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

208 Pages·2005·2.42 MB·English

by  Prof. Dr. Broder J. Merkel

Broder J. Merkel, Britta Planer-Friedrich

Edited by
Darrell Kirk Nordstrom

Groundwater Geochemistry

A Practical Guide to Modeling of Natural
and Contaminated Aquatic Systems
Broder J. Merkel
Britta Planer-Friedrich
Edited by
Darrell Kirk Nordstrom

Groundwater
Geochemistry

A Practical Guide to Modeling
of Natural and Contaminated Aquatic
Systems

With 76 Figures and a CD-ROM
PROF. DR. BRODER J. MERKEL
DR. BRITTA PLANER-FRIEDRICH
DEPARTMENT OF GEOLOGY
TECHNISCHE UNIVERSITAET
BERGAKADEMIE FREIBERG
GUSTAV ZEUNER STR. 12
09599 FREIBERG
GERMANY

DR. DARRELL KIRK NORDSTROM
U.S. GEOLOGICAL SURVEY
3215 MARINE ST., SUITE E-127
BOULDER, CO 80303
USA

E-mail: [email protected]
b. [email protected]
[email protected]

This book has been translated and updated from the German version
"Grundwasserchemie", ISBN 3-540-42836-4, published at Springer
in 2002.

ISBN 3-540-24195-7 Springer Berlin Heidelberg New York

Library of Congress Control Number: 2004117858

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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 data sets 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:
(cid:120) speciation
(cid:120) determination of saturation indices
(cid:120) adjustment of equilibria/disequilibria for minerals or gases
(cid:120) mixing of different waters
(cid:120) modeling the effects of temperature
(cid:120) stoichiometric reactions (e.g. titration)
(cid:120) reactions with solids, fluids, and gaseous phases (in open and closed
systems)
(cid:120) sorption (cation exchange, surface complexation)
(cid:120) inverse modeling
(cid:120) kinetically controlled reactions
(cid:120) reactive transport
Hydrogeochemical models are dependent on the quality of the chemical analyses,
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..............................................................4
1.1.2.1 Mass action law.............................................................................4
1.1.2.2 Gibbs free energy..........................................................................6
1.1.2.3 Gibbs phase rule............................................................................7
1.1.2.4 Activity..........................................................................................8
1.1.2.5 Ionic strength.................................................................................8
1.1.2.6 Calculation of activity coefficient...............................................10
1.1.2.6.1. Theory of ion dissociation.................................................10
1.1.2.6.2. Theory of ion interaction...................................................12
1.1.2.7 Theories of ion dissociation and ion interaction..........................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...................................18
1.1.4.1 Dissolution and precipitation.......................................................18
1.1.4.1.1. Solubility product..............................................................18
1.1.4.1.2. Saturation index.................................................................20
1.1.4.1.3. Limiting mineral phases....................................................22
1.1.4.2 Sorption.......................................................................................24
1.1.4.2.1. Hydrophobic /hydrophilic substances...............................24
1.1.4.2.2. Ion exchange......................................................................24
1.1.4.2.3. Mathematical description of the sorption..........................30
1.1.5 Interactions in the liquid phase...........................................................34
1.1.5.1 Complexation..............................................................................34
1.1.5.2 Redox processes..........................................................................36
1.1.5.2.1. Measurement of the redox potential..................................36
1.1.5.2.2. Calculation of the redox potential.....................................37
1.1.5.2.3. Presentation in predominance diagrams............................41
1.1.5.2.4. Redox buffer......................................................................45
1.1.5.2.5. Significance of redox reactions.........................................46
1.2 Kinetics.......................................................................................................49
1.2.1 Kinetics of various chemical processes...............................................49
1.2.1.1 Half-life.......................................................................................49
1.2.1.2 Kinetics of mineral dissolution....................................................50
1.2.2 Calculation of the reaction rate...........................................................51
1.2.2.1 Subsequent reactions...................................................................52
VIII Table ofcontents
1.2.2.2 Parallel reactions.........................................................................53
1.2.3 Controlling factors on the reaction rate...............................................53
1.2.4 Empiric approaches for kinetically controlled reactions.....................55
1.3 Reactive mass transport..............................................................................57
1.3.1 Introduction.........................................................................................57
1.3.2 Flow models........................................................................................57
1.3.3 Transport models................................................................................57
