High levels of uranium and radium in groundwaters at Canada's Underground Research Laboratory, Lac du Bonnet, Manitoba, Canada

Applied Geochemistry, Vol. 4, pp. 577 591, 1989

(18832927/89 $3.00 + .(~1 Pergamon Press pie

Printed in Great Britain

High levels of uranium and radium in groundwaters at Canada's Underground Research Laboratory, Lac du Bonnet, Manitoba, Canada M . GASCOYNE Applied Geoscience Branch, Whiteshell Nuclear Research Establishment, Atomic Energy of Canada Limited, Pinawa, Manitoba ROE 1LO, Canada

(Received 8 December 1988; accepted in revised form 24 July 1989)

Abstract--High concentrations of U and 226Ra, and elevated 234Uf138U activity ratios have been measured in groundwater samples collected from water supply wells and exploratory boreholes in the area surrounding the Underground Research Laboratory (URL) of Atomic Energy of Canada Limited, in southeastern Manitoba. All groundwaters come from the Lac du Bonnet granite batholith or sediments overlying the batholith. Uranium concentrations attain almost 1 mg/I in some shallow, low-salinity groundwaters, whereas 226Ra tends to be high (up to 38 Bq/l) in deeper, saline waters. The U concentrations are some of thc highest observed in global groundwaters, yet no significant ore body or mineralization is known in thc area. Analyses of unaltered rock samples of the Lac du Bonnet granite show slight U enrichment over average Canadian Shield granites (6.5 p,g/g vs 4 ~g/g), and altered wall rock in fracture zones is enriched in U by up to an order of magnitude compared to adjacent bedrock. Low 234U/238U activity ratios in this altered rock indicate active and recent leaching of U by groundwater. The key control on U concentration appears to be redox potential. Concentrations of U in rock~ residence time and groundwater composition are of lesser importance. Geochemical modelling of the shallower, oxidized waters indicates that U speciation consists mainly of anionic carbonate complexes of the uranyl ion. This is supported by the remarkable efficiency of an anionic filter developed to remove high levels of U from drinking water in the area. In more reducing groundwaters, U concentrations are similar to those determined in recent experimental work on uraninite solubility in the pH range 7-8.5. Colloidal U is <10% of total U and organic complexation is unlikely to be significant because of low dissolved organic concentrations. The results emphasize the significance of redox potential in controlling U mobility in both oxidizing and reducing environments and indicate the usefulness of U concentration in estimating groundwater Eh.

INTRODUCTION IN 1982, high levels of a l p h a and b e t a radioactivity were f o u n d in w a t e r samples t a k e n from eight household wells located close to the p r o p o s e d site of the U n d e r g r o u n d R e s e a r c h L a b o r a t o r y ( U R L ) of A t o mic E n e r g y of C a n a d a Limited ( A E C L ) , n e a r Lac du B o n n e t , s o u t h e a s t e r n M a n i t o b a (Fig. 1). T h e s e wells were s a m p l e d to provide a baseline of the naturally occurring radioactivity in the area as part of the p r o g r a m of the P r o v i n c e of M a n i t o b a to m o n i t o r possible e n v i r o n m e n t a l effects of the c o n s t r u c t i o n a n d o p e r a t i o n of the U R L . S u b s e q u e n t analyses s h o w e d this radioactivity to be due to the p r e s e n c e of dissolved 226Ra a n d U, which, in m a n y cases, were p r e s e n t in c o n c e n t r a t i o n s well in excess of federal guidelines for m a x i m u m acceptable c o n c e n t r a t i o n s in drinking w a t e r (1 Bq/1226Ra, 20/~g/1 U , see MINISTRY OF NATIONAL HEALTH AND WELFARE, 1978). T h e nuclides c o n t r i b u t i n g to these radioactivity levels indicated t h a t the radioactivity was entirely f r o m natural sources, principally R a a n d U in the granite b e d r o c k a n d glacial/lacustrine o v e r b u r d e n in the area. A s a result of this first survey, the G o v e r n m e n t

FIG. 1. Location of the Whiteshell Research Area (the outlined study area) and Underground Research Laboratory (URL) with respect to the Lac du Bonnet granite batholith. Private wells that were sampled in this study in the area are indicated. 577

578

M.

