Chapter 9 – The continental environment
Simpler Box Model of Hydrologic Cycle
Weathering: the various physical and chemical processes that lead to
the decomposition of minerals and the breakdown of rocks to form soil.
4 Types of Chemical Weathering
Oxidation of minerals containing reduced elements
4FeS2(pyrite) + 15O2 + 8H2O  2Fe2O3(hematite) + 8H2SO4
Congruent dissolution by water – ions go directly into solution
NaCl(halite)  Na+ + ClCongruent dissolution by acid (written as carbonic)
MgSiO4 (olivine) + 4H2CO3aq  2Mg2+ + 4HCO3- + H4SiO4aq
Incongruent dissolution by acid (breaks down to ion and a different mineral)
2NaAlSi3O8(Na-plagioclase) + 2H2CO3aq + 9H2O  Al2Si2O5(OH)4(kaolinite) + 2Na+ + 2HCO3- + 4H4SiO4aq
Primary
mineral
secondary
mineral
Equations for silica dissolution
Dissolution of Silica (quartz and amorphous silica)
SiO2 + H2O  H4SiO4 aq
Log Ksp = 1.8814 – 2.028 x 10-3T – 1560.46 / T ……quartz
Log Ksp = 0.338037 – 7.8896 x 10-4T – 840.075 / T……opal
SiT = [H4SiO4]
1+
Ka1
[H+]
+
Ka1 Ka2
[H+]2
Solubility of silica increases at higher pH
Opposite of CaCO3 containing minerals
Fossilization = SiO2 replacement of CaCO3
Figure 9-2. function of pH. pH = 9.83 and pH = 13.17 correspond to the first
and second dissociation constants, respectively, of silicic acid.
Dissolution of Al and Fe hydroxides
Al(OH)3  Al3+ + 3OH-
General eq.
Total solubility of Al is determined by the sum of free Al + all the Al-OH aqueous
complexes in solution. Complexes: AlOH2+, Al(OH)2+, Al(OH)3aq, Al(OH)-4
Overall Al solubility eq:
SAl = Ksp / K3w ([H+]3 + Kb1[H+]2 + Kb2[H+] + Kb3 + Kb4/[H+])
Ta ble 9-5. Constants for Al an d Fe33++ solu bility calcul ations at 25ooC*
Phase
p K sp
pK β
1
pK β
2
pK β
3
pK β
A l(O H)3 (am)
3 1.2
5.00
10.1
16.9
22.7
A l(O H)3 gib bsite
3 3.9
5.00
10.1
16.9
22.7
Fe(O H)3 (am)
3 7.1
2.19
5.67
12.56
21.6
Fe(O H)3 goethite
4 4.2
2.19
5.67
12.56
21.6
pK o
pK 1
p K3
p K4
-3.72
1.28
13.18
18.98
Kaolinite
p K2
6.38
*Data fro m Nordstro m et al. (1990) and M acalady et al. (1990)
4
Al hydroxide solubility depends on the exact mineral but generally has a near
mid pH minima increasing asymmetrically at lower and higher pHs.
Figure 9-3. Total concentration of aluminum in solution, as a function of pH, for a solution in equilibrium
with gibbsite.
Stablility diagrams
Represent equilibria between minerals and aqueous solutions
Derived from thermodynamic data
For the weathering of silicate
minerals
X and Y axis describe solution
Chemistry
Fields denote the ‘stable mineral’ in
Equilibria with particular solution chemistry
ABCetc… represents the
path of weathering starting with gibbsite
In a closed system
Flushing rates in non-closed systems
Will effect solution chemistry during weathering
And control final weathering product. Hence Figure 9-4b. Mineral stability fields as delineated by
equilibrium equations plotted in Figure 9-4a. The labeled
Dominance of different minerals in different
curve indicates the changes in chemistry of a solution in
Soil types
equilibrium with albite during weathering in a closed
system. See text for discussion.
Groundwaters
Water chemistry modified by
the vadose zone, and during
transport in the saturated zone
In general, the older the water
the higher the dissolved ion
Conc. = higher conductivity
Surface and Groundwaters
Graphical representations
Stiff diagram
Piper Diagram
Plots each ion as a value
normalized to 100%
Data on the 2 triangles is
Also projected on the quadrilateral
Data used to generate the Piper
Diagram is Found on Eby p. 324,
Table 9-7
Mg
SO4
Ca
Cl
Figure 9-7. Hydrochemical facies. After Back (1966).
