metals by boiler air pollution control (APC) equipment. 

(EPA, 2001) 

Mercury and other elements partition between the combustion gas, fly ash and scrubber residues. 

Depending upon the gas conditioning, presence or absence of NOx control and other air 

pollution control technology in use, there may be changes occurring to the fly ash that may affect 

the stability and mobility of mercury and other metals in the CCRs. Similarly, NOx control and 

SO2 scrubber technology may affect the content, stability and mobility of mercury and other 

metals in scrubber residues. 

In response to wider use of multi-pollutant control, changes are occurring in air pollution control 

at coal-fired power plants to reduce emissions of nitrogen oxides, sulfur oxides, and mercury. 

How these changes in air pollution control is the focus of this research. The fate of mercury and 

other COPCs in fly ash and scrubber residues is of interest as part of the overall lifecycle 

evaluation of impact of air pollution control technology and the management of CCRs either 

through disposal or beneficial use. This research is evaluating changes to air pollution control 

residues as a result of more widespread implementation of these multi-pollutant technologies, 

and the impacts of land disposal or commercial use of the residues. 

The specific objectives of the research reported here are to: 

1. Evaluate the potential for leaching of mercury and other COPCs removed from coalfired power plant air emissions by different types of air pollution control technology 

that includes acid gas scrubbers, particulate, and sorbents; 

2. Provide information to be used in separate reports to assess the fate of mercury and 

other COPCs from enhanced or expanded use multi-pollutant control technologies. 

This will include consideration of potential leaching of mercury and other COPCs 

during the life-cycle management of CCRs during storage, beneficial use and 

disposal; and 

3. Perform these assessments using the most appropriate evaluation methods currently 

available. The laboratory leach testing followed the approach developed by Kosson, 

et al. (2002), which considers the effects of varying environmental conditions on 

waste constituent leaching. 

This is the second of a series of reports that will address the potential for cross-media transfer of 

COPCs from CCRs. The first report focused on the use of sorbent injection (activated carbon 

and brominated activated carbon) for enhanced mercury control (Sanchez et al., 2006). 

Subsequent reports will address: 

 CCRs from coal-fired power plants that use air pollution control technologies other than 

evaluated in earlier reports necessary to span the range of coal-types and air pollution control 

technology configurations (report 3); 

 Assessment of leaching for COPCs under additional management scenarios, including 

impoundments and beneficial use on the land (report 4); and, 

3
Characterization of Coal Combustion Residues II 

 Broader correlation of CCR leaching characteristics to coal rank, combustion facility 

characteristics and geochemical speciation within CCRs supported by information and 

analysis on additional trace elements and primary constituents (report 4). 

Table 1 provides a summary of facility configurations, including samples and sample 

identification for testing, described in this report. For simplicity in presentation, the use of NOx 

controls is indicated as either “off” or “on” (i.e., SNCR Off, SNCR On, SCR Off, SCR On), 

recognizing that SCR not in use (SCR Off) reflects that either the system was bypassed or 

ammonia was not added, and SNCR not in use (SNCR Off) indicates that urea was not added. 

Sampled CCRs were subjected to multiple leaching conditions according to the designated 

leaching assessment approach. Leaching conditions included batch equilibrium5 extractions at 

acidic, neutral and alkaline conditions at an LS of 10 mL/g, and LS from 0.5 to 10 mL/g using 

distilled water as the leachant. The leach testing results are used to evaluate the likely range of 

leaching characteristics during land disposal (i.e., landfill or surface impoundment). Results of 

the laboratory leaching tests carried out in this study were compared to the range of observed 

constituent concentrations in field leachates reported in a U.S. EPA database (EPA, 2007) and an 

Electric Power Research Institute (EPRI) database (EPRI, 2006). The testing results presented 

here will be used for evaluating disposal and beneficial use scenarios in subsequent reports. 

As part of this research program, a QA/QC plan consistent with EPA requirements was 

developed for the leaching assessment approach and reported earlier (Sanchez et al., 2006). The 

QA/QC methodology included initial verification of acceptable mercury retention during 

laboratory testing through evaluation of a mass balance around testing procedures (Sanchez et 

al., 2006). Modifications to the QA/QC program to reduce the experimental and analytical 

burden while maintaining confidence in the resulting data, based on program results to date, are 

presented in this report. 

Laboratory testing for leaching assessment was carried out at the EPA National Risk 

Management Research Laboratory (Research Triangle Park, North Carolina) with technical 

assistance from Vanderbilt University and ARCADIS. 

5

 In the context of leaching tests, the term “equilibrium” is used to indicate that the test method result is a 

reasonable approximation of chemical equilibrium conditions even though thermodynamic equilibrium 

may not be approached for all constituents. 

4
Characterization of Coal Cum

5

Table 1. Summary of facility configurations and sample identification contained in this report. Facilities are idonly (e.g., “Facility A”) to preserve the confidentiality of the CCR source. All fly ash samples are ASTM Class F1Bit – bituminous; Sub-Bit – sub-bituminous 2SNCR – selective non-catalytic reduction; SNCR-BP – SNCR by-passed during winter months; SCR & SNCR 

from facility with both SCR and SNCR 

3ESP-CS – cold-side electrostatic precipitator; ESP-HS – hot-side electrostatic precipitator. 4FA – fly ash; Gyp-U – unwashed gypsum; Gyp-W – washed gypsum; ScS – scrubber sludge; FSS – fixat

mixture of fly ash and scrubber sludge); FSSL – fixated scrubber sludge with lime (a mixture of fly ash and additional lime added). The three-letter identification codes are shown for each facility and sample type (e.g., C

FGD Scrubber Type Sample Types and Identificat

Facility Code 

Coal Rank1

PostCombustion NOx Control2

Particulate Control3

Lime orMg Lime Oxidation FA Gyp-U Gyp-W Sc

A Bit SNCR-BP (off) Fabric Filter Limestone Natural CFA C

A Bit SNCR (on) Fabric Filter Limestone Natural AFA A

B Bit SCR-BP (off) ESP-CS Mg Lime Natural DFA D

B Bit SCR (on) ESP-CS Mg Lime Natural BFA B

K Sub-Bit SCR (on) ESP-CS Mg Lime Natural KFA K

M Bit SCR-BP (off) ESP-CS Limestone Inhibited 

M Bit SCR (on) ESP-CS Limestone Inhibited N Bit None ESP-CS Limestone Forced NAU NAW O Bit SCR (on) ESP-CS Limestone Forced OAU OAW P Bit SCR & SNCR ESP-CS Limestone Forced PAD Q Sub-Bit none ESP-HS Limestone Forced QAU 
Characterization of Coal Combustion Residues II 

1.1. REGULATORY CONTEXT 

1.1.1. Waste Management 

The management of coal combustion residues is subject to the Resource Conservation and 

Recovery Act (RCRA), which is the federal law regulating both solid and hazardous wastes, as 

well as state regulatory requirements. Subtitle C of RCRA pertains to hazardous waste; other 

solid, non-hazardous wastes fall under RCRA Subtitle D. Subtitle C wastes are federally 

regulated while Subtitle D wastes are regulated primarily at the state level. The original version 

of RCRA did not specify whether CCRs were Subtitle C or D wastes. In 1980, the Solid Waste 

Disposal Act (SWDA) amendments to RCRA conditionally excluded CCRs from Subtitle C 

regulation pending completion of a study of CCR hazards. Since that time, CCRs have been 

regulated at the state level under Subtitle D. 

The SWDA amendments to RCRA required EPA to prepare a report to Congress identifying 

CCR hazards and recommending a regulatory approach for CCRs. In this report (EPA, 1988) and 

the subsequent regulatory determination, EPA recommended that CCRs generated by electric 

utilities continue to be regulated under RCRA Subtitle D (See 58 FR 42466, August 9, 1993). 

Other residues generated at coal-fired electric utilities were not included in this 1993 decision. 

EPA conducted a follow-up study specifically aimed at low-volume, co-managed wastes6

 and 

issued another Report to Congress (EPA, 1999) with a similar recommendation. In April 2000, 

EPA issued a regulatory determination exempting these wastes from hazardous waste regulations 

(see 65 FR 32214, May 22, 2000). However, concern was expressed over the use of CCRs as 

backfill for mine reclamation operations, and it was determined that this practice should be 

regulated under a federal Subtitle D rule. It was also decided by EPA that federal regulations 

under Subtitle D are needed for CCR when they are disposed in surface impoundments and 

landfills. Currently, the agency is in the process of developing these regulations 

(http://www.epa.gov/epaoswer/other/fossil/noda07.htm). The results presented in this report, and 

subsequent reports, will help provide the information needed to identify the release potential of 

mercury and other metals that have been removed from stack gases into air pollution control 

residues, over a range of plausible management options. These data will help identify those 

conditions that will either reduce or enhance releases to the land so that the effects of different 

management conditions can be factored into any controls developed under the regulations. 

1.1.2. Air Pollution Control 

On March 10, 2005, EPA announced the CAIR (FR 25612, May 2005) which is expected to 

increase the use of wet scrubbers and selective catalytic reduction (SCR) units to help reduce 

sulfur dioxide and nitrogen oxides emissions from coal-fired power plants. On March 15, 2005, 

EPA announced the CAMR (FR 28606, May 2005) for reducing mercury emissions through the 

use of a cap and trade program. Power plants are the largest remaining source of anthropogenic 

mercury emissions in the county. The CAMR established “standards of performance” that limit 

mercury emissions from new [through new source performance standards (NSPSs)] and existing 

(through emission guidelines) coal-fired power plants through the creation of a market-based 

cap-and-trade program that will reduce mercury emissions in two phases. The first phase caps 

6

 Co-managed wastes are low-volume wastes that are co-managed with the high-volume CCRs. 

6
Characterization of Coal Cumbustion Residues II 

national annual mercury emissions at 38 tons through co-benefit reductions achieved through 

controlling sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions under CAIR. In the second 

phase, due in 2018, coal-fired power plants will be subject to a second cap, which will reduce 

mercury emissions to 15 tons per year upon full implementation. On February 8, 2008, the D.C. 

Circuit vacated EPA's rule removing power plants from the Clean Air Act list of sources of 

hazardous air pollutants. At the same time, the court vacated the CAMR. EPA is reviewing the 

court's decisions and evaluating its impacts. (http://www.epa.gov/mercuryrule/) On July 11, 

2008, the D.C. Circuit vacated EPA’s Clean Air Interstate Rule. EPA is reviewing the court’s 

decisions and evaluating its impacts. 

Congress has directed EPA to document that the Clean Air Act regulations are not trading one 

environmental burden for another. The Air Pollution Prevention and Control Division (APPCD) 

of EPA’s Office of Research and Development (ORD) is conducting the current research to help 

identify any potential pollutant transfers resulting from land disposal or beneficial use of 

mercury-enriched CCRs. The research results presented in this report are part of that effort. 

In response to the evolving implementation of advanced air pollution control technology for 

coal-fired power plants, this research is directed towards understanding changes in CCR 

characteristics that may increase environmental burdens from land disposal of CCRs or impact 

CCR usage in commercial applications. 

1.2. CONFIGURATIONS OF U.S. COAL FIRED POWER PLANTS AND 

MULTI-POLLUTANT CONTROL TECHNOLOGIES 

In the U.S., there are approximately 1,100 units at approximately 500 coal-fired electricity 

generating facilities. These facilities represent a range of coal ranks, boiler types, and air 

pollution control technologies. The current combined capacity of U.S. coal-fired power plants is 

307 GW (DOE-EIA, 2007). The coal rank burned and facility design characteristics affect the 

effectiveness of multi-pollutant control technologies that are or could be used at these plants. The 

U.S. coal-fired power plants typically burn one of three types of fuel: (1) bituminous coal (also 

referred to as “high rank” coal), (2) sub-bituminous coal, and (3) and lignite (sub-bituminous 

coal and lignite are referred to as “low rank” coals). Some of the characteristics of interest 

related to the possible environmental impacts of burning these different coal ranks are given in 

Table 2 (EPA, 2005). 

Table 2. General Characteristics of Coals Burned in U. S. Power Plants (EPA, 2005). 

Mercury Chlorine Sulfur Ash HHVa 

ppm (dry) ppm (dry) % (dry) % (dry) BTU/lb (dry) 

Coal Range Avg Range Avg Range Avg Range Avg Range Avg 

Bitu- 0.036 - 0.113 48 - 1033 0.55 - 1.69 5.4 - 11.1 8646 - 13203 

minous 0.279 2730 4.10 27.3 14014 

Sub- 0.025 - 0.071 51 - 158 0.22 - 0.50 4.7 - 8.0 8606 - 12005 

bitu- 0.136 1143 1.16 26.7 13168 

minous 

Lignite 0.080 - 

0.127 

0.107 133 -

233 

188 0.8 -

1.42 

1.30 12.2 -

24.6 

19.4 9487 -

10702 

10028 

a

 Higher Heating Value. 

7 
Characterization of Coal Combustion Residues II 

1.2.1. Current Air Pollution Control Technologies 

The key air pollutants of concern released by coal fired power plants include particulates, SO2, 

NOx, mercury and other metals7. A range of pollution control technologies is used to reduce 

particulate, SO2, and NOx and these technologies also impact the emission of mercury and other 

metals. The pollution control technology type and configurations vary across facilities. 

7

 Concerns regarding carbon dioxide emissions from coal fired power plants are beyond the scope of this 

report. 

8
Characterization of Coal Cumbustion Residues II 

Table 3 shows the current and projected coal-fired capacity by air pollution control technology 

configuration. This report emphasizes wet scrubbers since their use is expected to double or 

triple in response to implementation of CAIR. Post-combustion particulate matter controls used 

at coal-fired utility boilers in the United States can include electrostatic precipitators (ESPs), 

fabric filters (FFs), particulate scrubbers (PSs), or mechanical collectors (MCs). Post-combustion 

SO2 controls can consist of a wet scrubber (WS), spray dryer adsorber (SDA), or duct injection. 

Post-combustion NOx controls can involve selective catalytic reduction (SCR) or selective 

noncatalytic reduction (SNCR). 

In response to current and proposed NOx and SO2 control requirements, additional NOx control 

and flue gas desulfurization (FGD) systems for SO2 control are expected to be installed and more 

widely used in the future. Some estimates project a doubling or tripling of the number of wet 

scrubbers as a result of CAIR implementation. Over half of the U.S. coal-fired capacity is 

projected to be equipped with SCR and, or, FGD technology by 2020. 

The mercury capture efficiency of existing ESPs and FFs appears to be heavily dependent on the 

partitioning of mercury between the particulate and vapor phases and the distribution of mercury 

species (e.g., elemental or oxidized) in the vapor phase. In general, ESPs and FFs which are 

designed for particulate control are quite efficient at removing mercury in the particulate phase; 

however, the overall mercury removal efficiency in these devices may be low if most of the 

mercury entering the device is in the vapor phase (MTI, 2001). Many factors contribute to the 

observed differences in mercury removal efficiency, such as the mercury oxidation state. 

Differences in mercury contents of U.S. coals also result in a range of mercury concentrations in 

the flue gas from the boiler. In general, it is easier to achieve higher mercury percent removal 

with higher mercury inlet concentrations (MTI, 2001). Further, the chlorine content of the coal 

may have an impact on mercury removal because the oxidation state of mercury is strongly 

affected by the presence of halides in the flue gas. In general, the higher the chlorine content of 

the coal, the more likely the mercury will be present in its oxidized state, enhancing the 

likelihood of its removal from the gas stream. The addition of NOx controls may improve the 

mercury capture efficiency of particulate collection devices for some cases. (EPA, 2001) 

9
Characterization of Coal Combustion Residues II 

Table 3. Projected Coal-Fired Capacity by APC Configuration as per data collection in 1999 

(EPA, 2005). CCR samples evaluated in this report are from configurations indicated by shaded 

(light gray) rows. Current capacity reflects date of data collection for EPA report (EPA, 2005). 

Air Pollution Control Configuration Current 

Capacity, MW 

2010 Capacity, 

MW 

2020 Capacity, 

MW 

Cold-side ESP 111,616 75,732 48,915 

Cold-side ESP + Wet Scrubber 41,745 34,570 33,117 

Cold-side ESP + Wet Scrubber + ACI - 379 379 

Cold-side ESP + Dry Scrubber 2,515 3,161 5,403 

Cold-side ESP + SCR 45,984 35,312 22,528 

Cold-side ESP + SCR + Wet Scrubber 27,775 62,663 98,138 

Cold-side ESP + SCR + Dry Scrubber - 11,979 13,153 

Cold-side ESP + SNCR 7,019 4,576 2,534 

Cold-side ESP + SNCR + Wet Scrubber 317 2,830 6,088 

Fabric Filter 11,969 10,885 7,646 

Fabric Filter + Dry Scrubber 8,832 8,037 9,163 

Fabric Filter + Wet Scrubber 4,960 4,960 4,960 

Fabric Filter + Dry Scrubber + ACI - 195 195 

Fabric Filter + SCR 2,210 2,950 1,330 

Fabric Filter + SCR + Dry Scrubber 2,002 2,601 4,422 

Fabric Filter + SCR + Wet Scrubber 805 805 2,363 

Fabric Filter + SNCR 267 267 345 

Fabric Filter + SNCR + Dry Scrubber 559 557 557 

Fabric Filter + SNCR + Wet Scrubber 932 932 1,108 

Hot-side ESP 18,929 11,763 10,160 

Hot-side ESP + Wet Scrubber 8,724 10,509 10,398 

Hot-side ESP + Dry Scrubber - 538 538 

Hot-side ESP + SCR 5,952 3,233 1,847 

Hot-side ESP + SCR + Wet Scrubber 688 6,864 9,912 

Hot-side ESP + SNCR 684 1,490 1,334 

Hot-side ESP + SNCR + Wet Scrubber 474 474 627 

Existing or Planned Retrofit Units 304,955 298,263 297,161 

New Builds of Coal Steam Units 

Current 

Capacity, MW 

2010 Capacity, 

MW 

2020 Capacity, 

MW 

Fabric Filter + SCR + Wet Scrubber - 221 17,292 

Total All Units 304,955 298,484 314,453 

Note: IGCC units are not included as part of this list. 

Note: Current capacity includes some SCR and FGD projected to be built in 2005 and 2006. 

Note: 2010 and 2020 is capacity projected for final CAIR rule. 

Note: IPM projects some coal retirements and new coal in 2010 and 2020. 

10
Characterization of Coal Cumbustion Residues II 

1.2.2. Wet Scrubbers, NOx Controls and Multi-pollutant Controls 

Wet FGD scrubbers are the most widely used technology for SO2 control. Scrubbers are typically 

installed downstream of particulate control (i.e., ESP or FF). Removal of PM from the flue gas 

before it enters the wet scrubber reduces solids in the scrubbing solution and minimizes impacts 

to the fly ash that might affect its beneficial use. 

FGD technology uses sorbents and chemical reactants such as limestone (calcium carbonate) or 

lime (hydrated to form calcium hydroxide) to remove sulfur dioxide from the flue gas created 

from coal combustion. Limestone is ground into a fine powder and then combined with water to 

spray the slurry into combustion gases as they pass through a scrubber vessel. The residues are 

collected primarily as calcium sulfite (a chemically reduced material produced in natural 

oxidation or inhibited oxidation scrubbers), or can be oxidized to form calcium sulfate or FGD 

gypsum (using forced oxidation). The most widely used FGD systems use either forced oxidation 

scrubbers with limestone addition, or natural/inhibited oxidation scrubbers with lime or Mg-lime 

addition8

. Wet scrubbers that use forced oxidation produce calcium sulfate (gypsum) and are 

expected to be the most prevalent technology because of the potential beneficial use of gypsum 

and easier management and handling of the residues. There are also dry FGD systems that 

include spray dryer absorbers, usually in combination with a FF (EPA, 2001; Srivastava et al., 

2001). 

NOx emissions are controlled through the use of low NOx producing burners and use of a 

selective catalytic reduction (SCR) system in the flue gas that is capable of a 90% reduction of 

flue gas NOx emissions. SCR is typically installed upstream of the PM control device. 

Sometimes selective non-catalytic reduction (SNCR) is used for NOx control, although use of 

SNCR is less frequent. 

