Acid Rain Buffering Potential in Oregon Cascade Lakes :
Secondary Mineral Solubility Contro l
of Solution Ionic Compositio n
' Peter O . Nelso n
Water Resources Research Institute'
. Oregon State Universit y
Corvallis, Orego n
WRRI- 1 00
March 1985
ACID RAIN BUFFERING POTENTIAL IN OREGON CASCADE LAKES :
SECONDARY MINERAL SOLUBILITY CONTRO L
OF SOLUTION IONIC COMPOSITIO N
by
Peter O . Nelso n
Department of Civil Engineerin g
Oregon State Universit y
Final Technical Completion Repor t
Project Number G864-02
Submitted to
United States Department of the Interior
Geological Surve y
Reston, Virginia 22092
Project Sponsored by :
Water Resources Research Institut e
Oregon State University
Corvallis, Oregon 9733 1
The research on which this report is based was financed in part by the Unite d
States Department of the Interior as authorized by the Water Research an d
Development Act of 1978 (P .L . 95-467) .
Contents of this publication do not necessarily reflect the views an d
policies of the United States Department of the Interior, nor does mention o f
trade names or commercial products constitute their endorsement by the U .S .
Government .
WRRI-100
March 1985
ABSTRAC T
The results of a recent study by Nelson and Delwiche (1983) wer e
extended to determine possible geochemical controls on the chemica l
composition of Oregon's Cascade lakes . Two types of chemica l
equilibruim relationships were developed . Predominance-area diagram s
were constructed relating primary and secondary minerals to determin e
which phase was most likely to be in thermodynamic equilibrium with th e
lake water solutions . Chemical speciation calculations, emphasizin g
aluminum, were then made and interpreted in the context of th e
controlling geochemical phase and the susceptibility of the lake t o
acidification .
Comparison of the measured chemical composition of eight lakes for whic h
adequate data existed to the phase regions of the predominance diagram s
indicated that kaolinite was the secondary mineral phase controllin g
solution chemistry in most lakes investigated . Gibbsite controlle d
solution chemistry in one lake but was of subequal importance t o
kaolinite in others .
Speciation calculations showed aluminum to be predominantly in hydroxid e
complexes with the free aluminum ion concentration never exceeding 2% o f
monomeric aluminum . Flouride and sulfate complexes were generall y
negligible . Stability indices calculated from solution compositions wer e
in qualitative agreement with the predicted controlling secondar y
mineral phases from the predominance-area diagrams .
The Oregon Cascade lakes studied appear to be in equilibrium or derive d
from secondary mineral phases formed during the incongruent dissolutio n
of parent feldspar minerals . These geochemical forms, known for thei r
slow dissolution kinetics and resistance to chemical weathering, explai n
the dilute solution chemistry and weak buffering capacity of the lake s
and thus their susceptibility to acidification .
FOREWOR D
The Water Resources Research Institute, located on the Oregon Stat e
University campus, serves the State of Oregon . The Institute fosters ,
encourages, and facilitates water resources research and educatio n
involving all aspects of the quality and quantity of water available fo r
beneficial use . The institute administers and coordinates statewid e
and regional programs of multidisciplinary research in water and relate d
land resources . The Institute provides a necessary communications an d
coordination link between the agencies of local, state, and federa l
government, as well as the private sector, and the broad researc h
community at universities in the state on matters of water-relate d
research . The Institute also coordinates the interdisciplinary progra m
of graduate education in water resources at Oregon State University .
It is Institute policy to make available the results of significan t
water-related research conducted in Oregon's universities and colleges .
The Institute neither endorses nor rejects the findings of the author s
of such research . It does recommend careful consideration of th e
accumulated facts by those concerned with the solution of water-relate d
problems .
ACKNOWLEDGEMENT S
I thank Greg Delwiche, former graduate student, now with the Nort h
Pacific Division, U .S . Army Corps of Engineers, Portland Oregon, wh o
performed most of the work in the original investigation upon which thi s
study is based, and Everett Jenne, Battelle Pacific Northwes t
Laboratories, Richland Washington, for the original idea behind th e
approach used in this study .
11
TABLE OF CONTENTS
Pag e
List of Figures
iv
List of Tables
iv
Introduction
1
Study Rationale
1
Stability-Field Diagrams
2
Chemical Speciation Calculations 5
Summary and Conclusions
13
Bibliography
15
LIST OF FIGURES
Pag e
1.
Stability-Field Diagram for Sodium Feldspar System 7
2.
Stability-Field Diagram for Potassium Feldspar System
8
LIST OF TABLE S
Pag e
1.
2.
3.
4.
5.
