WATER RESOURCES RESEARCH, VOL. 27, NO. 10, PAGES 2543-2554, OCTOBER 1991

Hydrogeochemistry of a Clayey Till 1. Spatial Variability

C. KENT KELLER

Department of Geology, Washington State Universit3,, Pullman

G. VAN DER KAMP

National Hydrology Research Institute, Saskatoon, Saskatchewan

JOHN h. CHERRY

Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario

The spatial variability of hydrogeochemical processes was studied within an 18-m-thick unit of clayey till beneath a flat prairie landscape. Models of major element mass transfer, constrained by chemical, mineralogical, and isotopic measurements on waters and solids, were developed for two end-member water quality types occurring within the unit. The observed pattern of chemical variation, which is predominantly lateral on a scale of tens of meters, is caused by microtopographically controlled differences in the ratio of vertical water flux to oxidation rate associated with depression- focused recharge. Such variation is to be expected where dynamic flow regimes develop in thin surficial tills containing chemically reduced constituents. Observed depletions of major elements from oxidized zones of the till support the mass transfer models and demonstrate that the time scales of persistence of hydrogeochemical variability in the till and of water quality evolution in the aquifer beneath range from 10 4 tO 10 5 year. Na and S concentrations in such aquifers will depend primarily on ratios of influxes of end-member water quality types from the till above, while Ca, Mg, and inorganic C concentrations will remain high as long as partial pressures of CO2 in the till remain high. CO2 generation in the till is discussed in the companion paper (Keller, this issue).

INTRODUCTION

In the add and semiarid parts of the Western Glaciated Plains of North America [Lennox et al., 1988] the quality of much shallow groundwater is marginal to poor, and soil salinization has affected or threatens substantial percentages of otherwise arable lands. For these reasons it is important to understand the processes controlling the generation and distribution of dissolved mineral mass in the shallow subsur- face. From the perspective of chemical hydrogeology the problem is prediction of the occurrence of glacial-deposit groundwaters termed "type III" by Freeze and Cherry [1979] which are characteristically high in sodium, calcium, magnesium, bicarbonate, and sulfate [Meyboom, 1966; Roz- koaiski, 1967; Cherry at al., 1971; Cherry, 1972; Davison and Vo•hof, 1978; Moran et al., 1978; Wallick, 1981; Schwartz et a/.•19811.

An important step in understanding the genesis of type III waters was taken by Hendry et al. [1986], who developed a model for sulfate generation and distribution in a deposit of clayey till in southern Alberta. Hendry et al. [1986] found hig.•sulfate waters in weathered till overlying low-sulfate waters in unweathered till and explained this pattern as due to oxidation of organic sulfide by an infusion of atmospheric oxygen into the weathered till during a period of water table decline, corresponding to the dry Altithermal climatic inter- val which persisted in this region for several thousand years of Holocene time [Heusser, 1973; Sauchyn, 1990]. Useful as

Copyright 1991 by the American Geophysical Union. Paper number 91WR01586. 0043.1397/9 !/9 ! WR-01586505.00

it is, the model developed by Hendry et al. [1986] pertains to a low-permeability system which is hydrologically relatively static, in which recharge flows and net transport of dissolved S and other major elements are negligible on a time scale of 10 4 year. For this reason the model does not address water quality in the many Glaciated Plains aquifers which underlie and are recharged through tills [Lennox et al., 1988].

This study concerns hydrogeochemical processes in a relatively permeable, hydrologically dynamic till deposit through which an intertill aquifer is recharged. Previous work at the study site, named Dalmeny, has shown strong lateral variability of sulfate concentration and downward groundwater flow, with recharge to the underlying aquifer of low-sulfate water focused beneath subtle depressions [Keller et al., 1986, 1988; Keller and van der Kamp, 1988]. Study of aquifer water quality indicates that its chemistry reflects a simple mixture of waters from the till, and the rate of recharge to the aquifer as a whole is consistent with depres- sion-focused recharge rates through the till at the Dalmeny site [Fortin et al., 1991]. Thus the hydrogeochemical pro- cesses occurring within the till at Dalmeny control the water quality of the underlying aquifer. This is very likely true for many of the intertill aquifers of the glaciated interior plains. Therefore an understanding of these processes, and their spatial distribution in relation to depression-focused recharge, is essential to an understanding of the quality of much of the groundwater supply of the region. Elucidation of these pro- cesses and spatial distributions was the objective of this study.

Our strategy was to observe the chemical and isotopic composition of both aqueous and solid phases and to limit the domain of observation to a very small region, 300 m

2543

2544 KELLER ET AL.' HYDROGEOCHEMISTRY OF A CLAYEY TILL, 1

52"

\ I I

/ /

/

106" 106' 15'

(:::::::• Extent of Dolrt•ny Aquifer c.'" '" "• Extent of Tyner AqtJ•fer

Warm, Site

Site

\ \

io7ooo '

/ / \ \

/ /

/ /

! ,

106"45'

SASKATOON •

L I I I0 km

I 106'

52'30'

sZ'15'

500 -

450-

400 -

w

TILL •

? ø T••iLL SAND 6

BEDROCK

SILT t• CLAY

•' , DALMENY: AQUøIFER'

z

-JI-

6 eau}

,

SUTHERLAND

8

GROUP

I : ! ! , I

b o km 5

Fig. 1. Hydrogeologic setting. (a) Location of the Dalmeny site. (b) The Dalmeny aquifer (areal extent indicated) is overlain by and recharged via the clayey calcareous till deposit of this study, which is designated Saskatoon Group.

square by 18 m thick, of the lithologically uniform till at Dalmeny. This enabled development of well-constrained models of the mass-transfer paths underlying the locally distinct hydrochemistries, as well as mass balances on fluxes of several constituents from the till to the aquifer in recharg- ing groundwater. The overall consistency of the mass trans- fer models and the mass fluxes supports our interpretations

and yields general insights regarding hydrogeochemical e•- olution of till-aquifer systems in the region.