Figure 2 illustrates options for multi-pollutant control at power plants. 

8

 As of 1999: Total FGD units – 151; limestone forced oxidation (FO) - 38 units (25%); limestone natural/inhibited 

oxidation - 65 (43%); lime FO (all forms other than Mg-Lime) - 1 (<1%); lime natural/inhibited oxidation (all forms 

other than Mg-lime) - 23 (15%); Mg-lime FO - 0 (0%); Mg-lime natural/inhibited oxidation - 25 (17%) 

It is estimated that the numbers of natural/inhibited systems has remained nearly the same since 1999, and the 

limestone FO units have increased significantly. In the future, limestone FO units will increase significantly, and all 

types of natural/inhibited units will likely decrease (Ladwig, 2007). 

11
Characterization of Coal Combustion Residues II 

OxidizingOxidizing Wet Scrubber Wet Scrubber Stack

Coal & Air 

Oxidizing 

Chemicals 

SCR 

Catalysts 

Residue 

Ash 

PM Control 

Coal & Air

Oxidizing

Chemicals

SCR

Catalysts

Residue

Ash

PM Control

Figure 2. Multi-pollutant control systems in coal fired power plants. 

Improvements in wet scrubber performance to enhance mercury capture depend on oxidizing 

elemental mercury (Hg0) to Hg2+ by using additives to the flue gas or scrubber. A DOE-funded 

study found that wet scrubbers remove about 90% of the oxidized gaseous mercury (Hg2+) in the 

flue gas but none of the elemental mercury (Pavlish et al., 2003). The percentage of total Hg 

removed by multi-pollutant controls (particulate and scrubber devices) is influenced by coal 

chlorine content, which determines the Hg oxidation status exiting the particulate control and 

entering the scrubber. Mercury removal efficiency by wet scrubbers ranges from 30 to 60% for 

cold-side ESPs as coal chlorine content is increased from 50 to 1000 mg kg-1. Mercury removal 

efficiency for hot-side ESPs is less effective ranging from 20 to 50% as coal chlorine content is 

increased from 200 to 1000 mg kg-1 (Pavlish et al., 2003). Other factors that influence mercury 

capture are the amount of carbon and chlorine in the fly ash. Fuel blending, addition of oxidizing 

chemicals, controlling unburned carbon content in the fly ash, and addition of a mercury-specific 

oxidizing catalyst downstream of the particulate matter control can help improve mercury 

capture (Thorneloe, 2006; EPA, 2005). 

1.3. COAL COMBUSTION RESIDUES 

The range of air pollution control technologies and configurations determines the characteristics 

of the coal combustion residues. In 2006, 125 million tons of coal combustion residues were 

produced with ~53 million tons being used in commercial, engineering, and agricultural 

applications. (ACAA, 2007). CCRs result from unburned carbon and inorganic materials in 

coals that do not burn, such as oxides of silicon, aluminum, iron, and calcium. Fly ash is the 

unburned material from coal combustion that is light enough to be entrained in the flue gas 

stream, carried out of the process, and collected as a dry material in the APC equipment. APC 

can concentrate or partition metals in fly ash and scrubber sludge. Bottom ash and boiler slag are 

not affected by APC technology and, therefore, these materials are not being evaluated as part of 

this study. Bottom ash is the unburned material that is too heavy to be entrained in the flue gas 

12
Characterization of Coal Cumbustion Residues II 

stream and drops out in the furnace. Boiler slag, unburned carbon or inorganic material in coal 

that does not burn, falls to the bottom of the furnace and melts. 

Changes in multi-pollutant control in response to CAIR implementation, voluntary 

improvements by facilities, and changes in state requirements, will reduce air emissions of 

mercury and other pollutants that will be transferred from the flue gas to the APC residues. The 

purpose of this research is to evaluate the impact of these changes on CCRs, with a focus on 

changes in metals concentrations in CCRs, and the potential for subsequent release of these 

metals to the environment under different plausible management (disposal or reuse) conditions 

(Figure 3). The properties of fly ash and scrubber residues are likely to change as a result of 

changes in air pollution control to reduce emissions of concern from coal-fired power plants. 

Changes in CCRs that may occur include increased content of mercury and other metals (e.g., 

arsenic, selenium, chromium) The chemical and physical properties may also change as a 

results of sorbents and other additives being used to improve air pollution control. 

Figure 3. Life-cycle evaluation of coal combustion residues (EPA, 2002). 

1.4. RESIDUE MANAGEMENT PRACTICES 

CCRs can be disposed in landfills or surface impoundments or used in commercial applications 

to produce concrete and gypsum wallboard, among other products. The major pathway of 

concern for release from land disposal and some beneficial use applications is leaching to 

groundwater. Research on the impact of CCR disposal on the environment has been conducted 

by many researchers and has been summarized by the EPA (1988, 1999). However, most of the 

existing CCR data are for CCRs prior to implementation of mercury or multi-pollutant controls. 

1.4.1. Beneficial Use 

In the United States, approximately 40% percent (49.6 million tons) of all CCRs produced are 

reused in commercial applications or other uses that are considered beneficial and avoid 

13
Characterization of Coal Combustion Residues II 

landfilling. Forty-eight percent (23.8 million tons) of CCRs is fly ash which is used in 

commercial applications such as making concrete/grout, cement, structural fill, and highway 

construction (ACAA, 2005; Thorneloe, 2003). Eight million tons of the FGD gypsum that was 

produced (or 68%) was used in making wall board (ACAA, 2005). Table 4 and Figure 4 present 

the primary commercial uses of CCRs, and a breakdown of U.S. production and usage by CCR 

type. 

Some of the beneficial uses may have the potential to release mercury from the CCRs by several 

pathways. Of particular concern are high-temperature processes. In cement manufacturing, for 

example, CCRs are inputs to the cement kiln. Virtually all mercury will be volatilized from 

CCRs when CCRs are used as feedstock to cement kilns. Even where mercury can be captured 

by the controls on cement kilns, approximately two-thirds of cement kiln dust captured by the 

control devices is reintroduced into the kiln. Therefore, a significant fraction of the mercury in 

CCRs introduced into cement kilns may be emitted to the air at the cement plant. Some mercury 

may also be revolatilized when CCRs are used as filler for asphalt, or when FGD material is used 

in wallboard manufacturing. A separate report is being prepared to document the finding on the 

thermal stability of Hg and other metals when used in high-temperature processes. 

The fate of mercury and other metals is also a potential concern when CCRs are used on the land 

(mine reclamation, building highways, soil amendments, agriculture and in making concrete, 

cement) or to make products that are subsequently disposed (e.g., disposal of wallboard in 

unlined landfill). 

For several commercial uses, it appears less likely that mercury in CCRs will be reintroduced 

into the environment, at least during the lifetime of the product. However, the impact of 

advanced mercury emissions control technology (e.g., activated carbon injection) on beneficial 

use applications is uncertain. There is concern that the presence of increased concentrations of 

mercury, certain other metals, or high carbon content may reduce the suitability of CCRs for use 

in some applications (e.g., carbon content can limit use in Portland cement concrete). 

1.4.2. Land Disposal 

There are approximately 600 land-based CCR waste disposal units (landfills or surface 

impoundments) being used by the approximately 500 coal-fired power plants in the United States 

(EPA, 1999). About 60% of the 122 million tons of CCRs generated annually are land disposed. 

Landfills may be located either on-site or off-site while surface impoundments are almost always 

located on-site with the combustion operations. Although the distribution of units is about equal 

between landfills and surface impoundments, there is a trend toward increased use of landfills as 

the primary disposal method. 

14
Cha

C

Table 4. Beneficial uses of CCRs (ACAA, 2007). Total production of CCRs during 2006 was 124,795,124 

CCR Categories (Short Tons) Fly Ash Bottom Ash FGD Gypsum FGD Wet Scrubbers Boiler Slag1 FG

Sc

CCR Production Category Totals2 72,400,000 18,600,000 12,100,000 16,300,000 2,026,066 1,4

CCR Used Category Totals3 32,423,569 8,378,494 9,561,489 904,348 1,690,999 13

 CCR Use By Application4 

Fly Ash 

Bottom Ash 

FGD Gypsum 

FGD Wet Scrubbers 

Boiler Slag

1 

FGSc

1. Concrete/Concrete Products/Grout 15,041,335 597,387 1,541,930 0 0 9,6

2. Cement/Raw Feed for Clinker 4,150,228 925,888 264,568 0 17,773 0 

3. Flowable Fill 109,357 0 0 0 0 9,8

4. Structural Fills/Embankments 7,175,784 3,908,561 0 131,821 126,280 0 5. Road Base/Sub-base/Pavement 379,020 815,520 0 0 60 24

6. Soil Modification/Stabilization 648,551 189,587 0 0 0 29

7. Mineral Filler in Asphalt 26,720 19,250 0 0 45,000 0 

8. Snow and Ice Control 0 331,107 0 0 41,549 0 

9. Blasting Grit/Roofing Granules 0 81,242 0 232,765 1,445,933 0 

10. Mining Applications 942,048 79,636 0 201,011 0 1111. Wallboard 0 0 7,579,187 0 0 0 12. Waste Stabilization/Solidification 2,582,125 105,052 0 0 0 0 

13. Agriculture 81,212 1,527 168,190 0 0 84

14. Aggregate 271,098 647,274 0 0 416 0 15. Miscellaneous/Other 1,016,091 676,463 7,614 338,751 13,988 46 

CCR Category Use Tools 32,423,569 8,378,494 9,561,489 904,348 1,690,999 13

Application Use to Production Rate 44.8% 45.0% 79.0% 5.5% 83.5% 9.2

1 As submitted based on 54 percent coal burn. 2 CCR Production totals for Fly Ash, Bottom Ash, FGD Gypsum, and Wet FGD are extrapolated estimates rounded off to nearest 50,00

3 CCR Used totals for Fly Ash, Bottom Ash, FGD Gypsum, and Wet FGD are per extrapolation calculations (not rounded off). 4 CCR Uses by application for Fly Ash, Bottom Ash, FGD Gypsum, and Wet FGD are calculated per proportioning the CCR Used 

Totals by the same percentage as each of the individual application types' raw data contributions to the as-submitted raw data subm

(not rounded off). 

15 
Characterization of Coal Combustion Residues II 

0

2

4

6

8

10

12

14

16

18

Concrete/Grout

Structural Fill

Wallboard

Cement

Waste Stabilization/Solidification

Miscellaneous/Other

Blasting Grit/Roofing Granules

Road Base/Pavement

Mining Applications

Soil Modification/Stabilization

Aggregate

Snow and Ice Control

Agriculture

Mineral Filler in Asphalt

Flowable Fill

Q

u

a

ntity of CCRs (millio

n tons)

FGD Dry Scrubbers

Boiler Slag

FGD Wet Scrubbers

FGD Gypsum

Bottom Ash

Fly Ash

Figure 4. Uses of CCRs based on 2006 Industry Statistics (ACAA, 2007). 

1.5. LEACHING PROTOCOL 

One of the major challenges initially facing this research was identification of an appropriate test 

protocol for evaluating the leaching potential of CCRs that may have increased levels of several 

metals, particularly mercury. The goal of this research is to develop the most accurate estimates 

of likely constituent leaching when CCRs are land disposed. These estimates of leaching need to 

be appropriate for assessing at a national level the likely impacts through leaching of pollutants 

from CCRs that is a consequence of installation of enhanced mercury and, or, multi-pollutant 

controls. To achieve this goal requires that U.S. EPA evaluate leaching potential for CCRs as 

managed (to the degree this is known), and that the leach testing results can be appropriately 

extrapolated to a national assessment. A large part of the approach to achieving this goal has 

been to identify and evaluate CCR samples collected from the most prevalent combinations of 

power plant design (with a focus on air pollution control technology configurations) and coal 

rank used. In addition, the resulting data set is expected to serve as foundation for evaluation of 

CCR management options for different types of CCRs at specific sites. 

Data have been collected on the disposal conditions for CCRs. The conditions will vary over 

time which need to be considered when evaluating leaching. (EPA, 1999, 202, 2007). When 

16
Characterization of Coal Cumbustion Residues II 

disposed, CCRs are typically monofilled or disposed with other CCRs. However, CCR 

composition can change over time, due to reactions with the atmosphere (e.g., carbonation and 

oxidation), changes in the source of coal or coal rank burned, or due to installation of additional 

pollution control equipment. 

Many leaching tests have been developed by regulatory agencies, researchers, or third-party 

technical standards organizations, and are described in the published literature. States and others 

have expressed concern with the variety of leaching protocols in use, the lack of correlation of 

test results with field conditions and actual leaching, and lack of comparability of available data 

because of incomplete reporting of test conditions. There is also limited or no quality assurance 

(QA) information for many of these tests. Leaching tests such as the Toxicity Characterization 

Leaching Procedure (TCLP)9 (which reflects municipal solid waste co-disposal conditions) or 

the synthetic precipitation leaching procedure (SPLP), or any number of deionized water based 

tests may be inappropriate, or are at least not optimal for evaluating the leaching potential of 

CCRs as they are actually managed (i.e., monofilled or codisposed with other CCRs). These tests 

either presume a set of prevailing landfill conditions (which may or may not exist at CCR 

disposal sites; e.g., TCLP), try to account for an environmental factor considered to be important 

in leaching (e.g., SPLP), or presume that the waste as tested in the laboratory will define the 

disposal conditions (such as deionized (DI) water tests). Most existing leaching tests are 

empirical, in that results are presented simply as the contaminant concentrations leached when 

using the test, and without measuring or reporting values for factors that may affect waste 

leaching, or that provide insight into the chemistry that is occurring in leaching. Most tests are 

performed as a single batch test, and so do not consider the effect of variations in conditions on 

waste constituent leaching10. 

In searching for a reliable procedure to characterize the leaching potential of metals from the 

management of CCRs, EPA sought an approach that (i) considers key aspects of the range of 

known CCR chemistry and management conditions (including re-use); and (ii) permits 

development of data that are comparable across U.S. coal and CCR types. Because the data 

resulting from this research will be used to support regulations, scrutiny of the data is expected. 

9 The Toxicity Characterization Leaching Procedure (TCLP) was not included as part of this study for 

two reasons. First, EPA previously made a waste status determination under RCRA that coal combustion 

residues are non-hazardous (65 FR 32214, May 22, 2000). Therefore, use of TCLP was not required as 

indicated under the RCRA toxicity characteristic regulation for determination of whether or not CCRs 

were hazardous. Second, TCLP was developed to simulate co-disposal of industrial waste with municipal 

solid waste as a mismanagement scenario, and to reflect conditions specific to this scenario. However, the 

vast majority of CCRs are not being managed through co-disposal with municipal solid waste, and the test 

conditions for TCLP are different from the actual management practices for most CCRs. In seeking a 

tailored, “best-estimate” of CCR leaching, the leaching framework provides the flexibility to consider the 

effects of actual management conditions on these wastes, and so will be more accurate in this case. 

10 Many factors are known or may reasonably be expected to affect waste constituent leaching. The 

solubility of many metal salts is well known to vary with pH; adsorption of metals to the waste matrix 

varies with pH; redox conditions may determine which metal salts are present in wastes; temperature may 

affect reaction rates; water infiltration can affect the leaching rate, and also affect leaching chemistry and 

equilibrium. 

17 
Characterization of Coal Combustion Residues II 

Therefore, the use of a published, peer-reviewed protocol is also considered to be an essential 

element of this work. 

EPA ORD has worked closely with EPA’s Office of Solid Waste and Emergency Response 

(OSWER) to identify an appropriate leaching protocol for evaluating CCRs. The protocol that 

has been adopted is the “Integrated Framework for Evaluating Leaching in Waste Management 

and Utilization of Secondary Materials” (Kosson et al., 2002) and referred to here as the 

“leaching framework.” The leaching framework consists of a tiered approach to leaching 

assessment. The general approach under the leaching framework is to use laboratory testing to 

measure intrinsic leaching characteristics of a material (i.e., liquid-solid equilibrium partitioning 

as a function of pH and LS ratio, mass transfer rates) and then use this information in 

conjunction with mass transfer models to estimate constituent release by leaching under specific 

management scenarios (e.g., landfilling). Unlike other laboratory leaching tests, under this 

approach, laboratory testing is not intended to directly simulate or mimic field conditions. 

Development work to-date on the leaching framework has focused on assessing metals leaching, 

and this work includes equilibrium batch testing (over a range of pH and LS ratio values), 

diffusion-controlled mass transfer, and percolation-controlled (column) laboratory test methods 

in conjunction with mass transfer models, to estimate release for specific management scenarios 

based on testing results from a common set of leaching conditions. EPA OSWER and ORD 

believe that this approach successfully addresses the concerns identified above, in that it seeks to 

consider the effect of key disposal conditions on constituent leaching, and to understand the 

leaching chemistry of wastes tested. 

The following attributes of the leaching framework were considered as part of the selection 

process: 

 The leaching framework will permit development of data that are comparable across U.S. 

coal and CCR types; 

 The leaching framework will permit comparison with existing laboratory and field leaching 

data on CCRs; 

 The leaching framework was published in the peer-reviewed scientific literature (Kosson et 

al., 2002); 

 On consultation with EPA’s OSWER, it was recommended as the appropriate protocol based 

on review of the range of available test methods and assessment approaches; and 

 On consultation with the Environmental Engineering Committee of the Science Advisory 

Board (June 2003), the committee considered the leaching framework responsive to earlier 

SAB criticisms of EPA’s approach to leaching evaluation, and also was considered broadly 

applicable and appropriate for this study 

For this study, the primary leaching tests used from the leaching framework were Solubility and 

Release as a Function of pH (SR002.1) and Solubility and Release as a Function of the LiquidSolid Ratio (LS) (SR003.1)11. These tests represent equilibrium-based leaching characterization 

11 LS refers to liquid to solid ratio (mL water/g CCR or L water/kg CCR) occurring during laboratory 

leaching tests or under field conditions. SR002.1 is carried out at LS=10 with several parallel batch 

extractions over a range of pH, while SR003.1 is carried out using several parallel batch extractions with 

18
Characterization of Coal Cumbustion Residues II 

(Kosson et al., 2002). The range of pH and LS ratio used in the leaching tests includes the range 

of conditions (pH and LS ratio) observed for current CCR management practices. Results of 

these tests provide insights into the physical-chemical mechanisms controlling constituent 

leaching. When used in conjunction with mass transfer and geochemical speciation modeling, the 

results can provide conservative12 but realistic estimates of constituent leaching under a variety 

of environmental conditions (pH, redox, salinity, carbonation) and management scenarios. 

Laboratory testing for leaching assessment was carried out at the U.S. EPA National Risk 

Management Research Laboratory (Research Triangle Park, NC) with technical assistance from 

Vanderbilt University. 

deionized water at LS= 0.5, 1, 2, 5 and 10. Under field conditions, LS refers to the cumulative amount of 

water passing through the total mass of CCR subject to leaching. 

12 In this report, “conservative” implies that the constituent release estimates are equal to or greater than 

actual expected release under field conditions. 

19 
Characterization of Coal Combustion Residues II 

2. MATERIALS AND METHODS 

2.1. CCR MATERIALS FOR EVALUATION 

The 23 CCR materials tested in this study include five fly ash, four unwashed gypsum, two 

washed gypsum, five scrubber sludges, two fixated scrubber sludges and five fixated scrubber 

sludges with lime obtained from eight coal fired power plants (Table 1). The facilities and CCRs 

that were sampled were selected to allow comparisons 

(i) between different CCR types from a given facility (Facilities A, B and K), 

(ii) between CCRs of the same type from the same facility without and with postcombustion NOx control, either by SNCR (Facility A) or SCR (Facility B), 

(iii) the impact of different FGD scrubber types on scrubber sludge and “as managed” FSS or 

FSSL (Facilities A, B, K and M), 

(iv) the influence of coal rank (bituminous vs. sub-bituminous) being combusted in facilities 

with similar APC technology configurations (Facility B and K with SCR on), 

(v) unwashed and washed gypsum from the same facility (Facilities N and O), and 

(vi) unwashed gypsum from four facilities (Facilities N, O, P and Q). This set of 23 CCRs 

reflects 84 and 74 percent, respectively, of the current and expected future facility 

configuration types with acid gas scrubbers based on generating capacity, but only a 

limited number of facilities within each configuration type. Figure 5, Figure 6, 

(vii) Figure 7, and Figure 8 diagram the layout of comparisons that will be used in 

presentation of data for fly ash, gypsum, scrubber sludge, fixated scrubber sludge and 

fixated scrubber sludge with lime, respectively. 

