WETLANDS, Vol. 27, No. 4, December 2007, pp. 806–818
’ 2007, The Society of Wetland Scientists
USE OF SOLUTE MASS BALANCE TO QUANTIFY GEOCHEMICAL PROCESSES
IN A PRAIRIE RECHARGE WETLAND
Dru J. Heagle1, Masaki Hayashi1, and Garth van der Kamp2
1
Department of Geoscience, University of Calgary
2500 University Drive NW
Calgary, Alberta, Canada T2N 1N4
2
Water and Science Technology, Environment Canada
11 Innovation Blvd.
Saskatoon, Saskatchewan, Canada, S7N 3H5
Abstract: In the northern prairie region of North America, there are millions of small seasonal
wetlands. The aquatic ecology of these wetlands is partly controlled by the salinity of the wetland pond
water, which affects the vegetation and invertebrate communities. The objective of this study was to
identify the key geochemical processes affecting water chemistry in prairie wetlands. We used the
combined water and solute mass balance approach to quantify the rates of geochemical reactions in
a typical prairie recharge wetland in Saskatchewan, Canada. Sulfate reduction, carbonate mineral
dissolution, and processes adding carbon dioxide to the pond were identified as the key geochemical
reactions. Sulfate reduction removed more sulfate from the pond than infiltration in each of the four
years examined. The average rate of sulfate reduction, 0.07 g m22 d21, was greatest in spring and
decreased during the year. Reduced sulfate remains in the sediments but is re-oxidized when the pond
dries out and is dissolved into the pond water and sediment pore water when the pond re-floods. X-ray
diffraction analyses of wetland soil and mass balance calculations indicate magnesium-calcite is dissolved
into the pond water in the spring and precipitates out of solution later in the year, and dissolves into the
pond the following year.
Key Words: carbonate, chloride, Mg-calcite, prairie wetland, sulfate reduction, water balance
from the dissolution of carbonate minerals in glacial
till underlying much of the region (Hendry et al.
1986, Keller and van der Kamp 1988, Keller and van
der Kamp 1991, Van Stempvoort et al. 1994).
Therefore, by understanding the transport and
geochemistry of sulfate and carbonate species, we
can gain further insight into the ecology of prairie
wetlands.
Prairie wetlands can generally be grouped into
three classes according to their landscape position:
recharge, flow-through, and discharge (Lissey 1971,
Euliss et al. 2004). Recharge wetlands occupy
relatively high landscape positions and tend to have
seasonal ponds that dry out during the summer.
Recharge wetlands have the lowest salinity because
they receive water from precipitation and surface
runoff from the adjacent uplands and they have a net
outward flow of groundwater that removes solute
from the wetland (Hayashi et al. 1998a). Soil
formation and soil salinity around prairie wetlands
are strongly governed by the redistribution of salts
with groundwater flow (Arndt and Richardson
1989, Steinwand and Richardson 1989). Despite
INTRODUCTION
The semiarid northern prairie region of North
America is characterized by a rolling landscape that
contains numerous depressions with wetlands. These
wetlands collect and store surface water runoff,
connect surface water and groundwater through
processes known as depression-focused recharge and
discharge (Meyboom 1966, Lissey 1968, Mills and
Zwarich 1986, Hayashi et al. 1998a), and provide
critical habitat for migrating waterfowl (Swanson
and Duebbert 1989). Pond water chemistry affects
the species composition, structure, and plant and
animal communities that occupy the wetland basin
(Swanson and Duebbert 1989, Euliss et al. 2004). In
particular, pond salinity strongly influences the
vegetation (Stewart and Kantrud 1972) and invertebrate communities in the wetlands (Swanson et
al. 1988), which affects the suitability of these
wetlands for waterfowl habitat (Labaugh and
Swanson 1992). Salinity in the prairie region is
primarily due to sulfate derived from oxidation of
reduced sulfur compounds and carbonate derived
806
Heagle et al., SOLUTE MASS BALANCE
the importance of the pond water chemistry for
aquatic ecology, soil formation and salinity, and
groundwater quality through recharge, the geochemical processes affecting the pond water are
not well understood.
Studies integrating hydrology and geochemistry
require water and solute mass balances, and often
employ conservative tracers. Since point measurements of groundwater exchange with a pond are
difficult to extrapolate across the entire pond,
conservative tracers can be used to quantify
groundwater inflow and outflow (Krabbenhoft and
Webster 1995, Labaugh et al. 1997, Choi and
Harvey 2000, Gurrieri and Furniss 2004), or to
identify geochemical reactions such as denitrification
(Hill et al. 1998, Mengis et al. 1999) and sulfur
reduction-oxidation reactions (Hines et al. 1992,
Anisfeld and Benoit 1997). The objective of this
study was to identify the key geochemical processes
and to quantify their rates in a typical prairie
recharge wetland. This study focused on recharge
wetlands because these wetlands are the beginning of
the surface and subsurface flow paths and represent
initial water chemistry conditions that are modified
as water moves to lower landscape positions including flow-through and discharge wetlands.
