Reconnaissance Study of Strontium Isotopic Composition of Lake
Brines and Groundwater Associated with Sodium Sulphate
Deposits, Southern Saskatchewan I
Lynn l Kelley and Chris Holmden 2
Kelley, L.I. and Holmden, C. (1999): Reconnaissance study of strontium isotopic composition of lake brines and groundwater
associated with sodium sulphate deposits, southern Saskatchewan; in Summary of Investigations 1999, Volume 2, Saskatchewan
Geological Survey, Sask. Energy Mines, Misc. Rep. 99-4.2.
Natural sodium sulphate deposits occur in many
shallow saline lakes in southern Saskatchewan,
northwestern North Dakota, northeastern Montana, and
east-central Alberta. The origin of the deposits is
understood in a general way. Water and solutes enter a
closed basin, and water is removed by evaporation,
leaving solutes behind as evaporite deposits. The
various hypotheses presented in the literature regarding
potential flow paths and sources of dissolved ions in
the discharging groundwater have not been tested. The
more detailed, predictive model we hope to develop
will hinge upon understanding the mass balance of
water and solutes in closed basins.
Because groundwater continues to supply dissolved
ions to lake basins through spring discharge, our
approach is hydrogeological, with the major objective
of quantifying fluid and chemical mass balances.
Hydrogen and oxygen isotopes demonstrate that
relatively shallow (<200 m) flow systems are the
source of groundwater that discharges in springs near
sodium sulphate deposits. Major-ion chemistry
suggests that groundwater from multiple aquifers may
contribute to spring discharge.
Strontium isotopic compositions of lake brines and
groundwater are being examined as a potential tool to
identify and quantify aquifer inputs. Our preliminary
work shows that 87Sr/86Sr has the potential to be
sensitive to small aquifer inputs, and that the lake
brines are isotopically well mixed. In order to use
strontium isotopes to fin~erprint specific aquifer
inputs, evolution of 87Sr/ 6 Sr along groundwater flow
paths must be understood and constrained by physical
hydrogeology.
1. Introduction
The sodium sulphate deposits of southern
Saskatchewan are evaporites, dominated by mirabilite
(NaiS04• l OH20) deposited on the beds oflakes that
occupy endorheic (closed) drainage basins. Virtually
all previous workers acknowledge that groundwater
discharge, manifested by seeps and springs in and
around the lakes, supplies dissolved ions to the lakes
(Ricketts, 1888; Cole, 1926; Witkind, 1952; Grossman,
I
2
1968; Rueffel, 1970; Last and Slezak, 1987; Tompkins,
1954; McJlveen and Cheek, 1994). The ions are
concentrated by evaporation, reach saturation, and
salts, primarily mirabilite, are precipitated. Previous
workers have offered various hypotheses regarding the
nature of the groundwater discharging at the deposits
and the ultimate source of dissolved ions, but none of
the hypotheses have been tested.
Our initial focus has been on testing the hypotheses
presented by previous workers regarding groundwater
flow path and solute source in order to identify specific
aquifer inputs to, and groundwater seepage from, the
lake systems. These are key elements in the mass
balance calculations developed and used by Wood and
Sandford (l 990), Sanford and Wood (1991), and
Donovan ( 1994). A similar approach to mass balance
will be a principal component of a quantitative model
for the genesis of southern Saskatchewan sodium
sulphate deposits.
In Kelley et al. (1998), we showed that the hydrogen
and oxygen isotopic compositions for groundwater
associated with sodium sulphate deposits are similar to
those reported by McMonagle ( 1987) for typical
shallow (<200 m) Saskatchewan groundwater, and are
very different from the H and O isotopic compositions
of deeper Paleozoic brines reported in Rostron et al.
( 1998). We concluded from the isotopic results that
groundwater discharging at sodium sulphate deposits is
not in communication with deep Paleozoic brines,
contrary to the hypothesis of Grossman ( 1968), which
is based on the spatial coincidence of sodium sulphate
deposits and major structures in the Devonian Prairie
Evaporite.
Major.ion composition of waters sampled by Kelley et
al. ( 1998) plotted midway between shallow intertill
aquifers and the uppermost regional bedrock aquifer
(Cretaceous Judith River Formation) perhaps implying
mixing of waters from shallower and deeper aquifers.
This paper discusses progress in using strontium
isotopes as a tool to further identify and quantify the
source(s) of dissolved ions that are discharged into
saline lake basins in southern Saskatchewan.
Partially funded by the Saskatchewan Strategic Initiatives Fund.
Department of Geological Sciences, University of Saskatchewan, l 14 Science Place, Saskatoon, SK S7N 5E2.