Piper Diagram
Plots each ion as a value
normalized to 100%
Data on the 2 triangles is also
projected on the quadrilateral
1
mix
Piper diagrams also provide
indications of mixing of water
masses
Data used to generate the
Piper Diagram is Found on
Eby p. 324, Table 9-7
2
Straight line
= mixture
Rivers
Controls on river chemistry
Gibbs Approach
Precip – high (Na and Cl),
tropical rivers
Weathering – rock
dominance, depends on rock
type, climate, relief
Evaporation and fractional
crystallization
Stallard and Edmond approach
Controls on River
chemistry
Rock weathering
controls River
chemistry
Sources of ions in river water by general rock type
Ocean source
Ta ble 9-9. Sources of major ele ments in ri ver water (% )*
At mosphere
Species
Carbonates
Silicates
Evaporites
Pollution
65
18
8
9
<<1
61
37
0
2
Na+
8
0
22
42
28
Cl -
13
0
0
57
30
SO 4
2
0
0
22
54
M g 2+
2
36
54
<<1
8
Na+
1
0
87
5
7
<<1
0
> 99
0
0
Ca 2+
Cyclic Salt
W eathering
0.1

HC O3-
+
2
H 4SiO 4
*Fro m Berner and Berner (1996)
Ta ble 9-10. Origin of major aqueous species in ground water*
Aqueous species
Na+
Origin
NaCl dissolution (so me pollution)
Plagioclase weathering
Rainwater additio n
K+
Biotite weathering
K-feldspar weathering
Mg 2 +
A mphibole and pyro xen e weathering
Biotite (and chlorite ) weathering
Dolo mite weathering
Olivine weathering
Rainwater additio n
Ca2 +
Calcite weathering
Plagioclase weathering
Dolo mite weathering

HC O3
Calcite and dolo mite weathering
Silicate weathering
2
SO 4
Pyrite weathering (so me pollution)
CaSO 4 dissolution
Rainwater additio n
Cl -
NaCl dissolution (so me pollution)
Rainwater additio n
H 4SiO 4 (aq)
Silicate weathering
*Fro m Berner and Berner (19 96)
pH in groundwater and surface water controlled by:
Ion exchange
Carbonic acid system
Water-mineral interactions
Remember the carbonic acid
system? As pH decreases
buffering decreases
Water-mineral interaction
Calcite
H+ + CaCO3  Ca2+ + HCO3- …
replaces bicarbonate ion needed for buffering, increases calcium conc.
(and Mg is dolomite is present), raises pH
Silicates
Al2SiO5(OH)4kaolinite + 6H+  2Al3+ + 2H4SiO4aq + H2O
NaAlSi3O8albite + 4 H2O + 4H+  Na+ + Al3+ + 3H4SiO4aq
KMg1.5Fe1.5(AlSi3O10)(OH2)biotite + 10H+  K+ + 1.5Mg2+ + 1.5Fe2+ + Al3+
+ 3H4SiO4aq
Water-mineral interaction
Calcite
H+ + CaCO3  Ca2+ + HCO3- …
replaces bicarbonate ion needed for buffering, increases calcium conc.
(and Mg is dolomite is present), raises pH
Silicates
Al2SiO5(OH)4kaolinite + 6H+  2Al3+ + 2H4SiO4aq + H2O
NaAlSi3O8albite + 4 H2O + 4H+  Na+ + Al3+ + 3H4SiO4aq
KMg1.5Fe1.5(AlSi3O10)(OH2)biotite + 10H+  K+ + 1.5Mg2+ + 1.5Fe2+ + Al3+
+ 3H4SiO4aq
All the silicate weathering rxns release silicic acid and free aluminium.
High free Aluminum conc. in acid lakes. Free Al is toxic to aquatic
critters.
Read about the ‘Killer Lakes’ of Cameroon,
Eby p. 336, Case Study 9-4
Eutrophication: a process whereby water bodies, such as lakes, estuaries, or
slow-moving streams receive excess nutrients that stimulate excessive plant
growth (algae, periphyton attached algae, and nuisance plants weeds). This
enhanced plant growth, often called an algal bloom, reduces dissolved oxygen in
the water when dead plant material decomposes and can cause other organisms
to die. Nutrients can come from many sources, such as fertilizers applied to
agricultural fields, golf courses, and suburban lawns; deposition of nitrogen from
the atmosphere; erosion of soil containing nutrients; and sewage treatment plant
discharges. Water with a low concentration of dissolved oxygen is called hypoxic.
Structure and mixing of lake waters
CiFi + ChFh = CoFo + CeFe + RPe
where Ci is the concentration of the
substance in the inlet water, Fi is the flux
of water into the lake, Ch is the
concentration of the substance in the
hypolimnion, Fh is the flux of water from
the hypolimnion to the epilimnion, Co is
the concentration of the substance in the
outlet water, Fo is the flux of water out of
the lake, Ce is the concentration of the
substance in the epilimnion, Fe is the flux
of water from the epilimnion to the
hypolimnion, and RPe is the rate of
removal of the substance by particles in
the epilimnion.