STUDY SITE
The study was conducted at wetland S109 located
in the St. Denis National Wildlife Area (106u069W,
52u029N), 40 km east of Saskatoon, Saskatchewan,
Canada (Figure 1). Previous studies (Miller et al.,
1985, Hayashi et al. 1998a, Berthold et al. 2004,
Parsons et al. 2004) showed S109 was hydrologically
similar to other recharge wetlands in the northern
prairie region (Winter and Rosenberry 1995, Rosenberry and Winter 1997). S109 also had similar
pond water ion dominance (Ca . K . Mg . Na
and HCO3 . Cl . SO4) to temporary wetland
chemistry observed in the northern prairie region of
Canada (Driver and Peden 1977) and USA (Labaugh et al. 1987).
The 30-year mean annual temperature from 1971–
2000 in Saskatoon was 2uC and the mean annual
precipitation was 350 mm, with 97 mm falling as
snow (Environment Canada 2006). The annual lake
evaporation for the area is approximately 700 mm
(Morton 1983). The wetland was located in a topographically closed catchment, without surface water
inflow or outflow. In this study, the term ‘‘wetland’’
refers to a topographical depression having saturated soil for at least part of the year, ‘‘pond’’ refers to
the open water within the wetland, ‘‘upland’’ is the
area surrounding the wetland within the catchment
807
Figure 1. Location of the St. Denis National Wildlife
Area. The shaded area indicates the prairie wetland region
(after Hayashi et al. 2003, with permission from
Elsevier Science).
boundary, and the ‘‘willow ring’’ is the vegetated
area marking the boundary between the wetland and
the upland.
The catchment area was 24,000 m2 and the
wetland had an area of 2,400 m2 (Hayashi et al.
1998a). The hydraulic conductivity of the near
surface stratified silty sediments in the wetland was
approximately 1026 m s21 (Hayashi et al. 1998a).
The surface soils are underlain by the early to late
Wisconsinian Battleford and Floral tills (Christiansen 1992). Vegetation within the center of the
wetland included sedges (Carex spp.) and spike rush
(Eleocharis spp.). The willow ring contained willows
(Salix spp.), trembling aspen (Populus tremuloides),
and balsam poplar (Populus balsamifera). The study
was conducted from 1994–1997, and during this
time the wetland was surrounded by a cultivated
field planted with wheat in 1994, fallowed in 1995,
oil seed in 1996, and wheat in 1997. The pond water
level from 1994–1997 was moderate to high compared to the water level records for the 1970–2000
period (van der Kamp et al. 2003, Figure 2).
Snowmelt runoff (R in Figure 2) is the largest
single source of water to prairie wetlands increasing
the pond water level in the spring (Woo and Rowsell
1993, Labaugh et al. 1996, Hayashi et al. 1998a).
The runoff water partially leaches solute from the
upland soil and transports it to the wetland
(Hayashi et al. 1998b). Precipitation (P) is the
largest source of water to the wetland during the
summer, as evapotranspiration (ET) keeps upland
soils dry, inhibiting runoff except during large
808
WETLANDS, Volume 27, No. 4, 2007
Figure 2. Conceptual model for solute movement in
a prairie recharge wetland. Arrows indicate the predominant directions of hydrological fluxes.
precipitation events (Hayashi et al. 1998a). Evapotranspiration from the riparian zone and the upland
creates a hydraulic gradient from the pond to the
upland, resulting in pond water infiltration (I) and
groundwater movement from the pond to the edge
of the wetland (Millar 1971), and to the upland
(Hayashi et al. 1998a), causing high soil salinity
around the periphery of the wetland (Berthold et al.
2004). Evaporation (E) and infiltration remove
water from the open pond typically resulting in
wetland drying in the fall (although in wet years the
pond may exist through the winter). Evaporation of
soil moisture from the moist wetland soils may
deposit solute, as well as oxidize reduced species in
the aerated soils. Geochemical and biological reactions in the pond water and the shallow wetland
soils may add or remove solute from the wetland
pond.
chromatography, and samples for calcium (Ca) and
magnesium (Mg) were acidified and analyzed by
atomic absorption spectroscopy. Alkalinity was
determined by titration. The pH of the pond was
measured at the time of sampling using a pH probe
in 1994, but not in subsequent years. The results
from the chemical analyses, including the pond pH,
were input into PHREEQC (Parkhurst and Appelo
1999) to determine the charge balance, mineral
saturation indices, partial pressure of CO2 (PCO2),
and total dissolved carbonate species (SCO2). For
this study we define SCO2 as
SCO2 ~ CO2ðaqÞ z ½H2 CO3 z ½HCO3 { ð1Þ
z CO3 2{
where square brackets indicate molar concentration
and SCO2 is expressed as mg L21 CO2.
Three dry surface soil samples, from depths up to
15 cm below ground surface, were obtained from
a transect through the dry wetland in September
2004. The soil was air dried for 48 hours then oven
dried at 65uC for 24 hours. After drying, the soil was
ground for X-ray diffraction (XRD) analyses using
a Rigaku Multiflex X-Ray Diffractometer.
Water Balance
The daily water balance for the wetland pond is
approximated by
Vðt z 1Þ ~ VðtÞ z AðtÞ½PðtÞ z RðtÞ { EðtÞ
METHODS
Field and Laboratory Methods
The pond water level was recorded hourly using
a chart recorder (Stevens F-type) in 1994 and
a pressure transducer (Geokon, 4500ALV-5) from
1995–1997 (Hayashi et al. 1998a). Precipitation was
recorded using a tipping bucket rain gauge located
within the catchment. The pond volume and surface
area were calculated from the measured pond water
level using equations based on the bathymetry of
S109 (Hayashi and van der Kamp 2000).
