Table of Contents

LIST OF EXAMPLES XII

NOTATION XV

1 INTRODUCTION TO GROUNDWATER GEOCHEMISTRY 1

• 1.1 Groundwater as drinking water 1
• 1.1.1 Standards for drinking water 1
• 1.2 Units of analysis 3
• 1.3 Groundwater quality 7
• 1.4 Sampling of groundwater 10
• 1.4.1 Depth integrated or depth specific sampling 10
• 1.4.2 Procedures for sampling of groundwater 12
• 1.5 Chemical analysis of groundwater 15
• 1.5.1 Field analyses and sample conservation 15
• 1.5.2 Accuracy of chemical analysis 17
• Problems 20
• References 21

2 FROM RAINWATER TO GROUNDWATER 23

• 2.1 The hydrological cycle 23
• 2.2 The composition of rainwater 26
• 2.2.1 Sources and transport of atmospheric pollutants 31
• 2.3 Stable isotopes in rain 31
• 2.3.1 Isotopic ratios and the < notation 32
• 2.3.2 The Rayleigh process 33
• 2.3.3 The isotopic composition of rain 37
• 2.4 Dry deposition and evapotranspiration 41
• 2.5 Mass balances and ecosystem dynamics 46
• 2.5.1 Water quality profiles in the unsaturated soil 49
• 2.6 Overall controls on water quality 51
• Problems 58
• References 59

3 FLOW AND TRANSPORT 63

• 3.1 Flow in the unsaturated zone 63
• 3.2 Flow in the saturated zone 64
• 3.2.1 Darcy’s law 64
• 3.2.2 Flowlines in the subsoil 67
• 3.2.3 Effects of non-homogeneity 70
• 3.2.4 The aquifer as a chemical reactor 71
• 3.3 Dating of groundwater 72
• 3.4 Retardation 75
• 3.4.1 The retardation equation 76
• 3.4.2 Indifferent and broadening fronts 79
• 3.4.3 Sharpening fronts 82
• 3.4.4 Solid and solute concentrations 84
• 3.5 Diffusion 86
• 3.5.1 Diffusion coefficients 87
• 3.5.2 Diffusion as a random process 89
• 3.5.3 Diffusive transport 93
• 3.5.4 Isotope diffusion 96
• 3.6 Dispersion 99
• 3.6.1 Column breakthrough curves 102
• 3.6.2 Dispersion coefficients and dispersivity 105
• 3.6.3 Macrodispersivity 107
• Problems 113
• References 115

4 MINERALS AND WATER 119

• 4.1 Equilibria and the solubility of minerals 119
• 4.2 Corrections for solubility calculations 123
• 4.2.1 Concentration and activity 123
• 4.2.2 Aqueous complexes 127
• 4.2.3 Combined complexes and activity corrections 128
• 4.2.4 Calculation of saturation states 131
• 4.3 Mass action constants and thermodynamics 132
• 4.3.1 The calculation of mass action constants 132
• 4.3.2 Calculation of mass action constants at different temperature 133
• 4.4 Equilibrium calculations with PHREEQC 135
• 4.4.1 Speciation calculations using PHREEQC 135
• 4.4.2 The PHREEQC database 137
• 4.4.3 Mineral equilibration 141
• 4.5 Solid solutions 142
• 4.5.1 Basic theory 142
• 4.5.2 The fractionation factor for solid solutions 148
• 4.5.3 Kinetic effects on the fractionation factor 149
• 4.6 Kinetics of geochemical processes 152
• 4.6.1 Kinetics and equilibrium 152
• 4.6.2 Chemical reactions and rates 153
• 4.6.3 Temperature dependency of reaction rates 159
• 4.6.4 Mechanisms of dissolution and crystallization 160
• 4.6.5 Rate laws for mineral dissolution and precipitation 162
• Problems 169
• References 171

5 CARBONATES AND CARBON DIOXIDE 175

• 5.1 Carbonate minerals 176
• 5.2 Dissolved carbonate equilibria 178
• 5.2.1 The carbonic acid system 179
• 5.2.2 Determining the carbonate speciation in groundwater 183
• 5.3 Carbon dioxide in soils 186
• 5.4 Calcite solubility and PCO2 191
• 5.4.1 Calcite dissolution in systems open and closed for CO2 gas 193
• 5.4.2 Two field examples 195
• 5.5 Carbonate rock aquifers 197
• 5.5.1 Dolomite and dedolomitization 201
• 5.5.2 Pleistocene carbonate aquifers 205
• 5.6 Kinetics of carbonate reactions 210
• 5.6.1 Dissolution 210
• 5.6.2 Precipitation 217
• 5.7 Carbon isotopes 218
• 5.7.1 Carbon-13 trends in aquifers 221
• 5.7.2 14C and groundwater age 226
• 5.7.3 Retardation by sorption and stagnant zone diffusion 228
• Problems 232
• References 236