One constituent, organic carbon, was found to be anom- alously distributed in the till. This anomaly is related to the consistently high PCO 2 levels observed in the Dalmeny till waters. These issues are the focus of the companion paper [Keller, this issue].

KELLER ET AL.' HYDROGEOCHEMISTRY OF A CLAYEY TILL, 1 2545

HYDROGEOLOGIC SETTING

The study site is situated on a broad semiarid recharge u•and between the North and South Saskatchewan rivers {Figure ! a). Regionally, the glacial deposits consist of clayey tills and stratified drift of the Saskatoon and Sutherland groups [Christiansen, 1968] underlain locally by preglacial- valley sands and gravels of the Empress Group and else- where by Upper Cretaceous sediments of the Lea Park, Judith River, and Bearpaw Formations. Whitaker and Chris- tiansen [1972] provide an overview of the geologic history ,and stratigraphy; pertinent aspects are shown in Figure 1 b. This study is concerned with the clayey calcareous till of the Saskatoon Group which occurs between the ground surface and the intertill Dalmeny aquifer (Figure 1 b). Keller et al. [1986] identified the till at the Dalmeny site as Floral For- marion, deposited prior to 38,000 years B.P. in early Wis- consinan time [Christiansen, 1968]. The area was last degla- ciated about 11,500 years B.P. [Christiansen, 1979], after •vhich a relatively dry Altithermal interval persisted from about 9,000 to 3,000 years [Sauchyn, 1990].

Keller et al. [ 1986] found the hydraulic conductivity of the Floral till deposit at Dalmeny to be isotropic and about 5 x I0 -9 m s -• or approximately 2 orders of magnitude greater than till matrix material. They attributed this unit-scale permeability to the presence of a network of fractures which is hydraulically continuous throughout the unit's 18-m thick- hess. This permeability is substantially larger than those of thicker deposits: Hendry [1988] reported 10 -•ø m s -• for till in southern Alberta, and Keller et al. [1989] reported less than 10 -•ø m s -• for till at the Warman site in southern Saskatchewan.

The till at Da!meny is sufficiently permeable that substan- tial recharge to the underlying aquifer occurs locally beneath the nearly flat landscape in response to the depression focusing of surface water by subtle (5-50 cm) topographic variation. The result is a patchwork of"dry" (small recharge flux) and "wet" (large recharge flux) flow regimes, differen- tiated at a plan scale of tens of meters [Keller et al., 1988]. (The terms "wet" and "dry" referred to the relative mag- nitudes of the local recharge fluxes, not to the local moisture content of the till.) Keller and van der Kamp [1988] found that in "wet" or" depression-focused recharge" regimes the till did not contain gypsum; these regimes could be mapped by their relatively low electrical conductivity (EC) as mea- mred at ground surface by a noncontacting technique. Figure 2 shows areas with apparent EC less than 30 mS m -• which correspond approximately to "wet" flow regimes where gypsum is absent. "Dry" flow regimes underlie the remaining areas.

MEASUREMENTS

Access to the subsurface was by continuous-flight, solid- stem auger drilling. The till was sufficiently stiff that 15-cm- diameter boreholes typically remained open and dry during piezometer construction and solids sampling procedures.

Groundwater. Piezometers were fabricated from PVC pil•; screened sections were placed in silica sand gravel .packs typically about 1 m long. The gravel pack interval, i.e., the effective intake interval of each piezometer, was •asured carefully downhole. Borehole annular space was f•led by tremie pipe with a mixture of dry powdered bento- 're and sand. In some cases, multiple piezometers were

lOON

GYPSUM ABSENT

(LEACHED)

Fig. 2. Dalmeny site plan. Circular symbols indicate piezome- ters, "x" symbols indicate test holes for till solids sampling. Apparent EC is less than 30 mS m -1 in the hatched areas, which correspond to the areal distribution of "leached" hydrochemical settings where S oxidation is slow relative to rate of flushing by groundwater recharge, and gypsum is absent from the till. Remain- ing areas are "unleached" settihgs.

nested in single boreholes; Keller et al. [1986] describe the procedure. In these cases the letter "A" indicates the deepest piezometer. After installation, piezometers were repeatedly bailed and allowed to recover. At each bailing, field alkalinity and p H were measured. Bailing was repeated until these measurements varied by no more than 10% and 0.1 unit, respectively, for successive bailing cycles. Water samples for major element and isotope analysis were then pumped to the surface under positive pressure, filtered through 0.45-/•m membranes, and refrigerated in the field. For many piezometers the entire procedure was repeated at intervals of 3 months to 1 year. A portion of each major element sample was acidified for cation determination; sam- ples for 834S(SO• 2-) determination were treated with sodium azide to kill any bacteria present. Inorganic solutes' in water samples were determined by atomic absorption/flame emis-