Surface water samples were collected from the
pond bi-weekly in 1994 and 1995 and monthly in
1996 and 1997. A total of 11 runoff samples were
obtained by two methods on April 8, 1996, March
29, 2000, and March 1, 2001. First, a sample bottle
was immersed in overland flow from the soil surface
during runoff events. Second, water was collected
from small puddles in upland depressions within the
catchment. All samples were filtered using a 0.45 mm
filter and kept at 4uC prior to analysis. Chloride (Cl)
and sulfate (SO4) were analyzed by ion-exchange
{ IðtÞDt
ð2Þ
where V(t) is the pond volume (m3) at the beginning
of every day (i.e., 00:00 AM), A(t) is the daily
average pond area (m2), P(t) is precipitation, R(t) is
runoff, E(t) is evaporation, I(t) is infiltration, and Dt
is one day. All flux values (P, R, E, and I) represent
average rates (m day21) for each day. The start
dates for the water and mass balance calculations
were May 11, 1994, April 21, 1995, July 10, 1996,
and June 18, 1997 (Table 1). For 1994 and 1995,
sampling began immediately after snowmelt was
completed. The 1996 date was selected after a large
precipitation event on July 4 caused a large amount
of runoff (140 mm) into the pond, and in 1997
frequent small runoff events in the spring delayed
the beginning of the calculations until June 18. The
final dates were selected using the final chemistry
data before the pond dried up or froze. Only P was
directly measured in this study. V and A were
calculated from pond depth measurements, and the
remaining terms were estimated from the water
balance equation and the solute mass balance
equation using the following methods.
Heagle et al., SOLUTE MASS BALANCE
809
Table 1. Pond volume and concentration (mg L21) of dissolved species in the pond from 1994–1997, and average
concentration in runoff water.
Year
1994
1995
1996
1997
Sampling
Date
Volume
(m3)
11-May
27-May
9-Jun
23-Jun
30-Jun
14-Jul
17-Aug
30-Aug
13-Sep
21-Apr
4-May
1-Jun
17-Jun
1-Jul
16-Jul
10-Jul
14-Aug
18-Sep
16-Oct
18-Jun
3-Jul
25-Jul
8-Aug
11-Sep
29-Sep
896
955
1030
922
840
690
361
232
114
512
454
306
150
75
9
1170
621
209
98
1860
1631
1170
825
414
337
Runoff
a
b
Ca
Mg
Cl
SO4
Alka
SCO2b
pH
SO4:Cl
28
32
11
13
42
15
41
50
16
16
40
24
28
28
29
30
39
37
47
41
40
34
38
44
47
52
48
24.1
17
11
13
16
16
15
16
13
16
15
16
15
16
17
19
19
18
9.9
4.5
4.6
4.1
3.8
4.2
4.0
4.3
4.2
4.8
6.6
7.3
8.6
8.3
8.2
7.0
2.9
3.7
5.8
7.1
2.7
2.5
3.0
3.6
4.4
4.5
3.4
2.56
1.12
0.74
0.38
0.58
0.17
0.17
0.14
0.08
30
36
17
3.9
1.3
0.9
9.9
6.7
5.4
5.4
30.8
23.4
15.4
9.6
5.4
4.0
9.2
167
187
206
223
230
237
238
234
238
128
142
189
200
189
226
169
221
217
229
179
206
242
269
294
273
58.4
136
161
174
180
209
204
215
198
194
7.19
7.02
7.09
7.21
6.91
7.02
6.90
7.08
7.18
0.56
0.24
0.18
0.10
0.14
0.04
0.04
0.03
0.02
4.60
4.90
1.98
0.47
0.16
0.13
3.41
1.82
0.94
0.76
11.5
9.44
5.20
2.66
1.23
0.89
Alkalinity as HCO32.
SCO2 values as CO2.
Runoff (R) occurred during large precipitation
events or consecutive days with moderate precipitation, as indicated by a sharp increase in the pond
water level beyond what is reconciled by precipitation. The short time period required for the water
level increase suggests that the additional increase in
pond depth is due to surface runoff and perhaps
very shallow subsurface runoff near the pond.
Noting that
½Vðt z 1Þ { VðtÞ=½AðtÞ % hðt z 1Þ { hðtÞ
ð3Þ
where h(t) is the depth of the pond measured at
00:00AM, we can determine the runoff to the pond
from a rearranged water balance equation
DhðtÞ { PðtÞ % RðtÞ { ½EðtÞ z IðtÞ
ð4Þ
where Dh(t) 5 h(t + 1) 2 h(t). The left hand side of
Equation 4 is always negative if R 5 0. When the
left hand side is positive, which occurs during large
rain events, we assume that E + I is negligible
because the evaporation rate is expected to be small
during rain events, and groundwater flow away
from the pond is reduced or slightly reversed as
shown by piezometer records (Hayashi 1996).
Assuming E + I 5 0 in Equation 4, an estimate of
runoff is given by R 5 Dh 2 P.
For days without precipitation and runoff,
evaporation (E) from the pond surface and infiltration (I) lower the water level, Dh 5 2 [E + I].