208
Summary of Investigations 1999, Volum e 2
2. Strontium Isotopes
3. Methods
Attempts at solute source identification are often
frustrated by the non-conservative nature of most
solutes. For example, precipitation of minerals from
lake water removes solute(s) from the lake water and
complicates the recognition of chemical signatures of
contributing ground or surface waters.
In order to evaluate the utility of Sr isotopes as tracers
of solute source, lake brine, spring discharge, and
groundwater from shallow wells were sampled at five
lakes with documented sodium sulphate resources
(Figure I). The well samples are generally from depths
of 5 to 30 m in domestic or stock (mostly flowing)
wells within a few hundred metres of the respective
lake. Exceptions include a municipal well that is -50 m
deep on the south shore of Chain Lake and the
Whiteshore Lake wells, both of which sample the
Judith River Formation, a regional upper Cretaceous
aquifer that is over 150 m below surface at Whiteshore
Lake.
For light elements such as hydrogen and oxygen, the
large difference in relative mass among their respective
isotopes gives rise to mass dependent isotope
fractionation during chemical reactions in nature. The
relative mass difference between isotopes of heavier
elements, such as strontium, is small, so that mass
dependent isotope fractionation does not occur to any
great extent. Thus, the removal of Sr from water as a
result of mineral precipitation does not change the
isotopic composition of the dissolved Sr. In the context
of our study of sodium sulphate deposits, we expect
87
Sr/86 Sr to act as a tracer that is conservative, at least in
the Jake waters.
Strontium isotopes have been used by other workers as
natural flow tracers (Banner et al., 1989; Musgrove
and Banner, 1993). In general, the isotope ratios of
different solute sources are reflected in contrasting
ratios in groundwaters that traverse different flow paths
(Johnson and de Paolo, I 997a). Recently, however,
several workers have recognized that 87 Sr!86Sr can
change dramatically along groundwater flow paths, due
to water-rock interaction (Bullen and Kendall, 1998;
Johnson and DePaolo, I 997a, J 997b; Bullen et al.,
1996).
The 'spring' samples are from springs or seeps that
discharge near the lakeshore, generally at or above lake
level. The discharge water immediately mixes with the
lake brine.
Lake brines were sampled in locations remote from
spings and seeps. Where more than one sample was
collected from a lake (Vincent, Grandora, and
Whiteshore ), the sampling locations are up to several
kilometres apart, in order to capture any compositional
heterogeneity. Direct precipitation and surface runoff
(primarily snowmelt), both of which are expected to be
minor contributors to the strontium isotopic
composition of the lake brine, have not been sampled.
Water samples collected for measurement of Sr isotope
composition and Sr and Ca concentrations were filtered
in the field using 0.45 µm filters, acidifed to pH=2, and
stored in acid-cleaned HOPE bottles. Strontium was
purified using conventional cation chromatography.
Mass spectrometric analysis was performed on a
Finnigan Mat 261 thermal ionization mass
spectrometer using a
multidynamic peak hopping
routine in the Isotope Laboratory
of the Department of Geological
Sciences, University of
Saskatchewan. External precision
for the isotopic measurements is
±0.00002 based on multiple
52'
analysis of the SRM 987 SrC03
standard which yielded 0.71026
±0.00002 (2cr), during the course
of this study. Sr and Ca
concentrations were performed
51 "
by ICP-AES at the Saskatchewan
Research Council.
4. Results
·, ,,
·-..,
~
,\
,. .
····~.. .
'
•. :
I
'\
1•
49"
•.•,, •
"'1 .. ', .', . . •• ...,
!>· ·f. , '"\;'
17:1o~
, ----:-::--,;,.;..~..:_....:..,~;__;;,_,:;.:,.;~~;....-...;........:~:,_-~.;..J 49·
l 09"
l 08'
l Ot'
l 06''
l 05"
l 04
103'·'
102"
Figure 1 - Locations ofsodium sulphate deposits sampled in this study are in black
te.xt; -k=sodium sulphate mine/plant; and ()"'potassium sulphate plant.
Saskatchewan Geological Survey
Results for strontium isotopes are
summarized in Figure 2. Each of
the three lake brines that were
sampled in multiple locations
showed little spatial variation in
s·:sr/&6Sr. This indicates that the
lake brines are isotopically well-
209
mixed, despite being exceedingly shallow and dense.
Figure 2 also shows the affect of calcium precipitation
on lake water Sr/Ca ratios. Calcite and gypsum
discriminate against Sr during precipitation (Kushnir,
1984) causing the lake water to increase in Sr/Ca ratio.
For example, the 1000 (Sr/Ca ratio) of Vincent Lake
brine samples are all in excess of nine while the 1000
(Sr/Ca ratios) in the discharge spring waters (that have
87
Sr/86 Sr ratios similar to the lake brine) are about four.