Selected Free Energies of Formation for Mineral Phase s
and Species at 25-C and 1 Bar Pressure
4
Thermodynamic Constants Used in Calculation of Incongruen t
Dissolution Reactions for Stability-Field Diagrams
6
Predicted Secondary Minerals Controlling Solutio n
Chemistry of Selected Cascade Lakes 9
Measured Lake Chemical Composition Parameter s
Mean Values
10
Thermodynamic Constants Used in Solution Phas e
Speciation Calculations for Alumninum
11
6.
Solution Phase Speciation Percent of Monomeric Aluminum
7.
Stability Indices for Gibbsite and Kaolinite iv
.
.
12
14
INTRODUCTION
The sensitivity of aquatic ecosystems to acidified precipitation result s
from the interaction of hydrological and geochemical factors of th e
system's watershed with the acidified water . Hydrological factor s
include both precipitation characteristics (frequency, duration, an d
total quantity) and runoff characteristics (retention in soil an d
vegetation) . Geochemical factors are the mineralogy and chemica l
composition of the region's bedrock, soil, and sediments . Chemica l
changes resulting from these interactions determine the net chemica l
composition of runoff waters and the system's ability to neutralize aci d
precipitation .
Studies of the sensitivity of lakes to acidification have emphasize d
both the contributing hydrological and geochemical factors of th e
watershed (e .g ., Kramer,1975 ; Cronan and Schofield, 1979 ; Norton, 1980 ;
Johnson et al, 1981 ; Stumm, Morgan, and Schnoor, 1983 ;) and th e
resultant lake chemical composition (e .g ., Wright and Henriksen, 1978 ;
Henricksen, 1980 ; Seip, 1980 ; Driscoll, 1980 ; Logan, Derby, and Duncan ,
1982 ; Nelson and Delwiche, 1983) .
In a recent study of 63 lakes in the Oregon Cascade Mountains, Nelso n
and Delwiche (1983) documented the major ion chemistry, acid-bas e
buffering capacity, and selected trace component chemistry (e .g . ,
A1(III), F-, TOC) to determine the susceptibility of the lakes t o
acidification . Results indicated that most Cascade Lakes are ver y
dilute in chemical composition and thus extremely susceptible t o
acidification .
The objectives of this study were to extend the findings of the Nelso n
and Delwiche study to determine the geochemical controls on the chemica l
composition of Oregon's Cascade lakes . Specifically, two types o f
chemical equilibrium relationships were developed . Predominance-are a
diagrams were constructed relating primary and secondary minerals t o
determine which phase was most likely to be in thermodynamic equilibrium
with the lake water solutions . Chemical speciation calculations wer e
then made and interpreted in the context of the controlling geochemica l
phase and the susceptibility of the lake to acidification .
STUDY RATIONAL E
The composition of natural waters is controlled to a significan t
extent by weathering reactions of bedrock minerals . Chemical reaction s
in soils and vegetation also have some influence on surface wate r
chemistry . Natural weathering reactions are dominated by carbonic aci d
dissolution of basic minerals in aqueous solutions, yielding bas e
cations and alkalinity . Granitic and similar aluminosilicate mineral s
are principal among these .
An example of such a reaction is th e
incongruent dissolution of anorthite to yield kaolinite :
1
(1)
CaA1 2 S i 2 O 8 (s) + 2C O 2 + 3 H 2 0 = Ca(HC O 3 ) 2 + A 1 2 S 1 2 0 5 (OH ) 4
anorthite
kaolinit e
If limestone or other carbonate minerals are found in the terrain ,
weathering proceeds by direct dissolution :
(2)
CaCO 3 (s) + CO 2 + H 2 O = Ca(HCO
3)2
calcit e
When anthropogenic sources impart strong acids (H2SO4, HN03) to th e
precipitation, neutralization occurs through the buffering provided b y
the alkalinity in the water :
(3)
Ca(HCO 3 ) 2 + H 2 SO 4 = CaSO 4 + 2CO 2 + 2H 2 0
From the above reactions, and considering the abundance of these minera l
types in the lithosphere, it would appear that an essentially infinit e
reservoir exists to buffer natural waters against changes in pH cause d
by acidic inputs . Although thermodynamically favorable, the kinetics o f
the above reactions vary widely . Only carbonate minerals such a s
calcite react rapidly enough to neutralize acidity over short tim e
frames (days to years) . Thus in terrains devoid of limestone and othe r
carbonate minerals, surface waters are often susceptible t o
acidification because of low buffering capacity . Knowledge of minera l
phases controlling a natural water's composition thus provide s
supporting evidence of its susceptibility and enables predictions to b e
made of probable impacts .