Pond water infiltrates into the shallow groundwater
and moves radially outward from the pond in
response to a lower water potential created by
evapotranspiration from the vegetation surrounding
the wetland (Parsons et al. 2004). Since evaporation
increases the solute concentration and infiltration
does not change the concentration, the pond solute
mass balance can be used to estimate infiltration and
evaporation, which is written as
Mðt z 1Þ ~ MðtÞ z AðtÞ½PðtÞCP z RðtÞCR
{ IðtÞCðtÞ z BðtÞDt
ð5Þ
where M(t) is the pond solute mass (kg) at the
beginning of each day, Cp is the solute concentra-
810
WETLANDS, Volume 27, No. 4, 2007
tion (kg m23) in precipitation, CR is the solute
concentration (kg m23) in the runoff water, C(t) is
the pond solute concentration (kg m23) at the
beginning of each day, and B(t) is the daily average
rate (kg m22 day21) of addition or removal of mass
to the pond by chemical reactions. It was assumed
that 1) the pond was well mixed, 2) groundwater
solute input to the pond was negligible, 3) diffusive
exchange of solute between pond water and underlying groundwater was negligible, 4) solute
uptake by vegetation was negligible, and 5) runoff
solute concentration was constant.
For days without runoff, Equation 2 can be
written as
AðtÞIðtÞDt ~ f ½AðtÞPðtÞDt { DV
ð6Þ
where f ~ I=ðI z EÞ
ð7Þ
and DV 5 V(t + 1) 2 V(t). Equation 6 gives an
estimate of daily infiltration loss from the pond.
Since infiltration and evaporation are the only water
loss processes in the wetland, the remaining water
loss is accounted for by evaporation:
AðtÞEðtÞDt ~ ð1 { f Þ½AðtÞPðtÞDt { DV
ð8Þ
The water balance and solute mass balance equations (Equations 2 and 5) were coupled using f in
Equation 7 to estimate solute concentration in the
pond.
C(t z 1) ~
C(t)V (t) z A(t)½P(t)CP z R(t)CR
V (t) z A(t)½P(t) z R(t) { fI(t) z E(t)gDt
ð9Þ
{ f fI(t) z E(t)gC(t) z B(t)Dt
V (t) z A(t)½P(t) z R(t) { fI(t) z E(t)gDt
Equation 9 was applied to the Cl mass balance to
determine f, I, and E using Equations 7 and 8,
assuming B 5 0 (no reactions). The Cl concentration
in precipitation (0.1 mg L21) was estimated using
the 1993 data from three Canadian Air and
Precipitation Monitoring Network (CAPMoN) stations; each located approximately 500 km from the
study site, in Cree Lake, McCreary, and Esther in
1993 (Environment Canada 1993). The runoff
concentration (3.4 mg L21) was obtained from the
average of the 11 runoff samples (see Field and
Laboratory Methods above). The calculated Cl
concentrations using Equation 9 were fitted to the
observed concentrations using f as a fitting parameter. By this method, f is constant for each day
between sampling dates, although the magnitude of
infiltration and evaporation may change according
to the total water loss from the pond.
Estimation of Reaction Rates
Once all terms in the water balance were determined using the Cl balance, the mass balance for
other dissolved species was examined. Reactive
species were identified using two steps. In the first
step, the observed concentrations and pond volumes
were used to calculate the total dissolved masses in
the pond water for Ca, Mg, SO4, and alkalinity.
Previous studies showed that the alkalinity in the
pond water was almost entirely due to carbonate
species (Hayashi 1996). Therefore, alkalinity is
assumed to consist entirely of bicarbonate
(HCO32), because the pond had a relatively narrow
pH range from 6.9–7.2 (Table 1) (Stumm and
Morgan 1996, p. 154). The masses were normalized
by dividing the observed dissolved mass by the
dissolved mass on the initial observation date for
each year. Normalized solute masses that differ from
the normalized Cl mass indicate addition or removal
of mass to the pond by reactions because the effects
of hydrological processes are reflected in the
normalized Cl mass.
In the second step, the mass balance equation
(Equation 9) was applied without reaction (B 5 0)
for the reactive species. The runoff concentrations
for each species were obtained from the average of
the 11 runoff samples. The precipitation concentrations for Ca, Mg, and SO4 were estimated to be 0.5,
0.1, and 1.5 mg L21, respectively, from the average
of three CAPMoN monitoring stations (see above).
The alkalinity of the precipitation (3 3 1025 mg
L21) was estimated assuming pure water was in
equilibrium with an atmospheric PCO2 of 1023.5
atm (Appelo and Postma 1996, pg. 97). Pond water
concentrations were calculated using Equation 9. If
the calculated results were noticeably different from
the observed concentrations the reaction term (B)
was used in Equation 9 as a fitting parameter to
match the concentrations; where B , 0 indicates
reactions removing mass from the pond water and B
. 0 indicates reactions adding mass to the pond.
RESULTS
1994 Water Balance and Mass Balance
The pond depth in 1994 increased in the spring
due to snowmelt and precipitation, and remained
relatively steady in May and June, when evaporation
and infiltration were roughly equal to precipitation
(Figure 3A). The pond volume decreased steadily
after mid-June, as evaporation and infiltration
exceeded precipitation, and surface water disappeared in October. Runoff (Dh 2 P in Equation 4)
was a small component of the water balance in April
Heagle et al., SOLUTE MASS BALANCE
811
Figure 4. Normalized masses of Cl, SO4, alkalinity, and
Ca + Mg in 1994. Values are normalized to the first
sampling date (11 May). Tick marks indicate the first day
of each month.
and May, and did not occur after a small runoff
event on 4 June (Figure 3B).