1.3.3.1 Definition....................................................................................57
1.3.3.2 Idealized transport conditions.....................................................58
1.3.3.3 Real transport conditions.............................................................60
1.3.3.3.1. Exchange within double-porosity aquifers........................61
1.3.3.4 Numerical methods of transport modeling..................................63
1.3.3.4.1. Finite-difference / finite-element method..........................63
1.3.3.4.2. Coupled methods...............................................................65
2 Hydrogeochemical Modeling Programs.............................................67
2.1 General........................................................................................................67
2.1.1 Geochemical algorithms.....................................................................67
2.1.2 Programs based on minimizing free energy........................................69
2.1.3 Programs based on equilibrium constants...........................................70
2.1.3.1 PHREEQC...................................................................................70
2.1.3.2 EQ 3/6.........................................................................................72
2.1.3.3 Comparison PHREEQC – EQ 3/6...............................................73
2.1.4 Thermodynamic data sets....................................................................76
2.1.4.1 General........................................................................................76
2.1.4.2 Structure of thermodynamic data sets.........................................78
2.1.5 Problems and sources of error in geochemical modeling....................80
2.2 Use of PHREEQC.......................................................................................84
2.2.1 Structure of PHREEQC under the Windows surface..........................84
2.2.1.1 Input............................................................................................85
2.2.1.2 Thermodynamic data...................................................................93
2.2.1.3 Output..........................................................................................94
2.2.1.4 Grid.............................................................................................95
2.2.1.5 Chart............................................................................................95
2.2.2 Introductory Examples for PHREEQC Modeling...............................95
2.2.2.1 Equilibrium reactions..................................................................95
2.2.2.1.1. Example 1: Standard output – seawater analysis...............96
2.2.2.1.2. Example 2 equilibrium – solution of gypsum....................98
2.2.2.2 Introductory examples for kinetics..............................................99
2.2.2.2.1. Defining reaction rates....................................................100
2.2.2.2.2. BASIC within PHREEQC...............................................103
2.2.2.3 Introductory example for reactive mass transport.....................106
Table ofcontents IX
3 Exercises..............................................................................................111
3.1 Equilibrium reactions................................................................................112
3.1.1 Groundwater - Lithosphere...............................................................112
3.1.1.1 Standard-output well analysis...................................................112
3.1.1.2 Equilibrium reaction - solubility of gypsum.............................113
3.1.1.3 Disequilibrium reaction - solubility of gypsum.........................113
3.1.1.4 Temperature dependency of gypsum solubility in well water...113
3.1.1.5 Temperature dependency of gypsum solubility in distilled water
..............................................................................................................113
3.1.1.6 Temperature and P(CO ) dependent calcite solubility..............113
2
3.1.1.7 Calcite precipitation and dolomite dissolution..........................114
3.1.1.8 Calcite solubility in an open and a closed system.....................114
3.1.1.9 Pyrite weathering......................................................................114
3.1.2 Atmosphere – Groundwater – Lithosphere.......................................116
3.1.2.1 Precipitation under the influence of soil CO ............................116
2
3.1.2.2 Buffering systems in the soil.....................................................116
3.1.2.3 Mineral precipitates at hot sulfur springs..................................117
3.1.2.4 Formation of stalactites in karst caves.......................................117
3.1.2.5 Evaporation...............................................................................118
3.1.3 Groundwater.....................................................................................119
3.1.3.1 The pE-pH diagram for the system iron....................................119
3.1.3.2 The Fe pE-pH diagram considering carbon and sulfur..............122
3.1.3.3 The pH dependency of uranium species....................................122
3.1.4 Origin of groundwater.......................................................................123
3.1.4.1 Origin of spring water...............................................................124
3.1.4.2 Pumping of fossil groundwater in arid regions.........................125
3.1.4.3 Salt water / fresh water interface...............................................127