STABILITY-FIELD DIAGRAM S
Because of the extremely slow dissolution kinetics of aluminosilicat e
parent bedrock minerals, surface water chemical composition is mos t
likely reflected in equilibrium dissolution reactions of secondar y
mineral phases that are formed over relatively long time scales b y
incongruent dissolution, as in reaction 1 above . To investigate this ,
stability field diagrams were developed relating major primary an d
secondary minerals . As suggested by Garrels and Christ (1965) 4 for th e
dissolution of primary feldspars, the ratio of base cation (Na or K )
to H + is always unity . Therefore, a two-dimensional diagram involvin g
+
the ratio of [Na '- ] or [ K + ] to [ H ] as one axis, and [ H 4 Si O 4 ] the other ,
can describe the mineral relations .
Clayton (1985) has developed stability field diagrams for Na- and K feldspars upon which the following is based . For the sodium system, th e
following dissolution reactions were considered :
2
(4) 2NaAlSi 3 O 8 + 2H + + 9H 2 0 = Al 2 Si 2 O 5 (OH) 4 + 4H 4 SiO 4 + 2Na +
albite
kaolinit e
(5) 8NaAlSi 3 O 8 + 6H + + 28H 2 O = 3Na0
+
.66A12 .66S13 .33010(OH)2
albite
montmorillonite
14H 4 S10 4 + 6Na +
(6) 3Na0
.66A12 .66S13 .33010(OH)2 + 2H + + 8H 2 0 = 4A1 2 Si 2 O 5 (OH) 4 +
montmorillonite
kaolinite 2H 4 SiO 4 + 2Na +
(7) 3Na0
28H 2 0 = 8A1(OH) 3 +
.66A12 .66$i3 .33010(OH)2 + 2H + +
montmorillonite
gibbsite 10H 4 SiO 4 + 2Na +
(8) A1 2 Si 2 O 5 (OH) 4 + 5H 2 0 = 2A1(OH) 3 + 2H 4 S1O 4
kaolinite
gibbsit e
For the potassium system, the following dissolution reactions wer e
considered :
(9)
3KA1Si 3 O 8 + 2H + + 12H 2 0 = KA1 3 Si 3 0 10 (OH) 2 + 6H 4 S10 4 + 2K +
orthoclase
muscovit e
(10) 3KAlSi 3 O 8 + 2H + + 9H 2 0 = A1 2 Si 2 0 5 (OH) 4 + 4H 4 SiO 4 + 2K *
orthoclase
kaolinit e
(11) 2KA1 3 Si 3 0 10 (OH) 2 + 2H + + 3H 2 O = 3A1 2 Si 2 0 5 (OH) 4 + 2K +
muscovite
kaolinit e
+
(12) KA1 3 Si 3 0 10 (OH) 2 + H + + 9H 2 0 = 3A1(OH) 3 + 3H 4 SiO 4 + K
muscovite
gibbsit e
Thermodynamic data for the mineral phases and species in these reaction s
are summarized in Table 1 . They were obtained from Robie, Hemingway ,
and Fisher (1978), except for montmorillonite which was obtained fro m
Helgeson (1969) .
From the free energy of formation data in Table 1, equilibrium constant s
for the dissolution reactions were calculated using the followin g
relationship (valid at 25-C and 1 atm pressure) :
(13)
log 10 K = - 0 G° r /2 .303RT ; wit h
3
Table 1 . Selected Free Energies of Formation for Minera l
Phases and Species at 25-C and 1 Bar Pressure . *
Specie
State
20 f ,
kcal/mol e
Albite
Crystalline
-887 .1 2
Kaolinite
Crystalline
-908 .07
Montmorillonite
Crystalline
Gibbsite
Crystalline
-276 .0 3
Orthoclase
Crystalline
-894 .4 4
Muscovite
Crystalline
-1338 .5 9
Silica
Amorphous solid
-203 .2 9
H 4 Si0 4
Aqueous
-312 .6 2
H2O
Aqueous
-56 .6 8
Aqueous
-67 .5 2
Aqueous
-62 .6
K+
Na+
*Roble et al ., 1978, and Helgeson, 196 9
4
-171 9
(14 )
Go r
=
Go
f,products - O Go f,reactant s
Results of equilibrium constant calculations are summarized in Table 2 .
The equilibrium constants in Table 2 were used to develop th e
predominance-area diagrams . Figure 1 represents the Na-feldspar syste m
(reactions 4-8) and Figure 2 the K-feldspar system (reactions 8-12) .
Labeled regions on the diagram represent conditions in which that soli d
phase predominates or controls aluminum solubility .