Using Equation 9, the calculated pond Cl concentration was matched with the observed concentration (Figure 3C) to determine the value of f in
Equation 7, and then evaporation and infiltration
were estimated using Equations 6 and 8 (Figure 3D,
Table 2). Monthly average evaporation and infiltration rates had large variability, due partly to the
actual variability and also to error associated with
the Cl balance method. However, in general,
infiltration was greater than evaporation after July.
The normalized mass data (Figure 4) shows SO4
was removed from the pond water compared to Cl,
and alkalinity and Ca + Mg were added to the pond
water. This is corroborated by the calculated pond
water concentrations using Equation 9 without
including the reaction term B. Calculated SO4
concentrations were greater than those observed
(Figure 5), while calculated Ca + Mg and alkalinity
concentrations were less than those observed (Figure 6A).
Figure 3. The water balance computations for summer
field seasons from 1994–1997: A) daily precipitation and
pond depth, B) estimated daily runoff (values above
horizontal dashed line (0 mm) indicate amount of runoff
(mm), values below the dashed line indicate 0 mm runoff),
C) observed and calculated pond Cl concentration (data
points show observed concentrations and the line shows
the calculated concentration (mg L21) using f (Equation 7) as a fitting parameter), and D) cumulative water
level change from the wetland pond (solid lines represent
infiltration and dashed line shows evaporation). Vertical
dashed lines separate years, and tick marks indicate the
first day of each month.
Table 2.
Average monthly evaporation and infiltration rates (mm d21) for selected dates from 1994–1997.
Year
Process
1994
Infiltration
Evaporation
Infiltration
Evaporation
Infiltration
Evaporation
Infiltration
Evaporation
1995
1996
1997
April
May
June
July
Aug
Sept.
2.4
2.1
8.0
1.9
5.6
2.0
11.6
0.8
3.4
3.0
5.2
4.6
5.9
2.0
6.9
1.7
0.3
2.8
0.02
2.6
1.9
2.0
5.4
4.9
5.2
4.1
4.2
3.5
4.4
3.2
6.7
0
Oct.
2.3
1.4
Average
4.2
2.1
5.5
1.9
4.1
3.5
5.1
3.5
812
WETLANDS, Volume 27, No. 4, 2007
Figure 5. Pond SO4 concentration and the SO4 reduction rate in 1994. Data points show the observed
concentration, the dashed line shows the calculated
concentration without the reaction term, and the solid
line shows the calculated concentration including SO4
reduction. Bars are the SO4 addition/removal rates by
reactions with error bars indicating the uncertainty in
runoff inputs.
SO4 Reduction. As shown in Figures 4 and 5, as
well as in the decreasing SO4:Cl mass ratio over time
(Table 1), SO4 is removed from the pond by
reactions. Gypsum precipitation does not account
for the removal because gypsum was under-saturated in the pond. Adsorption is minimized in soils with
pH greater than 6 (Kamprath et al. 1956, Chao et al.
1964), which was the case for the pond water
(Table 1). Therefore, the removal of SO4 from the
pond was attributed to SO4 reduction. Labaugh et
al. (1996) hypothesized SO4 reduction was occurring
in prairie wetlands receiving groundwater discharge
in North Dakota, USA, and Jokic et al. (2003)
found soils in wetland S109 contained reduced sulfur
compounds. The SO4 reduction reaction (Stumm
and Morgan 1996, p. 466) was included in the
calculations assuming organic carbon, represented
by CH2O, was not limiting the reaction.
2CH2 O z 2Hz z SO4
2{
ð10Þ
? H2 S z 2CO2 z 2H2 O
H2S produced by the reaction is likely sequestered as
sulfide compounds in the soils including pyrite
(Jokic et al. 2003), and not allowed to escape to
the atmosphere. Sulfide oxidation was estimated
based on the oxidation of pyrite (FeS2) with an
unlimited supply of oxygen (Appelo and Postma
1996, p. 263) when the calculated pond mass
underestimated the observed value.
FeS2 z 3:5O2 z H2 O
? Fe2z z 2SO4
2{
z 2Hz
ð11Þ
SO4 was added or removed by adjusting B in
Figure 6. A) Pond alkalinity and Ca + Mg concentration
in 1994. Circles and squares represent observed Ca + Mg
and alkalinity, respectively. The solid and dashed lines
show calculated Ca + Mg and alkalinity, respectively.
Calculated concentrations do not include the reaction
term. B) Pond Ca + Mg concentration and Mg-Calcite
reaction rate. Data points represent observed concentration and the line shows calculated concentration including
Mg-calcite reactions. The bars show the rate of Mg-calcite
addition to the pond.
Equation 9 to match the calculated concentrations
to the observed concentrations (Figure 5). The
reaction rate, B (g m22 d21), is shown in Figure 5
for each period between sampling events. The mass
of SO4 added to the pond water was calculated using
the product of the average daily reaction rate (B)
and the average daily pond area for days when B .
0. The sum of these values was used to produce the
total mass of SO4 added to the pond by reactions.
Similarly, the total SO4 mass removed from the
pond was calculated for days when B , 0. Reactions
added 0.1 kg of SO4 to the pond in 1994 and
removed 3.4 kg. Runoff added 0.3 kg and infiltration removed 0.2 kg.