The amount of the substance that is ultimately stored in the sediment is:
Rs = RPe + RPh – Rd
where Rs is the rate at which the substance is sequestered in the sediment, RPh is
the rate of removal of the substance by particles in the hypolimnion and Rd is the
rate of re-solution of the substance.
Soufrière Hills volcano
(Montserrat)
Mt Ruapehu lahar event - New Zealand (March 18, 2007)
Total dissolved solids > 80%
Tephra: air-fall material produced by a volcanic eruption regardless of
composition or fragment size.
Metals sorption
Figure 9-16. Adsorption of metal cations as a function of pH. From AQUATIC CHEMISTRY, 3rd Edition by W.
Stumm and J. J. Morgan. Copyright © 1996. This material is used by permission of John Wiley & Sons, Inc.
Depends on pH and charge to size ratio for individual metal
Generally for the transition metals, the lower the pH the less sorption
Metals mobility depends on:
ion exchange / sorption-desorption
complex formation and chelation
Ion exchange
aAy + bBX  bBz + aAX…..’X’ denotes that A or B are stuck on a solid
The exchange ratio between a and b (Ka/b) is:
Ka/b = [Bz]b [AX]a / [Ay]a [BX]b
Example: Exchange between Na and Ca
2Na+ + Casolid  Ca2+ + 2Nasolid
KNa/Ca = [Ca2+]1 [Nasolid]2 / [Na1+]2 [Casolid]1
Metals chelation
Ligands – net negatively charge molecules that associate (attract) metals
M2n+aq + nC2O42-   M(C2O4)n aq
metal
Ligand
(oxalate)
Multidentate ligand has more than one binding atom
Chelation occurs when complexes are formed with a multidentate ligand
Oxalate is a common multidentate ligand produced biologically
Metal-oxalate chelates have low solubility and will precipitate out.
Therefore biological processes assist in metals precipitation through the
oxalate chelation process.
Metal Cycles
Geosphere  atmosphere via
Volcanoes, dust
Geosphere  hydrosphere via
ocean vents
Short residence time in
atmosphere
Hydrosphere transport
controlled by redox, pH,
mineralogy
Biosphere important in
establishing redox, pH, etc
Heavy metals
At # > 20
Fate and transport determined strongly by element-type
-Transition metals - Zn, Cd, Pb
-As, Se
-Hg
Transition metals
Usually occur at divalent, or trivalent cations
With the exception of V, these metals will ppt at high pH
(forming oxyhydroxides, or metal carbonates)
Complex with humic material
(Mn 2+ <Cd 2+ <Co 2+ <Ni 2+ <Zn 2+ <Pb 2+ <Cu 2+ <VO 2+)
Adsorption decreases with decreasing pH
Arsenic and Selenium
Occur primarily as neutral or negatively charged species
(H2AsO4-, HAsO42-, H3AsO3, H2AsO3-, SeO4-, HSeO3-, SeO32-)
Adsorption increases with decreasing pH
When reduced S is present, both As and Se incorporated into S minerals
Changing redox conditions can either immobilize or liberate As or Se depending
on the presence of other elements……difficult to predict mobility
Major anthropogenic source of Se is coal combustion
Mercury
Most redox conditions allow for elemental Hg
Elemental Hg is inert = not immediately hazardous, long half life
High redox = Hg 2+ or Hg(OH)2aq
Low redox + sulfur = HgS (cinnabar)
Global mercury cycling dominated by atm transport and deposition on land
Some anaerobic bacteria methylate
Hg
CH3Hg= methyl mercury,
(CH3)2Hg = dimethyl mercury
easy biotic uptake
bioconcentration
toxic
Radioactive materials
Making fissionable nuclear fuel 235U
Natural uranium ore = 99.275% 238U, 0.719 % 235U
Extraction enrichment (1.8 – 3.7% 235U)
Nuclear Fission
235U
+ 10n  fission fragments + 2 or 3 neutrons + energy
neutron
In order to get the reaction to go, the impacting neutron must be slowed down.
Neutrons are slowed either with water or with graphite (Chernobyl) depending
on the type of reactor.
