Environmental Applications of Geochemical Modeling

CONTENTS

Preface xiii

1 Introduction 1

1.1 Environmental Problems and the Need for Geochemical Modeling 1
1.1.1 High-Level Radioactive Waste Disposal 1
1.1.2 Mining Related Environmental Issues 4
1.1.3 Landfills 8
1.1.4 Deep Well Injection of Hazardous Wastes 8
1.1.5 Artificial Recharge to Aquifers 9
1.2 The Regulatory Framework 11
1.2.1 CERCLA or Superfund 11




1.2.2 RCRA 11
1.2.3 NEPA 11
1.2.4 Clean Water Act 12
1.2.5 Safe Drinking Water Act 12
1.3 The Role of Geochemical Modeling 12
1.3.1 Contamination Issues 12
1.3.2 Water Resource Issues 13
1.4 Current Practice 14
1.4.1 Model Usage 14
1.4.2 The State of the Art 16
1.5 Overview 16


2 Model Concepts 18


2.1 Model Definitions 18
2.2 A Holistic View of Geochemical Models 19
2.3 Types of Geochemical Models 23
2.3.1 Speciation–solubility Models 23
2.3.2 Reaction Path Models 24
2.3.3 Inverse Mass Balance Models 26
2.3.4 Coupled Mass Transport Models 27
2.4 Model Verification and Validation 28
2.5 Model Usefulness and Limitations 30


3 Thermodynamic Background 32

3.1 Systems and Equilibrium 32
3.1.1 Real and Model Systems 32
3.1.2 Equilibrium 33
3.1.3 The Role of Kinetics 34
3.2 Chemical Reactions 34
3.3 Gibbs Energy 35
3.3.1 Enthalpy and Entropy 36
3.4 Activity, Fugacity, and Chemical Potential 37
3.4.1 Activity and Fugacity 37


3.4.2 Activity Coefficients 38
3.4.3 Chemical Potential 42
3.5 The Equilibrium Constant 42
3.5.1 Direct and Indirect Determination of K values 44
3.5.2 Solubility Product and Saturation Index 44
3.5.3 Dependence of K on Temperature 45
3.6 Components and Species 46
3.6.1 Components and the Basis 46
3.6.2 Species 46
3.6.3 An Alternative Basis 47
3.7 The Phase Rule 51
3.7.1 The Extensive Phase Rule 53
3.8 Redox 54

3.8.1 Oxygen Fugacity, log fO2 55
3.8.2 Redox Potential, Eh 57

3.8.3 Electron Potential, pe 58
3.9 Alkalinity 58
3.9.1 The Carbonate Component 59
3.9.2 Carbonate Speciation 59
3.9.3 Titration Alkalinity 61
3.9.4 The Alkalinity to Carbonate Component Correction 63
3.10 Acidity 65


3.10.1 Titration Acidity 65
3.10.2 The Acidity to Carbonate Component Correction 66
3.10.3 Alkalinity and Acidity: A Summary 67
3.11 The Local Equilibrium Assumption 67
3.11.1 Scales of Interest 69
3.11.2 Calculation of teq and leq 69
3.12 Summary 73


4 Computer Programs for Geochemical Modeling 74

4.1 Codes, Databases, and Models 74
4.1.1 The Code 74
4.1.2 The Database 75
4.2 Review of Popular Computer Programs 76
4.3 Databases 79

4.3.1 A Typical Database 79
4.3.2 Data Quality 81
4.4 Chemical Concentration Units 83
4.5 Examples of Input/Output 83
4.5.1 Program Input 83
4.5.2 Program Output 90


5 Preparation and Construction of a Geochemical Model 92

5.1 Introduction 92
5.2 Establish the Goals 92
5.3 Learn the Groundwater Flow System 92
5.4 Collection of Field and Laboratory Data 93

5.4.1 Decide Which Parameters to Measure for Groundwater 93
5.4.2 Characterize the Solids 93
5.4.3 Evaluate Quality of Water Analyses. Charge Balance I 94
5.5 Decide What Types of Model to Construct 97
5.6 Gather Chemical Properties 101
5.7 Select a Computer Code 101

5.8 Set Up a Model 102
5.8.1 Basis Swapping 102
5.8.2 Charge Balance II 102
5.9 Interpretation of Modeling Results 103
5.9.1 Accuracy and Completeness of the Database 103
5.9.2 Input Constraints 104

