Groundwater Geochemistry: A Practical Guide to Modeling of Natural and Contaminated Aquatic Systems

237 Pages·2008·4.92 MB·English

by  Broder J. Merkel

Groundwater Geochemistry
·
Broder J. Merkel Britta Planer-Friedrich
Authors
Groundwater
Geochemistry
A Practical Guide to Modeling of Natural
and Contaminated Aquatic Systems
Edited by Darrell Kirk Nordstrom
2ndEdition
WithCD-ROM
123
Authors
Prof.Dr.BroderJ.Merkel Dr.BrittaPlaner-Friedrich
TUBergakademieFreiberg TUBergakademieFreiberg
Inst.Geologie Inst.Geologie
Gustav-Zeuner-Str.12 Gustav-Zeuner-Str.12
09599Freiberg 09599Freiberg
Germany Germany
[email protected] b.planer-friedrich@geo.
tu-freiberg.de
Editor
Dr.DarrellK.Nordstrom
U.S.GeologicalSurvey
3215MarineSt.
BoulderCO80303
USA
[email protected]
ISBN:978-3-540-74667-6 e-ISBN:978-3-540-74668-3
LibraryofCongressControlNumber:2008927060
(cid:2)c 2008, 2005 Springer-VerlagBerlinHeidelberg
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Foreword
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.
calculation of activity coefficients) and the thermodynamic data. Therefore it is
vital to check the results critically. For that, a basic knowledge about chemical and
thermodynamic processes is required and will be outlined briefly in the following
chapters on hydrogeochemical equilibrium (chapter 1.1), kinetics (chapter 1.2),
and transport (chapter 1.3). Chapter 2 gives an overview on standard
VI Foreword
hydrogeochemical programs, problems and possible sources of error for modeling,
and a detailed introduction to run the program PHREEQC, which is used in this
book. With the help of examples, practical modeling applications are addressed
and specialized theoretical knowledge is extended. Chapter 4 presents the results
for the exercises of chapter 3. This book does not aim to replace a textbook but
rather attempts to be a practical guide for beginners at modeling.
Table of Contents
1 Theoretical Background.........................................................................1
1.1 Equilibrium reactions....................................................................................1
1.1.1 Introduction...........................................................................................1
1.1.2 Thermodynamic fundamentals..............................................................5
1.1.2.1 Mass-action law............................................................................5
1.1.2.2 Gibbs free energy..........................................................................7
1.1.2.3 Gibbs phase rule............................................................................8
1.1.2.4 Activity..........................................................................................8
1.1.2.5 Ionic strength...............................................................................10
1.1.2.6 Calculation of activity coefficient...............................................10
1.1.2.6.1. Theory of ion-association..................................................10
1.1.2.6.2. Theory of ion-interaction...................................................13
1.1.2.7 Comparison ion-association versus ion-interaction theory..........14
1.1.3 Interactions at the liquid-gaseous phase boundary..............................17
1.1.3.1 Henry Law...................................................................................17
1.1.4 Interactions at the liquid-solid phase boundary...................................19
1.1.4.1 Dissolution and precipitation.......................................................19
1.1.4.1.1. Solubility-product..............................................................19
1.1.4.1.2. Saturation index.................................................................22
1.1.4.1.3. Limiting mineral phases....................................................22
1.1.4.2 Sorption.......................................................................................25
1.1.4.2.1. Hydrophobic/hydrophilic substances................................25
1.1.4.2.2. Ion exchange......................................................................25
1.1.4.2.3. Mathematical description of the sorption..........................30
1.1.5 Interactions in the liquid phase...........................................................35
1.1.5.1 Complexation..............................................................................35
1.1.5.2 Redox processes..........................................................................37
1.1.5.2.1. Measurement of the redox potential..................................37
1.1.5.2.2. Calculation of the redox potential.....................................38
1.1.5.2.3. Presentation in predominance diagrams............................43
1.1.5.2.4. Redox buffer......................................................................47
1.1.5.2.5. Significance of redox reactions.........................................47
1.2 Kinetics.......................................................................................................50
1.2.1 Kinetics of various chemical processes...............................................50
1.2.1.1 Half-life.......................................................................................50
1.2.1.2 Kinetics of mineral dissolution....................................................51
1.2.2 Calculation of the reaction rate...........................................................52
1.2.2.1 Subsequent reactions...................................................................53
VIII Table of Contents
1.2.2.2 Parallel reactions.........................................................................54
1.2.3 Controlling factors on the reaction rate...............................................54
1.2.4 Empirical approaches for kinetically controlled reactions..................55
1.3 Reactive mass transport..............................................................................58
1.3.1 Introduction.........................................................................................58
1.3.2 Flow models........................................................................................58
1.3.3 Transport models................................................................................59
1.3.3.1 Definition....................................................................................59
