Introduction to Hydrogeology (GEO 346C)

1
Introduction to Hydrogeology (GEO 346C) Lecture
6a Hydrogeochemistry
Instructor Bayani Cardenas TA Travis Swanson
and John Nowinski
www.geo.utexas.edu/courses/geo346c/
For this part of the course, we will use the
following text Fundamentals of Ground Water,
2003 by Schwartz and Zhang The relevant chapters
are Ch. 17-19.
2
GEO346C, UT_at_Austin, Cardenas
3
Measures of chemical constituents review
Mass solute per mass solvent
parts per million (ppm) parts per billion (ppb)
Mass solute per volume solvent
mg/L (mg solute/ L solvent) mg/L (mg solute/ L
solvent)
Mole-based concentration
molarity M (mole solute/ L solvent) molality m
(mole solute/ kg solvent)
Equivalents-based concentration
eqmol ? z, zabsolute value of charge eq/L N
(normality, equivalent per L of solvent) meq/ L
GEO346C, UT_at_Austin, Cardenas
4
Sources of chemicals in ground water

• Natural sources
• Rocks and minerals
• SiO2 2H2O -gt H4SiO40
• CaCO3 H -gt Ca2 HCO3-
• Atmosphere
• CO2 (g), O2 (g), N2 (g)
• CO2 (g) H2O lt-gt HCO3- H
• Organic carbon
• CH2O O2 -gt CO2(aq) H2O

• 2) Anthropogenic sources
• Waste leaching
• Landfills
• Hazardous waste disposal/ storage
• Industrial waste
• Mine waste
• Radioactive waste
• Spills
• Gasoline spills
• Acid and base reagent spills
• Organic chemical spills
• Atmospheric fallout
• Acid rain
• Radioactive elements (bomb testing)

GEO346C, UT_at_Austin, Cardenas
5
GEO346C, UT_at_Austin, Cardenas
6
How do natural waters get chemical constituents?
Chemical reactions in natural waters

1. Precipitation/ dissolution
2. Acid/ base reactions
3. Complexation
4. Reduction/ oxidation
5. Surface reactions (sorption/ desorption)
6. Microbial processes

GEO346C, UT_at_Austin, Cardenas
7
Precipitation/ Dissolution
Law of mass action and chemical equilibrium
cC dD yY zZ
C D are reactants Y Z are products c, d, y,
and z are number of moles for each
For dilute solutions, the equilibrium
distribution is described by
where K (aka Keq or Ksp) is the equilibrium
constant or solubility product and (Y), (Z), (C),
and (D) are the molal or molar concentrations for
reactants and products. Technically, the values
in parentheses are activities but we will
assume that these are equal to concentrations (ie
the solutions are dilute) Keq values are taken
from tables.
GEO346C, UT_at_Austin, Cardenas
8
Equilibrium versus kinetics
Kinetics-based approaches are used when the
reactions havent reached equilibrium.
We will only consider reactions that are at
equilibrium.
equilibrium
GEO346C, UT_at_Austin, Cardenas
9
Deviations from Equilibrium
Ion activity product (IAP)
where (Y), (Z), (C), and (D) are the reported
sample molal or molar concentrations
cC dD yY zZ
If IAP lt Keq, the reaction is progressing from
left to right. If IAP gt Keq, the reaction is
progressing from right to left. If IAP Keq, the
reaction is at equilibrium (reactions in both
directions occur at equal rates)
GEO346C, UT_at_Austin, Cardenas
10
Deviations from Equilibrium
Ion activity product (IAP)
cC dD yY zZ
If IAP/Keq lt 1, the water is undersaturated with
respect to the mineral. If IAP/Keq gt 1, the
water is supersaturated with respect to the
mineral. If IAP/Keq 1, the water is saturated
with respect to the mineral.
GEO346C, UT_at_Austin, Cardenas
11
Revisiting Thermodynamics
Gibbs free energy Gibbs free energy is the energy
needed by the reaction in order for it to take
place.
cC dD yY zZ
R is the gas constant (8.314x10-3 kJ/mol-K) T is
absolute temperature (Kelvin, K) DGr0 is the
Gibbs standard free energy for the reaction
(kJ/mol) DGr is the Gibbs free energy for the
reaction under actual conditions
standard -gt P1 atm, and T25C
GEO346C, UT_at_Austin, Cardenas
12
Gibbs free energy
cC dD yY zZ
O
DGr lt 0, the reaction proceeds to the right
(spontaneous) DGr gt 0, the reaction proceeds to
the left (non-spontaneous) DGr 0, the reaction
is at equilibrium
GEO346C, UT_at_Austin, Cardenas
13
Revisiting Thermodynamics
DGr0 is the Gibbs standard free energy for the
reaction DGf0 is the Gibbs free energy of
formation for the reactants and
products standard -gt P1 atm, and T25C
GEO346C, UT_at_Austin, Cardenas
14
Revisiting Thermodynamics
H enthalpy T temperature of the system S entropy
15
Enthalpy
DHr0 is the standard enthalpy for the reaction
(kJ/mol) (enthalpy is part of the internal energy
of a system heat gained or lost by a system
during a reaction at constant pressure)
DHr0 lt 0, exothermic, releases energy (heat) DHr0
gt 0, endothermic, takes in heat