Water-type reactors are self quenching because the ability of water to slow down
neutrons decreases as water heats up (negative feedback)
Graphite-type reactors behave the opposite
Radioactive wastes from fission
High level = very radioactive and long half-life
Low level = not so radioactive and short half-life
Ta ble 9-15. Re presentati ve ra dioacti ve isoto pes for nuclear wastes
Isotope
Half-life
Decay
mode
Iso tope
Fission products
Half-life
Decay
mode
Fission products
85
Kr
10.8 y
β
1 37
Cs
30 y
β
89
Sr
51 d
β
1 41
Ce
33 d
β
90
Sr
28 y
β
1 47
Pm
2.6 y
β
95
Zr
64 d
β
95
Nb
35 d
β
99
Tc
2.1 x 10 5 y
β
2 37
Np
2.1 x 1 0 6 y
α
Transuranics
10 6
Ru
1y
β
2 39
Pu
2.4 x 1 0 4 y
α
13 1
I
8d
β
2 40
Pu
6.6 x 1 0 3 y
α
13 3
Xe
5.2 d
β
2 41
Am
433 y
α
Yucca Mountain
Mobility of nuclear waste elements
Ta ble 9 -17. Retardation factors for ra dioactive isoto pes in various geo media*
Element
U is more mobile in oxidizing
conditions
Ta ble 9-16 . Esti mates of solu bilities ( mg L-1-1) of i mportant
ra dioisoto pes at 25oo C and 1 at m*
Element
Granite
Basalt
Volcan ic ash
Shale (or clay)
Sr
20 - 4000
50 - 3000
100 - 100000
10 0 - 10000 0
Cs
200 - 100000
200 - 100000
500 - 100000
20 0 - 10000 0
Tc
1 - 40
1 - 100
1 - 100
1 - 40
I
1
1
1
1
U
40 - 500
100 - 500
40 - 400
1 00 - 2000
Np
20 - 500
2 0 - 200
20 - 200
50 - 1000
Pu
20 - 2000
20 - 10000
2 0 - 5000
50 - 100000
Am
500 - 10000
10 0 - 1000
100 - 1000
50 0 - 10000 0
Reducing conditio ns
O xidizing conditio ns
Ra
50 - 500
5 0 - 500
100 - 1000
100 - 200
Eh = -0.2 V
Eh = +0.2 V
Pb
2 0 - 50
2 0 - 100
20 - 100
20 - 100
pH 9
pH 6
pH 9
pH 6
Sr
0.6
high
0.6
high
Cs
high
high
high
high
Tc
10 -10
high
high
high
I
high
high
high
high
U
10 -3
10 -6
high
high
Np
10 -4
10 -4
10 -2
10 -1
Pu
10 -5
10 -4
10 -5
10 -3
Am
10 -8
10 -5
10 -8
10 -5
Ra
10 -3
10 -1
10 -3
10 -1
Pb
10 -1
1
10 -1
1
*Fro m Krauskop f (1986)
*Fro m Krauskopf (1986)
Yucca mountain is made
of volcanic tuff
Oklo – the fossil natural nuclear reactor
Uranium ores in Oklo, Gabon contain little 235U because of prior fission rxns
Original U minerals (2 billion years ago) dispersed in a sandstone conglomerate.
Relative abundance of 235U vs 238U was much higher then the U minerals were
dissolved and redeposited as UO2 at a redox boundary
(U concentrated to 50-70%)
Interstitial water moderated the fission like a present day reactor. When the water
was steamed away due to the heat release from fission, reaction stopped.
More water infiltrated and reaction fired up again, on off on off etc. for 500,000 yrs
Large quantities of fission products exist today
Site used to study migration and retention of fission wastes
Nonmetals
Carbon – see chapter 5
Halogens
Negative charge so sorption is less important
Solubility controls mobility
Fluorine concentrations determined by availbility and solubility of fluorite (CaF2)
Chlorine and bromine used as hydrologic tracer because once dissolved they
largely behave conservatively
-no volatile derivatives
-little to no adsorption
-little to no biological uptake
-care taken to discount anthropogenic inputs
Nitrogen
Oxidized and mobile = NO3Reduced and sorptive = NH4+
Phosphorous
Source is weathering of P containing rocks (e.g. apatites)
Mobility and abundance controlled by solubility, adsorption, biological
uptake, redox
Phosphate (PO43-) strongly adsorbed to ferric (Fe3+) oxides and
oxyhydroxides (Fe(OH)3…..at high redox
Low redox (e.g. anoxia) desorbs phosphate and releases it in the water
Freshwater eutrophication due to phosphate
Anoxia associated with eutrophication keeps resupplying phosphate (+
feedback)
Sulfur
HSO4-, SO42-, H2S, HSMost natural Eh and pH waters sulfate dominates
Low Eh’s sulfide dominates. Which sulfide species at low Eh and high
pH?
Which sulfide at high Eh and low pH?
Very low Eh (deep sediments) metal sulfide minerals ppt out (e.g.
pyrite)
Sulfate mobility controlled by adsorption (divalent anions preferentially
adsorbed
over monovalent anions like Cl- or NO3-), and biology.
Similar to phosphate, sulfate is adsorbed by Al and Fe oxides