2546 K•L•R ET AL.i HYDROGEOCHF_.MISTRY OF A CLAYEY TmL, 1

,-.i ,J ,-i ,J ,J I I I I I

KELLER ET AL.' HYDROGEOCHEMISTRY OF A CLAYEY TILL, 1 2547

.4on (Na, K, Fe, Si); titration (Ca, Mg, C); and ion chroma- t0graphy (S, C1). K, Fe, and C1 contributions to ion loading ,ere less than 1% (milliequivalent basis); the results are repoged by Keller [1987] but are not discussed here. Dis- solved organic C (DOC) was determined by U-V •)xidation; aml•c• precision was approximately *-0.5 mg C/L. Values of •34S(SO•-) were determined by mass spectrom-

etry and reported using the familiar "per mil" convention:

= [Rsample _ 1 x 1000 • sample [R standard where R is the abundance ratio 34S/32S and •he standard is Canyon Diablo Troilite. Analytical precisiori was -*0.5 per

Till solids. Continuous vertical profiles of till material were collected by slowly screwing the leading auger through the desired depth interval, then withdrawing the augers without rotation and slicing a piece of till from each "wrap" of the ax•ger flights. The sum of these pieces constituted the sample for the given interval [Keller and van der Karnp, 1988]. Samples were immediately triple wrapped in plastic film,

,

then oven dried (103øC) the same night, crushed to pass a 2-ram screen, and homogenized using a mechanical sample splitter.

Total S was determined by titration using a Leco induction fm•ace; S present as •u!fate, or S(SO4), was determined by m chromatography on distilled water extracts using a 20:1 water:s0il ratio and 18-hour extraction period with gentle agitation. Relative anaiytica! precisiori for both determina- tions was approximately -*10%. This procedure closely foUowed that of Hendry et al. [ 1986]; Keller [1987] discusses its uncertainties.

For selected sampies, • 34s was determiried on S(SO4), on gypsum crystals, and on the "reduced. S'.' extracted from unoxidized till by combustion after S(SO4) had been re- moved by repeated washing with distilled water. Ca, Mg, and inorganic C ("SIC" •o distinguish from

"SOC," below) contents of till carbonaie minerals were determined on continuous vertical profiles of samples by the •cki digestion/atomic absorption method described by Ross [1984]. Results are reported as percentages of till solids mass. Analytical precision was -*0.1%. Solid organic C IS0C) was determined by combustion, with relative ana!yt- ical precision of _* 10%. Bulk till mineralogy was determined on two samples by X ray diffraction. The dete•tion limit for a given phase was approximately 1% by mass. Exchangeable cations were determined by the method of Neal [!977], morporating corrections for calcite dissoution and reagent ½ontarnination. Cation exchange capacity (CEC) was deter- mined by barium displacement [Page et al., 1982].

PATTERN OF VARIABILITY

Results of the chemical analyses of groundwater samples are shown in Table 1. Speciation and saturation indices were calculated using WATEQF [Piummer et al., 1984]. Piezom- eter locations appear in Figure 2. The waters are divided into two groups: a "leached" group, undersaturated with respect te gypsum, and an "unleached" group, saturated with respect to gypsum. (Here "saturation" is meant in the sense Of Langrnuir [1971], i.e., -0,1 < log S1 < 0.1.) In leached settings, gypsum is not locally present in the till; in un-

7'.0-

LEACHED (34) O-

14.C , 0,0

- "]•,..•. 8(TOTAL)

8(•04) 1 _.Oxid. '•' ;cl.

0:2'0:4 •

UNLEACHED (35)

S(S04) • J••(TOTAL)

i

0.0 0.2 0.4

Wt.% $

Fig. 3. Typical depth distributions of total-S and sulfate-S con- tent of till solids in the two hydrochemical settings (see also Figure 2), adapted from Keller and van der Karnp [1988]. In the leached settings dowhward flow flushes sulfate from the till as it is generated, while in unleached settings downward flow is very small such that sulfate precipitates in the profile as gypsum.

leached se•ings, gypsum is locally present. Leached waters have sum of ions less than 80 meq/L and typically about 30 meq/L; unleached waters have sum of ions greater than 80 meq/L and typically about 150 meq/L. Sodium and sulfate concentrations are greater in unleached than in leached waters; by contrast, bicarbonate concentrations differ little if at all between the two groups.

Spatial occurrence of leached waters is within "wet"- flow-regime till where recharge is depression-focused. In these .settings the S content of the till has been almost totally depleted through most of the depth interval of oxidation as identified in auger cuttings, and very low S(SO4) levels indicate the absence of gypsum (e.g., profile 34, Figure 3). Unleached waters occur within "dry"-flow-regime till where S remains in the oxidized zone, almost completely converted to S(SO4) and precipitated there as authigenic gypsum (e.g., profile 35, Figure 3) [Keller and van der Kamp, 1988]. Figure 4 is a schematic adaptation from Keller and van der Kamp [1988] illustrating the relationship. Detailed control on the spatial distribution of gypsum was obtained by test drilling at locations marked "x" in Figure 2. Unshaded and shaded regions indicate parts of the saturated zone from which leached and unleached water samples have been obtained from the piezometers listed. In the interiors of the respective settings, as at locations 10 and 35, there is little variation in water chemistry with depth. There are depth variations associated with the margins of the settings, as near location ! [Keller and van der Kamp, 1988], but the overall pattern is of lateral rather than vertical variability of water quality on the scale of microtopographic variability, i.e., tens of meters.

Figure 5 shows the results of SOC determinations. The shapes of the leached-setting profiles are similar to the shapes of the sulfur depth profiles in leached settings, where sulfur has been depleted from oxidized till material (Figure 3). In the unleached setting, SOC values for oxidized till are smaller than for underlying unoxidized till, but the contrast is not as great as in the leached settings.