How does enthalpy change with temperature?
Vant Hoff equation
T1 and T2 are two different temperatures
GEO346C, UT_at_Austin, Cardenas
16
(No Transcript)
17
Solubility- equilibrium concentration of a
dissolved species
What is the solubility of AgCl in pure water?
AgCl ? Ag Cl-
Ksp10-9.8 AgCl- AgCl
Ksp10-9.8 AgCl-
One equation, two unknowns!
Mass/ charge balance AgCl-
10-9.8AgAg AgCl-(10-9.8)1/210 -4.9
or 1.26 x10-5 mol/L
GEO346C, UT_at_Austin, Cardenas
18
Common-ion effect
What is the solubility of AgCl in 0.1 M NaCl?
For X moles of Ag, there are X0.1 moles of Cl-.
Ksp10-9.8 AgCl-
10-9.8 XX0.1
10-9.8 X20.1X
XgtgtX2
X 10-8.8
Ag10-8.8 or 1.58 x10-9 mol/L in 0.1 M NaCl In
pure water, it is 1.26 x10-5 mol/L
Common-ion effect the solubility of a salt
reduced when one of the ions ( or -) is already
present in solution
GEO346C, UT_at_Austin, Cardenas
19
(No Transcript)
20
Chemical reactions in natural waters

1. Precipitation/ dissolution
2. Acid/ base reactions
3. Complexation
4. Reduction/ oxidation
5. Surface reactions (sorption/ desorption)
6. Microbial processes

GEO346C, UT_at_Austin, Cardenas
21
Acid/ Base Reactions
Acid/ Base Reactions- involves the transfer of
hydrogen ion (H) and/ or (OH-) among the ions
present in the aqueous phase
The concentration of (H) determines the pH of
the solution.
pH-log(H)
A solution is acidic when pHlt7, basic when pHgt7,
and neutral when pH7.
Many processes (eg precipitation/ dissolution,
reduction/ oxidation) are pH dependent.
GEO346C, UT_at_Austin, Cardenas
22
Acid/ Base Reactions
Is water an acid or a base?
H2O ? H OH-
Its both a base and an acid, its an ampholyte.
What is the pH of pure water?
KeqKw 10-14 HOH- H2O
Charge balance or electrical neutrality
Charge from cations () charge from anions (-)
zi is absolute value of charge, mi is molal
concentration
GEO346C, UT_at_Austin, Cardenas
23
What is the pH of pure water?
Keq 10-14 HOH- H2O
HOH-
10-14 H2 HH
10-7 H
pH-logH
pH7
GEO346C, UT_at_Austin, Cardenas
24
Acid/ Base Reactions and Carbonate chemistry
K
10-14
10-1.46
10-6.35
10-10.33
CaCO3 ? Ca2 CO32- Ksp Ca2
CO32- 8.48 10-8.48