Facility A 

Coal: low sulfur bituminous 

APC: NO+SNCR+FF 

Facility B 

Coal: low sulfur bituminous

APC: NO+SCR+ESP(CS) [Mg lime]

Facility K 

Coal: sub- bituminous

APC: NO+SCR+ESP(CS) [Mg lime]

BFA 

(SCR On) 

DFA 

(SCR Off) 

AFA 

(SNCR On) 

CFA 

(SNCR Off) 

KFA 

(SCR On) 

Figure 5. Fly ash (FA) comparisons (CFA, AFA, DFA, etc refer to sample identification codes; 

see Table 1). Shorthand is used for when SCR is in use (“on”) or not in use (“off”). 

20
Facility N 

Coal: bituminous 

APC: FO+ESP(CS) 

Facility O 

Coal: bituminous 

APC: FO+SCR+ESP(CS) 

Gyp-U

OAU 

(unwashed) 

NAU 

(unwashed) 

Gyp-W

OAW 

(washed) 

NAW 

(washed) 

Facility P 

Coal: bituminous

APC: FO+ SCR & SNCR +ESP(CS)

PAD (U) 

(unwashed) 

QAU 

(unwashed) 

Facility Q 

Coal: sub-bituminous

APC: FO+SCR+ESP(CS)

Characterization of Coal Cumbustion Residues II 

Figure 6. Gypsum (Gyp-U, Gyp-W) comparisons (NAU, NAW, OAU, etc. are sample 

identification codes; see Table 1). 

Errata: Subsequent tables and figures indicate the Facility N has SCR in use. That is not correct. 

This will be corrected in Report 4 and does not change the leach testing results. 

21 
Facility A 

Coal: low sulfur bituminous

APC: NO+SNCR+FF

Facility B 

Coal: low sulfur bituminous

APC: NO+SCR+ESP(CS) [Mg lime]

BGD 

(SCR On) 

DGD 

(SCR Off) 

AGD 

(SNCR On) 

CGD 

(SNCR Off) 

KGD 

(SCR On) 

Facility K 

Coal: sub- bituminous

APC: NO+SCR+ESP(CS) [Mg lime]

Characterization of Coal Combustion Residues II 

Figure 7. Scrubber sludge (ScS) comparisons (CGD, AGD, DGD, etc. are sample identification 

codes; see Table 1). Shorthand is used for when SCR is in use (“on”) or not in (“off”). 

22
FSS: Fly Ash + Scrubber Sludge (FA+ScS) 

Facility A (FSS) 

Coal: low sulfur bituminous

APC: NO+SNCR+FF

ACC 

(SNCR On) 

CCC 

(SNCR Off) 

FSSL: Fly Ash + Scrubber Sludge + Lime (FA+ScS+lime) 

Facility B (FSSL) 

Coal: low sulfur bituminous

APC: NO+SCR+ESP(CS) [Mg lime]

Facility K (FSSL) 

Coal: sub-bituminous

APC: NO+SCR+ESP(CS) [Mg lime]

Facility M (FSSL) 

Coal: bituminous MAD MAS 

APC: IO+SCR+ESP(CS) (SCR Off) (SCR On) 

BCC 

(SCR On) 

DCC 

(SCR Off) 

KCC 

(SCR On) 

Characterization of Coal Cumbustion Residues II 

Figure 8. Fixated scrubber sludge (FSS) and fixated scrubber sludge with lime (FSSL) 

comparisons (DCC, BCC, KCC, etc. are sample identification codes; see Table 1). 

23 
Characterization of Coal Combustion Residues II 

2.1.1. Facilities Using Inhibited or Natural Oxidation of Scrubber Residues (Producing 

Scrubber Sludge or Fixated Scrubber Sludge) 

2.1.1.1. Facility A (Natural Oxidation and SNCR) 

Facility A is a 440-MW coal-fired power plant with a reverse-air fabric filter followed by a wet 

FGD system. The unit burns ~1 percent sulfur eastern bituminous coal. The unit operated at 

nominally full load for the duration of the test program. The unit is equipped with a pulverizedcoal boiler and in-furnace selective SNCR; urea was injected into the boiler during the course of 

operations within the duration of the initial part of this test program. However, urea was not 

injected into the boiler for the final comparison test (“SNCR off”). Gas exiting the furnace is 

split between two flues equipped with comparable control equipment. Particulate is removed 

with a reverse-air fabric filter. Flue gas is then scrubbed through a multiple tower wet FGD unit; 

FGD is a limestone natural-oxidation design. The two flues are joined prior to exhausting to a 

common stack. The annular stack rises 308 feet above the top of the incoming flue. The stack is 

operated in a saturated condition with no reheat. The fly ash and FGD waste are combined and 

then dewatered before landfill disposal. 

Facility A was sampled in September 2003. Three samples were collected in September 2003 

when the SCR was operating: one fresh fly ash sample collected from the ash hopper (sample 

AFA), one scrubber sludge filter cake sample collected after the centrifuge but before mixing 

with other materials in the pug mill (sample AGD), and one fixated scrubber sludge sample 

collected after mixing the scrubber sludge with fly ash and magnesium-enhanced lime in the pug 

mill (sample ACC). Three additional samples were collected from the same locations in February 

2004 when the SCR was not in use (samples CFA, CGD and CCC, respectively). Each sample 

consisted of two 5-gallon pails of the material, and all were collected by plant personnel. 

2.1.1.2. Facility B (Natural Oxidation and SCR) 

Facility B is a 640 MW coal-fired power plant with cold side ESP followed by a wet FGD 

system with Mg-lime. The unit burns medium to high sulfur eastern bituminous coals. The unit is 

equipped with a pulverized coal boiler and selective catalytic reduction composed of vanadium 

pentoxide (V2O5) and tungsten trioxide (WO3), on titanium dioxide (TiO2) supporting matrix. 

One set of samples was collected during the season of elevated ozone, when ammonia is injected 

into the ductwork in front of the SCR catalyst, resulting in a flue gas mixture with a 

concentration of 320 ppm ammonia as it enters the catalyst. Samples were also collected during 

the winter when ammonia was not being injected (“SCR off”). Particulate is removed with a 

cold-side ESP. Flue gas is then scrubbed through a wet FGD unit; FGD is an inhibited mag-lime 

design. The FGD sludge is thickened and then mixed with fly ash and magnesium-enhanced lime 

before landfill disposal in a clay-lined site. Refer to appendix G for a process flow-diagram for 

facility A. 

Three samples were collected in September, 2003 when the SCR was operating: one give gallon 

bucket of the fly ash (BFA) from the hoppers, partially dewatered scrubber sludge by 

centrifuging (BGD), and centrate cake (BCC) or scrubber sludge fixated with fly ash and Mg 

lime (MgO). Three additional samples were collected from the same locations in February, 2004 

when the SCR was not in use (samples DFA, DGD, and DCC, respectively). Each sample 

consisted of one 5-gallon bucket of the material, and all were collected a Natural Resource 

Technology contractor to EPRI. 

24
Characterization of Coal Cumbustion Residues II 

Coal analysis information for facility B was provided by EPRI. Results of the analysis on the 

medium to high sulfur bituminous coal for sulfur was 3.24% by weight, ash was 12.4%, moisture 

was 6.1%, heat of combustion was 12,000 BTU/lb, chloride was 615 ug/g, and mercury was 0.08 

ug/g. 

2.1.1.3. Facility K (Natural Oxidation and SCR) 

Facility K is two tangentially fired 400 MW coal-fired boilers with cold side ESP followed by a 

wet flue gas desulfurization system with wet Mg-lime natural oxidation. These units burn 

medium sulfur eastern bituminous coals from Ohio, Pennsylvania and West Virginia. Flue gas is 

scrubbed through a common wet FGD unit; FGD is a wet Mg-lime natural oxidation design. 

FGD sludge is mixed with fly ash and quicklime for stabilization prior to disposal. 

Two samples were collected on November 29, 2004: one scrubber sludge filter cake before 

mixing in the pug mill (sample KGD), and one fixated scrubber sludge collected after mixing the 

scrubber sludge with fly ash and 2-3% lime in the pugmill (sample KCC). On January 12, 2005, 

one fly ash sample was collected directly from the ESP before the ash storage silo (sample 

KFA). Each sample consisted of four 5-gallon pails of the material, and all were collected by 

plant personnel. 

2.1.1.4. Facility M (Inhibited Oxidation and SCR) 

Facility M is a 1,000+ MW power plant. The plant burns bituminous coal in a dry-bottom 

pulverizer boiler. Cold-side ESPs are used on all units for particulate control, and wet FGD 

systems are used to reduce SO2 emissions on two units. The wet FGD systems utilize limestone 

slurry and an inhibited oxidation process. The FGD sludge, consisting primarily of calcium 

sulfite, is pumped from the absorber to a thickener. Liquid overflow from the thickener is 

recycled back into the FGD system, and the thickened sludge is pumped to a series of drum 

vacuum filters for further dewatering. Water removed by the drum vacuum filters is recycled 

back into the FGD system, and the filter cake is taken by conveyor belt to a pug mill, where it is 

mixed with dry fly ash and dry quicklime for stabilization. The resulting scrubber FGD solids are 

taken by conveyor to a temporary outdoor stockpile, and then transported by truck either to a 

utilization site or to an on-site landfill. The currently active portion of the landfill is lined and 

includes leachate collection. An older inactive portion of the landfill is clay-lined but does not 

have leachate collection. 

Three samples were obtained from the Pug Mill Area by the EPRI contractor during the week of 

March 6, 2006 when the SCR was not operating: fly ash, vacuum drum filter cake, and fixated 

scrubber sludge with lime (only FSSL was evaluated as part of this study, sample MAD). In 

each case, the samples were collected daily during the four day sample collection (four daily 

samples of each), for compositing in the laboratory. All of the samples were collected into clean 

5 gallon plastic pails. Excess sample was containerized and discharged back into the appropriate 

system. The drum filter cake was sampled daily from the conveyor belt leading into the pug mill. 

Two of the three drum filters were running simultaneously; both were feeding the conveyor belt. 

The same drums were running each day of sampling. Each 5 gallon bucket was sealed 

immediately after collection and the lid secured with duct tape. The dry fly ash sample was 

obtained directly from the day tank via a hose connected to a sampling port. Each 5 gallon 

bucket was sealed immediately after collection and the lid secured with duct tape. FSS was 

25 
Characterization of Coal Combustion Residues II 

sampled from the conveyor belt on the outlet side of the pug mill on the first, third and fourth 

days. A clean, short handled spade was used to collect sample from the conveyor belt into a 2 

gallon bucket. The sample in the bucket was placed on a clean piece of 3 mm plastic sheeting; 

then more sample was collected from the conveyor belt into the bucket and added to the sheet 

until at least 6 gallons of sample was collected. Each sample was homogenized on the sheet 

using the spade and placed into a 5 gallon bucket, sealed immediately, and the lid secured with 

duct tape. A similar process was used to collect three more samples the week of May 9 when the 

SCR was in use (FSSL sample MAS). 

2.1.2. Facilities Using Forced Oxidation of Scrubber Residues (Producing FGD Gypsum) 

2.1.2.1. Facility N (Forced Oxidation) 

Facility N is a wall fired 715 MW coal-fired power plant with cold side ESP followed by a wet 

FGD system with wet limestone forced oxidation. The unit burns medium to high sulfur eastern 

bituminous coals approximately 3% sulfur. The gypsum is washed, dried and then sold to the 

wallboard industry. 

Facility N was sampled on June 1, 2006. Five gallon buckets of the washed and unwashed 

gypsum were collected by RMB Consulting & Research Inc. (Raleigh, NC) personnel and 

provided for analysis. 

2.1.2.2. Facility O (Forced Oxidation and SCR) 

Facility O is a tangentially fired 500 MW coal-fired plant with cold side ESP followed by a wet 

FGD system with wet limestone forced oxidation. The unit is equipped with a pulverized coal 

boiler and ammonia based SCR. This unit burns high sulfur eastern bituminous coals. Slurry 

from the absorber goes to a primary hydrocyclone for initial dewatering. The gypsum 

(hydrocyclone underflow) is dried on a vacuum belt and washed to remove chlorides, before use 

in wallboard. 

Two samples were collected from the FGD gypsum drying facility by compositing samples 

collected on June 10, 11, and 12, 2006 when the SCR was operating. On each day, two gallon 

pails of unwashed gypsum and washed/dried gypsum were collected. The unwashed gypsum was 

collected from the vacuum belt prior to the chloride spray wash. The washed/dried gypsum was 

collected from the end of the vacuum belt. The three daily samples were sent to Arcadis for 

compositing to form sample OAU (unwashed gypsum) and sample OAW (washed gypsum). All 

samples were collected by plant personnel. 

2.1.2.3. Facility P (Forced Oxidation and SCR and SNCR) 

Facility P is two wall fired 200 MW coal-fired boilers with cold side ESP followed by a wet 

FGD system with wet limestone forced oxidation. Unit 1 is equipped with SNCR and Unit 2 is 

equipped with SCR. These units burn medium sulfur eastern bituminous coals. Particulate is 

removed with a cold-side ESP. Flue gas is then scrubbed through a common wet FGD unit; FGD 

is a wet limestone forced oxidation design. The gypsum provided was not washed. 

Facility P was sampled in October 2006 when both SCR and SNCR were operating and the 

residues from Unit 1 and Unit 2 were commingled during collection. A five gallon bucket of the 

washed gypsum was collected by plant personnel. 

26
Characterization of Coal Cumbustion Residues II 

2.1.2.4. Facility Q (Forced Oxidation and SCR) 

Facility Q is a 1800 MW coal fired plant with hot side ESP followed by a wet flue gas 

desulfurization system with wet limestone forced oxidation. This unit burns sub-bituminous 

coals. Particulate is removed with a hot-side ESP. Flue gas is then scrubbed through a wet FGD 

unit; FGD is a wet limestone forced oxidation design that includes the addition of dibasic acid to 

the absorber13. The gypsum provided was not washed and was sampled on Oct. 30, 2006 by an 

EPRI contractor (Natural Resource Technology). A five gall bucket of the unwashed gypsum 

was shipped to ARCADIS for analysis on May 4, 2007. 

2.2. LEACHING ASSESSMENT PROTOCOLS 

Laboratory testing for this study focused on leaching as a function of pH and LS ratio as defined 

by the leaching framework. This test set is considered Tier 2 testing (equilibrium-based) for 

detailed characterization, which was selected to develop a comprehensive data set of CCR 

characteristics. Mass transfer rate testing (Tier 3, detailed characterization) may be carried out in 

the future for specific cases where results from equilibrium-based characterization indicate a 

need for detailed assessment. 

2.2.1. Alkalinity, Solubility and Release as a Function of pH (SR002.1) 

Alkalinity, solubility and release as a function of pH were determined according to method 

SR002.1 (Kosson et al., 2002). This protocol consists of 11 parallel extractions of particle size 

reduced material, at a LS ratio of 10 mL extractant/g dry sample. In this method, particle-size 

reduction is used to prepare large-grained samples for extraction so that the approach toward 

liquid-solid equilibrium is enhanced and mass transport through large particles is minimized. 

For the samples evaluated in this study, minimal size reduction was required. Each extraction 

condition was carried out in triplicate using 40 g of material for each material evaluated. In 

addition, three method blanks were included, consisting of the DI water, nitric acid and 

potassium hydroxide used for extractions. Typical particle size of the tested materials was less 

than 300 μm using standard sieves according to ASTM E-11-70(1995). An acid or base addition 

schedule is formulated based on initial screening for eleven extracts with final solution pH values 

between 3 and 12, through addition of aliquots of nitric acid or potassium hydroxide as needed. 

The exact schedule is adjusted based on the nature of the material; however, the range of pH 

values includes the natural pH of the matrix that may extend the pH domain (e.g., for very 

alkaline or acidic materials). The final LS ratio is 10 mL extractant/g dry sample which includes 

DI water, the added acid or base, and the amount of moisture that is inherent to the waste matrix 

as determined by moisture content analysis. The eleven extractions were tumbled in an end-overend fashion at 28 ± 2 rpm for 24 hours followed by filtration separation of the solid phase from 

the extract using a 0.45 μm polypropylene filter. Each extract then was analyzed for constituents 

of interest. The acid and base neutralization behavior of the materials is evaluated by plotting the 

pH of each extract as a function of equivalents of acid or base added per gram of dry solid. 

Concentration of constituents of interest for each extract is plotted as a function of extract final 

pH to provide liquid-solid partitioning equilibrium as a function of pH. Initially, the SR002.1 test 

was carried out in triplicate; however, replication was reduced to two replicates of the test 

13 Dibasic acid (DBA) is a commercial mixture of glutaric, succinic, and adipic acids: 

HOOC(CH2)2-4COOH. 

27 
Characterization of Coal Combustion Residues II 

method for later samples based on good replication and consistency amongst the early results 

(Sanchez et al., 2006). 

2.2.2. Solubility and Release as a Function of LS Ratio (SR003.1) 

Solubility and release as a function of LS ratio was determined according to method SR003.1 

(Kosson et al., 2002). This protocol consists of five parallel batch extractions over a range of LS 

ratios (i.e., 10, 5, 2, 1, and 0.5 mL/g dry material), using DI water as the extractant with aliquots 

of material that has been particle size reduced. Typical particle size of the material tested was 

less than 300 μm. Between 40 and 200 g of material were used for each extraction, based on the 

desired LS ratio. All extractions are conducted at room temperature (20 ± 2 °C) in leak-proof 

vessels that are tumbled in an end-over-end fashion at 28 ± 2 rpm for 24 hours. Following gross 

separation of the solid and liquid phases by centrifuge or settling, leachate pH and conductivity 

measurements are taken and the phases are separated by pressure filtration using 0.45-μm 

polypropylene filter membrane. The five leachates are collected, and preserved as appropriate for 

chemical analysis. Initially, the SR003.1 test was carried out in triplicate; however, replication 

was reduced to two replicates of the test method for later samples based on good replication and 

consistency amongst the early results. 

2.3. ANALYTICAL METHODS 

2.3.1. Surface Area and Pore Size Distribution 

A Quantachrome Autosorb-1 C-MS chemisorption mass spectrometer was used to perform 5-

point Brunauer, Emmett, and Teller (BET) method surface area, pore volume, and pore size 

distribution analysis on each as-received and size-reduced CCR. A 200 mg sample was degassed 

under vacuum at 200 oC for at least one hour in the sample preparation manifold prior to analysis 

with N2 as the analysis gas. Standard materials with known surface area were routinely run as a 

QC check. 

2.3.2. pH and Conductivity 

pH and conductivity were measured for all aqueous extracts using an Accumet 925 pH/ion 

meter. The pH of the leachates was measured using a combined pH electrode accurate to 0.1 pH 

units. A 3-point calibration was performed daily using pH buffer solutions at pH 4.0, 7.0 and 

10.0. Conductivity of the leachates was measured using a standard conductivity probe. The 

conductivity probe was calibrated using appropriate standard conductivity solutions for the 

conductivity range of concern. Conductivity meters typically are accurate to ± 1% and have a 

precision of ± 1%. 

2.3.3. Moisture Content 

Moisture content of the “as received” CCRs was determined using American Society for Testing 

and Materials (ASTM) D 2216-92. This procedure supersedes the method indicated in the 

version of the leaching procedure published by Kosson et al. (2002). 

