Journal of Geochemical Exploration 89 (2006) 170 – 173
www.elsevier.com/locate/jgeoexp
Bromine and stable isotopic profiles of formation waters from
potash mine-shafts, Saskatchewan, Canada
G.K.S. Jensen a,⁎, B.J. Rostron a , M.J.M. Duke b , C. Holmden c
a
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E3
b
SLOWPOKE Nuclear Reactor Facility, University of Alberta, Edmonton, AB, Canada, T6G 2E3
c
Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada, S7N 5E2
Received 16 August 2005; accepted 24 November 2005
Available online 20 March 2006
Abstract
Sixty-five inflow samples from access shafts were collected at three separate potash mines in order to construct three 1000 m
deep hydrochemical profiles. Bromine concentrations, and δD and δ18O stable isotopic compositions, increase with depth in each
case. Measured isotopic ratios have not changed in 15+ years since the mine-inflows were first sampled, implying little change in
the hydraulic regimes at the mines over time. However, the bromine concentrations are typically a factor of five lower than
previously reported. Newer analytical techniques have improved the accuracy, precision and resolution of the hydrochemical
profiles. Results indicate that the salinity of the inflow waters originated as mixtures of evaporatively concentrated seawater,
meteoric water, and brine derived from halite dissolution. Extremely concentrated brines (TDS N 525 g/L) were found at the Cory
and Allan potash mines some 55 km apart, but their role in the paleohydrogeology of the basin remains uncertain.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Potash; Brine; Isotopes; Bromide
1. Introduction
The Prairie Evaporite Formation in the Elk Point
Basin in Saskatchewan, Canada contains some of the
largest deposits of potash in the world. Problematic
mine-level flooding has threatened the longevity of
potash mines in Saskatchewan almost since their construction (Wittrup et al., 1987). Therefore, it is essential
to determine the origin of inflows to remediate them or to
plan preventative maintenance. Previously, hydrochemical tracers such as chloride or major ions have had
⁎ Corresponding author.
E-mail address: [email protected] (G.K.S. Jensen).
0375-6742/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.gexplo.2005.11.071
limited success because the inflows pass through, and
dissolve, evaporites of the Prairie Evaporite Formation.
Isotopic tracers (e.g., δ18O, δD) have met with more
success at fingerprinting waters in the potash mines
(Wittrup et al., 1987; Wittrup and Kyser, 1990) and oil
fields in the basin (Rostron and Holmden, 2000). In
addition, bromine has worked well in oilfield applications (Iampen and Rostron, 2000), however, the application of bromine in potash mine-inflow fingerprinting
was hampered by analytical interferences.
A recently developed analytical technique using
Epithermal Neutron Activation Analysis (ENAA) for
determining bromine concentrations in brines (Duke and
Rostron, in press) facilitates the re-examination of
G.K.S. Jensen et al. / Journal of Geochemical Exploration 89 (2006) 170–173
Fig. 1. Location of the sampled potash mines.
bromine as a tracer for fingerprinting inflows in potash
mines. That, combined with newer continuous flow
stable isotope methods that allow measurements on
small (b 5 mL) samples, and the more than 15+ year
elapsed time since the pioneering fingerprinting work by
Wittrup and Kyser (1990), prompted a re-examination of
the chemistry of the inflows into potash mine shafts in
Saskatchewan. Thus, the primary objective of this study
was to obtain new hydrochemical and stable isotope
vertical-profiles at the Potash Corporation of Saskatchewan (PCS) Rocanville, Cory, and Allan potash mines in
Saskatchewan (Fig. 1), and to compare these results to
previously published data.
2. Methods
Sixty-five inflow samples from access shafts were
collected at three separate mines (Fig. 1) in order to
construct three 1000 m deep hydrochemical profiles for
each mine. Sample points were identified by visual
identification of water seeping into the dry mine shafts.
Samples were collected into sealed plastic containers,
and the depth below ground was calculated from the
amount of cable dispensed to support the elevator car.
