Groundwater geochemistry a practical guide to modeling of natural and contaminated aquatic systems

To understand hydrochemistry and to analyze natural as well as man-made impacts on aquatic systems, hydrogeochemical models have been used since the 1960's and more frequently in recent times. Numerical groundwater flow, transport, and geochemical models are important tools besides classical deterministic and analytical approaches. Solving complex linear or non-linear systems of equations, commonly with hundreds of unknown parameters, is a routine task for a PC. Modeling hydrogeochemical processes requires a detailed and accurate water analysis, as well as thermodynamic and kinetic data as input. Thermodynamic data, such as complex formation constants and solubility-products, are often provided as databases within the respective programs. However, the description of surface-controlled reactions (sorption, cation exchange, surface complexation) and kinetically controlled reactions requires additional input data. Unlike groundwater flow and transport models, thermodynamic models, in principal, do not need any calibration. However, considering surface-controlled or kinetically controlled reaction models might be subject to calibration. Typical problems for the application of geochemical models are: * speciation * determination of saturation indices * adjustment of equilibria/disequilibria for minerals or gases * mixing of different waters * modeling the effects of temperature * stoichiometric reactions (e.g. titration) * reactions with solids, fluids, and gaseous phases (in open and closed systems) * sorption (cation exchange, surface complexation) * inverse modeling * kinetically controlled reactions * reactive transport Hydrogeochemical models depend on the quality of the chemical analysis, the boundary conditions presumed by the program, theoretical concepts (e.g.

Author Notes

Broder J. Merkel (born 1949) studied geology and hydrogeology in Munich, where he got his master's and PhD degree. He worked as a scientist at the department of water chemistry in Munich and as a hydrogeologist in his own business in Germany and various foreign countries. After his habilitation in 1992 from the Christian-Albrechts University in Kiel he was offered the chair of hydrogeology at the Technische Universitaomlet;t Bergakademie Freiberg that he holds since 1993. His main research interests are on hydrogeochemical and reactive modeling, the influence of acid mine drainage on groundwater systems, and the application of GIS and remote sensing in hydrogeology.
Britta Planer-Friedrich (born 1975) studied geology in Wuomlet;rzburg and Freiberg where she obtained her master's degree in hydrogeology and environmental geology in 2000 and her PhD on volatile metals in 2004 as a stipend of the German National Merit Foundation. Her current work as postdoctoral researcher at Trent University, Canada, focuses on analytical chemistry and speciation of trace elements. Since 1999 she teaches computer courses in hydrogeochemical and reactive transport modeling as well as basics of groundwater chemistry and analytics at the Technische Universitaumlet;t Bergakademie Freiberg.
Darrell Kirk Nordstrom (born 1946) studied chemistry at Southern Illinois University (BA), geology at University of Colorado (MS), and environmental geochemistry (hydrogeology, surface chemistry, and microbiology) at Stanford University (PhD). He was an Assistant Professor at University of Virginia for four years before accepting a permanent position with the US Geological Survey in 1980. His primary research is on the effect of mining on water quality, especially acid mine drainage, and the application of geochemical models to the interpretation of water-rock interactions, but he has also been involved with research related to radioactive waste disposal and to geothermal chemistry.