2.3.4. Carbon Content Organic Carbon/Elemental Carbon Analyzer 

Organic carbon (OC) and elemental carbon (EC) content of each CCR tested was measured using 

a Sunset Lab thermal-optical EC/OC analyzer using the thermal/optical method (NIOSH Method 

5040). The sample collected on quartz fiber filters is heated under a completely oxygen-free 

28
Characterization of Coal Cumbustion Residues II 

helium atmosphere in a quartz oven in four increasing temperature steps (375 °C, 540 °C, 670 °C 

and 870 °C) at 60 second ramp times for the first three temperatures and a ramp time of 90 

seconds for the final temperature. The heating process removes all organic carbon on the filter. 

As the organic compounds are vaporized, they are immediately oxidized to carbon dioxide in an 

oxidizer oven which follows the sample oven. The flow of helium containing the produced 

carbon dioxide then flows to a quartz methanator oven where the carbon dioxide is reduced to 

methane. The methane is then detected by a flame ionization detector (FID). After the sample 

oven is cooled to 525 oC, the pure helium eluent is switched to an oxygen/helium mixture in the 

sample oven. At that time, the sample oven temperature is stepped up to 850 oC. During this 

phase, both the original elemental carbon and the residual carbon produced by the pyrolysis of 

organic compounds during the first phase are oxidized to carbon dioxide due to the presence of 

oxygen in the eluent. The carbon dioxide is then converted to methane and detected by the FID. 

After all carbon has been oxidized from the sample, a known volume and concentration of 

methane is injected into the sample oven. Thus, each sample is calibrated to a known quantity of 

carbon as a means of checking the operation of the instrument. 

The calibration range for these analyses was from 10 to 200 μg/cm2 of carbon using a sucrose 

solution as the standard. The detection limit of this instrument is approximately 100 ng/cm2 with 

a linear dynamic range from 100 ng/cm2

 to 1 g/cm2. 

2.3.5. Dissolved Inorganic Carbon (DIC) and Dissolved Organic Carbon (DOC) 

Analyses of total organic carbon and inorganic carbon were performed on a Shimadzu model 

TOC-V CPH/CPN. Five-point calibration curves, for both dissolved inorganic carbon (DIC) and 

non-purgeable dissolved organic carbon (DOC) analyses, were generated for an analytical range 

between 5 ppm and 100 ppm and are accepted with a correlation coefficient of at least 0.995. An 

analytical blank and check standard at approximately 10 ppm were run every 10 samples. The 

standard was required to be within 15% of the specified value. A volume of approximately 16 

mL of undiluted sample is loaded for analysis. DIC analysis is performed first for the analytical 

blank and standard and then the samples. DOC analysis is carried out separately after completion 

of DIC analysis. DOC analysis begins with addition of 2 M (mole/L) of hydrochloric acid to 

achieve a pH of 2 along with a sparge gas flow rate of 50 mL/min to purge inorganic carbon 

prior to analysis. Method detection limit (MDL) and minimum level of quantification (ML) are 

shown in Table 5. 

Table 5. MDL and ML of analysis of DIC and DOC. 

MDL (μg/L) ML (μg/L) 

DIC 70 200 

DOC 90 200 

2.3.6. Mercury (CVAA, Method 3052, and Method 7473) 

Liquid samples were preserved for mercury analysis by additions of nitric acid and potassium 

permanganate and then prepared prior to analysis according to the following method. For each 

87 mL of sample, 3 mL of concentrated nitric acid and 5 mL of 5 wt% aqueous potassium 

permanganate solution were added prior to storage. Immediately before cold vapor atomic 

absorption (CVAA) analysis, 5 mL of hydroxylamine were added to clear the sample and then 

the sample was digested according to ASTM Method D6784-02 (Ontario Hydro) as described for 

29 
Characterization of Coal Combustion Residues II 

the permanganate fraction (ASTM, 2002). On completion of the digestion, the sample was 

analyzed for mercury by CVAA. Samples with known additions of mercury for matrix analytical 

spikes also were digested as described above prior to CVAA analysis. 

Sample preparation of the solids and filters was carried out by HF/HNO3 microwave digestion 

according to Method 3052 (EPA, 1996) followed by CVAA analysis as indicated above. No 

additional preservation or digestion was carried out prior to CVAA analysis. 

Mercury analysis of each digest, extract and leachate was carried out by CVAA according to 

EPA SW846 Method 7470A “Mercury in Liquid Waste (Manual Cold Vapor Technique)” (EPA, 

1998a). A Perkin Elmer FIMS 100 Flow Injection Mercury System was used for this analysis. 

The instrument was calibrated with known standards ranging from 0.025 to 1 μg/L mercury. 

Solids also were analyzed by Method 7473 “Mercury in Solids and Solutions by Thermal 

Decomposition, Amalgamation, and Atomic Absorption Spectrophotometry” (EPA, 1998b). A 

Nippon MD-1 mercury system was used for this analysis. The instrument was calibrated with 

known standards ranging from 1 to 20 ng of mercury. The method detection limit for mercury in 

solids is 0.145 μg/kg. 

2.3.7. Other Metals (ICP-MS, Method 3052, and Method 6020) 

Liquid samples for ICP-MS analysis were preserved through addition of 3 mL of concentrated 

nitric acid (trace metal grade) per 97 mL of sample. Known quantities of each analyte were also 

added to sample aliquots for analytical matrix spikes. Solid samples were digested by EPA 

Method 3052 (EPA, 1996) prior to ICP-MS analysis. 

2.3.7.1. ICP-MS Analysis 

ICP-MS analyses of aqueous samples from laboratory leaching tests were carried out at 

Vanderbilt University (Department of Civil and Environmental Engineering) using a Perkin 

Elmer model ELAN DRC II in both standard and dynamic reaction chamber (DRC) modes. 

Standard analysis mode was used for all analytes except for As and Se, which were run in DRC 

mode with 0.5 mL/min of oxygen as the reaction gas. Seven-point standard curves were used for 

an analytical range between approximately 0.5 μg/L and 500 μg/L and completed before each 

analysis. Analytical blanks and analytical check standards at approximately 50 μg/L were run 

every 10 samples and required to be within 15% of the specified value. Samples for analysis 

were diluted gravimetrically to within the targeted analytical range using 1% v/v Optima grade 

nitric acid (Fisher Scientific). Initially, analyses for 10:1 dilutions were performed to minimize 

total dissolved loading to the instrument. Additional dilutions at 100:1 and 1000:1 were analyzed 

if the calibration range was exceeded with the 10:1 dilution. 50 μL of a 10 mg/L internal 

standard consisting of indium (In) (for mass range below 150) and holmium (Ho) (for mass 

range over 150) was added to 10 mL of sample aliquot prior to analysis. Analytical matrix spikes 

were completed for one of each of the replicate extracts from SR002.1. For each analytical 

matrix spike, a volume between 10 μL and 100 μL of a 10 mg/L standard solution was added to 

10 mL of sample aliquot. Table 6 provides the element analyzed, method detection limit (MDL) 

and minimum level of quantification (ML). Analyte concentrations measured that are less than 

the ML and greater than the MDL are reported as estimated value using the instrument response. 

The values reflect the initial 10:1 dilution used for samples from laboratory leaching tests. 

30
Characterization of Coal Cumbustion Residues II 

Table 6. Method detection limits (MDLs) and minimum level of quantification (ML) for ICPMS analysis on liquid samples.* 

Symbol Units MDL ML 

Al μg/L 1.25 2.00 

Sb μg/L 0.60 2.00 

As μg/L 1.09 2.00 

Ba μg/L 0.75 2.00 

Be μg/L 0.85 2.00 

B μg/L 0.63 2.00 

Cd μg/L 0.40 1.00 

Ca μg/L 1.86 5.00 

Cs μg/L 0.60 2.00 

Cr μg/L 0.47 1.00 

Co μg/L 0.51 2.00 

Cu μg/L 0.87 2.00 

Fe μg/L 1.55 5.00 

Pb μg/L 0.28 1.00 

Li μg/L 0.80 2.00 

Mg μg/L 1.17 2.00 

Mn μg/L 0.47 1.00 

Mo μg/L 0.75 2.00 

Ni μg/L 0.90 2.00 

K μg/L 1.86 5.00 

Re μg/L 0.30 1.00 

Rb μg/L 0.70 2.00 

Se μg/L 0.78 2.00 

Si μg/L 1.85 5.00 

Ag μg/L 2.10 5.00 

Na μg/L 1.12 2.00 

Sr μg/L 0.47 1.00 

Tl μg/L 0.63 2.00 

Sn μg/L 0.87 2.00 

Ti μg/L 0.61 2.00 

U μg/L 0.36 1.00 

V μg/L 0.79 2.00 

Zn μg/L 1.15 2.00 

Zr μg/L 0.60 2.00 

* All elements indicated in Table 6 have been analyzed, however, only elements indicated in 

bold are reported as part of the leaching studies. The elements that were included in the leaching 

studies were selected based on input from EPA program offices due to potential concern for 

human health and the environment. 

31 
Characterization of Coal Combustion Residues II 

2.3.8. X-Ray Fluorescence (XRF) 

XRF analysis was performed on each CCR to provide additional information on each CCR total 

elemental composition. For each CCR two pellets were prepared as follows. 3000 mg of material 

was weighed and mixed with 1.5 mL (100 mg dry solids) of liquid binder to give a 32 mm 

diameter pellet weighing 3150 mg with a material-to-diluent ratio of 0.05. For high carbon 

content samples 3.0 ml (100 mg dry solids) of liquid binder was used to give a 32 mm diameter 

pellet weighing 3300 mg with a material to diluent ratio of 0.1. XRF intensities were collected on 

each side of each pellet using Philips SuperQ data collection software and evaluated using 

Omega Data System’s UniQuant 4 XRF “standardless” data analysis software. The UQ/Fly ash 

calibration was used to analyze the samples. The pellets were evaluated as oxides. Known fly ash 

Standard Reference Materials (SRMs) were also run to assess the accuracy of the analysis. This 

information is useful in supplementing CVAA and ICP results. 

2.3.9. MDL and ML for Analytical Results 

The MDL is defined by 40 CFR Part 136, Appendix B, July 1, 1995, Revision 1.11 as “the 

minimum concentration of a substance that can be measured and reported with 99% confidence 

that the analyte concentration is greater than zero and is determined from analysis of a sample in 

a given matrix containing the analyte.” 

The MDL was determined statistically from data generated by the analysis of seven or more 

aliquots of a spiked reagent matrix14 and verified by the analysis of calibration standards near the 

calculated MDL according to EPA (2003). The MDL then was determined by multiplying the 

standard deviation of the replicate measurements by the appropriate Students t value for a 99% 

confidence level (two tailed) and n-1 (six) degrees of freedom and also multiplying by the 

minimum dilution factor required for matrix preservation and analysis. 

The ML is defined by 40 CFR Part 136, 1994 as “the lowest level at which the entire analytical 

system must give a recognizable signal and acceptable calibration point for the analyte.” 

According to EPA (2003), the ML is intended to be the nearest integer value (i.e., 1, 2 or 5x10n, 

where n is an integer) to 10 times the standard deviation observed for determination of the MDL. 

This value is also multiplied by the minimum dilution factor required for preservation and 

analysis of the sample matrix to obtain the ML reported here. 

Mercury, as measured by CVAA, required modification of the calculation of the MDL and ML 

because very consistent replication resulted in calculation of a MDL lower than the instrument 

detection limit. For this case, the standard deviation of seven replicate analyses of 0.025 μg/L 

was 0.00069. Therefore, the MDL was set equal to the instrument detection limit of 0.001 μg/L 

times the minimum dilution factor from sample preparation (3.59) to result in an MDL of 0.0036 

μg/L. The ML was set to 10 times the instrument detection limit and rounded to the nearest 

integer value as above. The resulting ML was 0.01 μg/L. 

14 Establishing spikes in an actual leaching extract matrix is not possible because the sample being 

extracted dictates the matrix composition by virtue of the constituents that partition into the resulting 

aqueous extract, which varies by test position and material being tested. However, the extract aliquots are 

diluted at least 10:1 with 1% nitric acid (prepared from Optima grade nitric acid, Fisher Scientific), and 

the COPCs are dilute in the resulting analytical sample. Therefore, the 1% nitric acid solution was used 

as the matrix for MDL and ML determinations. 

32
Characterization of Coal Cumbustion Residues II 

2.4. QUALITY ASSURANCE ASSESSMENT

2.4.1. Homogenization of Individual CCR Samples and Aliquots for Analyses 

To ensure sample homogeneity the fly ashes were mixed using a Morse single can tumbler 

model 1-305 as described in Sanchez et al. (2006). Scrubber sludges that were flowable slurries 

were mixed using a paddle mixer. Gypsum and fixated scrubber sludge samples were mixed by 

repetitively coning 15 and quartering while passing through a mesh screen. After mixing, ten 

Subsamples were taken from FSSL sample MAD and analyzed by XRF to evaluate the 

homogeneity of the resultant material; Figure 9 presents the coefficient of variation for the XRF 

results. These results indicate that total content variability for primary and most trace 

constituents is less than 20% for this set of samples. 

0

5

10

15

20

25

30

35

40

C.V. (%) 

K P S V Y

F Si Ti

Al Cl Ni Sr Zr

As Br Cr Fe La Sc Se W Ba Ca Zn

Cd Ce Co Cu Ga Ge Na Nb Pb Rb

Mg

Mn Mo

Fac. M, SCR off, FSSL (MAD)

Figure 9. Coefficient of variation (C.V.) from XRF elemental analysis of 10 subsamples of 

FSSL sample MAD after mixing by coning and quartering. 

2.4.2. Leaching Test Methods and Analytical QA/QC 

One of the requirements of this project was to establish a QA/QC framework for the leaching 

assessment approach developed by Kosson et al. (2002). The developed QA/QC framework 

incorporates the use of blanks, spiked samples, and replicates. Appendix A provides the 

complete Quality Assurance Project Plan, as updated for this phase of the study. For each 

designated leaching test condition (i.e., acid or base addition to establish end-point pH values 

and LS value), triplicate leaching test extractions were completed (i.e., three separate aliquots of 

CCR were each extracted at the designated test condition) for early samples, while duplicate 

extractions were used after evaluation of initial results. The three types of method blanks were 

the deionized water case, the most concentrated nitric acid addition case, and the most

15 "Coning and quartering" is a term used to describe how the material is mixed. The approach is to pass 

the material through a screen so that a "cone" forms in the collection container. Then the cone is bisected 

twice into quarters (quarter sections of the cone) and each section then is passed sequentially through the 

screen again to form a new cone. This sequence is repeated several times to achieve desired mixing. 

33 
Characterization of Coal Combustion Residues II 

concentrated potassium hydroxide addition case. Each method blank was carried through the 

entire protocol, including tumbling and filtration, except an aliquot of CCR was not added. 

During analysis for mercury and elemental species by ICP-MS, analytical spikes for the 

constituents of interest were carried out for one replicate of each test case to assess analytical 

recoveries over the complete range of pH and liquid matrix conditions. Multipoint calibration 

curves using at least seven standards and an initial calibration verification (ICV) using a standard 

obtained from a different source than the calibration standards were completed daily or after 

every 50 samples, whichever was more frequent. In addition, instrument blanks and continuing 

calibration verification (CCV) standards were analyzed after every 10 analytical samples and 

required to be within 10 percent of the expected value. Samples are rerun if they are not within 

10 percent of the expected value. CCV standards and instrument blanks also were run at the end 

of each batch of samples. 

For both ICP-MS and CVAA analyses, each sample was analyzed along with a matrix spike, 

which is an aliquot of the sample plus a known spike concentration of the element of interest. 

The “spike recovery” was required to be within 80 – 120% of the expected value for an 

acceptable analytical result. 

2.4.3. Improving QA/QC efficiency 

Throughout the study, the approach to QA/QC was regularly reviewed to seek out opportunities 

for increased evaluation efficiency without unacceptable degradation of precision or accuracy in 

results. Based on evaluation of results from the first several facilities (Sanchez et al, 2006), the 

number of replicates for Method SR002.1 (solubility as a function of pH) and Method SR003.1 

(solubility as a function of liquid/solid ratio) was reduced from three to two. Results from this 

study (Sanchez et al., 2006 and this report) show that the precision between duplicate analyses is 

acceptable and that the triplicate set does not significantly increase the quality of the data set. 

This finding follows from recognition that (i) the data sets generated by Method SR002.1 and 

SR003.1 must provide both consistency between replicate extractions and analyses, and internal 

consistency between results at different pH and LS ratio, and (ii) precision is controlled primarily 

by the degree of homogeneity of the CCR under evaluation and representative sub-sampling, 

rather than by the intrinsic variability of the leaching test methods. There were a total of 11,743 

observations for the 14 parameters evaluated in detail in this report (pH and 13 constituents of 

interest). Review of the resulting data sets indicated 15 outlier pH values of 846 measurements16 

and an additional 19 outliers out of 10,897 measurements of specific constituents. Thus, the 

overall error rate was less than 2%. Implementation of a reduction in the number of replicates 

has greatly improved laboratory efficiency without compromising data quality. 

Data were screened for outliers based on comparison of individual data points (i) relative to 

replicate extractions (i.e., parallel extractions of aliquots of the same material under the same 

extraction conditions), and (ii) relative to the other data points in the extraction series (i.e., 

parallel extractions of aliquots of the same material at different pH (SR02) and LS conditions 

16 When a pH measurement is determined to be an outlier, then all constituent measurements associated 

with the particular extract sample are also considered outliers because they would be incorrectly evaluated 

as release as a function of pH. This resulted in excluding (15)×(13)=195 individual constituent 

measurements. 

34
Characterization of Coal Cumbustion Residues II 

(SR03)) because of the expected systematic response behavior. The pH was considered an 

outlier when the final pH of the extract deviated from the other replicates by more than 0.5 pH 

units and corresponding constituent analyses did not follow systematic behavior indicated by 

other extracts across multiple constituents. Individual constituents were considered outliers when 

results of constituent analyses deviated from the systematic behavior indicated by results in the 

extraction series (as a function of pH or as a function of LS) by more than one-half to one order 

of magnitude. Results were screened through inspection of the appropriately plotted results. 

Data quality indicators (DQIs) were measured for all parameters continuously during the 

leaching experiments and during analytical tasks. Chemical (ICP, CVAA, XRF, IC, EC/OC) and 

physical (surface area, pore size distribution and density) characterization data were reduced and 

reports were generated automatically by the instrument software. The primary analyst reviewed 

100% of the report data for completeness to ensure that quality control checks met established 

criteria. Sample analysis was repeated for any results not meeting acceptance criteria. A 

secondary review was performed by the Inorganic Laboratory Manager to validate the analytical 

report. A data quality report for the CCR leach testing results will be provided in the fourth and 

final report of this research. The data quality report will cover the leach test results documented 

in Report 1 (Sanchez et al., 2006), Report 2 (this report), and a third report (in preparation). The 

fourth report summarizes the data from the first three reports and provides probabilistic 

assessment of the potential release rates of mercury and other metals based on plausible 

management practices. 

2.5. INTERPRETATION AND PRESENTATION OF LABORATORY 

LEACHING DATA 

Complete laboratory leaching results for each CCR type and test method are presented in 

Appendices D and E. Appendix D presents results for Solubility and Release as a Function of pH 

(SR002.1). Appendix E presents results for Solubility and Release as a Function of LS ratio 

(SR003.1). Results are organized by CCR type (fly ash, gypsum, scrubber sludge, fixated 

scrubber sludge), with pH results followed by mercury and then other constituents of interest 

(aluminum, antimony, arsenic, barium, boron, cadmium, chromium, cobalt, lead, molybdenum, 

selenium and thallium). For SR002.1, pH results are a titration curve of pH as a function of 

milliequivalents of acid or base, with acid additions considered positive (+) and base additions 

considered negative (-). For SR003.1, pH results are a curve of pH as a function of LS ratio. 

For Solubility and Release as a Function of pH (SR002.1), results for each CCR type are grouped 

as described in Figure 5 through Figure 8. Results are presented as extract concentrations as a 

function of pH. The “natural” pH17 of the system is indicated as a vertical line to the average pH 

and a horizontal line to the y-axis indicating the corresponding extract concentration. Included 

with each figure are horizontal lines at the drinking water maximum concentration level (MCL) 

or drinking water equivalent level (DWEL)18, and analytical limits (ML and MDL) to provide a 

frame of reference for the results. Also included with each figure are the 5 and 95 percentile for 

pH (vertical lines) from field observations of leachate from landfills and surface impoundments 

17 “Natural pH” or “own pH” of a material refers to the equilibrium pH when the material is placed in 

deionized water at a ratio of 10 g CCR per 100 mL of water. 