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Field filtration was done using 0.45 μm PES filters to
remove any suspended solids. Oxygen and hydrogen
stable isotopes were determined at the University of
Saskatchewan. An aliquot of brine was injected directly
into a continuous flow (CF) Delta plus XL isotope ratio
mass spectrometer. Isotopic values were reported in delta
(δ) notation as 18O/16O and D/H relative to Vienna
Standard Mean Ocean Water (VSMOW). Isotopic
measurements have an uncertainty of ± 3‰ and ± 0.3‰
for δD and δ18O, respectively. ENAA was completed at
the SLOWPOKE Reactor at the University of Alberta to
determine the sodium, chloride, bromide and iodine
concentrations of each sample. Bromide concentrations
varied from 17.9 to 6520 mg/L with uncertainties ranging from ± 3.9% and ± 0.6%, for this concentration range.
Ion chromatography was not used for bromide analysis
as a dilution factor of ∼10,000 would have been required, due to the high TDS contents of the samples.
Major ions, trace metals, and alkalinity data were determined at a commercial laboratory in Edmonton, Alberta
using ICP-OES, ICP-MS, and titration techniques. Analytical uncertainties for the commercially determined
ions varied with the different techniques, and details are
available upon request of the authors.
3. Results and discussion
Each of the 65 samples collected for this study were
fully analyzed for their major and minor ion and
isotopic composition. Space limitations preclude a
display and discussion of most of these data, instead, a
comparison between data collected previously by
Wittrup et al. (1987) and the newly collected data is
provided. The two most important parameters are the
bromine and stable isotopic compositions of the potash
shaft inflows.
Bromine concentrations versus depth for the Cory,
Allan, and Rocanville potash mines are shown in Fig. 2.
Bromine increases with depth at all three mines and
Fig. 2. Bromine concentrations versus depth for Cory, Allan and Rocanville potash mines. Note that bromine is a logarithmic scale.
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G.K.S. Jensen et al. / Journal of Geochemical Exploration 89 (2006) 170–173
Fig. 3. δD (VSMOW) versus depth for Cory, Allan and Rocanville potash mines.
range from approximately 2 mg/L in near-surface aquifers (58 m depth), to a maximum of approximately
5500 mg/L in Devonian carbonate aquifers (1025 m
depth) just above the mining horizon. Inflow samples
with bromine concentrations of 5500 mg/L are found at
the Cory and Allan mines some 55 km apart. These
calcium-chloride brines are the most concentrated
waters found in the Williston basin to date, with total
dissolved solids of 525,000 mg/L. They contain
elevated chloride (350,000 mg/L), calcium
(136,000 mg/L), and magnesium (22,000 mg/L), yet
relatively low sodium (6400 mg/L) and their widely
spaced nature implies that they are more than just an
isolated occurrence at each mine. The origin and
implication of these extremely bromine-rich brines is
the subject of on-going research.
There are significant differences between the newly
determined bromine profiles and those previously
published. Firstly, there are more samples in these new
profiles because the analytical techniques require less
sample (i.e. 0.25 mL), hence smaller inflows can be
analyzed. This gives our new profiles a finer resolution.
Secondly, at any given depth the bromine concentrations
in this study are significantly lower than measured in the
previous study, with the difference becoming larger at
greater depths, that is, higher concentrations. This difference is attributed to the analytical method by Wittrup
et al. (1987) (ion selective electrode) being affected by
interference with increasingly high chloride concentrations with depth increases. In some cases the bromine
concentrations obtained using ENAA in this study
were an order of magnitude less than previously determined. Thirdly, samples with such elevated bromine
(N5000 mg/L) were not previously reported at the Allan
mine.
Isotopic compositions of the mine inflows are illustrated by means of plots of δD versus depth for each
mine (Fig. 3). Isotopic compositions of δ18O are not
shown because they follow a similar pattern to δD and
previous work has demonstrated a linear relationship
between δD and δ18O for the Williston Basin (Wittrup
and Kyser, 1990).