Ca, Mg, and SIC contents of carbonate minerals are shown in Figure 6. Some depletion of these elements in

2548 KELLER ET AL.: HYDROGEOCHEMISTRY OF A CLAYEY TILL, 1

•

7 I 10 35 i ! i I

. '•,. ß Wet-.•.,,,•,-.••, _"Wet" • ..... , ,

9A '. "•'?•1B ';•'"•."•':".9:'¾ •. ' I 1 C , '. 'q '2A '•:•:•':"'•'*:' ' ' ' '.." ::•3 B ::::• •.•:•:•:•:"' 3 3 A

ß aqbifer

20 m

Fig'. 4. Cross section (line of section showfi on Figure 2) simplified to illustrate how "dry" flow regimes correspond to unleached setlings while "wet" flow i'egimes (where depression-focused recharge occurs) correspond to leached settitles. Spatial hydrogeochemical variation is predominantly late. ral, occurring at the scale of the microtopography. Topographic variation is exaggerated with respect to the vertical scale by 2 times.

oxidized zones is indicated, but variaiions (1) between settings and (2) with depth in the leached setting are not as large as for S or SOC.

CONTROLLING PROCESSES

A model of hydrochemical evolution in the till must .explain the evolution of two Water types, unleached and leached, as well as their later'ally intimate association in the till. Identification of the controlling processes in each setting is approached as a reaction simulation problem, in which .hypothesized major element mass transfers are subjected to lithologic, isotopic, stoichiometric, and thermodynamic con- str•ihts [P!umrner et al., 1983]. The pai-ticular method used is a variation of that pioneered by Garrels and MacKenzie [1967]. Conceptually, we dissolve and precipitate minerals known to be present in the till, in amounts which yield the observed solute distribution. Such a simulation is thermody- namically reasonable if the .water is at or approaching equilibrium with the minerals proposed to be the solute sources. It is isotopically reasonabli if the isotopic compo- sitions of the minerals are reflected in the solutei.

Unleached waters. Processes resulting in the observed evolution of unleached waters are shown in Table 2a, where

the composition of a typical sample (25A, August 1986. Table 1) is modeled. Process 1 represents generation of the CO2 and carbonic acid which causes dolomite dissoht• and calcite precipitation (processes 2 and 4). The CO,

.

generation process itself is examined by Keller [this issue]. Processes 2 and 4 proceed approximately to equilibrium, as indicated by the slight supersaturation of the water with respect to calcite and dolomite (Table 1). Process 3 also goes to equilibrium as indicated by gypsum saturation. The sim- ilarity of •34S($O4) values for gypsum crystals (Table 3)to •34S(SO42-) values for unleached groundwaters (the hypoth- esis that the means are the same was accepted with 2% risk of type I error) supports the derivation of aqueous sulfate from gypsum. Process 5 represents the only reasonable source for the high levels of sodium in unleached waters: while exchangeable sodium is small on a mass solids basis (Table 4) it is sufficient to cause observed Na + charging. since small masses of the major elements have been flushed away in the very small downward flushing fluxes of these settings (discussion below).

The oxidation of reduced sulfur in the till to suEate aM precipitation of gypsum in till oxidized zones are evidenced by vertical profiles like 35 (Figure 3). This process is

LEACHED UNLEACHED

3.5

Depth 7 (m)

10.5

location 32 location 34

location 35 I

I 0 0.2 0.4 0.6 0 0,2 0,4 0.6

Wt. % Organic C

Fig. 5. Depth distributions of solid organic carbon (SOC) cori- tents of till samples, showing depletion of SOC in oxidized zones. Compare with Figure 3.

0

3.5

Depth 7 (m)

10.5

LEACHED (34) UNLEACHED (35)

II ji

0 1 2 3 4 0 1

Wt.%

2 3 4

Fig. 6. Typical depth distributions of the Mg, Ca, and C ("SIC") contents of carbonate minerals in the two hydrog eøchem• ical settings.

KELLER ET AL.: HYDROGEOCHEMiSTRY OF A CLAYEY TILL, 1 2549

TABLE 2a. Reaction Models

Process Ca 2+ Mg 2+ Na + HCO•- SO•-

1. Generate CO2: CH2 ¸ + 02 '-) H2CO3 ø

2. Dissolve dolomite:

15.7 Ca3tlg(CO3) 2 q- 31.4 H2CO 15.7 Ca 2+ + 15.7 Mg 2+ + 62.8 HCO•-

3. Dissolve gypsum: 29.2 CaSO4 ß 2H20 29.2 Ca2* + 29.2 SO42-

4. Precipitate calcite: 24.1 Ca 2+ + 48.2 HCO•- --• 24.1 CaCO 3 + 24.1 H2CO3 ø

5. Cation exchange: 16.6 NaX + 8.3 Ca 2+ • 16.6 Na* + 8.3 CaX

Observed (25A, August 1986)

15.7 15.7 0 62.8 0

44.9 15.7 0 62.8 29.2

20.8 15.7 0 14.6 29.2

12.5 15.7 16.6 14.6 29.2

12.5 15.7 16.6 14.2 29.2

Concentrations in millimoles.