18 MCL and DWEL values used are as reported in (EPA, 2006). 

35 
Characterization of Coal Combustion Residues II 

for combustion residues (see Section 2.5.1). An annotated example of the results is provided as 

Figure 10. 

For Solubility and Release as a Function of LS ratio (SR003.1), results are presented as extract 

concentrations as a function of LS ratio. Also indicated are the relevant MCL, ML and MDL or 

DWEL. 

2.5.1. Interpretation of Mechanisms Controlling Constituent Leaching 

Constituent (e.g., mercury, arsenic, and selenium) concentrations observed in laboratory leach 

test extracts and in field leachate samples may be the result of several mechanisms and factors. 

The discussion presented here focuses on constituent leaching and source term modeling 

approaches. Source term is defined here as the flux or amount of constituent released from the 

waste or secondary material (e.g., CCRs). Factors controlling constituent release and transport in 

and within the near field of the CCRs are often distinctly different from the factors and 

mechanisms which are important for subsequent vadose zone or groundwater transport outside of 

the near field area. 

In general, constituents are present in the waste or secondary material either as adsorbed species, 

co-precipitated as amorphous or crystalline solid phases, or incorporated as trace components in 

solid phases. If chemical equilibrium conditions are approached (as is the approximate case for 

the laboratory and field sample conditions discussed in this report), then the functional behavior 

of the aqueous solution concentration reflects the nature of the constituent species in the waste or 

secondary material, the presence of any co-constituents in the aqueous phase influencing 

aqueous solution speciation (e.g., effects of high ionic strength, chelating or complexing 

constituents), and the presence of species in the solution that may compete for adsorption sites if 

adsorption is the controlling solid phase mechanism. If the constituent is present in the waste or 

secondary material as an adsorbed species, many different adsorption/desorption characteristic 

patterns are possible (Ruthven, 1984; Duong, 1998). 

The simplest case is when the constituent of interest is present at very low concentration in the 

waste or secondary material, relatively weakly adsorbed, and the presence of complexing and, or, 

competing species in solution is at a constant concentration. For this case, leaching test results 

will indicate a constant concentration as a function of pH at a fixed LS ratio, and linearly 

increasing concentration as LS ratio decreases at constant pH. This case is represented 

mathematically as a linear equilibrium partitioning function, where the critical constant of 

proportionality is the partitioning coefficient, commonly known as Kd. Linear partitioning and 

use of Kd values is a common approach for mathematically modeling contaminant transport at 

low contaminant concentrations in soils. Assumption of linear partitioning is a valid and useful 

approach when the necessary conditions (discussed above) are fulfilled19. 

For mercury adsorbed on activated carbon or char particles in fly ash, a complex combination of 

adsorption mechanisms is indicated. During laboratory leaching tests, mercury concentrations in 

the leaching test extracts are relatively constant over the pH range and LS ratio of interest, and 

19 Often specific Kd values are a function of pH because of competition for adsorption sites by hydrogen 

ions. However, often a single Kd or range of Kd values are used in contaminant fate and transport models, 

without specific relationship between pH and Kd which can result in misrepresentation of actual 

contaminant behavior. 

36
MDL

ML

10.3

46.8

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

As [μg/L]

pH

SR2 DFA A SR2 DFA - B

SR2 DFA C

SR2-DFA-A

SR2=SR002.1

Sample name

Replicate A

Characterization of Coal Cumbustion Residues II 

independent of total mercury content in the CCR. In addition, the total mercury content in the 

CCR is very low. These results are indicative of adsorption phenomena where, in the adsorbed 

state, interactions between adsorbed mercury species are stronger (thermodynamically) than the 

interactions between the adsorbed mercury species and carbon surface20. This observation has 

been supported by the observation of mercury dimer formation during sorption (Munro et al, 

2001) and the occurrence of chemisorption as the dominant adsorption mechanism at 

temperatures above 75 oC (consistent with conditions in air pollution control devices; Vidic, 

2002). In other studies, this phenomenon has been observed as the formation of molecular 

clusters on the adsorbent surface (Ruthven, 1984; Duong, 1998; Rudzinski et al., 1997). For this 

case, use of a Kd approach would underestimate release because desorption is best represented as 

a constant aqueous concentration until depletion occurs. 

A third case is encountered when the constituent of interest is present in the waste or secondary 

material (e.g., CCR) as a primary or trace constituent in either an amorphous or crystalline solid 

phase and there may be complexing or chelating co-constituents in the aqueous phase. Observed 

aqueous concentrations are a non-linear function of pH and LS ratio, and reflect aqueous 

saturation with respect to the species of interest under the given conditions (pH, co-constituents). 

For these cases, an approximation of field conditions can be made empirically based on 

laboratory testing and observed saturation over the relevant domain (as applied in this report) or 

geochemical speciation modeling coupled with mass transfer modeling can be used to assess 

release under specific field scenarios (the subject of a future report). Use of a Kd approach would 

not be appropriate for these cases because constituent concentrations will remain relatively 

constant at a given pH until the controlling solid phase is depleted and control is shifted to a new 

solid phase or mechanism. 

- ­ -

- ­

95 5 th %tile for field pH th %tile for field pH 

Own pH 

Concentration 

at own pH 

MCL 

MDL 

ML 

20 For this case, the first mercury molecule is adsorbed more weakly than subsequent mercury molecules 

because the adsorbed mercury-mercury interaction is stronger than the adsorbed mercury-carbon surface 

interaction [see Sanchez et al. (2006) for further discussion]. 

37 

Figure 10. An example of extract concentrations as a function of pH from SR002.1.
Characterization of Coal Combustion Residues II 

38

2.6. FIELD pH PROBABILITY DISTRIBUTION 

A probability distribution of field leachate pH values from coal combustion waste landfills was 

derived, as described below, from the set of field pH observations included in the EPA Risk 

Report (EPA, 2007). The pH probability distribution used in this report considers additional data 

beyond the pH probability distribution used in Report 1 (Sanchez et al, 2006), which was based 

solely on relevant data from the EPA Office of Solid Waste database (EPA, 2000) and included 

158 observations from six CCR disposal facilities. The data set developed for the EPA Risk 

Report included (i) observations from the comprehensive database of landfill leachate 

characteristics developed by the EPA’s Office of Solid Waste (EPA, 2000), (ii) field 

observations from literature, primarily from EPRI reports, (iii) additional data reported to EPA, 

and (vi) pH observations from laboratory leaching tests. Only pH measurements from field 

samples (i.e., leachate, pore water) were selected for use in development of the resulting pH 

probability distribution. The resulting data set included 580 observations from 42 CCR landfill 

disposal facilities and was highly unbalanced, with some sites having only a few (e.g., less than 

five) observations and some sites having many observations (e.g., greater than 20). To prevent 

the unbalanced data from skewing the resulting probability distribution, the minimum, 25th, 50th, 

75th percentile, and maximum values of observations for each individual facility were compiled 

into a single data set. For facilities with fewer than five observations, all observations for that 

facility were included. This data set then served as the basis for fitting a statistical distribution 

function. For each data set, different distribution functions were used to fit the data and the one 

providing the best data fit based on the chi-square test was selected. The resulting field pH 

probability distribution was truncated and normalized to the pH range of the field data. 

The resulting pH probability distribution developed in this report is compared in Figure 11 and 

Table 7 to the pH probability distribution used in Report 1. The new pH probability distribution 

reflects a similar range of pH, but has a more alkaline median value, and in general, has greater 

weighting in the pH range between 8 and 12. The probability distribution used in this report is 

considered more representative than what was used in Report 1 because of a larger dataset using 

results from recent studies by EPA and EPRI. (EPA, 2006; EPRI, 2006) 

Field pH observations were also evaluated for surface impoundments receiving CCRs from coal 

combustion facilities with FGD scrubbers in use. Pore water pH values measured in samples 

obtained from within the settled CCRs were selected from the EPRI database. Resulting pH 

observations were across the same range as the landfill field pH observations, but were 

insufficient to develop an independent pH probability distribution. Therefore, the same pH 

probability distribution was used for landfill and surface impoundment facilities in this report.
Characterization of Coal Cumbustion Residues II 

39 

0

0.2

0.4

0.6

0.8

1

2 3 4 5 6 7 8 9 10 11 12 13

 pH

 Probability

EPA leach 2000

Fitted distribution (LogLogistic, (1))

EPA Risk database & EPA leach 2000

Fitted distribution (BetaGeneral, (2))

(1)

(2)

Distributions

LogLogistic(4.7386, 2.9538, 3.4815)

BetaGeneral(9.0369, 1.5076, -7.4214, 12.814)

pH Field data

Fitted 

distribution Simulated

Min 2.75 -7.42 2.76

5th percentile 5.40 5.84 5.85

50th percentile 10.53 10.38 10.24

90th percentile 12.20 12.18 11.94

95th percentile 12.40 12.43 12.43

Max 12.80 12.81 12.43

Figure 11. Probability distributions for field pH used in Report 1 (LogLogistic) and this report 

(BetaGeneral). Summary statistics for the field data and the probability distribution used in this 

report (BetaGeneral) are provided to the right of the graph.. 

Table 7. Comparison of summary statistics for field pH data and pH probability distributions 

used in Report 1 and this report. 

Field Data 

Distribution

Estimates 

 Used in Release 

Report 1 This Report Report 1 This Report 

Minimum 5.40 2.75 4.92 2.76 

5th percentile 5.80 5.40 5.97 5.85 

50th percentile 7.70 10.53 7.63 10.24 

90th percentile NR 12.20 NR 11.94 

95th percentile 12.09 12.40 10.63 12.43 

Maximum 12.80 12.80 12.50 12.43 

NR=not reported. 

2.7. ESTIMATED LEACHATE CONCENTRATION AS A FUNCTION OF 

pH 

For each CCR tested, results from SR002.1 (Alkalinity, Solubility and Release as a Function of 

pH) were used to develop an empirical functional relationship between solution pH and expected 

concentration for each constituent of interest. For each constituent within each CCR case, a 

polynomial function was regressed to the results from SR002.1 (Alkalinity, Solubility and
Characterization of Coal Combustion Residues II 

40

Release as a Function of pH) to provide the expected leachate concentration as a function of 

solution pH. Up to a fifth order polynomial was used for the regression. An example of a 

regression fit and corresponding equation for solubility and release as a function of pH is 

presented in Figure 12. The coefficients provided in the table reflect the order of the polynomial 

used. For all cases, the lowest order polynomial possible based on the R-square (no further 

increase for higher order) was used. Also included with each figure of regression fit is the 5th

and 95th percentile for pH (vertical line) from field observations of leachate from landfills and 

surface impoundments for combustion residues. Regression fit results are provided in Appendix 

F for each case examined (i.e., for each constituent in each CCR tested). 

Antimony

log (μg/L) Number 

of points

pH range of 

validity

0.0000 pH5 33 2.2-12.4

0.0008 pH4

-0.0399 pH3

0.5519 pH2

-2.7978 pH

6.031866 pH0

0.97 R2

5% 95%

0.001

0.01

0.1

1

10

100

1000

10000

100000

2 4 6 8 10 12 14

pH

Sb [μg/L]

SR2-DFA - A SR2-DFA - B

SR2-DFA - C Fit curve

Figure 12. Example of regression fit and corresponding coefficients for a 5th order polynomial 

equation used to represent solubility and release as a function of pH (antimony for fly ash from 

Facility B with SCR bypassed (DFA)).
Characterization of Coal Cumbustion Residues II 

3. RESULTS AND DISCUSSION 

The EPA Risk Report (EPA, 2007) identified the following COPCs based on the potential for 

either human health or ecological impacts using a screening risk assessment: aluminum (Al), 

arsenic (As), antimony (Sb), barium (Ba), boron (B), cadmium (Cd), cobalt (Co), chromium (Cr), 

lead (Pb), mercury (Hg), molybdenum (Mo), selenium (Se), and thallium (Tl).21 Thus, the 

evaluation provided here focuses on the same thirteen constituents and can be used in future risk 

and environmental assessments. 

3.1. TOTAL ELEMENTAL CONTENT BY DIGESTION 

Total elemental compositions using digestion22, on a dry weight basis, for COPCs for the CCRs 

evaluated are compared by constituent in Figures 13 through 24; tabular results are provided in 

Appendix B. Boron was not analyzed because it is used in the digestion process and therefore 

analysis would not yield meaningful results. 

Mercury. Mercury content exhibited a similar range (0.01 to 1.0 μg/g) for all CCR types. Use of 

SNCR (Facility A) resulted in increased mercury content in the fly ash and decreased mercury 

content in the scrubber sludge, compared to when the SNCR was not in use. In contrast, use of 

SCR (Facility B) resulted in decreased mercury content in the fly ash and increased mercury 

content in the scrubber sludge, compared to when the SCR was bypassed. For all three 

comparative cases (Facilities A, B and M) use of the NOx control increased the total mercury 

content in the fixated scrubber sludge. Mercury content in gypsum was significantly lower in the 

washed gypsum than in the unwashed gypsum for comparative cases (Facilities N and O). 

Aluminum. Aluminum content was approximately an order of magnitude greater in the fly ash 

samples than in the scrubber sludge samples from facilities without SCR and than in gypsum 

samples with and without SCR. Facilities with SCR operating (samples BGD, KGD) had greater 

aluminum content in scrubber sludge than the other scrubber sludges, likely because of the 

addition of aluminum with the SCR catalyst. Fixated scrubber sludge samples had intermediate 

Al content, reflective of the blending of fly ash with scrubber sludge. 

Antimony. Antimony content ranged over similar levels for fly ash, scrubber sludge and fixated 

scrubber sludge; gypsum had lower antimony content except for the sample from Facility Q. 

Samples from both Facilities K and Q had greater antimony content than comparative FGD 

residues from other facilities (scrubber sludges and fixated scrubber sludges for Facility K, 

gypsum for Facility Q) perhaps as a consequence of greater antimony content in sub-bituminous 

coal burned by these facilities than bituminous coal burned by the other facilities. The SNCR 

samples had higher antimony contents in the fly ash, scrubber sludge and fixated scrubber sludge 

for Facility A than the samples collected with SNCR off. The SCR samples had higher antimony 

contents in the fly ash from Facility B and the fixated scrubber sludges from Facilities A, B and 

21 The database used in the EPA Risk Report (EPA, 2007) for the assessment was based on both 

measurements of field samples (e.g., leachate, pore water) and single point laboratory leaching tests (e.g., 

TCLP, SPLP). The database was sparse with respect to measurements of field samples for many 

constituents. 

22 Digestion Method 3052 and ICP-MS analysis by Method 6020; see Section 2.3.7. 

41
Characterization of Coal Combustion Residues II 

M, although the reason for the high level of antimony in sample BCC cannot be explained by the 

relative antimony content in samples BFA and BGD. 

Arsenic. Gypsum had very low arsenic content (<5 μg/g) compared to the other residue types. 

Arsenic content in scrubber sludge was significantly less than in fly ash for residue from the 

same facility. Use of NOx controls resulted in decreased arsenic content in the fly ash, increased 

content in the scrubber sludge (Facilities A and B) and decreased content in the fixated scrubber 

sludge (Facilities A, B and M; reflecting the relative contributions of fly ash and scrubber sludge 

in fixated scrubber sludge) for comparative samples. 

Barium. Barium content was similar in scrubber sludge and gypsum for all facilities except for 

sample BGD (unexplained), and lower than barium content in fly ash by approximately an order 

of magnitude. The relatively low barium content in unwashed gypsum from Facility O is also 

unexplained. 

Cadmium. Cadmium content was low in the gypsum (<0.69 ng/g) and scrubber sludges (<1.72 

ug/g) and generally total content of the gypsum was half that in fly ash (<1.51 ng/g). The greater 

cadmium content in sample BGD relative to sample DGD may be a consequence of the use of 

post-combustion NOx control using SCR. 

Chromium. Chromium content in gypsum (< 19.3 ng/g) and scrubber sludges (< 139 ng/g) was 

low and approximately an order of magnitude less than in fly ash samples (<194). Scrubber 

sludge samples without SCR in operation also had chromium content similar to that of gypsum. 

Scrubber sludge samples without SCR in use also had chromium content similar to that of 

gypsum. Elevated chromium content in scrubber sludge samples BGD and KGD may be 

associated with the use of post-combustion NOx control using SCR. 

Cobalt. Cobalt content in gypsum was low and approximately an order of magnitude less than 

in fly ash samples. Scrubber sludge samples without SCR in operation also had cobalt content 

similar to that of gypsum. Elevated cobalt content in scrubber sludge samples BGD and KGD 

may be associated with SCR catalyst addition. The relatively low cobalt content in fixated 

scrubber sludge samples BCC and KCC relative to corresponding fly ash and scrubber sludge 

samples is unexplained. 

Lead. Lead content in gypsum was low and less than one third of the lead content in fly ash 

samples. Scrubber sludge samples without SCR in operation also had lead content similar to that 

of gypsum. The relatively low lead content in fixated scrubber sludge samples BCC and KCC in 

relationship to corresponding fly ash and scrubber sludge samples is unexplained. 

Molybdenum. A similar range in molybdenum content was found in fly ash, scrubber sludge 

and fixated scrubber sludge samples, with lower content by approximately one third in gypsum 

samples, with the exception of gypsum sample QAU. 

Selenium. All samples were less than 5 μg/g, except for samples for fly ash and fixated scrubber 

sludge from Facility A and unwashed gypsum from Facilities P and Q. The upper bound for 

these samples was less than 30 μg/g. 

Thallium. All gypsum samples had thallium content less than 3 μg/g. Fly ash, scrubber sludge, 

and fixated scrubber sludge samples all had a similar range of thallium content between 2 and 5 

μg/g, except scrubber sludge and fixated scrubber sludge from Facility B with SCR and fly ash 

and fixated scrubber sludge from Facility K. Both of these observations of greater thallium 

content (between 5 and 13 μg/g) may result from the SCR catalyst addition. 

42
Characterization of Coal Cumbustion Residues II 

43 

0.001

0.01

0.1

1

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Mercury (Hg)

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

Fly Ash            |  Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

100

1,000

10,000

100,000

1,000,000

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Aluminum(Al)

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

Fly Ash            |  Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

Figure 13 and Figure 14. Mercury and Aluminum. Comparison of total elemental content by 

digestion (key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed 

gypsum; Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited 

oxidation).
Characterization of Coal Combustion Residues II 

0

5

10

15

20

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Antimony (Sb)

Fly Ash            |  Scrubber Sludge |         Gypsum | Fixated Scrubber Sludge

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

0

20

40

60

80

100

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Arsenic (As)

Fly Ash            |  Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

44

Figure 15 and Figure 16. Antimony and Arsenic. Comparison of total elemental content by 

digestion (key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed 

gypsum; Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited 

oxidation).
Characterization of Coal Cumbustion Residues II 

45 

1

10

100

1,000

10,000

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Barium(Ba)

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

Fly Ash            |  Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

0.0

0.5

1.0

1.5

2.0

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Cadmium(Cd)

Fly Ash            |  Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

Figure 17 and Figure 18. Barium and Cadmium. Comparison of total elemental content by

digestion (key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed 

gypsum; Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited 

oxidation).
Characterization of Coal Combustion Residues II 

46

0

100

200

300

400

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Chromium(Cr)

Fly Ash            |  Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

0

20

40

60

80

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Cobalt (Co)

Fly Ash            |  Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

Figure 19 and Figure 20. Chromium and Cobalt. Comparison of total elemental content by 

digestion (key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed 

gypsum; Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited 

oxidation).
Characterization of Coal Cumbustion Residues II 

47 

0

20

40

60

80

100

120

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Lead (Pb)

Fly Ash            | Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

0

10

20

30

40

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Molybdenum(Mo)

Fly Ash            |  Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

Figure 21 and Figure 22. Lead and Molybdenum. Comparison of total elemental content by 

digestion (key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed 

gypsum; Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited 

oxidation).
Characterization of Coal Combustion Residues II 

48

0

5

10

15

20

25

30

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Selenium(Se)

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

Fly Ash            |  Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

0

5

10

15

Fac.