The runoff water concentration is the largest
source of uncertainty in our mass balance calculation. Therefore, we used the standard deviation of
the 11 runoff samples for SO4 (6 9.4 mg L21) to
change the SO4 runoff concentration to 18.6 and
Heagle et al., SOLUTE MASS BALANCE
813
0 mg L21 to examine the effect of runoff variability
on the reaction rate. The error bars on the reaction
rate in Figure 5 show the results of varying the SO4
concentration. The standard deviation in the runoff
concentration only affected the first two calculated
time periods because runoff did not occur in 1994
after June 4. The total SO4 reduced from the pond
water with the 0 and 18.6 mg L21 runoff concentrations were 2.4 and 3.2 kg, respectively. The small
range in the reaction rate is due to the limited role
runoff plays in the pond water balance.
The net increase in alkalinity caused by the SO4
reduction and oxidation was calculated using
Equations 10 and 11 to be 4 kg as HCO32. This is
negligible compared to the maximum alkalinity in
the pond of 212 kg, and was insufficient to match
calculated and observed alkalinities (Figure 6A).
Carbonate Addition and Removal. The mismatch
between the calculated versus observed Ca + Mg
concentration and alkalinity (Figure 6A) was addressed by including reactions involving carbonate
in the mass balance equation (Equation 9). Ca,
alkalinity, and SCO2 are linked through the
equilibrium reaction; the simplest reaction is the
dissolution of calcite (CaCO3) (Stumm and Morgan
1996, p. 387).
CaCO3 z H2 O z CO2 < Ca2z z 2HCO3
{
ð12Þ
Calcite in Equation 12 can be replaced by Ca-Mgcarbonate (Stumm and Morgan 1996, p. 388).
Cax Mg1{x CO3 z H2 O z CO2
< xCa2z z ð1 { xÞMg2z z 2HCO3
{
ð13Þ
The XRD analyses of the wetland soils indicated
that the Ca-Mg-carbonate mineral was Mg-calcite.
Using the method of Goldsmith et al. (1955), the
Mg-calcite in the wetland soils contained 12 mol%
MgCO3, which is above the 8 mol% MgCO3 found
by St. Arnaud and Herbillon (1973) in upland soils
in central Saskatchewan. Based on these data, x 5
0.88 was used in Equation 13 for all four years. The
solubility equilibrium constant of the Mg-calcite (K
5 1028.39) was determined using the data for Group
I solids with calcite and disordered dolomite as the
end-members, found in Busenberg and Plummer
(1989). Although a single composition of Mg-calcite
was used in the calculations in this study, the actual
composition likely varied within the wetland, and
also within and between years.
Due to the uncertainty in the composition of the
Mg-calcite, alkalinity was used to match the
calculated and observed mass balance by adjusting
B (Equation 9). Before the Mg-calcite addition,
alkalinity and Ca + Mg concentrations were under-
Figure 7. SCO2 (mg L21 as CO2) in 1994. Data points
represent the observed values, the dashed line represents
the calculated values without the reaction term, and the
solid line shows the calculated values with SO4 reduction
and Mg-calcite reactions.
estimated by approximately 47% and 40% (Figure 6A), respectively. The addition of Mg-calcite fit
the alkalinity, and improved the match between
calculated and observed Ca + Mg concentrations
(Figure 6B). The total mass added and removed
from the pond was calculated using the same
methods described above to calculate the addition
and removal of SO4. Approximately 100 kg of
alkalinity (as HCO32) was added to the calculated
pond mass through Mg-calcite addition and 40 kg
was removed despite the under saturation of the
Mg-calcite. Temperature was not accounted for in
the saturation calculations, and daily temperature
variability in the pond was significant. Carbonate
precipitation may have occurred during days with
elevated temperature, or near the edges of the pond
where the temperature was higher than the middle of
the pond. Runoff added 4 kg of alkalinity to the
pond and infiltration removed approximately
200 kg.
SO4 reduction and Mg-calcite dissolution added
approximately 110 kg of SCO2 to the pond, but it
was insufficient to match the calculated to observed
concentrations (Figure 7), indicating other processes
may be adding SCO2 to the pond. With the
available data we are only able to examine net
addition or removal of SCO2 from the pond water,
while SCO2 was likely added and removed simultaneously from the pond. The CO2 may be added to
the pond by oxidation of organic carbon, with
vegetation litter and dissolved organic carbon
(Waiser 2006) as the source of carbon. CO2 removal
from the pond may occur due to photosynthesis by
algae and aquatic vegetation. Also, the computed
PCO2 ranged from 1022.0 to 1021.5 atm, indicating
814
WETLANDS, Volume 27, No. 4, 2007
emission of CO2 to the atmosphere (PCO2 5 1023.5
atm) contributes to the CO2 removal. The discrepancy between the calculated and observed SCO2
shows reactions adding CO2 to the pond are greater
than those removing CO2. This agrees with the
results of Waiser and Robarts (2004), where net
heterotrophy and bacterial carbon demand were
shown to be high in wetland S109 in 1998 and 1999.