1 Theoretical Background	p. 1
1.1 Equilibrium reactions	p. 1
1.1.1 Introduction	p. 1
1.1.2 Thermodynamic fundamentals	p. 5
1.1.2.1 Mass-action law	p. 5
1.1.2.2 Gibbs free energy	p. 7
1.1.2.3 Gibbs phase rule	p. 8
1.1.2.4 Activity	p. 8
1.1.2.5 Ionic strength	p. 10
1.1.2.6 Calculation of activity coefficient	p. 10
1.1.2.6.1 Theory of ion-association	p. 10
1.1.2.6.2 Theory of ion-interaction	p. 13
1.1.2.7 Comparison ion-association versus ion-interaction theory	p. 14
1.1.3 Interactions at the liquid-gaseous phase boundary	p. 17
1.1.3.1 Henry Law	p. 17
1.1.4 Interactions at the liquid-solid phase boundary	p. 19
1.1.4.1 Dissolution and precipitation	p. 19
1.1.4.1.1 Solubility-product	p. 19
1.1.4.1.2 Saturation index	p. 22
1.1.4.1.3 Limiting mineral phases	p. 22
1.1.4.2 Sorption	p. 25
1.1.4.2.1 Hydrophobic/hydrophilic substances	p. 25
1.1.4.2.2 Ion exchange	p. 25
1.1.4.2.3 Mathematical description of the sorption	p. 30
1.1.5 Interactions in the liquid phase	p. 35
1.1.5.1 Complexation	p. 35
1.1.5.2 Redox processes	p. 37
1.1.5.2.1 Measurement of the redox potential	p. 37
1.1.5.2.2 Calculation of the redox potential	p. 38
1.1.5.2.3 Presentation in predominance diagrams	p. 43
1.1.5.2.4 Redox buffer	p. 47
1.1.5.2.5 Significance of redox reactions	p. 47
1.2 Kinetics	p. 50
1.2.1 Kinetics of various chemical processes	p. 50
1.2.1.1 Half-life	p. 50
1.2.1.2 Kinetics of mineral dissolution	p. 51
1.2.2 Calculation of the reaction rate	p. 52
1.2.2.1 Subsequent reactions	p. 53
1.2.2.2 Parallel reactions	p. 54
1.2.3 Controlling factors on the reaction rate	p. 54
1.2.4 Empirical approaches for kinetically controlled reactions	p. 55
1.3 Reactive mass transport	p. 58
1.3.1 Introduction	p. 58
1.3.2 Flow models	p. 58
1.3.3 Transport models	p. 59
1.3.3.1 Definition	p. 59
1.3.3.2 Idealized transport conditions	p. 61
1.3.3.3 Real transport conditions	p. 61
1.3.3.1 Exchange within double-porosity aquifers	p. 62
1.3.3.4 Numerical methods of transport modeling	p. 63
1.3.3.4.1 Finite-difference/finite-element method	p. 65
1.3.3.4.2 Coupled methods	p. 66
2 Hydrogeochemical Modeling Programs	p. 69
2.1 General	p. 69
2.1.1 Geochemical algorithms	p. 69
2.1.2 Programs based on minimizing free energy	p. 71
2.1.3 Programs based on equilibrium constants	p. 72
2.1.3.1 Phreeqc	p. 72
2.1.3.2 EQ 3/6	p. 74
2.1.4 Thermodynamic databases	p. 76
2.1.4.1 General	p. 76
2.1.4.2 Structure of thermodynamic databases	p. 78
2.1.5 Problems and sources of error in geochemical modeling	p. 81
2.2 Use of Phreeqc	p. 85
2.2.1 The structure of Phreeqc and its graphical user interfaces	p. 85
2.2.1.1 Input	p. 88
2.2.1.2 Database	p. 95
2.2.1.3 Output	p. 96
2.2.1.4 Grid	p. 97
2.2.1.5 Chart	p. 97
2.2.2 Introductory Examples for Phreeqc Modeling	p. 97
2.2.2.1 Equilibrium reactions	p. 97
2.2.2.1.1 Example 1a standard output - seawater analysis	p. 98
2.2.2.1.2 Example 1b equilibrium - solution of gypsum	p. 100
2.2.2.1.3 Example 1c equilibrium - solution of calcite with CO2	p. 101
2.2.2.1.4 Example 1d: Modeling uncertainties - Ljungskile	p. 103
2.2.2.2 Introductory example for sorption	p. 107
2.2.2.3 Introductory examples for kinetics	p. 114
2.2.2.3.1 Defining reaction rates	p. 115
2.2.2.3.2 Basic within Phreeqc	p. 117
2.2.2.4 Introductory example for isotope fractionation	p. 122
2.2.2.5 Introductory example for reactive mass transport	p. 126
2.2.2.5.1 Simple 1D transport: Column experiment	p. 126
2.2.2.5.2 1D transport, dilution, and surface complexation in an abandoned uranium mine	p. 130
2.2.2.5.3 3D transport with Phast	p. 134
3 Exercises	p. 141
3.1 Equilibrium reactions	p. 143
3.1.1 Groundwater - Lithosphere	p. 143
3.1.1.1 Standard output well analysis	p. 143
3.1.1.2 Equilibrium reaction - solubility of gypsum	p. 144
3.1.1.3 Disequilibrium reaction - solubility of gypsum	p. 144
3.1.1.4 Temperature dependency of gypsum solubility in well water	p. 144
3.1.1.5 Temperature dependency of gypsum solubility in pure water	p. 144
3.1.1.6 Temperature-and P(CO2)-dependent calcite solubility	p. 144
3.1.1.7 Calcite precipitation and dolomite dissolution	p. 145
3.1.1.8 Calcite solubility in an open and a closed system	p. 145