antecedent to process 3, having gone largely to completior• in the past [Keller and van der Karnp, 1988], and so is not represented in Table 2a. The source of reduced sulfur was inorganic sulfides because the total organic matter content of unoxi•dized ti!l is about 0.5%, and the Corganic: S mole ratio of tinoxidized till solids is therefore of the order of 6: 1 (Figures 3 and 5), which is improbably low for natural organic matter [Anderson and Schiff, 1986]. An inorganic source in un- Hched settings is consistent with the findings of Mermut et d. [!985], who found pyrite in the Upper Cretaceous marine shales which constitute part of the provenance of Saskatch- ewan tills. Hendry et al. [1986] determined that the source of reduced S in their till was organic sulfides. Apart from this •difference, the mass transfer model for unleached waters is similar to that of Hendry et al. [1986]. he model shown in Table 2a is the result of application of

m inverse technique and is therefore nonunique in principle, because of the possible presence of unaccounted-for phases ß and processes providing major elements in solution. This can be a serious difficulty, particularly in the modeling of silicate waters where reaction equilibration rates are slow or un- known [e.g., Wood and Low, 1986]. The possibility of silicate dissolution contributing substantially to observed

major ion loading is discussed and dismissed below, and we conclude that the large observed aqueous concentrations of the major elements must be attributed to the few relatively soluble phases identified.

Leached waters. Within the leached group of waters sodium results vary by a factor of 16 (Table 1). This is a considerably greater range of variation than exhibited by the other major elements in this group. Part of the reason for this variation is suggested by Table 5, which shows results of salt extractions and exchangeable cations determinations on bentonitc sealing materials used in piezometer construction. Sodium dominates the exchangeable cation population and sodium sulfate is the dominant soluble salt. The exchange- able cation determination was performed on a particular sample only after extraction of salts from that sample [Neal, 1977] so reported exchangeable Na should be independent of Na salts. The sealing materials are therefore a potential source of contaminating sodium from both exchangeable and salt sources, as well as sulfate.

Sodium contamination from bentonitc, which would de- crease over time, is suggested by some of the Na* time series for individual piezometers, particularly 7B, 8A, and 33 (Table 1). Lack of stoichiometric correlation between Na +

TABLE 2b. Reaction Models

Process Ca 2+ Mg 2+ Na + HCO•'

1. Generate CO2: CH20 + 02 --• H2CO3 ø

2. Dissolve dolomite:

2.3 CaMg(CO3) 2 + 4.6 H2CO• ) --• 2.3 Ca 2+ + 2.3 Mg 2+ + 9.2 HCO•-

3. Oxidize S, generate CO2: !.4 FeS2 + 5.25 02 + 3.5 H20 + 5.6 HCO•- --• 1.4 FeOOH + 2.8 SO•- + 5.6 H2CO3 ø

4. Dissolve calcite:

3.3 CaCO3 + 3.3 H2CO30 --• 3.3 Ca 2+ + 6.6 HCO•-

5. Cation exchange: 0.6 NaX + 0.3 Ca 2+ --• 0.3 CaX + 0.6 Na +

Observed (7B, October 1987)

2.3 2.3 0 9.2 0

SO•-

5.6 2.3 0 10.2 2.8

5.3 2.3 0.6 10.2 2.8

5.3 2.3 0.7 10.0 2.8

Concentrations in millimoles.

2.3 2.3 0 3.6 2.8

255O KELLER ET AL.: HYDROGEOCHEMISTRY OF A CLAYEY TILL, 1

TABLE 3. Determinations of $34S on Till Solids

Location/Depth, (•34S(SO4) /534S(Reduced S) m Material per mil per mil

Unleached, Oxidized 31/3.1-3.8 bulk till -7.8 "' 35/3.1-3.8 bulk till -8.0 "' !40N/1.5-3.1 gypsum -8.5 "' 100N/4.6•. 1 gypsum -5.0 "'

Unleached, Unoxidized 120N/12.2 gypsum -8.1 "' 31/12.2-13.7 bulk till - 12.9 "' 35/12.2-13.7 bulk till - 12.9 "' 160N/10.7-11.0' bulk till -18.2 '"

Leached, Unoxidized 32/9.2-10.7 bulk till - 15.3 - 13.4 34/9.2-10.7 bulk till - 15.7 -9.7

*Submitted for analysis as undisturbed core sealed in microcrys- talline wax. All other samples submitted oven dried.

and SO42- in leached waters indicates that salt in bentonite is not the primary SO•- source. Another potential reason for elevated Na + and SO¬- in some leached water samples is mixing with formation waters from nearby unleached re- gions. This is almost certainly the case for piezometer 9A.

A follow-up study using bromide tracer salts in bentonite seal mixtures (V. Remenda, University of Waterloo, per- sonal communication, 1991) confirms that solute contamina- tion from bentonite can occur in these kinds of piezometer installations. "End-member" representatives of the leached class at Dalmeny, i.e., unaffected by contamination or mixing, are calcium-bicarbonate waters characterized by milliequivalents Na +/(Ca 2+ + Mg 2+) < 0.2 and milliequiv- alents SO42- < 10. The waters fulfilling both criteria are from piezometers 33 and 7B.

Simulation of the composition of a 7B sample is shown in Table 2b. As before, process 1 represents generation of carbonic acid; this causes dolomite and calcite dissolution (processes 2 and 4) to approximate equilibrium, as indicated by the slight undersaturation to slight supersaturation with respect to both minerals in leached waters (Table 1).

Leached waters are undersaturated with respect to gyp- sum (Table 1) and no gypsum is found in the till in this setting; the only other reservoir and plausible source of

however, the stoichiometry of process 3 is valid for our purposes, because the ratio of moles reactant S to moles product SO42- and H* is the same [Anderson and Schiff. 1986]. The visually detected presence of a fine-grained iron oxide precipitate on till fracture surfaces is consistent with generation of an iron hydroxide phase.