To determine the mineral phase predicted to control solution chemistry ,
mean values of measured water chemistry data for individual Cascad e
Lakes under study were compared to the diagrams . Table 3 summarizes th e
predicted controlling mineral phases, and Table 4 contains the measure d
lake chemistry data from Nelson and Delwiche (1983) for those lakes wit h
sufficient aluminum measurements .
CHEMICAL SPECIATION CALCULATION S
The predominance-area diagrams enabled prediction of the secondar y
aluminosilicate mineral phase controlling the solution chemistry of th e
lakes investigated . These diagrams, however, are not quantitative i n
depicting actual concentrations of species in solution . To gain a
quantitative understanding of the solution chemistry, equilibriu m
chemical speciation calculations were performed using the compute r
program MICROQL (Westall, 1979) . Emphasis was placed on aluminu m
chemistry because of the equilibria between aluminum species an d
controlling mineral phases .
Table 5 is a summary of formation reactions and thermodynamic constant s
for the aluminum species considered . Johnson et al (1981) was used a s
the primary reference in selecting pertinent aluminum species for thes e
calculations . Together with the measured lake chemical composition dat a
of Table 4, these data formed the major information input to the compute r
program .
A summary of the solution phase speciation calculations is contained i n
Table 6 . Results are summarized in terms of percent of monomeri c
aluminum and as such are independent of the mineral phase controllin g
solubility . To determine species molar concentrations, only the fre e
ionic aluminum concentration (A13+) need be computed for the controllin g
solid phase from which other species are directly calculated .
Polyfluoride complexes were negligible, while the maximum monofluorid e
complex concentration was only 1 .5 % of monomeric aluminum . All sulfat e
complexes were negligible . Organic complexes of aluminum were no t
included in the calculations and may be a significant source of error i n
some lakes by comparison with the results of Driscoll (1980) .
Equilibrium speciation results also enable
to be made of the mineral phase controlling
calculation of the ion activity product (Q)
compared to the solubility product constant
5
an alternative determinatio n
solubility throug h
. This product is then
(Kso) for the mineral phas e
Table 2 . Thermodynamic Constants Used in Calculatio n
of Incongruent Dissolution Reactions for
Stability-Field Diagrams .
Equatio n
Number
Solid Phase s
Reactant
Product
log lo K
(I=0,25-C) ,
(4)
albite
kaolinite
(5)
albite
montmorillonite
(6)
montmorillonite
kaolinite
(7)
montmorillonite
gibbsite
-38 .5 5
(8)
kaolinite
gibbsite
-10 .39
(9)
orthoclase
muscovite
-10 .3 6
(10)
orthoclase
kaolinite
-3 .97
(11)
muscovite
kaolinite
8 .8 2
(12)
muscovite
gibbsite
6
-0 .45
-188 .37
3 .0 1
-11 .17
HOTS SnOHddOW V
31 T 81d
E
a
E
L
W
Q --P
o
d
d-
c:) cn
0
L
U)
-
u
U
-D
-P
U-
~-- ~
E
W
0 -)
CD
o
.H
W
F--
co
L'--.
0
Cn CI)
CO
CD
CO
CO
Lf7
D
-0
U)
CO
r--l
d-
CO
Cl!
W
L
Zn
LL
[+HJ / C+ D N]
8
7
°1
CV
ZOTS
SOOHddOW V
L
C r7
3Sd1SOH1d 0
0
CI-
E
a (n
L
MU)
f
w
0_
L
0
0
o
- J
uI)
U
U)
-0 C
O
aJ
u
LL
LL
V /
- CO
f
0
-1->
0
U) o_
oa
ci
I
CO
L'--
LO
t!-7
m
cv
6° 1
8
Table 3 . Predicted Secondary Minerals Controlling Solutio n
Chemistry of Selected Cascade Lakes .
Lake
Controlling Phas e
Waldo
Kaolinite
Bull Run
Kaolinit e
Lost
Kaolinit e
N . Summit
Kaolinit e
Olathe
Kaolinite
Betty
Kaolinit e
Lucky
Gibbsit e
Summit
Kaolinit e
9
Table 4 . Measured Lake Chemical Compositio n
Parameter Mean Values .