This is indicative of in situ precipitation of calcite
and/or gypsum within Vincent Lake. Therefore, lake
brines with 87Sr/86Sr ratios similar to those of aquifer
inputs, but higher Sr/Ca ratios, suggest Ca-mineral
precipitation.
The 87Sr/ 86Sr for groundwater discharging from springs
and seeps near the shores of lakes was generally
similar in isotor.ic composition to the Jake brine. For
examfcle, 87Sr/8 Sr in Grandora Lake brine (mean
87
Sr/8 Sr of0.70824) is virtually identical to that of
discharge spring waters (0.70830). The 87Sr/86Sr for
Vincent Lake brine (0.70855) is close to and bracketed
by that of two discharge springs (0. 70836 and
0.70938).
One notable exception is Chain Lake, where
groundwater discharging into the lake from one site
had lower 87Sr/86 Sr ratios than the lake brine.
5. Discussion
In general, shallow groundwater sampled from wells
near the lakes had lower 87Sr/86 Sr ratios than either lake
brine or groundwater discharging from springs. Again,
Chain Lake is the exception. The 87 Sr/86Sr of
groundwater sampled from a spring that discharges
directly into the lake is lower than the lake water, and
very close to that of groundwater sampled from a
nearby municipal well that is -50 m deep. The 87 Sr/86Sr
ratio of water sampled from a domestic well, in which
the water level is approximately equal to the level of
the Jake, was intermediate between the compositions of
the Jake brine and the spring discharge.
Except for Chain Lake, 87Sr/86 Sr ratios for the lake
brines were similar to that of groundwater sampled
from springs and seeps that discharge directly into the
lakes. The discharge waters had generally higher
87
Sr/86Sr ratios than groundwater sampled from intertill
aquifers at depths of 5 to 30 m. We offer two possible
explanations for this observation.
First, there may be no connection between the
groundwater discharging through springs and the
shallow groundwater sampled in wells near sodium
sulphate deposits. The spring discharge may be the
result of very shallow local flow cells that are charged
by precipitation and snowmelt from the highlands
around the lake basin. We consider this hypothesis
unlikely because the springs flow year-around, and
some have flowed at the same location for generations,
based on the recollections of
landowners and deposit
descriptions of Cole ( 1926) and
Tompkins (1954). Shallow local
flow cells seem unlikely to be the
source of such persistent
discharge over such long periods
of time.
Although the Sr isotope systematics of Chain Lake are
at first glance more complicated than the other lakes,
the utility of using Sr isotopes as a sensitive tracer of
the relative contributions of water and salts from
specific aquifers is amply demonstrated.
0.7105
0 .7100
0 .7095
0 .7090
Ch
~ 0.7 085
ti)
I.
0.7080
0 .7075
0 .707 0
0 .7065
•
•
• • ••
-
411
•
An alternative explanation is that
the discharge springs are
connected to the larger and
deeper flow systems that are
sam~led by the wells, but that
87
Sr/ 6Sr undergoes considerable
evolution due to rock (till)-water
interaction as the water moves
toward the discharge springs.
Johnson and DePaolo 997a)
used the evolution of 8 Sr/86Sr
along flow paths to infer fluid
flow rates and preferential flow
paths as well as solute source.
V
The anomalous results from
Chain Lake may be a reflection of
0.7060 L_--------- - - -- ~ - - - - - - -- - -~
14
unusual hydrogeological
12
10
conditions. Landowners report
1000(Sr/Ca molar ratio)
that abundant potable
Figure 2- Plot of 17Sr/ 66Sr vs. JOOO(Sr/Ca molar ratio). The number in the lower right groundwater, at depths of IO to
of each box in the legend indicates the number ofsamples analyzed from thaJ class.
I 00 m, exists west of the lake, but
210
Summary of Investigations 1999, Volume 2
no potable groundwater has been found for several
kilometres east of the lake. The large variations seen
thus far in Sr isotope compositions indicates that there
is sufficient isotopic sensitivity to be of use in
elucidating the nature of this potentially unusual flow
regime.
6. Conclusion
Strontium isotopes hold promise for fingerprinting
aquifer inputs to the saline lake basins that host sodium
sulphate deposits. However 87 Srr86Sr must be examined
in the context of flow-path evolution, constrained by
physical hydrogeology, to be ultimately useful in
identifying solute source(s) and quantifying aquifer
input(s) to the lake basins that host sodium sulphate
deposits.
7. References
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(1996): Kinetic and mineralogic controls on the
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- ~ - -_ (1997b) : Rapid exchange effects on
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211
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212
Summary of Investigations /999, Volume 2