The results of 334S(SO¬ -) determinations of leached and unleached waters (Table 1) are different, as confirmed by "t" test in which the hypothesis that the population means are the same was rejected with 2% risk of type I error. This difference, which may reflect an isotopically heavier organic sulfide source in the leached setting, is also consistent with oxidation of a pyrite S source in which kinetic S isotope fractionation occurs (fractionations for this process as large as - 14 per mil have been reported in the literature; Pearson and Rightmire [1980] provide an overview). Recharging groundwater would remove relatively light sulfate from the till as oxidation progresses, with remaining sulfide progres. sirely enriched in the heavy isotope. The remnant sulfide currently subject to oxidation in the leached setting would be sufficiently enriched in 34S that the sulfate now produced from it is also heavy relative to the unleached case where sulfide has been completely oxidized and precipitated in place (and •534S(SO42-) and 334S(SO4) (Tables 1 and 4)are the same as •534S of the original sulfide). Early in the history of S oxidation in the leached setting, dissolved sulfate would have been correspondingly enriched in the lighter isotope.

The results of /534S(SO4) determinations on till solids (Table 3) probably illustrate this process. Values for unoxi- dized till are significantly smaller than values for oxidized till and gypsum, because of enrichment of the lighter isotope in a sulfate fraction which was generated during handling, drying, and extraction of unoxidized samples. Comparison of S(SO4) from extracts of unoxidized till from the bottom of test hole 34 (Figure 3) with piezometer water su!fate from this setting (7B, 33, Table 1) shows that the extract-sulfate values are too high by a factor of 2 to 3. This indicates that some oxidation of sulfide did indeed occur during unoxi- dized-sample processing.

Given the similarity of the •534S(SO• -) signatures of leached waters and su!fate from bentonite (Tables I and it might be argued that the leached/unleached contrast is due to bentonite contamination of leached waters. However, the

sulfate is reduced S in the till. Oxidation of reduced S to lack of Na+/SO42- stoichiometric correlation mentioned SO42- is represented by the oxidation of pyrite to SO42- and ..... above is not consistent with such an argument; moreover. goethite (process 3). In this setting the Corganic' S ratio of the most aquifer waters have/334S(SO42-) values in the -5 to -6 till is much larger than in the unleached setting and not per mil range [Fortin et al., 1991], which is similar to inconsistent with an S source from organic sulfides; even so, leached-setting values. For these reasons the effect of sulfate

TABLE 4. Exchangeable Cations Determinations

Sample Location/

Depth, m Setting Method Ca 2+

Cations, meq/100g (/.•g/g) . Total (CEC),*

Mg 2+ Na + meq/100g

34/5.0 leached 1 4.5(910) 34/5.0 leached 2 24/9.1 unleached 1 4.8(! 100) 24/9.1 unleached 2 160N/9.9 unleached 1 6.6(1500) 160N/3.8 unleached 1 4.6(1100)

2.7(330) 0.02(5) 7.5 8.4

3.5(430) 0.1(20) 8.8 9.2

2.7(330) 0.2(40) 9.8 4.0(480) 0.1(30) 9.0

Methods: 1 is of Neal [1977], 2 is the barium displacement method. *Includes exchangeable K [Keller, 1987].

KELLER ET AL ' HYDROGEOCHEMISTRY OF A CLAYEY TILL, 1 2551

TABLE 5. Bentonitc Sealing Materials

Type

Exchangeable Cations,* meq/100g Dry Solids Ions in 60' 1 Extract, meq/100g Dry Solids

$34S(SO4), Ca 2+ Mg 2+ Na + Ca 2+ Mg 2+ Na + K + CI- SO•- per mil

Potyg½lt (Powder) 7.1 Coregelt (Powder) 18.8 Peters 5.3 Avonlea (Granular) 5.2

6.5 37.9 0.1 0.4 25.0 0.2 1.4 11.6 +4.9 3.7 48.9 .................. +2.8

12.6 47.6 0.4 0.4 36.5 0.2 1.0 27.3 +0.9 0.4 50.0 ................... 3.2

*Method of Neal [1977]. tTraxte name.

contamination on leached/534S(SO42-) values is believed to be small.

As in the unleached setting, cation exchange is taken to be the only plausible source of observed Na concentrations (Table 2b). The importance of silicate dissolution as a source, as has been suggested for some shallow Glaciated Plains groundwaters [e.g., Wallick, 1981], may be assessed as fogows. Available models [e.g., Stumrn and Morgan, 1981, pp. 542-547] suggest that primary silicates such as plagio- rases (abundant in the till, Table 6) should dissolve incon- gruently to yield Na + and H4SiO4 ø in a sodium:silicon molar ratio which is 1:2 or less. This ratio should pertain in solution unless and until increasing solute activity results in precipitation of some silica phase. However, in the leached setting sodium:silicon major ratios are much greater than 1:2, and there is no evidence of silica precipitation, either in the solution chemistry (Table 1) or in the solids themselves. As a check, simple reaction progress trajectories for incon- gruent plagioclase dissolution were calculated with ?HREEQE [Parkhurst et al., 1980] assuming open-system equilibrium with calcite at the PCO 2 of the 7B sample simulated in Table 2b. Thermodynamic data for the silicates co.mpiled by Drever [1988, chap. 6] were used in the calcu- lations. The results confirmed that Na + concentrations in leached samples are at least an order of magnitude greater than can be explained by this model. While processes such as selective dissolution, precipitation of unidentifiable silica phases, and silica sorption have not been considered here, it must be concluded that available evidence does not support silicates as a significant source of Na in solution.