1.3.3.2 Idealized transport conditions.....................................................61
1.3.3.3 Real transport conditions.............................................................61
1.3.3.3.1. Exchange within double-porosity aquifers........................62
1.3.3.4 Numerical methods of transport modeling..................................63
1.3.3.4.1. Finite-difference/finite-element method............................65
1.3.3.4.2. Coupled methods...............................................................66
2 Hydrogeochemical Modeling Programs.............................................69
2.1 General........................................................................................................69
2.1.1 Geochemical algorithms.....................................................................69
2.1.2 Programs based on minimizing free energy........................................71
2.1.3 Programs based on equilibrium constants...........................................72
2.1.3.1 PHREEQC...................................................................................72
2.1.3.2 EQ 3/6.........................................................................................74
2.1.4 Thermodynamic databases..................................................................76
2.1.4.1 General........................................................................................76
2.1.4.2 Structure of thermodynamic databases........................................78
2.1.5 Problems and sources of error in geochemical modeling....................81
2.2 Use of PHREEQC.......................................................................................85
2.2.1 The structure of PHREEQC and its graphical user interfaces.............85
2.2.1.1 Input............................................................................................88
2.2.1.2 Database......................................................................................95
2.2.1.3 Output..........................................................................................96
2.2.1.4 Grid.............................................................................................97
2.2.1.5 Chart............................................................................................97
2.2.2 Introductory Examples for PHREEQC Modeling...............................97
2.2.2.1 Equilibrium reactions..................................................................97
2.2.2.1.1. Example 1a standard output – seawater analysis...............98
2.2.2.1.2. Example 1b equilibrium – solution of gypsum................100
2.2.2.1.3. Example 1c equilibrium – solution of calcite with CO ..101
2
2.2.2.1.4. Example 1d: Modeling uncertainties – LJUNGSKILE...103
2.2.2.2 Introductory example for sorption.............................................107
2.2.2.3 Introductory examples for kinetics............................................114
2.2.2.3.1. Defining reaction rates....................................................115
2.2.2.3.2. BASIC within PHREEQC...............................................117
2.2.2.4 Introductory example for isotope fractionation.........................122
2.2.2.5 Introductory examples for reactive mass transport....................126
Table of Contents IX
2.2.2.5.1. Simple 1D transport: column experiment........................126
2.2.2.5.2. 1D transport, dilution, and surface complexation in an
abandoned uranium mine.................................................................130
2.2.2.5.3. 3D transport with PHAST...............................................134
3 Exercises..............................................................................................141
3.1 Equilibrium reactions................................................................................143
3.1.1 Groundwater – Lithosphere..............................................................143
3.1.1.1 Standard output well analysis....................................................143
3.1.1.2 Equilibrium reaction – solubility of gypsum.............................144
3.1.1.3 Disequilibrium reaction – solubility of gypsum........................144
3.1.1.4 Temperature dependency of gypsum solubility in well water...144
3.1.1.5 Temperature dependency of gypsum solubility in pure water...144
3.1.1.6 Temperature- and P(CO )-dependent calcite solubility.............144
2
3.1.1.7 Calcite precipitation and dolomite dissolution..........................145
3.1.1.8 Calcite solubility in an open and a closed system.....................145
3.1.1.9 Pyrite weathering......................................................................145
3.1.2 Atmosphere – Groundwater – Lithosphere.......................................146
3.1.2.1 Precipitation under the influence of soil CO ............................146
2
3.1.2.2 Buffering systems in the soil.....................................................147
3.1.2.3 Mineral precipitates at hot sulfur springs..................................147
3.1.2.4 Formation of stalactites in karst caves.......................................148
3.1.2.5 Evaporation...............................................................................149
3.1.3 Groundwater.....................................................................................150
3.1.3.1 The pE-pH diagram for the system iron....................................150
3.1.3.2 The Fe pE-pH diagram considering carbon and sulfur..............152
3.1.3.3 The pH dependency of uranium species....................................152
3.1.4 Origin of groundwater.......................................................................153
3.1.4.1 Pumping of fossil groundwater in arid regions.........................155
3.1.4.2 Salt water/fresh water interface.................................................156
3.1.5 Anthropogenic use of groundwater...................................................157
3.1.5.1 Sampling: Ca titration with EDTA............................................157
3.1.5.2 Carbonic acid aggressiveness....................................................157
3.1.5.3 Water treatment by aeration – well water..................................158
3.1.5.4 Water treatment by aeration – sulfur spring..............................158
3.1.5.5 Mixing of waters.......................................................................159