5.9.3 Who Produced the Model? 104
5.10 Reporting and Presentation of Modeling Results 105


6 Speciation and Solubility Modeling 106


6.1 Introduction 106
6.2 A Uranium Mill Tailings Impoundment 107
6.2.1 The Site 107
6.2.2 The Purpose of Geochemical Modeling 108
6.2.3 Site Geology and Data 111
6.2.4 Selection of Modeling Code and Model Input 112
6.2.5 Geochemical Modeling 113
6.2.6 Modeling Results 114


6.2.7 Analysis of Mineral Saturation Indices 114
6.2.8 Activity–Activity Diagrams 121
6.2.9 Geochemical Evolution Along A Flow Path 123
6.2.10 Comments on the Bear Creek Site 125
6.3 Applications to Bioavailability and Risk Assessment Studies 126
6.4 Interpretations of Column Experiments 128


7 Modeling Surface Adsorption 133


7.1 Introduction 133
7.1.1 The Solid–Water Interface 133
7.2 Ion-exchange 134
7.2.1 Cation-exchange Capacity 134
7.2.2 Exchange Reactions 135
7.2.3 Isotherms 136

7.2.4 Ion-exchange vs. Surface Complexation 138
7.3 Surface Complexation 138
7.3.1 The Electrical Double layer 139
7.3.2 Other Surface Models 142

7.4 Sorption Implementation in Computer Programs 142
7.4.1 Examples 143
7.4.2 Why Surface Modeling is Not Perfect 148
7.5 Retardation of Radionuclides at Oak Ridge 148
7.6 Mobility of Radionuclides at a Uranium Mill Tailings Impoundment 151
7.6.1 Why Geochemical Modeling? 152
7.6.2 Modeling Approach 152
7.6.3 Modeling Results 153
7.6.4 Comparison with Field Data 153
7.6.5 Discussion of Modeling Results 155
7.7 Adsorption of Arsenic in Smelter Flue Dust 155



8 Reaction Path Modeling 157


8.1 Introduction 157
8.2 Alkalinity Titration 159
8.3 Acidity of Acid Mine Water 161
8.4 pH Buffering 164
8.5 Deep Well Injection of Hazardous Wastes 167
8.5.1 Background 167

8.5.2 A Case Study 168
8.6 Pit Lake Chemistry 174
8.7 Artificial Recharge 177
8.8 Applications to Natural Background Studies 178



9 Inverse Mass Balance Modeling 180


9.1 Introduction 180
9.2 Model Assumptions 181
9.3 Groundwater Genesis, Black Mesa, Arizona 183
9.4 Acid Mine Drainage, Pinal Creek, Arizona 187

9.5 14C dating, Black Mesa, Arizona 192
9.6 Estimate of Microbial Metabolism Rates in Deep Aquifers 195
9.6.1 Chapelle and Lovley (1990) 195
9.6.2 Murphy and Schramke (1998) 197

10 Coupled Reactive Transport Models 199


10.1 Introduction 199
10.2 Multi-component Reactive Transport Models 200
10.3 Isotherm-based Reactive Transport Models 201
10.3.1 Linear Isotherm, Kd 201
10.3.2 Freundlich Isotherm 202
10.3.3 Langmuir Isotherm 202
10.3.4 Applicability of the Isotherm or Retardation-factor-based
Reactive Transport Models 202
10.4 A Simple Example 205

10.5 Buffering in Reactive Transport 211
10.5.1 The Buffer Concept 211
10.5.2 Application of the Buffer Concept 212
10.6 Migration of an Acid Plume at a Uranium Mill Tailings Site 215
10.6.1 Model Description 215
10.6.2 Modeling Results 218

10.7 Remedial Design of a Uranium Tailings Repository 225
10.8 Summary and Comments 229


11 Kinetics Modeling 230

11.1 Introduction 230
11.2 Some Basic Theory 230
11.2.1 The Progress Variable 230
11.2.2 The Reaction Rate 232
11.2.3 Rate Laws 233

11.2.4 Temperature Dependence of Rate Constants 235
11.3 Kinetics of Precipitation and Dissolution Reactions 237
11.4 Kinetics of Acetate Decomposition 241
11.5 Coupled Aqueous Speciation and Biological Processes 247
11.6 Application to Landfill Leachate into Aquifers 249
11.7 Conclusions 251

Appendix 253
A Modifying a Database 253
A.1 Why Modify a Database? 253
A.1.1 Adding Arsenic Data to phreeqc 254
References 261
Index 281

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Environmental Applications of Geochemical Modeling