Preface xi 1 Introduction 1 1.1 Environmental Problems and 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 19 2.1 Model Definitions 19 2.2 A Holistic View of Geochemical Models 20 2.3 Types of Geochemical Models 24 2.3.1 Speciation solubility Models 24 2.3.2 Reaction Path Models 25 2.3.3 Inverse Mass Balance Models 27 2.3.4 Coupled Mass Transport Models 28 2.4 Model Verification and Validation 29 2.5 Model Usefulness and Limitations 31 v
vi Contents 3 Thermodynamic Background 33 3.1 Systems and Equilibrium 33 3.1.1 Real and Model Systems 33 3.1.2 Equilibrium 34 3.1.3 The Role of Kinetics 35 3.2 Chemical Reactions 35 3.3 Gibbs Energy 36 3.3.1 Enthalpy and Entropy 37 3.4 Activity, Fugacity, and Chemical Potential 38 3.4.1 Activity and Fugacity 38 3.4.2 Activity Coefficients 39 3.4.3 Chemical Potential 43 3.5 The Equilibrium Constant 43 3.5.1 Direct and Indirect Determination of K values 45 3.5.2 Solubility Product and Saturation Index 45 3.5.3 Dependence of K on Temperature 46 3.6 Components and Species 47 3.6.1 Components and the Basis 47 3.6.2 Species 47 3.6.3 An Alternative Basis 48 3.7 The Phase Rule 52 3.7.1 The Extensive Phase Rule 54 3.8 Redox 55 3.8.1 Oxygen Fugacity, log f O2 56 3.8.2 Redox Potential, Eh 58 3.8.3 Electron Potential, pe 59 3.9 Alkalinity 59 3.9.1 The Carbonate Component 60 3.9.2 Carbonate Speciation 60 3.9.3 Titration Alkalinity 62 3.9.4 The Alkalinity to Carbonate Component Correction 64 3.10 Acidity 66 3.10.1 Titration Acidity 66 3.10.2 The Acidity to Carbonate Component Correction 67 3.10.3 Alkalinity and Acidity: A Summary 68 3.11 The Local Equilibrium Assumption (LEA) 68 3.11.1 Scales of Interest 70 3.11.2 Calculation of t eq and l eq 70 3.12 Summary 74 4 Computer Programs for Geochemical Modeling 77 4.1 Codes, Databases, and Models 77 4.1.1 The Code 77 4.1.2 The Database 78 4.2 Review of Popular Computer Programs 79 4.3 Databases 82
vii 4.3.1 A Typical Database 82 4.3.2 Data Quality 84 4.4 Chemical Concentration Units 86 4.5 Examples of Input/Output 86 4.5.1 Program Input 86 4.5.2 Program Output 93 5 Preparation and Construction of a Geochemical Model 95 5.1 Introduction 95 5.2 Establish the Goals 95 5.3 Learn the Groundwater Flow System 95 5.4 Collection of Field and Laboratory Data 96 5.4.1 Decide Which Parameters to Measure for Groundwater 96 5.4.2 Characterize the Solids 97 5.4.3 Evaluate Quality of Water Analyses. Charge Balance I 97 5.5 Decide What Types of Model to Construct 100 5.6 Gather Chemical Properties 103 5.7 Select a Computer Code 104 5.8 Set Up a Model 104 5.8.1 Basis Swapping 104 5.8.2 Charge Balance II 105 5.9 Interpretation of Modeling Results 106 5.9.1 Accuracy and Completeness of the Database 106 5.9.2 Input Constraints 107 5.9.3 Who produced the model? 107 5.10 Reporting and Presentation of Modeling Results 107 6 Speciation and Solubility Modeling 109 6.1 Introduction 109 6.2 A Uranium Mill Tailings Impoundment 110 6.2.1 The Site 110 6.2.2 The Purpose of Geochemical Modeling 111 6.2.3 Site Geology and Data 114 6.2.4 Selection of Modeling Code and Model Input 115 6.2.5 Geochemical Modeling 116 6.2.6 Modeling Results 117 6.2.7 Analysis of Mineral Saturation Indices 117 6.2.8 Activity Activity Diagrams 124 6.2.9 Geochemical Evolution Along A Flow Path 126 6.2.10 Comments on the Bear Creek Site 128 6.3 Applications to Bioavailability and Risk Assessment Studies 129 6.4 Interpretations of Column Experiments 131