The CEC and exchangeable-sodium of Dalmeny till (Table 41 are smaller by factors of 5 to 5000 than the values to which Hendry et al. [ 1986] attributed sodium loading of pore water in southern Alberta till. Presumably, this is due in part to

TABLE 6. Bulk Till Mineralogy

Sample Depth Interval,* m

Phase 3.1-4.6 9.2-10.7

,Quiz 43% 30% K-feldspar 7% 9% Hagioclase 18% 12% C•cite 4% 6% Bok•te 11% 15 %

l•e 14% 23% Tric• .h_lofite 2% 4% Mixed.layer clay 1% 1%

!)etermined by X ray diffraction. *'Collected at location 36.

differing clay mineralogies (illite dominated at Dalmeny, Table 6; smectite dominated in southern Alberta).

MASS TRANSPORT AND MASS DEPLETION

IN THE TILL

The processes in Table 2 represent both depletion of element mass from the till and transfer of that mass to the

aqueous phase, i.e., the local recharge flux to the aquifer below. In this scenario, assuming dominantly vertical flow, element depletion and aqueous flux are related by the mass balance

M d = QrCr r [ML -2] (1)

where Me is local depletion of mass per unit area of till, C r is concentration [ML -3] in the recharge flux Qr [LT-1], and T is time. If the assumption is made that mass transfer and mass transport have been steady at the present rate over the past 11,500 year since deglaciation, present-day esti- mates of Q r and C r may be used in (1) to estimate mass depletion during that interval. The steady state assumption is probably reasonable: the fact that the top of the unoxidized till lies only a few meters below the present-day water table [Keller et al., 1988] shows that the recharge rate must have been only slightly less during the Altithermal interval than today.

Ma may be calculated for leached location 34, where Keller et al. [1988] estimated Qr as 0.035 m yr -• . Cr for Ca, Mg, Na, S, and organic C (OC) are taken as the mean of the concentrations in the most recent water analyses for piezom- eters 7B and 33 (Table 1). CF for inorganic C (IC) is taken as the IC contribution from dolomite and calcite dissolution

(processes 2 and 4, Table 2b). These concentrations and the results of the Me calculations are shown in Table 7.

M a values should be consistent with observed local de- pletions of mass in the till. The observed local mass deple- tion per unit area, Md(obs), may be defined as

TABLE 7. Mass Depletion From the Till

Qr, Cr, Me x 10 4, Constituent m yr -1 mg L -1 g m -2

Ca 0.035 198 8.0 Mg 0.035 68 2.7 Na 0.035 30 1.2 S 0.035 96 3.9 IC 0.035 95 3.8 OC 0.035 8 0.3

Md(obs) _X 2104, gm

6.8 2.8 0.4* 2.1

3.3

5.0

*By difference from unleached setting, see text.

2552 KELLER ET AL.' HYDROGEOCHEMISTRY OF A CLAYEY TILL, 1

Md(obs) = (Munox- Mox)Ps(1 - n)Zox [ ML-2] (2) where Munox is the mean solid-phase concentration in unox- idized till, representing elemental concentration in the till as a whole before oxidation and depletion began ([M M -1 ]); Mox is the mean solid-phase concentration in oxidized and

THIN

, ,

depleted till ([M M-i]) ß 9s is solids density, taken as 2.7 x : ....... . ...... . ' 'AQUIFER .... ' ß ' ß ' .... ' ' ' ß ' ' ' ß ß

106 g m-3; n is porosity, 0.26 [Keller et al., 1986]; and z is :""'": "'"' '""" ": "-"'"' ": "'" ': :- depth of the oxidized and depleted zone, taken as 6.1 m at location 34. The respective mean solid-phase concentrations are calculated from the unoxidized- and oxidized-zone ana- lytical results shown for location 34 in Figures 3, 5, and 6. Equation (2) could not be used for Na because of its depletion throughout the till. M d(obs) for Na is estimated as the sum of the differences between the exchangeable (Table 4) and aqueous pools for the leached and unleached settings. Mr(ohs) results are shown in Table 7.

Agreement between M d(obs) and M a, for each the inor- ganic solutes except Na, is within a factor of 2. The M d(obs) value for Na is compromised by the paucity of exchange- able-Na data and is underestimated in any case if any Na has been depleted from unleached settings. This agreement supports the mass transfer model of Table 2b, particularly the representation of the gradual release of S from the till by process 3. Apparently, in this setting the rate of sulfide oxidation was never large enough to saturate recharging water with respect to sulfate minerals, in contrast to the unleached setting where most of the S was oxidized to sulfate in the past and is now stored in the solids as gypsum. M d(obs) and Ma calculations for the unleached setting are too unreliable to be meaningful, due to large relative errors in estimates of Qr and (Munox - Mox) terms. Mass fluxes from unleached settings are clearly much smaller than in leached settings as evidenced by the presence of gypsum and the fact that measured hydraulic gradients are much smaller than in leached settings [Keller et al., 1986, 1988].

Mr(ohs) and Ma for organic C differ by more than an order of magnitude. This indicates that SOC depletion cannot be attributed to dissolution and advective transport to the aquifer in recharging groundwater. This and related issues are explored by Keller [this issue].

DISCUSSION AND CONCLUSIONS

Spatial variability. Hendry et al. [1986] discuss a two- layer pattern of variability (high-sulfate oxidized till overly- ing low-sulfate unoxidized till). The pattern at Dalmeny, by contrast, is one of dominantly lateral variability (unleached/ sulfate settings adjacent to leached/bicarbonate settings).