6 ION EXCHANGE 241

• 6.1 Cation exchange at the salt/fresh water interface 242
• 6.2 Adsorbents in soils and aquifers 247
• 6.2.1 Clay minerals 248
• 6.3 Exchange equations 251
• 6.3.1 Values for exchange coefficients 254
• 6.3.2 Calculation of exchanger composition 255
• 6.3.3 Calculation of exchanger composition with PHREEQC 257
• 6.3.4 Determination of exchangeable cations 260
• 6.4 Chromatography of cation exchange 262
• 6.4.1 Field examples of freshening 263
• 6.4.2 Salinity effects on cation exchange 268
• 6.4.3 Quality patterns with salinization 271
• 6.4.4 Fronts and chromatographic sequences 272
• 6.4.5 Modeling chromatographic sequences with PHREEQC 275
• 6.5 Physical non-equilibrium 283
• 6.5.1 Modeling stagnant zones 285
• 6.6 The Gouy-Chapman theory of the double layer 288
• 6.6.1 Numerical integration of the double layer equations 293
• 6.6.2 Practical aspects of double layer theory 296
• 6.7 Irrigation water quality 299
• Problems 303
• References 306

7 SORPTION OF TRACE METALS 311

• 7.1 The origin and occurrence of heavy metals in groundwater 311
• 7.2 Sorption isotherms and distribution coefficients 315
• 7.2.1 Distribution coefficients from ion exchange 318
• 7.3 Variable charge surfaces 322
• 7.3.1 Titration curves with suspended oxide particles 322
• 7.3.2 Surface charge and point of zero charge, PZC 324
• 7.3.3 Sorption edges 328
• 7.3.4 Sorption, absorption, and coprecipitation 333
• 7.4 Surface complexation 334
• 7.4.1 Surface complexation models 338
• 7.4.2 The ferrihydrite (Fe(OH)3) database 340
• 7.4.3 Diffuse double layer concentrations in surface complexation models 343
• 7.5 Complexation to humic acids 344
• 7.5.1 The ion association model 346
• 7.5.2 Tipping and Hurley’s discrete site model “WHAM” 348
• 7.5.3 Distribution models 354
• 7.5.4 Humic acids as carriers of trace elements 356
• 7.6 Kinetics of surface complexation 358
• 7.6.1 Extrapolation of adsorption kinetics for other metal ions 363
• 7.7 Field applications 363
• Problems 367
• References 369

8 SILICATE WEATHERING 375

• 8.1 Weathering processes 375
• 8.2 The stability of weathering products 380
• 8.3 Incongruent dissolution of primary silicates 383
• 8.4 The mass balance approach to weathering 389
• 8.5 Kinetics of silicate weathering 395
• 8.6 Field weathering rates 400
• 8.7 Acid groundwater 404
• 8.7.1 Buffering processes in aquifers 405
• Problems 410
• References 412

9 REDOX PROCESSES 415

• 9.1 Basic theory 415
• 9.1.1 The significance of redox measurements 420
• 9.1.2 Redox reactions and the pe concept 422
• 9.2 Redox diagrams 423
• 9.2.1 Stability of water 424
• 9.2.2 The stability of dissolved species and gases: Arsenic 425
• 9.2.3 The stability of minerals in redox diagrams 432
• 9.3 Sequences of redox reactions and redox zoning 438
• 9.3.1 Decomposition of organic matter 442
• 9.4 Oxygen consumption 446
• 9.4.1 Pyrite oxidation 450
• 9.4.2 Kinetics of pyrite oxidation 450
• 9.4.3 Oxygen transport and pyrite oxidation 453
• 9.5 Nitrate reduction 458
• 9.5.1 Nitrate reduction by organic matter oxidation 459
• 9.5.2 Nitrate reduction by pyrite and ferrous iron 462
• 9.6 Iron reduction and sources of iron in groundwater 465
• 9.6.1 Iron in aquifer sediments 465
• 9.6.2 Reductive dissolution of iron oxides 466
• 9.7 Sulfate reduction and iron sulfide formation 472
• 9.7.1 The formation of iron sulfides 476
• 9.8 The formation of methane 477
• Problems 479
• References 480

10 POLLUTION BY ORGANIC CHEMICALS 489

• 10.1 Gas-water exchange 489
• 10.1.1 Evaporation of a pure organic liquid 494
• 10.2 Transport of pure organic liquids through soil 496
• 10.3 Sorption of organic chemicals 499
• 10.3.1 Sorption of charged organic molecules 504
• 10.3.2 Sorption in stagnant zones 506
• 10.3.3 Release from stagnant zones and blobs 510
• 10.4 Transformation reactions of organic chemicals 516
• 10.4.1 Monod biotransformation kinetics 518
• 10.5 Kinetic complexation of heavy metals on organics 529
• Problems 535
• References 537

11 NUMERICAL MODELING 541

• 11.1 Numerical modeling of transport 543
• 11.1.1 Only diffusion 543
• 11.1.2 Advection and diffusion/dispersion 550
• 11.1.3 Non-linear reactions 558
• 11.2 Examples of hydrogeochemical transport modeling 560
• 11.2.1 Tritium-Helium age dating 561
• 11.2.2 Toluene degradation in an aquifer 565
• 11.2.3 Remediation of a BTEX polluted site 568
• 11.2.4 Acid drainage from a Uranium mine 570
• 11.2.5 In-situ iron removal from groundwater 579
• 11.2.6 Arsenic in Bangladesh groundwater 585
• 11.2.7 Fractionation of isotopes 590
• References 595

APPENDIX A: HYDROGEOCHEMICAL MODELING WITH PHREEQC 599

APPENDIX B: ANSWERS TO PROBLEMS 617

INDEX 635