A (CFA) ‐

Fac.

A (AFA) +

Fac. B (DFA) ‐

Fac. B (BFA) +

Fac. K (KFA) +

Fac.

A (CGD)  ‐

Fac.

A (AGD) +

Fac. B (DGD)  ‐

Fac. B (BGD) +

Fac. K (KGD) +

Fac.

N (NAU) +U

Fac.

N (NAW) +W

Fac.

O (OAU) +U

Fac.

O (OAW) +W

Fac. P (PAD) +U

Fac.

Q (QAU) +U

Fac.

A (CCC)  ‐

Fac.

A (ACC) +

Fac. B (DCC) ‐

Fac. B (BCC) +

Fac. K (KCC) +

Fac.

M (MAD) ‐

Fac.

M (MAS) +

μg/g

Thallium(Tl)

Nat. Ox. Nat. Ox. Forced Ox. Nat. Ox. I. Ox.

Fly Ash            |  Scrubber Sludge  |         Gypsum | Fixated Scrubber Sludge

Figure 23 and Figure 24. Selenium and Thallium. Comparison of total elemental content by 

digestion (key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed 

gypsum; Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited 

oxidation).
Characterization of Coal Cumbustion Residues II 

3.2. TOTAL ELEMENTAL CONTENT BY XRF 

Total elemental compositions, on a dry weight basis, for major and some trace constituents in 

each of the CCRs evaluated are compared in grouping by CCR type (i.e., fly ash, gypsum, 

scrubber sludge, fixated scrubber sludge) in Figure 25 through Figure 29; tabular results are 

provided in Appendix C. Major elemental constituents present in CCRs but not detected by XRF 

analysis include oxygen and carbon. Elements that may be analyzed by XRF but were below the 

detection limit are indicated in the figures on the x-axis but without any reported value 

represented. Separate analyses were carried out for carbon and are also included in Appendix C. 

Fly Ash. Elemental constituents typically present in fly ash at concentrations greater than 1 

percent (10,000 mg/kg) are aluminum, calcium, iron, potassium, and silicon. Elemental 

constituents typically present at concentrations between 0.1 and 1 percent are barium, chloride, 

magnesium, sodium, phosphorus, strontium, sulfur, and titanium. 

Gypsum. Elemental constituents typically present in gypsum at concentrations greater than 1 

percent (10,000 mg/kg) are calcium and sulfur. Elemental constituents typically present at 

concentrations between 0.1 and 1 percent are chloride (unwashed gypsum), fluoride (unwashed 

gypsum), iron, magnesium, sodium (unwashed gypsum) and silicon. 

Scrubber Residue. Elemental constituents typically present in scrubber residue at 

concentrations greater than 1 percent (10,000 mg/kg) are aluminum (SCR on), calcium, iron 

(SCR on), magnesium, silicon (SCR on) and sulfur. Elemental constituents typically present at 

concentrations between 0.1 and 1 percent are aluminum (except with SCR on), chloride, fluoride, 

iron (except with SCR on), potassium, sodium (SCR on), silicon (except with SCR on), and 

titanium (SCR on). 

Fixated Scrubber Sludge. Elemental constituents typically present in fixated scrubber sludge at 

concentrations greater than 1 percent (10,000 mg/kg) are aluminum, calcium, iron, potassium, 

silicon and sulfur. Elemental constituents typically present at concentrations between 0.1 and 1 

percent are barium, chloride, sodium, phosphorus and strontium. 

Fixated Scrubber Sludge with Lime. Elemental constituents typically present in fixated 

scrubber sludge with lime at concentrations greater than 1 percent (10,000 mg/kg) are aluminum, 

calcium, iron, magnesium (Mg lime scrubbers), silicon, and sulfur. Elemental constituents 

typically present at concentrations between 0.1 and 1 percent are chloride, potassium, 

magnesium (non-Mg lime scrubbers), sodium, and titanium. 

49
Characterization of Coal Combustion Residues II 

50

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac. A.(NO+SNCR+FF) ‐ Fly Ash CFA (SNCR Off) AFA (SNCR On)

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac. B. (NO+SCR+ESP, Mg lime) ‐ Fly Ash DFA (SCR Off) BFA (SCR On)

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

Mn

Mo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac. K. (NO+SCR+ESP, Mg lime) ‐ Fly Ash KFA (SCR On)

a) 

b) 

c) 

Figure 25. Fly Ash - Total content by XRF.
Characterization of Coal Cumbustion Residues II 

51 

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac.N.(FO+SCR+ESP) ‐ Gypsum NAU (Unwashed) NAW (Washed)

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac.O. OAU(Unwashed) OAW (Washed) (FO+SCR+ESP) ‐ Gypsum

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

PAD(U) (Unwashed) QAU(Unwashed)

Fac. P. (FO+SCR&SNCR+ESP) ‐ Gypsum

Fac.Q. (FO+SCR+ESP) ‐Gypsum

a) 

b) 

c) 

Figure 26. Gypsum – Total content by XRF.
Characterization of Coal Combustion Residues II 

52

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac. A.(NO+SNCR+FF) ‐ Scrubber Sludge CGD (SNCR Off) AGD(SNCR On)

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac. B. (NO+SCR+ESP, Mg lime) ‐ Scrubber Sludge DGD (SCR Off) BGD (SCR On)

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac. K. (NO+SCR+ESP, Mg KGD (SCR On) lime) ‐ Scrubber Sludge

a) 

b) 

c) 

Figure 27. Scrubber Sludge – Total content by XRF.
Characterization of Coal Cumbustion Residues II 

53 

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

Mn

Mo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac. A.(NO+SNCR+FF) ‐ Fixated Scrubber Sludge CCC (SNCR Off) ACC (SNCR On)

Figure 28. Fixated Scrubber Sludge – Total content by XRF.
Characterization of Coal Combustion Residues II 

54

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac. B. (NO+SCR+ESP, Mg lime) ‐ FSSL DCC (SCR Off) BCC (SCR On)

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac. K. (NO+SCR+ESP, Mg lime) ‐ FSSL KCC (SCR On)

1

10

100

1,000

10,000

100,000

1,000,000

Al

As

Ba

Br

Ca

Cd

Ce

Cl

Co

Cr

Cu

F

Fe

Ga

Ge

K

La

Mg

MnMo

Na

Nb

Ni

P

Pb

Rb

Sc

Se

Si

Sr

S

Ti

V

W

Y

Zn

Zr

μg/g

Fac. M. (IO+SCR+ESP)‐ FSSL MAD(SCR Off) MAS (SCR On)

a) 

b) 

c) 

Figure 29. Fixated Scrubber Sludge with Lime – Total content by XRF.
Characterization of Coal Cumbustion Residues II 

3.3. LABORATORY LEACHING TEST RESULTS 

Appendix D and Appendix E provide graphical presentation of the results of Solubility and 

Release as a Function of pH (SR002.1) and Solubility and Release as a Function of LS 

(SR003.1), respectively, for the 13 constituents of interest in this report. Within each appendix, 

results are grouped by CCR type (fly ash, gypsum, scrubber sludge, FSS, FSSL) and with each 

CCR type grouping, comparisons are made by constituent of interest. First discussed below are 

typical characteristic results for pH and each of the 13 constituents of interest (Section 3.3.1) 

followed by a discussion (Section 3.3.2) comparing the ranges of observed constituent 

concentrations (from both test methods) with measurements reported elsewhere on field leachate 

and pore water samples for CCR disposal sites and the database used in the EPA Risk Report 

(EPA, 2007). Complete data also have been developed for other constituents to facilitate 

evaluation of geochemical speciation of constituents of concern and provide more thorough 

evaluation of leaching under alternative management scenarios in the future if warranted. 

For each CCR evaluated, results of the leaching tests provide the following information: 

 Leachate concentrations for the constituents of interest as a function of pH over the range of 

reported field management conditions (from test method SR002.1); 

 pH titration curves (from test method SR002.1). This information is useful in characterizing 

the CCR and assessing how it will respond to environmental stresses and material aging (e.g., 

carbon dioxide uptake, acid precipitation, co-disposal, mixing with other materials); and, 

 Leachate concentrations for the constituents of interest, pH and electrical conductivity as a 

function of LS ratio when contacted with distilled water (from test method SR003.1). This 

information provides insight into the initial leachate concentrations expected during land 

disposal and effects of pH and ionic strength at low LS ratio. Often these concentrations can 

be either greater than or less than concentrations observed at higher LS ratio (i.e., LS=10 

mL/g as used in SR002.1) because of ionic strength and co-constituent concentration effects. 

The MCL is used as a reference threshold for the constituent of interest. However, releases 

identified here are estimates of concentrations potentially leaching from landfills. Any 

assessment of the environmental impact of these releases needs to consider the dilution and 

attenuation of these constituents in ground water, and the plausibility of drinking water well 

contamination resulting from the release. Dilution and attenuation factors for metals (DAFs) 

have been estimated to be potentially as low as 2 to 10 on a national basis or as high as 8,000 at a 

particular site with hydrogeology that indicated low transport potential23. Therefore, comparison 

with thresholds greater than the MCL and developed for specific scenarios may be appropriate. 

23 See 60 FR 66372, Dec. 21, 1995, for a discussion of model parameters leading to low DAFs, 

particularly the assumption of a continuous source landfill. Implied DAFs for the metals of interest here 

can be found at 60 FR 66432-66438 in Table C-2. Site specific high-end DAFs are discussed in 65 FR 

55703, September 14, 2000. 

55
Characterization of Coal Combustion Residues II 

3.3.1. Typical Characteristic Leaching Behavior as a Function of pH and LS 

Figure 30 through Figure 45 provide comparisons of typical leaching behavior as a function of 

pH for each constituent, as examples reproduced from Appendix D. These comparisons illustrate 

on an empirical basis some of the differences in leaching behavior for different CCRs that result 

from the combination of the coal rank combusted, combustion conditions and specific 

combustion facility design and operation. Also noted but not shown is the observed behavior for 

each constituent as a function of LS at the material’s natural pH (see Appendix E). Elements 

with predominantly oxyanionic species (e.g., boron, molybdenum, selenium, etc.) typically 

exhibited non-linearly increasing extract concentrations as LS was decreased from 10 mL/g to 

0.5 mL/g, in many cases increasing by a factor of 5 or 10 or greater. 

These figures illustrate that for a particular constituent, the chemistry controlling release or 

aqueous-solid equilibrium may be similar within a material type (i.e., mercury behavior for fly 

ash or scrubber sludge) or across material types (i.e., the same behavior for aluminum in fly ash 

and fixated scrubber sludge) but that there are not necessarily generalized behaviors present for 

each constituent across all samples within a material type or between material types. The most 

robust groupings of leaching behavior will result from the development of geochemical 

speciation models of the materials that account for the underlying solid phase speciation (e.g., 

solid phases, adsorption behavior) and modifying solution characteristics (e.g., dissolved organic 

matter, pH, ionic strength, co-dissolved constituents). Development of the needed geochemical 

speciation models, and associated leaching behavior groupings as a function of coal rank, 

combustion facility design, and CCR type, will be the basis of a subsequent report. The resulting 

models and groupings, in turn, are expected to allow for more detailed constituent release 

predictions based on limited testing for a broader set of facilities. 

Mercury. Figure 30 (a, b) compares the impact of SNCR usage (Facility A) on the release 

behavior of mercury from fly ash. The increased mercury release, reaching a maximum at pH~8, 

when SNCR is in use, is likely a consequence of additional ammonium present and consequent 

formation of an ammonium mercury complex in solution (Wang, 2007). For all fly ash samples 

except AFA (Facility A, SNCR[+]), the mercury release indicated apparently random scatter 

with solution concentrations ranging from 0.01 to 0.1 μg/L. This behavior is similar to that 

reported earlier in this study (Sanchez et al., 2006) and is indicative of adsorption of elemental 

mercury as the primary release mechanism. For gypsum samples, mercury release was either (i) 

similar to that observed for fly ash (Figure 30d, Facility P), (ii) at very low concentration near or 

below the MDL (Facility O, unwashed and washed; Facility N washed) potentially indicative of 

adsorption onto carbon char, or (iii) had increased release at pH<6 potentially indicative of 

adsorbed ionic mercury being displaced by hydrogen ions at acidic pH (Facility A, unwashed; 

Facility Q). For scrubber sludge, mercury release was either (i) similar to that observed for fly 

ash, (ii) had increasing release concurrent with decreasing pH at pH<8 (Facility B, Figure 30c), 

or (iii) had increased release at pH~8 (local peak, indicative of ammonium complexation) and 

then increasing release with decreasing pH at pH<6 (Facility K). For FSS, behavior was either (i) 

analogous to that observed for fly ash (Facility A), or (ii) analogous to that observed for scrubber 

sludge illustrated in Figure 30c (Facilities B, K, M). Mercury extract concentrations were not 

significantly affected by LS. 

Aluminum. Figure 31 (a, b) compares the impact of SNCR usage (Facility A) on the release 

behavior of aluminum from fly ash and illustrates one of the three types of aluminum behavior 

observed across the CCR types. One type of observed aluminum behavior was amphoteric 

56
Characterization of Coal Cumbustion Residues II 

behavior (minimum release at pH~6-8, with increasing release as pH decreases and increases 

outside of this range) with a local maximum of approximately 10,000 μg/L at pH~12 and a 

maximum of ca. 100,000 to 1,000,000 μg/L at pH~3. The concentration of the minimum at 

6<pH<8 is typically influenced by the amount of DOC complexing aluminum in solution 

(increased complexation increases dissolved aluminum). This behavior was present for fly ash 

from Facilities A and K. Other samples exhibited increasing aluminum release with decreasing 

pH at pH<8 but without substantially increased release at pH>8 (fly ash from facility B; scrubber 

sludge from facilities B, K). Several samples exhibited increasing release with decreasing pH for 

pH<6 and a local maximum at pH~10 (Figure 31c, scrubber sludge from Facility A; FSS from 

Facility B). In addition, several samples exhibited relatively constant release (ca. 100 μg/L) at 

pH>8 with increasing release with decreasing pH at pH<8 (Figure 31d, scrubber sludge for 

facility B with SCR[-]; FSS from facilities A, B, K). Aluminum extract concentrations typically 

were either relatively constant or decreasing (salting out) with decreasing LS. The notable 

exception was the scrubber sludge from facility B with SCR off (DGD) and FSS from facility M 

with SCR on (MAS), where aluminum concentration increased with decreasing LS. 

Antimony. Figure 32a illustrates antimony behavior with local maxima at pH~8 and pH<3, 

which was observed for fly ash from facilities A and B, and FSS from facility A. Figure 32b 

illustrates behavior that was observed for gypsum from Facility Q and scrubber sludge from 

Facility A. For gypsum samples other than from Facility Q, antimony release appeared random at 

concentrations of <MDL to 3 μg/L at pH>4 with a general slight increase in concentration at 

pH<4. Figure 32c and d illustrates behavior that was observed for the remaining CCR samples. 

For some samples, antimony concentrations in extracts increased by up to a factor of 5 with 

decreasing LS, while it remained constant or decreased for other samples. 

Arsenic. Figure 33 illustrates the four typical release behaviors observed for arsenic release as a 

function of pH. Each of these four behaviors was observed for a least one of the samples from 

each material type (fly ash, gypsum, scrubber sludge, FSS). Leaching from gypsum generally 

was less than 10 μg/L for pH>6 and reached a maximum of approximately 30-100 μg/L at pH<5. 

Arsenic concentrations in extracts were either constant or increased by up to a factor of 2 with 

decreasing LS. 

Barium. Figure 34 illustrates the four typical release behaviors observed for barium. For fly ash, 

barium was either relatively constant at approximately 100 μg/L as a function of pH or exhibited 

increases at pH< 4 (Figure 34a) and pH>9 to approximately 1000 μg/L (i.e., facility K). For 

gypsum, Figure 35d illustrates the typical behavior. Behaviors illustrated by Figure 34b and c are 

typical of that observed for scrubber sludge and FSS, with both behaviors observed for both 

material types. Barium extract concentrations remained constant with decreasing LS, with the 

exception of fly ash from facility A, where barium concentrations increased by up to a factor of 5 

with decreasing LS. 

Boron. Figure 35 (a, b, c) illustrates the three typical release behaviors for boron: (i) decreasing 

concentration with increasing pH for pH>8 (Figure 35a, fly ash from facilities A, B; scrubber 

sludge from facility K; FSS from facilities A, M), (ii) relatively constant concentration with a 

slight decrease at 8<pH<10 (Figure 35b, scrubber sludge from facility B, FSS from facility K), 

(iii) relatively constant or with a slight increase at pH>10 (Figure 35d). Comparison of Figure 

35c and d illustrates the reduction in leachable boron achieved through the washing step (facility 

N, unwashed and washed). Boron concentrations in extracts were either constant or increased by 

up to a factor of 10 with decreasing LS. 

57
Characterization of Coal Combustion Residues II 

Cadmium. Figure 36 illustrates typical release behavior for cadmium, which is similar for all 

cases. The maximum concentration at pH<4 reflects the total content in the sample, and the slope 

and pH shift in the increasing cadmium concentration with decreasing pH typically reflects the 

presence or absence of complexants in solution (e.g., chloride or DOC). Cadmium concentrations 

in extracts were either constant or increasing with decreasing LS. 

Chromium. Figure 37 and Figure 38 illustrate typical release behavior for chromium. 

Comparison of the pairs Figure 37a and b, Figure 37c and d, and Figure 38a and b illustrate the 

effect of NOx control on chromium release from fly ash, scrubber sludge, and FSS respectively 

for Facility B. Use of SCR at this facility appears to result in a larger fraction of the chromium in 

the residue being oxidized (Cr+6), forming chromate, which is more soluble at neutral pH. The 

SCR catalyst used for Facility B may have contributed to increased overall total chromium 

present in the CCRs from that facility when SCR was in use. Increased leachability of chromium 

as a consequence of NOx control was also observed for Facility A (comparing results for 

SNCR[-] with SNCR[+]), even though total chromium content was similar for cases with and 

without NOx control. Figure 38c and d illustrates the typical amphoteric behavior for reduced 

chromium (Cr+3), which was observed for gypsum samples. Chromium concentrations in extracts 

were either constant (for cases with low leachable chromium), or increasing by up to a factor of 

10 (cases with Cr+6 as the apparently dominant chromium form) with decreasing LS. 

Cobalt. Figure 39 illustrates typical release behavior for cobalt, which is similar for all CCRs 

tested. Cobalt release increases with decreasing pH at pH<8, with the maximum concentration 

reflecting the total leachable content and the slope and relative pH shift in the slope typically 

reflecting the presence or absence of complexants in solution. Cobalt concentrations in extracts 

(where greater than the MDL) were typically increasing by up to a factor of 5 with decreasing 

LS. 

Lead. Figure 40 illustrates typical release behavior for lead, which generally is expected to be 

amphoteric (Figure 40a, c). However, many samples did not have sufficient lead content or 

complexants present in solution to produce amphoteric behavior at pH<12 (Figure 40b, d). Lead 

concentrations in extracts were not significantly changed at decreasing LS. 

Molybdenum. Figure 41 and Figure 42 illustrate typical release behaviors for molybdenum. 

Figure 41a illustrates increased concentration peaking at pH~8 most likely from complexation 

with ammonium present from use of SNCR (Facility A). Note the similar release behavior 

observed for fly ash, FSS and gypsum illustrated by Figure 41b and Figure 42b, c, albeit at 

different orders of magnitude in concentration. Figure 41c, d and Figure 42b, d illustrate three 

additional observed behaviors, present across multiple CCR types. Molybdenum concentrations 

typically increased with decreasing LS, in some cases by a factor much greater than 10 (e.g., FSS 

from Facility A). 

Selenium. Figure 43 and Figure 44 illustrate typical selenium release behaviors observed for fly 

ash, scrubber sludge and FSS. For gypsum, selenium release was either constant as a function of 

pH (facilities O, P) or amphoteric (facilities N, Q). Selenium concentrations in extracts typically 

increased by up to a factor of 6 with decreasing LS. 

