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 Invest igations 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/ 6Sr 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,

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.

I Partially funded by the Saskatchewan Strategic Initiatives Fund. 2 Department of Geological Sciences, University of Saskatchewan, l 14 Science Place, Saskatoon, SK S7N 5E2.

208 Summary of Investigations 1999, Volume 2

2. Strontium Isotopes 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.

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 87Sr/86Sr 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 87Sr!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).

3. Methods In order to evaluate the util ity 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.

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

52'

51 "

I

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 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 by ICP-AES at the Saskatchewan Research Council.

4. Results

·-.., ,\ ,. . ' \ ·, ,, ····~.. . •. : '

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-

49" 1 • ~ •.• ,, • "'1 .. ',.', .. •• ... , !>· ·f. , '"\; ' 17:1 o~,,----:-::--,;,.;..~..:_....:..,~;__;;,_,:;.:,.;~~;....-...;........:~:,_-~.;..J 49·

l 09" l 08' l Ot' l 06'' l 05" l 04 103'·' 102"

Figure 1 - Locations of sodium sulphate deposits sampled in this study are in black te.xt; -k=sodium sulphate mine/plant; and ()"'potassium sulphate plant.

Saskatchewan Geological Survey 209

mixed, despite being exceedingly shallow and dense.

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 87Sr/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/86Sr ratios than the lake brine.

In general, shallow groundwater sampled from wells near the lakes had lower 87Sr/86Sr ratios than either lake brine or groundwater discharging from springs. Again, Chain Lake is the exception. The 87Sr/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 87Sr/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.

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 ~ 0.7085 ti)

I.

0.7080

0 .7075

0 .7070

0 .7065

• • • • •• - 411

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 87Sr/86Sr 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.

5. Discussion Except for Chain Lake, 87Sr/86Sr 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 87Sr/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.

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 87Sr/ 6Sr undergoes considerable evolution due to rock (till)-water interaction as the water moves toward the discharge springs. Johnson and DePaolo V 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.

The anomalous results from 0.7060 L_--------- ---- ~ - - - - - --- - -~ Chain Lake may be a reflection of

unusual hydrogeological conditions. Landowners report that abundant potable groundwater, at depths of IO to

10 12 14

1000(Sr/Ca molar ratio)

Figure 2- Plot of17Sr/ 66Sr vs. JOOO(Sr/Ca molar ratio). The number in the lower right of each box in the legend indicates the number of samples 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 87Srr86Sr 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 Banner, J.L, Wasserburg, G.J., Dobson, P.F.,

Carpenter, A.B., and Moore, C. H. (l 989): Isotopic and trace element constraints on the origin and evolution of saline groundwaters from central Missouri; Geochim. Cosmochim. Acta, v53, p383-398.

Bullen, T.D, Krabbenhoft, D.P., and Kendall, C. (1996): Kinetic and mineralogic controls on the evolution of groundwater chemistry and 87Sr/86Sr in a sandy silicate aquifer, northern Wisconsin, USA; Geochim. Cosmochim. Acta, v60, pl 807-1821.

Bullen, T.D. and Kendall, C. (1998): Tracing of weathering reactions and water tlowpaths: A multi-isotope approach; in Kendall, C. and McDonnell (eds.), Isotope Tracers in Catchment Hydrology, Elsevier Science B.V., p611-646.

Cole, L.H. (1926): Sodium sulphate of western Canada-Occurrence, uses, and technology; Can. Dept. Mines, Bull. 646, l 55p.

Donovan, J.J. (1994): On the measurement ofreactive mass fluxes in evaporative groundwater-source lakes; in Renaut, R.W. and Last, W.M. (eds.), Sedimentology and Geochemistry of Modem and Ancient Saline Lakes, SEPM Spec. Publ. 50, p33-50.

Grossman, I.G. (1968): Origin of the sodium sulfate deposits of the northern Great Plains of Canada and the United States; U.S. Geol. Surv., Prof. Pap. 600-B, pB104-BI09.

Johnson, T.M. and DePaolo, D.J. (I 997a): Rapid exchange effects on isotope ratios in groundwater systems 2. Flow investigations using Sr isotope ratios; Water Resour. Resear. , v33, pl97-209.

- ~--_ (1997b) : Rapid exchange effects on isotope ratios in groundwater systems I .

Saskatchewan Geological Survey

Development of a transport-dissolution-exchange model; Water Resour. Resear., v33, pl 87-195.

Kelley, L.l., Smith, J.J., and Holmden, C. (1998): Stable isotope and chemical composition of groundwater associated with sodium sulphate deposits, southern Saskatchewan; in Summary of Investigations 1998, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 98-4, pl36-141.

Kushnir, K. (1984): The coprecipitation of strontium, magnesium, sodium, potassium, and chloride ions with gypsum: An experimental study; Geochim. Cosmochim . Acta, v44, p1471-1482.

Last, W.M. and Slezak, L.A. (1987): Sodium sulphate deposits of western Canada: Geology, mineralogy, and origin; in Gilboy, C.F. and Vigrass, L. W. (eds.), Economic Minerals of Saskatchewan, Sask. Geo I. Soc., Spec. Pub I. No. 8, p 197-205.

Mcllveen, S. and Cheek, R.L. (1994): Sodium sulfate resources; in Carr, D.D. (ed.), Industrial Minerals and Rocks, 6th ed., Soc. Mining, Metal. Explor., pll29-1158.

McMonagle, A.L. (1987): Stable isotope and chemical compositions of surface and subsurface waters in Saskatchewan; unpubl. M.Sc. thesis, Univ. Sask., 108p.

Musgrove, M. and Banner, J.L. (1993): Regional groundwater mixing and the origin of saline fluids: Midcontinent, United States; Sci., v259, pl877-1882.

Ricketts, L.C. ( 1888): Annual Report of the Territorial Geologist, Wyoming.

Rostron, B.J., Holmden, C., and Kreis, L.K. (1998): Hydrogen and oxygen isotope compositions of Cambrian to Devonian formation waters, Midale area, Saskatchewan; in Proceedings, 8th International Williston Basin Symposium, Sask. Geol. Soc., Spec. Publ. No. 14, p267-273.

Rueffel, P.G. ( 1970): Natural sodium sulfate in North America; Northern Ohio Geological Society, 3rd Symposium on Salt, v I , p429-45 l.

Sandford, W. and Wood, W.W. (1991): Brine evolution and mineral deposition in hydrologically open evaporite basins; Amer. J. Sci., v291, p687-710.

Tompkins, R.V. (1954): Natural sodium sulphate in Saskatchewan; Sask. Dep. Miner. Resour., Rep. 6, 7lp.

Wood, W.W. and Sandford, W. (1990): Groundwater control of evaporite deposition; Econ. Geol. , v85, pl226- 1335.

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Witkind, I.J. (1952): The localization of sodium sulfate deposits in northeastern Montana and northwestern North Dakota; Amer. J. Sci., v250, p667-676.

Summary of Investigations /999, Volume 2