1995 to 1997 Water Balance and Mass Balance
The water balances for 1995, 1996, and 1997 were
completed following the same methods as the 1994
water balance (Figure 3). The total water loss from
the pond was greatest in June, July, and August,
although the rates of water loss varied from year to
year. Hayashi et al. (1998a) estimated average rates
of evaporation and infiltration using a submerged
evaporation pan and the pond level data over
a two weeks period from June 28 to July 12,
1995. The evaporation rate was 3.1 mm day21 and
the infiltration rate was 9.4 mm day21. Over the
same period, Equation 9 gave an evaporation
rate of 1.2 mm day21 and infiltration rate of
9.1 mm day21. 1995 was a dry year with little snow
melt runoff and precipitation. A large rain event on
July 4, 1996, was responsible for the large pond
volume that year, which was carried forward to 1997
due to supplementary snowmelt.
The small amount (6 m3) of runoff in 1995
contributed a negligible amount of SO4 to the pond,
while infiltration removed 4.5 kg of SO4. The
reaction term (B) in Equation 9 was adjusted to
match the calculated and observed SO4 mass in the
pond (Figure 8A). The observed increase in SO4
mass required approximately 1 kg of SO4 to be
added to the calculated mass. Equation 9 estimated
that approximately 12 kg of SO4 was removed from
the pond by reaction from May 4, 1995 until the
pond dried up. The SO4 removal resulted in 15 kg of
alkalinity being added to the pond. Mg-calcite
dissolution and precipitation added a total of 1 kg
of alkalinity in 1995, while 0.7 kg of alkalinity was
added by runoff and approximately 80 kg was
removed by infiltration.
In 1996, runoff added 0.3 kg of SO4 to the pond,
infiltration removed 5 kg, and reactions removed
6.7 kg. Approximately 0.5 kg of SO4 remained in
the pond on the final day of the calculations in 1996
(Figure 8A). The reaction term (B) in Equation 9
was adjusted to match the calculated and observed
SO4 mass in the pond (Figure 8A). This resulted in
approximately 8 kg of alkalinity added to the
calculated mass, in addition to the 4 kg added by
runoff. The overestimated alkalinity was also
Figure 8. A) Pond SO4 concentrations in 1995, 1996,
and 1997. Data points indicate the observed concentration, the lines indicate the calculated concentrations
including the reaction term, and the bars indicate the
reaction rate. B) Ca + Mg concentration. Data points
show the observed pond Ca + Mg concentrations, the
solid lines show the calculated concentrations including
the reaction term, and the bars show the Mg-calcite
reaction rate. Vertical dashed lines separate years.
addressed by adjusting the reaction term (B) in the
calculated pond water (Figure 8B). Equation 9
estimated 45 kg was removed over the remainder
of the modeled year by precipitation of Mg-calcite.
Calculations indicate infiltration removed approximately 150 kg of alkalinity in 1996, with 22 kg
remaining in the pond on the final day of calculations.
In 1997, runoff added 2.5 kg of SO4 to the pond,
infiltration removed 23 kg, and reactions removed
37 kg from the pond. The removal of SO4 according
to Equation 10 resulted in 46 kg of alkalinity added
to the pond. Carbonate dissolution and precipitation added a net 3 kg of alkalinity to the pond in
1997. Calculations show 32 kg of alkalinity was
added by runoff and 325 kg was removed by
infiltration. The large pond volume at the end of
the study in 1997 (337 m3, Table 1) contained 92 kg
of alkalinity.
Heagle et al., SOLUTE MASS BALANCE
DISCUSSION
Water Balance
We used the Cl mass balance to complete the
water balance for the wetland pond. Among many
assumptions made in this method, Cl concentration
in runoff water is the largest source of uncertainty.
This was due to the runoff concentration calculation
representing an average of only 11 samples. However, the effect of the variation of runoff chemistry
on the calculated pond water concentrations is
minimized due to the low frequency of runoff events
as shown by the small variability in the SO4
reduction rates for 1994 when using a range of
runoff concentrations.
The water balance results showed a similar trend
in April and May for both 1994 and 1995.
Evaporation was greater than infiltration until early
June, when infiltration became greater. This change
is close to the time when leaves reach their maximum
size on the trees in the willow ring, and perhaps the
willow ring acted as a wind barrier and decreased
evaporation. Also, emergent wetland and upland
vegetation began to transpire and may have decreased the water potential in the soil and shallow
groundwater surrounding the wetland causing pond
water to infiltrate and move radially outward from
the pond. Transpiration by upland vegetation
probably caused infiltration to be a dominant water
loss process for the remainder of the year. However,
some inter-annual variability was observed. Evaporation and infiltration were nearly equal towards the
end of the summer in 1996 and 1997, while
infiltration was much greater than evaporation in
1994 and 1995. The difference was probably related
to a smaller depth (and area) of the pond in 1994
and 1995, which would enhance the effects of
infiltration induced by the transpiration due to the
riparian vegetation (Millar 1971).
815
focused on hydrological processes affecting pond
water chemistry (for example Stewart and Kantrud
1972, Swanson and Duebbert 1989, Labaugh and
Swanson 1992). However, calculations showed the
amount of SO4 removed by reactions was greater
than the amount removed by infiltration, demonstrating the importance of SO4 reduction on pond
water chemistry. SO4 was generally removed from
the pond, and the reaction rate removing SO4 from
the pond was highest in spring. The average
calculated rate of SO4 reduction for wetland S109
for four years (Figures 5 and 8A) was
0.07 g m22 d21. This average is similar to the results
from other studies, including Meier et al. (2004) who
used 35S–SO4 to estimate the rate of SO4 reduction
in a neutral pH mining lake in Germany to be
0.19 g m22 d21. Urban et al. (2001) calculated SO4
reduction in freshwater lake sediments in Wisconsin
USA to be up to 0.015 g m22 d21. Oremland et al.