Number o f
Samples
SO 2
mgll
ug/l
14
0 .11
Bull Run
6
Lost
Lake
F
SiO 2
Al
Fiel d
_pH___
Cond .
uS
mg/I
ug/T
2 .05
0 .62
9
6 .32
3 .3 9
0 .14
7 .43
6 .89
11
6 .75
16 .7 8
7
0 .23
6 .08
4 .67
13
6 .85
13 .1 4
N .Summit
3
0 .60
1 .46
0 .50
1 .3
5 .81
5 .4 1
01allie
5
0 .11
2 .58
1 .07
13
6 .51
4 .0 5
Betty
3
0 .23
4 .06
4 .32
14
6 .75
4 .9 0
Lucky
3
0 .11
1 .32
0 .05
3
5 .99
2 .7 1
Summit
5
0 .14
2 .90
1 .32
9
6 .35
2 .86
Waldo
10
Table 5 . Thermodynamic Constants Used in Solution Phas e
Speciation Calculations for Aluminum .
Reactio n
191.01S
A1 3+ + H 2 O = A10H 2+ + H +
-4 .99
A 1 3+ + 2H 2 0 = Al(0H) 2 + + 2H +
-10 .1 3
A 1 3+ + 4H 2 0 = Al(OH)4 - + 4H +
-22 .1 6
A1 3+ + F = AlF 2
+
7 .02
A1 3+ + 2F - = AlF 2 +
12 .7 6
A1 3+ + 3F - = A1F 3 (aq )
17 .0 3
A1 3+ + 4F = A1F 4 -
19 .7 3
A1 3+ + 5F - = A1F 5
2
20 .9 2
A1 3+ + 6F - = A1F 6
3
20 .8 7
A1 3+ + SO 4 2
= AlSO 4
+
A1 3+ + 250 4 2- = A1(S0 4 )
3 .2 1
2-
5 .22
Sourc e
May (1979 )
Hem (1968 )
Table 6 . Solution Phase Speciation Percen t
of Monomeric Aluminum .
Species
Lak e
Waldo
Bull Run Lost
N .Summit Olathe
Betty
Lucky
Summi t
1 .95
0 .04
0
0 .69
0 .05
2 .09
12 .86
0 .95
0 .08
6 .87
1 .07
97 .86
60 .15
16 .13
3 .27
48 .61
17 .41
0
23 .40
82 .83
96 .63
42 .55
81 .43
0 .01
0
1 .53
0 .06
0
0 .50
0 .07
0
0
0
0 .06
0
0
0 .02
0
A1 F3 (aq)
0
0
0
0
0
0
0
0
A1F 4 -
0
0
0
0
0
0
0
0
A1F 52
0
0
0
0
0
0
0
0
A1F 63
0
0
0
0
0
0
0
0
AlSO4 +
0
0
0
0 .02
0
0
0
0
Al(S04) 2 - 0
0
0
0
0
0
0
0
A1 3+
0 .06
0
0
AlOH 2+
1 .28
0 .08
Al(OH) 2+
19 .43
3 .27
Al(OH) 4
79 .16
96 .62
A1F 2+
0 .07
AlF 2+
12
of interest . The logarithm of the ratio of Q to Kso is called th e
stability index (SI) . If SI is greater than zero, the solution i n
oversaturated with respect to that phase, and conversely, it i s
undersaturated if SI is less than zero . Furthermore, the solid phas e
with the greater SI value is the controlling solid phase . Table 7 i s
shows stability indices calculated for gibbsite and kaolinite . Bot h
gibbsite and kaolinite are oversaturated for most of the lake waters .
In agreement with the predominance-area diagrams, kaolinite is th e
controlling solid phase in nearly all waters . However, Driscoll et a l
(1984) have suggested, based on similar calculations for Adirondac k
lakes in New York State, that solution conditions would not favo r
equilibrium with kaolinite but that gibbsite, which is als o
oversaturated, is the more likely controlling solid phase for aluminum .
SUMMARY AND CONCLUSION S
Examination of water chemistry data for the Oregon Cascade lakes ha s
given strong indication that they are in equilibrium with secondar y
mineral phases formed during the incongruent dissolution of bedroc k
feldspar minerals . Secondary mineral phases controlling solutio n
chemistry are kaolinite and gibbsite . These geochemical forms, known
for their slow dissolution kinetics and resistance to chemica l
weathering, explain the dilute solution chemistry and weak bufferin g
capacity of the lakes and thus their susceptibility to acidification .
13
Table 7 . Stability Indices for Gibbsite and Kaolinite .
Lake
*
Stability Index (SI )
Gibbsite
Kaolinit e
Waldo
0 .49
0 .9 8
Bull Run
0 .24
2 .7 6
Lost
0 .21
1 .5 7
N .Summit
1 .25
1 .6 4
0lallie
0 .62
1 .3 4
Betty
0 .34
1 .6 7
Lucky
0 .08
-0 .4 0
Summit
0 .47
1 .28
*
SI = log
10 Q - 1og 10 K s o
where Q = ion molar produc t
and K so = solubility product constan t
14
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15
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14
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15
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16
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16