At all three mines, δD compositions in the mine
inflows increase with depth (Fig. 3). At shallow depths,
water samples have an isotopic composition (− 160 ‰)
similar to that of meteoric water in Saskatchewan
(Wittrup et al., 1987). At the greatest depths in the
mines, isotopic compositions in Devonian strata reach
− 45‰, similar to that reported previously (Wittrup and
Kyser, 1990; Rostron and Holmden, 2000). Variations in
the isotopic compositions of the inflows as a function of
depth are interpreted to reflect differences caused by the
origin and/or mixing history of the fluids in different
aquifers at each level of the shaft. Using these data, an
isotopic fingerprint has been obtained for the aquifers
above the mine providing excellent tracers of the origin
(s) of the shaft inflows.
Current isotopic compositions versus depth compare
very closely to those published previously (Wittrup et al.,
Fig. 4. Na / Br versus Cl / Br (mg/L) of shaft inflow samples.
G.K.S. Jensen et al. / Journal of Geochemical Exploration 89 (2006) 170–173
1987; Wittrup and Kyser, 1990), with a few differences.
Firstly, as mentioned above, these isotopic profiles of
this study are more detailed than previously shown as a
result of an increased number of samples made possible
by the CF analytical techniques. Continuous flow can
measure the isotopic composition on small (b2 mL)
brine samples, and thus small shaft inflows can be included in the vertical profiles. This has resulted, for
example, in data for the deep aquifers above the Cory and
Allan mines (Fig. 3A, B) and in other places in the
profiles that were not available previously. Secondly,
with the exception of data from 600 to 800 m in
Rocanville mine, the isotopic data from 1987 overlap
those collected in this study. This indicates there has
been no change over time in the isotopic composition of
the shaft inflows. Observed differences in isotopic
compositions at the Rocanville mine are attributed to
sample collection at or near pumping stations where
inflows are collected for pumping to the surface, and
hence are not considered representative.
4. Origin of formation water
Revised bromine concentration data from the 3 mine
shafts provide additional insight into the origin of the
formation waters in the basin. A plot of Na / Br vs. Cl / Br
(Fig. 4) illustrates a linear relationship in the data but
with a wide variation compared to seawater. Variations in
the Na / Br ratios of the samples can be used to infer the
origin of the salinity in the samples (Walter, 1990).
Samples from Rocanville all plot above and to the right
of seawater on the Na / Br vs. Cl / Br plot, indicating the
salinity in these samples originated from the dissolution
of halite. In contrast, samples from Cory and Allan, plot
over a wide range on the Na/Br vs. Cl/Br plot, both above
and to the right of seawater (indicating dissolution of
halite) and below and to the left of seawater indicating
the presence of an evaporated end-member brine. Samples with Br N 5000 mg/L found above the potash mining
horizon at Cory and Allan are thought to represent the
evaporated end-member brine. As mentioned above, the
recognition of the evaporatively concentrated end-member brine at the Allan potash mine, and the fact that it is
173
found 55 km away from the previously known
occurrence at the Cory mine, suggests that this formation
water is more widespread than previously known. This
indication bears further investigation.
5. Conclusions
Sixty-five samples of inflows into access shafts at
PCS Cory, Allan, and Rocanville potash mines were
analyzed and used to construct hydrochemical profiles
through the aquifers in the Williston Basin. There have
been no significant change in the isotopic compositions
of the shaft inflows in the 15+ years since they were
initially studied, indicating that the origin of the inflows
have not changed. Newer analytical techniques for both
bromine (ENAA) and stable isotopes (CF) enabled the
analysis of smaller samples than in previous studies in
creation of higher resolution depth profiles which were
both more accurate and precise for bromine. Sampling
identified an evaporatively concentrated brine at the
Allan mine, previously only recorded at the Cory mine.
In addition to stable isotopes of oxygen and hydrogen,
bromine concentration data can be used to better define
the source of mine-level flooding brines.
References
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and sodium in highly saline formation waters by Epithermal NAA,
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