(2000) used 35S–SO4 to calculate a rate of SO4
reduction of 0.12 g m22 d21 in a hypersaline lake in
California USA. The low SO4 concentrations, ,
1 mg L21 in 1994 and 1995 (Table 1), created by
SO4 reduction in the wetland sediments, combined
with high organic carbon content of the soil (Jokic et
al. 2003), can create conditions conducive for
methanogenesis, which has been observed in other
prairie wetlands (Adedeji et al. 2006).
SO4 reduction removed the majority of SO4 from
the pond water (Figures 5 and 8A), and the presence
of reduced sulfur compounds in the wetland soil
(Jokic et al. 2003) indicates H2S produced from the
SO4 reduction reaction (Equation 10) is likely
transformed into more stable compounds, including
organic sulfur and pyrite (Jokic et al. 2003). The
reduced sulfur compounds sequestered in the
wetland soil would likely re-oxidize in the aerated
soil after the pond has dried and be available for
dissolution the following year (Labaugh et al. 1996).
Sulfur Cycle
The initial concentrations of dissolved species in
the wetland pond were different in each of the four
years examined (Table 1). The variability in the
spring pond water concentrations is likely due to the
annual variation in the volume of snowmelt runoff,
differences in the mass of salt stored in the upland
soils, and the storage of solute within the wetland.
Despite the differences of the concentrations of
reactive species, the mass balance calculations show
reactions affecting SO4, Ca + Mg, alkalinity, and
SCO2 occurred in all four years.
Previous studies investigating the relationship
between pond water chemistry and aquatic ecology
Carbonate Geochemistry Buffering of Pond pH
The 1994 results showed the pond water had
much higher PCO2 than the atmosphere, and the
addition of SCO2 through SO4 reduction and
carbonate dissolution estimated from Equation 9
was insufficient to account for the elevated values of
PCO2 or SCO2 concentration (Figure 7). These
calculations agree with the biomass and productivity
results of Waiser and Robarts (2004), who showed
that high PCO2 in wetland S109 in 1998 and 1999
was due to high bacterial carbon demand adding
more CO2 than photosynthesis or atmospheric
exchange were able to remove.
816
WETLANDS, Volume 27, No. 4, 2007
The pH of water can affect invertebrate communities and productivity of freshwater ecosystems
(Rosenberg and Resh 1993, p. 88). The addition of
CO2 to the pond, primarily by oxidation of organic
carbon may increase dissolved CO2 and drive the
CO2-H2O equilibrium (Equation 14) from left to
right (Appelo and Postma 1996, pg. 99) and lower
the pH.
CO2 z H2 O < H2 CO3 < Hz z HCO3
< Hz z CO3
2{
{
the fall. This reaction coupled with carbonate
equilibrium appears to play an important role in
buffering pond water pH against the addition of
CO2 by the oxidation of dissolved organic carbon.
This study demonstrated that hydrological and
geochemical processes can have strong influences
on pond water chemistry, which could have
implications for aquatic ecosystem functioning in
prairie wetlands.
ð14Þ
However, the pH of the pond water in S109
remained near neutral in 1994 (Table 1), likely
because it was buffered by the dissolution of
carbonate in the wetland soil (Equation 12), and
the removal of CO2 by photosynthesis or by
degassing to the atmosphere, all of which could
drive Equation 14 from right to left.
Alkalinity in the shallow (2–6 m) groundwater
beneath the wetland had a range of 400–600 mg L21
as HCO3 (Hayashi 1996), which was higher than
alkalinity of pond water (Table 1). Miller et al.
(1985) showed carbonate was leached to a depth of
1 m beneath the wetland pond. This suggests that
the infiltrating pond water is commonly undersaturated with respect to calcium carbonate minerals, as was observed in 1994 (Table 2). The undersaturated pond water could dissolve and transport
carbonate minerals beneath the wetland either into
the underlying aquifer or laterally to the upland
(Hayashi et al., 1998b).
CONCLUSIONS
We found that SO4 reduction, carbonate dissolution, and reactions adding CO2 to the pond water
were the key geochemical reactions affecting pond
water chemistry in a typical prairie recharge
wetland. These reactions were identified using the
normalized mass of dissolved species calculated
from the concentration and pond volume. These
normalized masses were calculated for non-reactive
Cl and also for Ca + Mg, SO4, and alkalinity.
Deviations of the latter species from Cl indicated
that Cl was participating in geochemical reactions.
Once the reactive species were identified, we could
estimate reaction rates from the combined water and
solute mass balance equations for the wetland pond.
SO4 reduction rates were highest in the spring and
lowest in the fall. However, direct evidence of SO4
reduction occurring in prairie wetlands was still
lacking. XRD analyses of the soil samples, as well as
mass balance calculation, showed Mg-calcite dissolved into the pond in the spring and precipitated in
ACKNOWLEDGMENTS
We thank Canadian Wildlife Service for site
access, Institute for Wetland and Waterfowl Research (Ducks Unlimited Canada), and the Natural
Sciences and Engineering Research Council of
Canada for funding support, Randy Schmidt and
Brad Fahlman for field assistance and Ken Supeene
for chemical analysis. Constructive comments by
two anonymous reviewers and the Associate Editor
greatly improved the manuscript.
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