As in the case of Hendry et al. [1986], the penetration of atmospheric 02 into highly reactive materials, recently ex- humed by glaciation, is a key process. However, at Dalmeny the local rate of flushing by groundwater recharge, relative to the rate of sulfide oxidation, is an important variable. In the unleached setting, oxidation has been rapid relative to flushing, leading to gypsum precipitation and a solid-phase sulfate "reservoir" in a deep vadose zone. Here gypsum solubility, calcium-for-sodium in exchange, and small re- charge flux limit S depletion in the till and S transport to the aquifer below. In the leached setting, by contrast, depres- sion-focused recharge causes seasonal oscillation of the water table about a shallower mean depth (Figure 4) [Keller et al., 1988]. Here oxidation is slow relative to flushing such

THICK

TILL

Fig. 7. Types of hydrogeochemical evolution in clayey tills. Dominantly lateral chemical differentiation (this study! occurs in thin tills with relatively large vertical permeabilities. Dominantly vertical chemical differentiation [e.g., Hendry et al., 1986] occurs in thick tills where vertical permeability is small.

that the oxidation rate limits S depletion and transport; the shallower mean water table position also limits the depth of infusion of 02 and sulfate generation.

The development of these distinct mass transport/mass transfer paths and consequent hydrogeochemical variability depend upon the localized differentiation of recharge flows to the underlying aquifer, relative to oxidation rates. This kind of dominantly lateral variability therefore cannot de- velop unless the till is permeable enough for substantial through the till to exist. Studies of the hydraulic properties of surficial deposits of clayey till in the Western Glaciated Plains have been reviewed by Keller et al. [1988, 1989]. They indicate that such permeability is much more likely to exist in deposits <20 m thick, such as at Dalmeny, than in deposits of greater thickness. It is in the latter geologic setting, where lateral differentiation of flow through the till cannot occur, that the vertically stratified kind of variabilit.• studied by Hendry et al. [1986] develops. This same pattern is present at the Warman site (Figure 1) where the till is about 65 m thick [Keller et al., 1988; Keller and van der Kamp, 1988].

We propose that style of hydrogeochemical evolution in clayey till and associated aquifers be conceptualized in terms of two types: one, as discussed herein, associated with relatively thin tills; and one, as first discussed by Hendr3.' al. [1986], associated with relatively thick tills. These two types are contrasted schematically in Figure 7. Lateral f10• may cause some spatial variability in the weathered zones thick tills [e.g., Keller and van der Kamp, 1988]; such variability is secondary to the primary vertical pattern.

Evolution in time. While groundwater flow through thick systems, as at Warman, is permeability limited to a small fraction of one pore volume since the last deglaciati0n. at Dalmeny groundwater throughflow on the same time scale ranges from a few pore volumes (unleached settings)to many tens or even hundreds of pore volumes {leached settings). Calculated and observed depletions of till constit- uents indicate exhaustion of exchangeable Na and S, which are small on a mass fraction basis, on a time scale of t04 years. Since exchangeable Na and S are the principal com- ponents of hydrogeochemical spatial variability, this is als0

KELLER ET AL.: HYDROGEOCHEMISTRY OF A CLAYEY TILL, 1 2553

the time scale of the "decay" of this variability. Exhaustion of the more abundant constituents from carbonate minerals •ill occur on a time scale of 105 year. For other till-aquifer systems these times would vary depending on (1) the mag- nitude of recharge, (2) the extent to which recharge is depression focused, and (3) the masses in till of carbonate minerals, sulfide, exchangeable sodium, and (as discussed further by Keller [this issue]) organic carbon. In the glaciated interior plains we can expect to encounter thin-tillJdrift- •uifer systems which range from strongly leached (with little spatial variability in the till) to nearly unleached (with variability in various stages of decay).

Consequences for aquifer water quality. Sulfate and so- dium concentrations in glacial drift aquifers underlying thin tills will depend, in a given system and point in time, on the relative proportions of leached and unleached waters reach- • the aquifer. By contrast, concentrations of bicarbonate, and to a lesser extent calcium and magnesium, will vary primarily with PCO2 in the till. As long as till PCO 2 levels remain high in both settings (as is currently the case at Dalmeny), Ca-Mg-HCO- will remain high in the aquifer lalso currently the case) [Fortin et al., 1991] regardless of proportions of leached and unleached recharge. Character- istically high Ca-Mg-HCO•- in Western Glaciated Plains drift aquifers [Freeze and Cherry, 1979] suggest persistently high till PC02 levels throughout the region. Controls on CO2 generation are discussed by Keller [this issue].

Acknowledgments. This study benefitted by discussions with H. Maathuis, B. Zebarth, G. Buckland, R. Farvolden, S. Schiff, M. J. Hendry, D. van Stempvoort, and D. Abbott. D. Abbott also consulted on water sampling techniques; E. Jaworski, D. Zlipko, V. Remenda, and D. Legault provided field assistance. W. Yuen, D. Tomasiewicz, B. Ellen, and R. Kellerman consulted on various analytical tasks. This study was supported by Washington State Urdversity, the Saskatchewan Research Council, Environment Can- ada Water Resources Research Support grant RA-85-10, NSERC Strategic grant G1372, and NSERC Operating grant A8184.

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G. van der Kamp, National Hydrology Research Institute, Innovation Blvd., Saskatoon, Saskatchewan S7N 3H5.

(Received February 15, 1991- revised June 6, 1991;

accepted June 12, !991.)