Thallium. Figure 45 illustrates typical release behavior for thallium. Most cases were either 

analogous to Figure 45a or Figure 45c (increasing concentration with decreasing pH at pH<9) or 

Figure 45d (relatively constant as a function of pH). For gypsum, washing resulted in at least an 

58
Characterization of Coal Cumbustion Residues II 

59 

order of magnitude reduction in the observed leaching concentrations (facilities N, O). Thallium 

concentrations typically increased by up to a factor of 5 with decreasing LS. 

MDL

ML

10.3

0.044

MCL

0.001

0.01

0.1

1

2 4 6 8 10 12 14

Hg [μg/L]

pH

SR2-CFA - A SR2-CFA - B

SR2-CFA - C

FlyAsh ‐ Fac. A. (NO+SNCR[‐]+FF)

MDL

ML

10.5

0.116

MCL

0.001

0.01

0.1

1

2 4 6 8 10 12 14

Hg [μg/L]

pH

SR2-AFA - A SR2-AFA - B

SR2-AFA - C

FlyAsh ‐ Fac. A. (NO+SNCR[+]+FF)

MDL

ML

9.1

0.040

MCL

0.001

0.01

0.1

1

10

2 4 6 8 10 12 14

Hg [μg/L]

pH

SR2-DGD - A SR2-DGD - B

SR2-DGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

6.7

0.029

MCL

0.001

0.01

0.1

1

10

2 4 6 8 10 12 14

Hg [μg/L]

pH

SR2-PAD - A SR2-PAD - B

Gypsum ‐ Fac. P. (FO+SCR[+]&SNCR[+]+ESP)

a) b)

c) d)

Figure 30. Mercury - Examples of characteristic leaching behavior as a function of pH (SR002.1 

results).
Characterization of Coal Combustion Residues II 

60

MDL

1000000

100000

10000

1804 ]L

1000

/g [μ

100

Al

10

1

0.1 10.3

2 4 6 8 10 12 14

ML pH

SR2-CFA - A SR2-CFA - B

SR2-CFA - C

FlyAsh ‐ Fac. A. (NO+SNCR[‐]+FF)

MDL

1000000

100000

13427

10000

]L

1000

/g [μ

100

Al

10

1

0.1 10.5

2 4 6 8 10 12 14

ML pH

SR2-AFA - A SR2-AFA - B

SR2-AFA - C

FlyAsh ‐ Fac. A. (NO+SNCR[+]+FF)

MDL

1000000

100000

10000

]L

1000

/g

496

 [μ

100

Al

10

1

0.1 7.3

2 4 6 8 10 12 14

ML pH

SR2-CGD - A SR2-CGD - B

SR2-CGD - C

ScrubberSludge ‐ Fac. A. (NO+SNCR[‐]+FF)

MDL

1000000

100000

10000

]L

1000

/g [μ

100 37.0

Al

10

1

0.1 9.1

2 4 6 8 10 12 14

ML pH

SR2-DGD - A SR2-DGD - B

SR2-DGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[‐]+ESP)

a) b)

c) d)

Figure 31. Aluminum. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Cumbustion Residues II 

61 

MDL

ML

10.3

6.43

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Sb [μg/L]

pH

SR2-DFA - A SR2-DFA - B

SR2-DFA - C

FlyAsh ‐ Fac. B. (NO+SNCR[‐]+FF)

MDL

ML

6.9

2.53

MCL

0.1

1

10

100

2 4 6 8 10 12 14

Sb [μg/L]

pH

SR2-QAU - A SR2-QAU - B

Gypsum ‐ Fac. Q. (FO+SCR[+]+ESP)

MDL

ML

9.1

5.44

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Sb [μg/L]

pH

SR2-DGD - A SR2-DGD - B

SR2-DGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[‐]+ESP, Mg Lime)

MDL

ML

12.2

1.76

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Sb [μg/L]

pH

SR2-DCC - A SR2-DCC - B

SR2-DCC - C

FSSL ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

a) b)

c) d)

Figure 32. Antimony. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Combustion Residues II 

62

MDL

ML

10.3

46.8

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

As [μg/L]

pH

SR2-DFA - A SR2-DFA - B

SR2-DFA - C

FlyAsh ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

10.1

29.0

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

As [μg/L]

pH

SR2-BFA - A SR2-BFA - B

SR2-BFA - C

FlyAsh ‐ Fac. B. (NO+SCR[+]+ESP,Mg Lime)

MDL

ML

9.1

0.545

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

As [μg/L]

pH

SR2-DGD - A SR2-DGD - B

SR2-DGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

12.2

2.09

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

As [μg/L]

pH

SR2-DCC - A SR2-DCC - B

SR2-DCC - C

FSSL ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

d)

a) b)

c) 

Figure 33. Arsenic. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Cumbustion Residues II 

63 

MDL

ML

10.3

192

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Ba [μg/L]

pH

SR2-DFA - A SR2-DFA - B

SR2-DFA - C

FlyAsh ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

11.0

113

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Ba [μg/L]

pH

SR2-KGD - A SR2-KGD - B

ScrubberSludge ‐ Fac. K. (NO+SCR[+]+ESP,Mg Lime)

MDL

ML

12.2

2286

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Ba [μg/L]

pH

SR2-DCC - A SR2-DCC - B

SR2-DCC - C

FSSL ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

7.3

80.0

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Ba [μg/L]

pH

SR2-OAW - A SR2-OAW - B

Gypsum (Gyp‐W)‐ Fac.O. (FO+SCR[+]+ESP)

d)

a) b)

c) 

Figure 34. Barium. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Combustion Residues II 

64

MDL

ML

10.3

3551

DWEL

0.1

1

10

100

1000

10000

100000

2 4 6 8 10 12 14

B [μg/L]

pH

SR2-DFA - A SR2-DFA - B

SR2-DFA - C

FlyAsh ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

9.1

3331

DWEL

0.1

1

10

100

1000

10000

100000

2 4 6 8 10 12 14

B [μg/L]

pH

SR2-DGD - A SR2-DGD - B

SR2-DGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

7.2

2214

DWEL

0.1

1

10

100

1000

10000

100000

2 4 6 8 10 12 14

B [μg/L]

pH

SR2-NAU - A SR2-NAU - B

Gypsum(Gyp‐U) ‐ Fac.N. (FO+SCR[+]+ESP)

MDL

ML

7.1

48.6

DWEL

0.1

1

10

100

1000

10000

100000

2 4 6 8 10 12 14

B [μg/L]

pH

SR2-NAW - A SR2-NAW - B

Gypsum(Gyp‐W) ‐ Fac.N. (FO+SCR[+]+ESP)

c) d)

b) a) 

Figure 35. Boron. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Cumbustion Residues II 

65 

MDL

ML

10.3

0.795

MCL

0.1

1

10

100

2 4 6 8 10 12 14

Cd [μg/L]

pH

SR2-DFA - A SR2-DFA - B

SR2-DFA - C

FlyAsh ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

11.0

0.200

MCL

0.1

1

10

100

2 4 6 8 10 12 14

Cd [μg/L]

pH

SR2-KGD - A SR2-KGD - B

ScrubberSludge ‐ Fac. K. (NO+SCR[+]+ESP,Mg Lime)

MDL

ML

6.9

6.03

MCL

0.1

1

10

100

2 4 6 8 10 12 14

Cd [μg/L]

pH

SR2-QAU - A SR2-QAU - B

Gypsum(Gyp‐U) ‐ Fac.Q. (FO+SCR[+]+ESP)

MDL

ML

8.4

0.634

MCL

0.1

1

10

100

2 4 6 8 10 12 14

Cd [μg/L]

pH

SR2-ACC - A SR2-ACC - B

SR2-ACC - C

FSSL‐ Fac. B. (NO+SCR[+]+ESP,Mg Lime)

c) d)

a) b)

Figure 36. Cadmium. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Combustion Residues II 

66

MDL

ML

10.3

132

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Cr [μg/L]

pH

SR2-DFA - A SR2-DFA - B

SR2-DFA - C

FlyAsh ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

10.1

851

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Cr [μg/L]

pH

SR2-BFA - A SR2-BFA - B

SR2-BFA - C

FlyAsh ‐ Fac. B. (NO+SCR[+]+ESP,Mg Lime)

MDL

ML

9.1

2.98

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Cr [μg/L]

pH

SR2-DGD - A SR2-DGD - B

SR2-DGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

10.1

228

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Cr [μg/L]

pH

SR2-BGD - A SR2-BGD - B

SR2-BGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[+]+ESP,Mg Lime)

c) d)

b) a) 

Figure 37. Chromium. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Cumbustion Residues II 

67 

MDL

ML

12.2

11.2

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Cr [μg/L]

pH

SR2-DCC - A SR2-DCC - B

SR2-DCC - C

FSSL ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

8.0

715

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Cr [μg/L]

pH

SR2-BCC - A SR2-BCC - B

SR2-BCC - C

FSSL ‐ Fac. B. (NO+SCR[+]+ESP,Mg Lime)

MDL

ML

7.2

2.87

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Cr [μg/L]

pH

SR2-NAU - A SR2-NAU - B

Gypsum (Gyp‐U)‐ Fac.N. (FO+SCR[+]+ESP)

MDL

ML

7.1

0.235

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Cr [μg/L]

pH

SR2-NAW - A SR2-NAW - B

Gypsum (Gyp‐W) ‐ Fac.N. (FO+SCR[+]+ESP) c) d)

a) b)

Figure 38. Chromium. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Combustion Residues II 

68

MDL

ML

10.3

0.255

0.1

1

10

100

1000

2 4 6 8 10 12 14

Co [μg/L]

pH

SR2-CFA - A SR2-CFA - B

SR2-CFA - C

FlyAsh ‐ Fac. A. (NO+SNCR[‐]+FF)

MDL

ML

10.5

7.72

0.1

1

10

100

1000

2 4 6 8 10 12 14

Co [μg/L]

pH

SR2-AFA - A SR2-AFA - B

SR2-AFA - C

FlyAsh ‐ Fac. A. (NO+SNCR[+]+FF)

MDL

ML

9.1

0.255

0.1

1

10

100

1000

2 4 6 8 10 12 14

Co [μg/L]

pH

SR2-DGD - A SR2-DGD - B

SR2-DGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

7.5

1.65

0.1

1

10

100

1000

2 4 6 8 10 12 14

Co [μg/L]

pH

SR2-OAU - A SR2-OAU - B

Gypsum(Gyp‐U) ‐ Fac.O. (FO+SCR[+]+ESP)

d) c) 

a) b)

Figure 39. Cobalt. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Cumbustion Residues II 

69 

MDL

ML

10.5

0.339

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Pb [μg/L]

pH

SR2-AFA - A SR2-AFA - B

SR2-AFA - C

FlyAsh ‐ Fac. A. (NO+SNCR[+]+FF)

MDL

ML

6.8

0.354

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Pb [μg/L]

pH

SR2-AGD - A SR2-AGD - B

SR2-AGD - C

ScrubberSludge ‐ Fac. A. (NO+SNCR[+]+FF)

MDL

ML

8.4

0.569

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Pb [μg/L]

pH

SR2-ACC - A SR2-ACC - B

SR2-ACC - C

FSSL‐ Fac. A. (NO+SCR[+]+ESP,Mg Lime)

MDL

ML

7.5

0.140

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Pb [μg/L]

pH

SR2-OAU - A SR2-OAU - B

Gypsum (Gyp‐U)‐ Fac.O. (FO+SCR[+]+ESP)

c) d)

a) b)

Figure 40. Lead. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Combustion Residues II 

70

MDL

ML

10.5

587

DWEL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Mo [μg/L]

pH

SR2-AFA - A SR2-AFA - B

SR2-AFA - C

FlyAsh ‐ Fac. A. (NO+SNCR[+]+FF)

MDL

ML

10.3

1956

DWEL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Mo [μg/L]

pH

SR2-DFA - A SR2-DFA - B

SR2-DFA - C

FlyAsh ‐ Fac. B. (NO+SCR[‐]+FF)

MDL

ML

7.3

10.2

DWEL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Mo [μg/L]

pH

SR2-CGD - A SR2-CGD - B

SR2-CGD - C

ScrubberSludge ‐ Fac. A. (NO+SNCR[‐]+FF)

MDL

ML

9.1

135

DWEL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Mo [μg/L]

pH

SR2-DGD - A SR2-DGD - B

SR2-DGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[‐]+ESP, Mg Lime)

c) d)

a) b)

Figure 41. Molybdenum. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Cumbustion Residues II 

71 

MDL

ML

8.4

241

DWEL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Mo [μg/L]

pH

SR2-ACC - A SR2-ACC - B

SR2-ACC - C

FSSL‐ Fac. A. (NO+SCR[+]+ESP,Mg Lime)

MDL

ML

12.2

138

DWEL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Mo [μg/L]

pH

SR2-DCC - A SR2-DCC - B

SR2-DCC - C

FSSL ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

7.5

18.8

DWEL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Mo [μg/L]

pH

SR2-OAU - A SR2-OAU - B

Gypsum(Gyp‐U) ‐ Fac.O. (FO+SCR[+]+ESP)

MDL

ML

6.9

14.3

DWEL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Mo [μg/L]

pH

SR2-QAU - A SR2-QAU - B

Gypsum(Gyp‐U) ‐ Fac.Q. (FO+SCR[+]+ESP)

d) c) 

a) b)

Figure 42. Molybdenum. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Combustion Residues II 

72

MDL

ML

10.3

9.27

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Se [μg/L]

pH

SR2-DFA - A SR2-DFA - B

SR2-DFA - C

FlyAsh ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

9.2

123

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Se [μg/L]

pH

SR2-KFA - A SR2-KFA - B

FlyAsh ‐ Fac. K. (NO+SCR[+]+ESP,Mg Lime)

MDL

ML

9.1

21.9

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Se [μg/L]

pH

SR2-DGD - A SR2-DGD - B

SR2-DGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

10.1

2.31

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Se [μg/L]

pH

SR2-BGD - A SR2-BGD - B

SR2-BGD - C

ScrubberSludge ‐ Fac. B. (NO+SCR[+]+ESP,Mg Lime)

c) d)

a) b)

Figure 43. Selenium. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Cumbustion Residues II 

73 

MDL

ML

12.2

16.3

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Se [μg/L]

pH

SR2-DCC - A SR2-DCC - B

SR2-DCC - C

FSSL ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

8.2

22.6

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Se [μg/L]

pH

SR2-KCC - A SR2-KCC - B

FSSL ‐ Fac. K. (NO+SCR[+]+ESP,Mg Lime)

MDL

ML

7.2

18.5

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Se [μg/L]

pH

SR2-NAU - A SR2-NAU - B

Gypsum (Gyp‐U)‐ Fac.N. (FO+SCR[+]+ESP)

MDL

ML

6.9

325

MCL

0.1

1

10

100

1000

10000

2 4 6 8 10 12 14

Se [μg/L]

pH

SR2-QAU - A SR2-QAU - B

Gypsum(Gyp‐U) ‐ Fac.Q. (FO+SCR[+]+ESP)

c) d)

b) a) 

Figure 44. Selenium. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Combustion Residues II 

74

MDL

ML

10.3

0.773

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Tl [μg/L]

pH

SR2-DFA - A SR2-DFA - B

SR2-DFA - C

FlyAsh ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

9.2

38.0

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Tl [μg/L]

pH

SR2-KFA - A SR2-KFA - B

FlyAsh ‐ Fac. K. (NO+SCR[+]+ESP,Mg Lime)

MDL

ML

12.2

15.6

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Tl [μg/L]

pH

SR2-DCC - A SR2-DCC - B

SR2-DCC - C

FSSL ‐ Fac. B. (NO+SCR[‐]+ESP,Mg Lime)

MDL

ML

8.2

2.98

MCL

0.1

1

10

100

1000

2 4 6 8 10 12 14

Tl [μg/L]

pH

SR2-KCC - A SR2-KCC - B

FSSL ‐ Fac. K. (NO+SCR[+]+ESP,Mg Lime)

c) d)

a) b)

Figure 45. Thallium. Examples of characteristic leaching behavior as a function of pH.
Characterization of Coal Cumbustion Residues II 

3.3.2. Comparisons of the Ranges of Constituent Concentrations from Laboratory Testing 

with Measurements of Field Samples and the EPA Risk Report Database 

Figure 46 through Figure 59 present comparisons of the range of constituent concentrations 

observed in laboratory extracts from testing as a function of pH and LS (SR002.1 and SR003.1) 

over the pH range from 5.4 to 12.4 and LS ratios from 0.5 to 10. This pH range represents the 5th 

and 95th percentiles of pH observed in field samples from CCR landfills and surface 

impoundments, as discussed in Section 2.6. For laboratory leaching test extracts, the presented 

data represent the observed minimum, natural pH, and maximum concentration, considering all 

results from both test methods. Including results from testing as a function of LS allows 

consideration of potentially higher concentrations observed for initial releases that may occur at 

low LS ratios in the field. The MCL or DWEL is included in each figure as a dashed horizontal 

line to provide a reference value. However, the concentration ranges indicated in the figures as 

results of this study are direct measurements of laboratory extracts and do not consider 

attenuation that may occur in the field. Ranges of field observations are included for comparison 

as derived from the EPRI database, considering only observations from disposal sites associated 

with facilities that have wet FGD scrubbers. The 5th, median, and 95th percentile of field data is 

presented for surface impoundments [“Surface Imp. (EPRI)”] and landfills [“Landfill (EPRI)”]. 

Surface impoundment data are compared with scrubber sludge results because scrubber sludges 

are most likely to be disposed in this manner (see Section 1). Landfill data are compared with 

FSS and FSSL because these blended materials are the likely to be disposed in landfills. Also 

included for comparison is the 5th, median, and 95th percentile of the database used to carry out 

human and ecological health risk evaluations in the EPA Risk Report (EPA, 2007) (“CCW Ash,” 

“CCW FGD,” and “CCW Ash and Coal Waste” referring to monofilled fly ash, disposed FGD 

scrubber sludge, and combined CCR disposal, respectively). 

pH. The natural pH of the fly ash samples evaluated in this report was alkaline24, scrubber 

sludge samples were neutral to alkaline, gypsum samples were neutral and FSS samples were 

mildly alkaline (pH ~8) to very alkaline (pH>11), most likely depending on the amount of lime 

added. 

Mercury. Laboratory extract concentration ranges for two of the scrubber sludge samples 

(Facility B) and four of the FSS samples (facilities B, K, M) exceeded the MCL for mercury. The 

mercury concentration ranges indicate that the greatest leaching concentrations are expected 

from scrubber sludge and blending with fly ash and lime to produce FSS does not substantially 

decrease, and may increase, mercury leaching. All natural pH samples were less than the MCL, 

most by more than an order of magnitude. All fly ash and gypsum data were less than the MCL, 

regardless of pH. Mercury field data were very sparse. 

Aluminum. There was no reference limit available for aluminum. The range of laboratory 

extract concentrations from scrubber sludge samples agreed well with field observations. The 

range of field observations for landfills tended to be similar to somewhat lower than the values 

observed for laboratory extracts. However, the range used in the EPA Risk Report (EPA, 2007) 

had an upper bound approximately one order of magnitude greater than the field data and the 

laboratory extracts. 

24 Some fly ash samples reported on in Sanchez et al. (2006) were acidic. 

75
Characterization of Coal Combustion Residues II 

Antimony. Fly ash and FSS had the highest concentrations of antimony in laboratory extracts. 

The range of field observations for surface impoundments (EPRI) was much greater than for 

laboratory extracts of scrubber sludge, but also likely reflects co-disposal of fly ash with 

scrubber sludge in surface impoundments. If the field data includes co-disposal data, then the 

concentration range reported for field observations is entirely consistent with the laboratory 

testing. The range of field observation for landfills (EPRI) was lower than observed in laboratory 

extracts. This comparison suggests the potential for attenuation or immobilization under field 

conditions, many of which may be reducing in contrast to the oxidizing conditions used in the 

laboratory testing. For gypsum, all natural pH results were less than the MCL. 