3.1.5 Anthropogenic use of groundwater...................................................127
3.1.5.1 Sampling: Ca titration with EDTA............................................127
3.1.5.2 Carbonic acid aggressiveness....................................................128
3.1.5.3 Water treatment by aeration - well water..................................128
3.1.5.4 Water treatment by aeration - sulfur spring...............................128
3.1.5.5 Mixing of waters.......................................................................129
3.1.6 Rehabilitation of groundwater...........................................................129
3.1.6.1 Reduction of nitrate with methanol...........................................129
3.1.6.2 Fe(0) barriers.............................................................................130
3.1.6.3 Increase in pH through a calcite barrier....................................130
3.2 Reaction kinetics.......................................................................................130
3.2.1 Pyrite weathering..............................................................................130
3.2.2 Quartz-feldspar-dissolution...............................................................131
3.2.3 Degradation of organic matter within the aquifer on reduction of redox
sensitive elements (Fe, As, U, Cu, Mn, S).................................................132
3.2.4 Degradation of tritium in the unsaturated zone.................................133
3.3 Reactive transport.....................................................................................137
X Table ofcontents
3.3.1 Lysimeter..........................................................................................137
3.3.2 Karst spring discharge.......................................................................137
3.3.3 Karstification (corrosion along a karst fracture)...............................138
3.3.4 The pH increase of an acid mine water.............................................139
3.3.5 In-situ leaching..................................................................................140
4 Solutions...............................................................................................143
4.1 Equilibrium reactions................................................................................143
4.1.1 Groundwater- Lithosphere................................................................143
4.1.1.1 Standard-output well analysis...................................................143
4.1.1.2 Equilibrium reaction- solubility of gypsum..............................145
4.1.1.3 Disequilibrium reaction – solubility of gypsum........................146
4.1.1.4 Temperature dependency of gypsumsolubility in well water....146
4.1.1.5 Temperature dependency of gypsum solubility in distilled water
..............................................................................................................146
4.1.1.6 Temperature and P(CO ) dependent calcite solubility..............147
2
4.1.1.7 Calcite precipitation and dolomite dissolution..........................148
4.1.1.8 Comparison of the calcite solubility in an open and a closed
system...................................................................................................149
4.1.1.9 Pyrite weathering......................................................................150
4.1.2 Atmosphere – Groundwater – Lithosphere.......................................152
4.1.2.1 Precipitation under the influence of soil CO ............................152
2
4.1.2.2 Buffering systems in the soil.....................................................152
4.1.2.3 Mineral precipitations at hot sulfur springs...............................152
4.1.2.4 Formation of stalactites in karst caves.......................................153
4.1.2.5 Evaporation...............................................................................154
4.1.3 Groundwater.....................................................................................155
4.1.3.1 The pE-pH diagram for the system iron....................................155
4.1.3.2 The Fe pE-pH diagram considering carbon and sulfur..............156
4.1.3.3 The pH dependency of uranium species....................................157
4.1.4 Origin of groundwater.......................................................................159
4.1.4.1 Origin of spring water...............................................................159
4.1.4.2 Pumping of fossil groundwater in arid regions.........................159
4.1.4.3 Salt water / fresh water interface...............................................160
4.1.5 Anthropogenic use of groundwater...................................................161
4.1.5.1 Sampling: Ca titration with EDTA............................................161
4.1.5.2 Carbonic acid aggressiveness....................................................162
4.1.5.3 Water treatment by aeration - well water..................................162
4.1.5.4 Water treatment by aeration - sulfur spring...............................162
4.1.5.5 Mixing of waters.......................................................................164
4.1.6 Rehabilitation of groundwater...........................................................165
4.1.6.1 Reduction of nitrate with methanol...........................................165
4.1.6.2 Fe(0) barriers.............................................................................166
4.1.6.3 Increase in pH through a calcite barrier....................................167
4.2 Reaction kinetics.......................................................................................168