CaCO3
PCO2 is partial pressure of CO2, it is convenient
to express this in atm
GEO346C, UT_at_Austin, Cardenas
25
What is the pH of water in equilibrium with the
atmosphere?
Conditions Temperature 25C PCO210-3.5 atm (at
sea level), PCO2 is partial pressure of CO2
GEO346C, UT_at_Austin, Cardenas
26
The Keeling Curve
27
CO2 and natural waters
GEO346C, UT_at_Austin, Cardenas
28
CO2 and natural waters
GEO346C, UT_at_Austin, Cardenas
29
What is the pH of river water running through a
channel incised in limestone?
Conditions Temperature 25C PCO210-3.5 atm (at
sea level), PCO2 is partial pressure of CO2
GEO346C, UT_at_Austin, Cardenas
30
Carbonate chemistry and pH
HCO3-1
CO3-2
H2CO3
GEO346C, UT_at_Austin, Cardenas
31
Carbonate chemistry and pH
GEO346C, UT_at_Austin, Cardenas
32
Solubility of carbonates
GEO346C, UT_at_Austin, Cardenas
33
Solubility of metal oxides and hydroxides (e.g.,
Al(OH)3 and Fe(OH)3, PbO)
GEO346C, UT_at_Austin, Cardenas
34
Soil and river water chemistry in area with
volcanic rocks
GEO346C, UT_at_Austin, Cardenas
35
Spring water chemistry in area with carbonate
rocks
GEO346C, UT_at_Austin, Cardenas
36
(No Transcript)
37
Chemical reactions in natural waters

1. Precipitation/ dissolution
2. Acid/ base reactions
3. Complexation
4. Reduction/ oxidation
5. Surface reactions (sorption/ desorption)
6. Microbial processes

GEO346C, UT_at_Austin, Cardenas
38
Complexation Reactions
A complex is an ion that forms by combining
simpler cations, anions, and sometimes,
molecules. In complexes, the anions are referred
to as ligands including many of the common
inorganic species found in natural waters such as
Cl-, F-. Br-, SO42-, PO42- and CO32-. Organic
compounds may also act as ligands. The cations
are typically metals.
Simple complex Mn2 Cl- MnCl
The difference between a complex and salt is that
a complex is in solution while salts precipitate
as solids.
GEO346C, UT_at_Austin, Cardenas
39
Complexation Reactions
Complexes are important because they facilitate
the dissolution of metals and transport of
metals. Some metals may be immobile as simple
cations, but they may be more mobile when part of
a complex. This results in good and bad things.
Some metal deposits, e.g., Pb, Zn and U,
accumulate as mineral deposits from complexes.
However, some metals which would normally be
bound in minerals and sediments and not be in
aqueous phase, may be mobile and spread in
pristine water resources when as a complex.
GEO346C, UT_at_Austin, Cardenas
40
Complexation Reactions
Formation of inorganic complexes are fast and we
dont need to worry about kinetics. Therefore, we
can apply equilibrium thermodynamics concepts.
Mn2 Cl- MnCl
KMnCl MnCl Mn2Cl-
GEO346C, UT_at_Austin, Cardenas
41
Complexation Reactions
Complexation reactions occur in series with the
minor species typically neglected.
Cr3 OH- Cr(OH)2 Cr(OH)2 OH-
Cr(OH)2 Cr(OH)2 OH- Cr(OH)30 Cr(OH)30 OH-
Cr(OH)4- and so on
GEO346C, UT_at_Austin, Cardenas
42
Chromium Complexation Reactions
Cr3 OH- Cr(OH)2 b1 Cr(OH)21010.0
Cr3OH-
Cr3 2OH- Cr(OH)2 b2 Cr(OH)21018.3
Cr3OH-2
Cr3 3OH- Cr(OH)30 b3 Cr(OH)301024.0
Cr3OH-3
43
Complexation Reactions
GEO346C, UT_at_Austin, Cardenas
44
Complexation Reactions
In reality, multiple metals (cations) form
multiple complexes with different ligands.
(Pb)T(Pb2) (PbCl20) (PbCl3-) (PbOH)
(PbCO30)
Solubility enhancement
45
(No Transcript)