3.1.1.9 Pyrite weathering	p. 145
3.1.2 Atmosphere - Groundwater - Lithosphere	p. 146
3.1.2.1 Precipitation under the influence of soil CO2	p. 146
3.1.2.2 Buffering systems in the soil	p. 147
3.1.2.3 Mineral precipitates at hot sulfur springs	p. 147
3.1.2.4 Formation of stalactites in karst caves	p. 148
3.1.2.5 Evaporation	p. 149
3.1.3 Groundwater	p. 150
3.1.3.1 The pE-pH diagram for the system iron	p. 150
3.1.3.2 The Fe pE-p-H diagram considering carbon and sulfur	p. 152
3.1.3.3 The pH dependency of uranium species	p. 152
3.1.4 Origin of groundwater	p. 153
3.1.4.1 Pumping of fossil groundwater in arid regions	p. 155
3.1.4.2 Salt water/fresh water interface	p. 156
3.1.5 Anthropogenic use of groundwater	p. 157
3.1.5.1 Sampling: Ca titration with Edta	p. 157
3.1.5.2 Carbonic acid aggressiveness	p. 157
3.1.5.3 Water treatment by aeration - well water	p. 158
3.1.5.4 Water treatment by areation - sulfur spring	p. 158
3.1.5.5 Mixing of waters	p. 159
3.1.6 Rehabilitation of groundwater	p. 159
3.1.6.1 Reduction of nitrate with methanol	p. 159
3.1.6.2 Fe(0) barriers	p. 160
3.1.6.3 Increase in pH through a calcite barrier	p. 160
3.2 Reaction kientics	p. 160
3.2.1 Pyrite weathering	p. 160
3.2.2 Quartz-feldspar-dissolution	p. 161
3.2.3 Degradation of organic matter within the aquifer on reduction of redox-sensitive elements (Fe, As, U, Cu, Mn, S)	p. 162
3.2.4 Degradation of tritium in the unsaturated zone	p. 163
3.3 Reactive transport	p. 166
3.3.1 Lysimeter	p. 166
3.3.2 Karst spring discharge	p. 167
3.3.3 Karstification (corrosion along a karst fracture)	p. 168
3.3.4 The pH increase of an acid mine water	p. 169
3.3.5 In-situ leaching	p. 170
3.3.6 3D Transport - Uranium and arsenic contamination plume	p. 171
4 Solutions	p. 173
4.1 Equilibrium reactions	p. 173
4.1.1 Groundwater - Lithosphere	p. 173
4.1.1.1 Standard output well analysis	p. 173
4.1.1.2 Equilibrium reaction - solubility of gypsum	p. 175
4.1.1.3 Disequilibrium reaction - solubility of gypsum	p. 175
4.1.1.4 Temperature dependency of gypsum solubility in well water	p. 176
4.1.1.5 Temperature dependency of gypsum solubility in pure water	p. 177
4.1.1.6 Temperature- and P(CO2)-dependent calcite solubility	p. 177
4.1.1.7 Calcite precipitation and dolomite dissolution	p. 178
4.1.1.8 Comparison of the calcite solubility in an open and a closed system	p. 179
4.1.1.9 Pyrite weathering	p. 179
4.1.2 Atmosphere - Groundwater - Lithosphere	p. 181
4.1.2.1 Precipitation under the influence of soil CO2	p. 181
4.1.2.2 Buffering systems in the soil	p. 181
4.1.2.3 Mineral precipitations at hot sulfur springs	p. 182
4.1.2.4 Formation of stalactites in karst caves	p. 183
4.1.2.5 Evaporation	p. 183
4.1.3 Groundwater	p. 184
4.1.3.1 The pE-pH diagram for the system iron	p. 184
4.1.3.2 The Fe pE-pH diagram considering carbon and sulfur	p. 186
4.1.3.3 The pH dependency of uranium species	p. 187
4.1.4 Origin of groundwater	p. 188
4.1.4.1 Pumping of fossil groundwater in arid regions	p. 188
4.1.4.2 Salt water/fresh water interface	p. 189
4.1.5 Anthropogenic use of groundwater	p. 190
4.1.5.1 Sampling: Ca titration with Edta	p. 190
4.1.5.2 Carbonic acid aggressiveness	p. 191
4.1.5.3 Water treatment by aeration - well water	p. 191
4.1.5.4 Water treatment by aeration - sulfur spring	p. 191
4.1.5.5 Mixing of waters	p. 193
4.1.6 Rehabilitation of groundwater	p. 194
4.1.6.1 Reduction of nitrate with methanol	p. 194
4.1.6.2 Fe(0) barriers	p. 195
4.1.6.3 Increase in pH through a calcite barrier	p. 196
4.2 Reaction kinetics	p. 197
4.2.1 Pyrite weathering	p. 197
4.2.2 Quartz-feldspar-dissolution	p. 199
4.2.3 Degradation of organic matter within the aquifer on reduction of redox-sensitive elements (Fe, As, U, Cu, Mn, S)	p. 201
4.2.4 Degradation of tritium in the unsaturated zone	p. 203
4.3 Reactive transport	p. 205
4.3.1 Lysimeter	p. 205
4.3.2 Karst spring discharge	p. 205
4.3.3 Karstification (corrosion along a karst fracture)	p. 207
4.3.4 The pH increase of an acid mine water	p. 208
4.3.5 In-situ leaching	p. 210
4.3.6 3D Transport - Uranium and arsenic contamination plume	p. 212
References	p. 215
Index	p. 221

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