viii Contents 7 Modeling Surface Adsorption 137 7.1 Introduction 137 7.1.1 The Solid Water Interface 137 7.2 Ion Exchange 138 7.2.1 Cation Exchange Capacity 138 7.2.2 Exchange Reactions 139 7.2.3 Isotherms 140 7.2.4 Ion Exchange vs. Surface Complexation 142 7.3 Surface Complexation 142 7.3.1 The Electrical Double layer 143 7.3.2 Other Surface Models 146 7.4 Sorption Implementation in Computer Programs 146 7.4.1 Examples 147 7.4.2 Why Surface Modeling is Not Perfect 152 7.5 Retardation of Radionuclides at Oak Ridge 152 7.6 Mobility of Radionuclides at a Uranium Mill Tailings Impoundment 155 7.6.1 Why Geochemical Modeling? 156 7.6.2 Modeling Approach 156 7.6.3 Modeling Results 157 7.6.4 Comparison with Field Data 157 7.6.5 Discussion of Modeling Results 159 7.7 Adsorption of Arsenic in Smelter Flue Dust 159 8 Reaction Path Modeling 161 8.1 Introduction 161 8.2 Alkalinity Titration 163 8.3 Acidity of Acid Mine Water 165 8.4 ph Buffering 168 8.5 Deep Well Injection of Hazardous Wastes 171 8.5.1 Background 171 8.5.2 A Case Study 173 8.6 Pit Lake Chemistry 178 8.7 Artificial Recharge 181 8.8 Applications to Natural Background Studies 182 9 Inverse Mass Balance Modeling 185 9.1 Introduction 185 9.2 Model Assumptions 186 9.3 Groundwater Genesis, Black Mesa, Arizona 188 9.4 Acid Mine Drainage, Pinal Creek, Arizona 192 9.5 14 C dating, Black Mesa, Arizona 197 9.6 Estimate of Microbial Metabolism Rates in Deep Aquifers 200 9.6.1 Chapelle and Lovley (1990) 200 9.6.2 Murphy and Schramke (1998) 202
ix 10 Coupled Reactive Transport Models 205 10.1 Introduction 205 10.2 Multi-component Reactive Transport Models 206 10.3 Isotherm-based Reactive Transport Models 207 10.3.1 Linear Isotherm, K d 207 10.3.2 Freundlich isotherm 208 10.3.3 Langmuir isotherm 208 10.3.4 Applicability of the Isotherm-Retardation- Factor Based Reactive Transport Models 208 10.4 A Simple Example 211 10.5 Buffering in Reactive Transport 217 10.5.1 The Buffer Concept 217 10.5.2 Application of the Buffer Concept 218 10.6 Migration of an Acid Plume at a Uranium Mill Tailings Site 221 10.6.1 Model Description 221 10.6.2 Modeling Results 224 10.7 Remedial Design of a Uranium Tailings Repository 231 10.8 Summary and Comments 235 11 Kinetics Modeling 237 11.1 Introduction 237 11.2 Some Basic Theory 237 11.2.1 The Progress Variable 237 11.2.2 The Reaction Rate 239 11.2.3 Rate Laws 240 11.2.4 Temperature Dependence of Rate Constants 242 11.3 Kinetics of Precipitation and Dissolution Reactions 244 11.4 Kinetics of Acetate Decomposition 248 11.5 Coupled Aqueous Speciation and Biological Processes 254 11.6 Application to Landfill Leachate into Aquifers 256 11.7 Conclusions 258 Appendix 260 A Modifying a Database 261 A.1 Why Modify a Database? 261 A.1.1 Adding Arsenic Data to phreeqc 262 References 268