3.1.6 Rehabilitation of groundwater...........................................................159
3.1.6.1 Reduction of nitrate with methanol...........................................159
3.1.6.2 Fe(0) barriers.............................................................................160
3.1.6.3 Increase in pH through a calcite barrier....................................160
3.2 Reaction kinetics.......................................................................................160
3.2.1 Pyrite weathering..............................................................................160
3.2.2 Quartz-feldspar-dissolution...............................................................161
3.2.3 Degradation of organic matter within the aquifer on reduction of
redox-sensitive elements (Fe, As, U, Cu, Mn, S).......................................162
X Table of Contents
3.2.4 Degradation of tritium in the unsaturated zone.................................163
3.3 Reactive transport.....................................................................................166
3.3.1 Lysimeter..........................................................................................166
3.3.2 Karst spring discharge.......................................................................167
3.3.3 Karstification (corrosion along a karst fracture)...............................168
3.3.4 The pH increase of an acid mine water.............................................169
3.3.5 In-situ leaching..................................................................................170
3.3.6 3D Transport – Uranium and arsenic contamination plume.............171
4 Solutions...............................................................................................173
4.1 Equilibrium reactions................................................................................173
4.1.1 Groundwater – Lithosphere..............................................................173
4.1.1.1 Standard output well analysis....................................................173
4.1.1.2 Equilibrium reaction – solubility of gypsum.............................175
4.1.1.3 Disequilibrium reaction – solubility of gypsum........................175
4.1.1.4 Temperature dependency of gypsum solubility in well water....176
4.1.1.5 Temperature dependency of gypsum solubility in pure water...177
4.1.1.6 Temperature- and P(CO )-dependent calcite solubility.............177
2
4.1.1.7 Calcite precipitation and dolomite dissolution..........................178
4.1.1.8 Comparison of the calcite solubility in an open and a closed
system...................................................................................................179
4.1.1.9 Pyrite weathering......................................................................179
4.1.2 Atmosphere – Groundwater – Lithosphere.......................................181
4.1.2.1 Precipitation under the influence of soil CO ............................181
2
4.1.2.2 Buffering systems in the soil.....................................................181
4.1.2.3 Mineral precipitations at hot sulfur springs...............................182
4.1.2.4 Formation of stalactites in karst caves.......................................183
4.1.2.5 Evaporation...............................................................................183
4.1.3 Groundwater.....................................................................................184
4.1.3.1 The pE-pH diagram for the system iron....................................184
4.1.3.2 The Fe pE-pH diagram considering carbon and sulfur..............186
4.1.3.3 The pH dependency of uranium species....................................187
4.1.4 Origin of groundwater.......................................................................188
4.1.4.1 Pumping of fossil groundwater in arid regions.........................188
4.1.4.2 Salt water/fresh water interface.................................................189
4.1.5 Anthropogenic use of groundwater...................................................190
4.1.5.1 Sampling: Ca titration with EDTA............................................190
4.1.5.2 Carbonic acid aggressiveness....................................................191
4.1.5.3 Water treatment by aeration – well water..................................191
4.1.5.4 Water treatment by aeration – sulfur spring..............................191
4.1.5.5 Mixing of waters.......................................................................193
4.1.6 Rehabilitation of groundwater...........................................................194
4.1.6.1 Reduction of nitrate with methanol...........................................194
4.1.6.2 Fe(0) barriers.............................................................................195
4.1.6.3 Increase in pH through a calcite barrier....................................196
Table of Contents XI
4.2 Reaction kinetics.......................................................................................197
4.2.1 Pyrite weathering..............................................................................197
4.2.2 Quartz-feldspar-dissolution...............................................................199
4.2.3 Degradation of organic matter within the aquifer on reduction of
redox-sensitive elements (Fe, As, U, Cu, Mn, S).......................................201
4.2.4 Degradation of tritium in the unsaturated zone.................................203
4.3 Reactive transport.....................................................................................205
4.3.1 Lysimeter..........................................................................................205
4.3.2 Karst spring discharge.......................................................................205
4.3.3 Karstification (corrosion along a karst fracture)...............................207
4.3.4 The pH increase of an acid mine water.............................................208
4.3.5 In-situ leaching..................................................................................210
4.3.6 3D Transport – Uranium and arsenic contamination plume.............212
References...............................................................................................215
Index........................................................................................................221