Arsenic. Leachable arsenic appears to be distributed between fly ash and scrubber sludge. 

Results for Facility A (fly ash, scrubber sludge, and FSS), Facility B (scrubber sludge and FSS) 

and Facility M (FSS) suggest that NOx controls increase the leachability of arsenic at the 

material’s natural pH. Results from field observations indicate narrower ranges than laboratory 

testing. Ranges used in the EPA Risk report (EPA, 2007) appear to be much higher than the 

laboratory testing results, except for Facility M. All results for gypsum were less than the MCL. 

Barium. Laboratory testing results indicate a much broader range of concentrations than field 

observations. Two samples of FSS, both with NOx controls off (Facility B and Facility M), had 

much greater laboratory extract concentrations than the other samples. All natural pH results 

were less than the MCL for fly ash, scrubber sludge and gypsum. 

Boron. Most notable for boron is a reduction of approximately an order of magnitude in 

leachable boron from gypsum as a consequence of washing (facilities N, O). There is no clear 

trend amongst the material types for boron. Laboratory results are consistent with field 

observations. All gypsum natural pH results were less than the DWEL. 

Cadmium. Cadmium concentrations in laboratory extracts were generally lower for the 

scrubber sludge than for the other materials. For scrubber sludge, cadmium concentrations 

observed in laboratory extracts were less than the concentrations reported for field observations 

for surface impoundments. For all cases, cadmium concentrations in laboratory extracts were 

consistent with field observations for landfills. In addition, the measured concentrations in 

laboratory extracts from this study and reported for field results are approximately an order of 

magnitude less than the upper bound reported for CCW and coal waste in the Risk Report (EPA 

2007). 

Chromium. The range of field observations appears low in comparison to the laboratory testing 

results. The increase in chromium leaching, apparently as a consequence of NOx controls, is 

evident for facilities A, B and M. Six cases exceed the MCL by greater than one order of 

magnitude. Test results for all of the gypsum samples are well below the MCL. The field 

observations for both surface impoundments and landfills are up to two orders of magnitude less 

than the laboratory testing results. 

Cobalt. The use of NOx controls appears to increase cobalt leaching from fly ash based results 

for facilities A and B. The data range used by the EPA risk report (EPA, 2007) for combined 

management of ash and coal waste is one to two orders of magnitude greater than the 

observations for laboratory extracts and field observations. 

76
Characterization of Coal Cumbustion Residues II 

77 

Lead. Laboratory test results indicate concentration ranges generally lower than or consistent 

with field observations for surface impoundments and landfills and much lower than the values 

used in the EPA risk report (EPA, 2007). All fly ash and gypsum samples were less than the 

MCL. 

Molybdenum. Leachate molybdenum concentrations exceeding the DWEL by up to two orders 

of magnitude were observed for fly ash and FSS. Leachate molybdenum concentrations 

generally less than the DWEL were observed for gypsum. 

Selenium. Similar ranges of selenium concentrations are expected for all materials. 

Thallium. Laboratory test results indicate a concentration range in excess of two orders of 

magnitude for all materials and the observed concentration can exceed the MCL for thallium by 

more than one order of magnitude. Washed gypsum had significantly lower extract 

concentrations of thallium than unwashed gypsum. 

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

pH (SI)

Fly  Ash                 |   Scrubber Sludge           |             Gypsum                 | Fixated Scrubber Sludge

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Figure 46. Natural pH (pH in distilled water at LS=10) observed in SR02 extracts.
Characterization of Coal Combustion Residues II 

78

0.001

0.010

0.100

1.000

10.000

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Hg (μg/L)

Max

Min

own pH

MCL = 2 μg/L

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated Scrubber Sludge

0.1

1.0

10.0

100.0

1,000.0

10,000.0

100,000.0

1,000,000.0

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Al (μg/L)

Max

Min

own pH Fly  Ash               |   Scrubber Sludge           |             Gypsum                | Fixated Scrubber Sludge

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Figure 47 and Figure 48. Mercury and Aluminum. Comparison of maximum, minimum and 

natural pH concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤ pH≤

12.4. (key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed 

gypsum; Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited 

oxidation).
Characterization of Coal Cumbustion Residues II 

79 

0.1

1.0

10.0

100.0

1,000.0

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Sb (μg/L)

Max

Min

own pH

MCL = 6 μg/L

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated Scrubber Sludge

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

0.1

1.0

10.0

100.0

1,000.0

10,000.0

100,000.0

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

As (μg/L)

Max

Min

own pH

MCL = 10 μg/L

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated Scrubber Sludge

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Figure 49 and Figure 50. Antimony and Arsenic. Comparison of maximum, minimum and 

natural pH concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤ pH≤

12.4. (key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed 

gypsum; Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited 

oxidation).
Characterization of Coal Combustion Residues II 

80

1

10

100

1,000

10,000

100,000

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Ba (μg/L)

Max

Min

own pH

MCL = 2000 μg/L

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Fly  Ash                 |   ScrubberSludge           |                Gypsum              | Fixated Scrubber Sludge

1

10

100

1,000

10,000

100,000

1,000,000

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

B (μg/L)

Max

Min

own pH

DWEL= 7000 

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated Scrubber Sludge

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Figure 51 and Figure 52. Barium and Boron. Comparison of maximum, minimum and natural 

pH concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤ pH≤ 12.4. 

(key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed gypsum; 

Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited oxidation).
Characterization of Coal Cumbustion Residues II 

81 

0.01

0.1

1

10

100

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Cd (μg/L)

Max

Min

own pH

MCL = 5 μg/L

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated Scrubber Sludge

0.1

1.0

10.0

100.0

1,000.0

10,000.0

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Cr (μg/L)

Max

Min

own pH

MCL = 100 μg/L

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated Scrubber Sludge

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Figure 53 and Figure 54. Cadmium and Chromium. Comparison of maximum, minimum and 

natural pH concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤ pH≤

12.4. (key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed 

gypsum; Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited 

oxidation).
Characterization of Coal Combustion Residues II 

82

0.1

1.0

10.0

100.0

1,000.0

10,000.0

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Co (μg/L)

Max

Min

own pH

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated Scrubber Sludge

0.01

0.10

1.00

10.00

100.00

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Pb (μg/L)

Max

Min

own pH

MCL = 15 μg/L

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated Scrubber Sludge

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Figure 55 and Figure 56. Cobalt and Lead. Comparison of maximum, minimum and natural pH 

concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤ pH≤ 12.4. (key: - 

= NOx control off; + = NOx control on; U = unwashed gypsum; W = washed gypsum; Nat. Ox. 

= natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited oxidation).
Characterization of Coal Cumbustion Residues II 

83 

0.1

1.0

10.0

100.0

1,000.0

10,000.0

100,000.0

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Mo (μg/L)

Max

Min

own pH

DWEL= 200 μg/L

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated Scrubber Sludge

1.0

10.0

100.0

1,000.0

10,000.0

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Se (μg/L)

Max

Min

own pH

MCL = 50 μg/L

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated Scrubber Sludge

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Figure 57 and Figure 58. Molybdenum and Selenium. Comparison of maximum, minimum and 

natural pH concentrations observed in SR02 and SR03 extracts over the pH domain 5.4≤ pH≤

12.4. (key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed 

gypsum; Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited 

oxidation).
Characterization of Coal Combustion Residues II 

84

0.1

1.0

10.0

100.0

1,000.0

Fac. A (CFA) -

Fac. A (AFA) +

Fac. B (DFA) -

Fac. B (BFA) +Fac. K (KFA) +

CCW Ash [EPA]

Fac. A (CGD) -

Fac. A (AGD) +

Fac. B (DGD) -

Fac. B (BGD) +Fac. K (KGD) +

Surface Imp. [EPRI]

Fac. N (NAU) +U

Fac. N (NAW) +W

Fac. O (OAU) +U

Fac. O (OAW) +W

Fac. P (PAD) +U

Fac. Q (QAU) +U

Fac. A (CCC) -

Fac. A (ACC) +

Fac. B (DCC) -

Fac. B (BCC) +Fac. K (KCC) +

Fac. M (MAD) -

Fac. M (MAS) +

Landfill [EPRI]

CCW FGD [EPA]

Ash & Coal Waste [EPA]

Tl (μg/L)

Max

Min

own pH

MCL = 2 μg/L

Fly  Ash                 |   Scrubber Sludge           |                Gypsum              | Fixated ScrubberSludge

Nat.Ox. Nat.Ox. ForcedOx. Nat. Ox.      | I. Ox.

Figure 59. Thallium. Comparison of maximum, minimum and natural pH concentrations 

observed in SR02 and SR03 extracts over the pH domain 5.4≤ pH≤ 12.4. 

(key: - = NOx control off; + = NOx control on; U = unwashed gypsum; W = washed gypsum; 

Nat. Ox. = natural oxidation; Forced Ox. = forced oxidation; I.Ox. = inhibited oxidation).
Characterization of Coal Cumbustion Residues II 

4. SUMMARY OF RESULTS, CONCLUSIONS AND RECOMMENDATIONS 

The following sections present conclusions from the results presented in this report. 

Tables 8 through 11 summarize the results obtained from this study for fly ash, FGD gypsum, 

scrubber sludge, and FSS, respectively. The set of 23 CCRs evaluated in this report reflects 84 

and 74 percent, respectively, of the current and future facility configuration types with acid gas 

scrubbers based on generating capacity, but only a limited number of facilities within each 

configuration type. Each table provides the following attributes, and associated ranges, related to 

each material type and constituent examined: 

1. Total content (mg/kg, dw) from acid digestion. 

2. The minimum and maximum elemental concentrations measured in laboratory leaching 

test extracts over the domain of 5.4≤ pH≤ 12.4 from leaching evaluation as a function of 

pH at LS=10 mL/g (SR002.1) and as a function of 0.5≤ LS≤ 10 mL/g dw (SR003.1). This 

range is intended to represent the potential range of leachate concentrations expected to 

be observed in the field from management of each of the material types in monofilled 

management conditions. Concentration ranges for individual samples of each material 

type are compared in Section 3.3.2. 

3. The minimum and maximum elemental concentrations measured in laboratory leaching 

test extracts when extracted with deionized water only (“natural pH”) and 0.5≤ LS≤ 10 

mL/g dw (SR003.1). The resulting pH range is also indicated. 

4. The MCL or DWEL and TC (as available) for each constituent to provide reference 

concentrations for evaluation of the concentration results summarized as described above. 

However, the expected leachate concentration ranges derived directly from laboratory 

testing and probabilistic assessments do not include any dilution and attenuation that may 

occur prior to impacting water resources. Previous studies have indicated dilution and 

attenuation factors of as low as 2 to 10 on a national basis or as high as 8,000 at a 

particular site25. Thus, comparisons with the MCL, DWEL or TC for any constituent 

must be done with caution. 

5. Variability in extract concentration as a function of pH based on results from laboratory 

leaching testing (SR002.1). Classification of variability is as follows: (a) Low = 

concentration range ≤ 1 order of magnitude, (b) Med. = concentration range of 1 to 2 

orders of magnitude, (c) High = concentration range of >2 orders of magnitude. 

In addition, results are emphasized through coding as follows: 

1. Expected concentration values that exceed either the MCL or DWEL for the given 

constituent are in red bold typeface. 

2. Constituents are underlined in the column heading (e.g., Cd in Table 9) when one of the 

expected concentration ranges exceeds either the MCL or DWEL. 

25 See Section 3.3. 

85
Characterization of Coal Combustion Residues II 

Based on the results of testing and evaluations in this study, the following conclusions are drawn: 

1. For each CCR type the following constituents exceeded either the maximum contaminant 

level (MCL) or drinking water equivalent level (DWEL) in at least one laboratory 

leaching test condition over the range of pH and LS ratios considered, and therefore 

potentially may present unacceptable environmental risks under some management 

scenarios. These cases warrant more detailed evaluation, including consideration of sitespecific conditions. 

a. Fly ash – antimony, arsenic, boron, cadmium, chromium, molybdenum, selenium 

and thallium. 

b. FGD gypsum – boron, cadmium, molybdenum, selenium and thallium. 

c. Scrubber sludge – mercury, antimony, arsenic, boron, chromium, lead, 

molybdenum, selenium and thallium. 

d. Fixated scrubber sludge – mercury, antimony, arsenic barium, boron, cadmium, 

chromium, lead, molybdenum, selenium and thallium. 

However, (i) typically, evaluation results from only a subset of samples of a given 

material type exceeded the indicated criteria, (ii) never did the full range reported 

exceed the indicated threshold, and (iii) this analysis does not account for additional 

dilution and attenuation processes that may occur under field management scenarios. 

These results suggest that (i) consideration of dilution and attenuation factors for 

specific management scenarios may indicate that release concentrations may be 

higher than exposure concentrations, and that release concentrations above the MCL 

or DWEL may not result in exposure concentrations above those levels; (ii) linear 

partition coefficients (Kds) are not appropriate for representing source term release 

from CCRs for a range of constituents and materials; and, (iii) evaluation of 

individual CCR sources may indicate that the environmental compatibility of specific 

types of CCRs with general management scenarios will depend on the source 

(reflected through leaching characteristics) of the material. 

2. Leaching of individual constituents may vary over several orders of magnitude, 

depending on the conditions of the management scenario. Thus, these results can be used 

to suggest design conditions that would reduce or minimize constituent release (e.g., pH, 

and other conditions). 

3. Leaching concentrations do not correlate with total content except for specific 

constituents in selected materials where the constituent (a) is weakly retained, and (b) 

leaching concentrations have a low variability relative to pH. Thus, total content is not a 

good indicator of leaching. 

4. Results of this study suggest that it appears that Cr leachability is associated with the use 

of post-combustion NOx controls. This is based on a limited set of paired samples from 

the same facility operating with and with SCR or SNCR in use. This finding will be 

further evaluated as additional data are collected. 

5. The systematic leaching behavior of COPCs observed in the range of samples evaluated 

suggests that the geochemical mechanisms controlling leaching can be discerned and 

86
Characterization of Coal Cumbustion Residues II 

quantified using geochemical speciation modeling, which in turn, can serve as the basis 

for evaluating and improving design of CCR management scenarios. Development of 

generalized geochemical speciation models for the materials evaluated in this study is 

recommended. 

6. Ranges of concentrations of some constituents in laboratory leaching test extracts and 

field data included in this study suggest applicable concentration ranges for risk 

evaluation are different from the concentration ranges used in a recent report by USEPA 

(EPA, 2007). The new information reported here will help provide a more up-to-date and 

comprehensive dataset for future risk assessments. 

87
Characterization of Coal Combustion Residues II 

Table 8. Fly Ash. Summary of results. 

Hg Sb As Ba B Cd Cr Co Pb Mo 

Total in Material (mg/kg) 

0.04 -0.6 3 - 15 70 - 90 600 - 1,500 NA 0.7 1.5 - 100 - 200 20 - 50 40 - 90 10 - 2

Leaching test and assessment results 

Conc. Range for 5.4≤pH≤12.4 

<0.01 - 0.5 

<0.3 -200 7 - 300 90 - 4,000 200 - 300,000 <0.2 30 - 1 - 4,000 <0.3 - 200 <0.2 - 2 100 

40,00

(μg/L) Conc. range at natural pH (7.7≤pH≤11.1) 

<0.01 – 0.4 

<0.3 -50 7 - 75 90 – 4,000 300 – 300,000 <0.2 30 - 20 – 4,000 <0.3 - 20 <0.2 – 0.7 400 

40,00

MCL (or DWEL) (μg/L) 2 6 10 2,000 

7,000 DWEL 

5 100 - 15 

200 DWE

TC (μg/L) 200 - 5,000 100,000 6,500 1,000 5,000 5,000 5,000 - Variability relative to pH 

Low to High 

Med. To High 

Low to Med. Low Med. to High High Low to Med. High Med. Low 

Med

NA – not analyzed (total content for boron estimated based on leaching test results); ND – not determined. 

88
Characterization of Coal Cum

Table 9. FGD Gypsum. Summary of results. 

Hg Sb As Ba B Cd Cr Co Pb Mo 

Total in Material (mg/kg) 

0.01 -0.5 2 - 6 2 - 4 3 - 60 NA 0.3 0.5 - 6 -20 1 - 4 1 -12 2 - 12 

Leaching test and assessment results 

Conc. Range for 5.4≤pH≤12.4 

(μg/L) 

<0.01 - 0.1 

<0.3 -9 

<0.5 -10 

40 -400 

40 -70,000 

<0.2 50 

- <0.3 -

50 

<0.6 -50 

<0.2 -10 1 - 40

Conc. range at natural pH (3.8≤pH≤8.1) 

<0.01 – 0.1 

<0.3 -15 

<0.5 -9 

40 -150 

40 – 70,000 

<0.2 15 

- <0.3 -

20 1 - 50 <0.2 - 12 3 - 40

MCL (or DWEL where indicated) (μg/L) 

2 6 10 2,000 

7,000 DWEL 

5 100 - 15 

200 DWE

TC (μg/L) 200 - 5,000 100,000 6,500 1,000 5,000 5,000 5,000 - Variability relative to pH 

Low -Med. Low Low - Med. Low Low - Med. High Med. - High Low Low Low 

NA – not analyzed (total content for boron estimated based on leaching test results); ND – not determined. 

89
Characterization of Coal Combustion Residues II 

Table 10. Scrubber Sludge. Summary of results. 

Hg Sb As Ba B Cd Cr Co Pb Mo 

Total in Material (mg/kg) 

0.04 – 0.6 4 - 15 4 - 40 80 – 2,500 NA 0.3 – 1.5 9 - 350 1 - 40 2 - 30 9 - 30 

Leaching test and assessment results 

Conc. Range for 5.4≤pH≤12.4 

(μg/L) 

<0.01 - 9 

<0.3 -20 

0.6 -100 

20 – 2,000 

20 – 200,000 

<0.2 -4 

<0.8 -800 

<0.3 -250 

<0.2 -25 

0.4 1,500 

Conc. range at natural pH (6.5≤pH≤11.0) <0.01 – 0.07 <0.3 - 10 0.6 - 20 30 - 950 20 – 200,000 <0.2 - 4 2 - 800 <0.3 - 60 <0.2 - 25 10 

1,500 

MCL (or DWEL where indicated) (μg/L) 

2 6 10 2,000 

7,000 DWEL 

5 100 - 15 

200 DWE

TC (μg/L) 200 - 5,000 100,000 6,500 1,000 5,000 5,000 5,000 - 

Variability relative to pH 

Low -High 

Low -Med. 

Med. -High 

Low -Med. 

Low -High High Low High Low Low 

High 

NA – not analyzed (total content for boron estimated based on leaching test results); ND – not determined. 

90
Characterization of Coal Cum

Table 11. Fixated Scrubber Sludge. Summary of results. 

Hg Sb As Ba

 B

 Cd Cr Co Pb Mo 

Total in Material (mg/kg) 

0.02 -1.0 5 - 20 3 - 70 80 - 1,000 NA 0.7 - 2 40 - 150 2 - 50 4 - 100 9 - 30 

Leaching test and assessment results 

Conc. Range for 5.4≤pH≤12.4 

(μg/L) 

<0.01 - 30 

<0.3 -200 

0.5 -4,000 

9 -10,000 

6 -200,000 

<0.2 -20 

<0.3 -2,000 

<0.3 150 

- <0.2 -

50 

10 40,00

Conc. range at natural pH (5.8≤pH≤12.3) 

<0.01 – 0.7 

<0.3 -60 

0.5 – 4,000 

15 – 10,000 

100 – 200,000 

<0.2 -20 

<0.3 – 2,000 

<0.3 50 

- <0.2 -

50 

10 40,00

MCL (or DWEL where indicated) (μg/L) 

2 6 10 2,000 

7,000 DWEL 

5 100 - 15 

200 DWE

TC (μg/L) 200 - 5,000 100,000 6,500 1,000 5,000 5,000 5,000 - Variability relative to pH 

Low -High 

Low -High 

Med -High 

Low -Med. High Low - High Low High Low Low 

NA – not analyzed (total content for boron estimated based on leaching test results); ND – not determined. 

91
Characterization of Coal Combustion Residues II 

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