Gascoyne

Table 1. Examples of worldwide groundwater radioactivity levels Maximum

Location

Rock type

Sample depth ( m )

Bath, England E . England Cornwall, England Altnabreac, Scotland Helsinki, Finland

limestone limestone granite granite mainly granite

0 -241) <300 9(1

0.05 4 3 15 14870*

Portugal East Germany Stripa, Sweden

granite sandstone granite

5(1 1) < 150 < 1611 300-12110 ---

71311* 1 17 90 <0.1-35 - 300

U (/xg/I)

USSR

--

Judean Hills, Israel

limestone

Hirabara, Japan

granite

0

23

U.S.A. U.S.A. U.S.A.

all

--

12(1

mixed

--

90

mixed

--

582

U.S.A. (Canadian Shield) Central U . S . A , Nevada Test Site, U . S . A .

mainly till dolomite/sandstone rhyolite

Maine, U . S . A . New Jersey, U . S . A . Illinois, U . S . A .

granite sandstone/siltstone sandstone

Washington,

Minnesota, U . S . A . South Carolina, U . S . A . A t i k o k a n , Ontario

organic Holocene soil igneous/metamorphic sedimentary granite

British Columbia

mixed

Chalk River, Ontario Okanagan, British Columbia

gneiss sedimentary

<60 11

Ottawa region, Ontario South-central British Columbia Southwest Saskatchewan

igneous/sedimentary mixed igneous overburden/mixed sediments granite, overburden

<81) ---

granite

11511

U.S.A.

Southeastern Manitoba Whiteshell Research Area, Lac du Bonnet, Manitoba

--< 15(X)

6,(1

4 0.6 2I

---

27011

--

I

1

318

--

~211

--

<1211 --

<200

60

--

I11 128

2 8000 73 11)6 240 211211 837

* See comments in text.

of Manitoba expanded its well monitoring program to almost 300 private wells in southeastern Manitoba. High levels of U were found in more than half these wells. The distribution and possible causes of such concentrations have been examined in detail by BETCHERet al. (1989). These radioactivity concentrations are compared with data on the global occurrence of Ra and U in groundwater in Table 1. In Canada, high levels have been observed in groundwater from across the country. Published examples are included in Table 1. In the U . S . A . , a detailed review of radioactivity levels in drinking water sources has been given by HESS et al. (1985). The presence of U, Ra and Rn in drinking water was found to be confined mainly to groundwater in the western mountainous areas (for U), the Upper Coastal Plain and Central Platform (for Ra) and elevated areas in the east (for Rn). Few

of these data, however, show U concentrations exceeding the levels found in groundwaters in southeastern Manitoba. This occurrence becomes even more anomalous when comparisons are made with groundwaters found in other granitic bodies, e.g. Stripa (Sweden), Scotland, Cornwall (UK), Portugal and Minnesota, where maximum Ra and U concentrations seldom exceed 1 Bq/1 and 20/xg/1, respectively (Table 1). The data from wells in the Helsinki a r e a (ASIKAINEN and KAHLOS, 1979) applies to unfiltered water samples and so dissolved U concentrations may be less than these values. This paper describes the results of measurement of levels of Ra and U in groundwaters collected from more than 50 intervals within exploratory boreholes drilled in granite on the URL lease area and from seven other boreholes in granite within the surrounding Whiteshell Research Area (WRA) (Fig. 1), and

U and Ra in groundwater, Manitoba, Canada

234U/238U

Maximum 226Ra

Range

(Bq/1)

1.7-3.5 t).8-1.9 1.0-2.8 0.7-10.1 1.18 1.39

0.4

1.0~6.4 2.2-4.8 2-11.2 4.3 11.9-2,2

27 1.0 1.0

<0.1 1.3

----5.4-10.6 2.8~.9

1.1 1.0 24 0.1 11.3 0.1

-2-40

0.8 2.0

1.4 m -1. ( ~ . 8

0.0l

1.0 <0.1 1.4

1.6-3.4

-1.8-7.11

4.1 38

Reference ANDREWS et al. (1982a) ANDREWSand KAY (1982) EDMUNDSe t a l . (1984) IVANOV1CHand KAY (1983) ASIKAINENand KAHLOS (1979) DEKKERSet al. (1986) FR6HIJCH et al. (1984) NORDSTROMel al. (1985) OSMOND and COWART(1976) GUTTMANAND KRONFELD (1982) Dol et al. 11975) SCOTt and BARKER(1962) HESS e t a l . (1985) COTHERNet al. (1983) Scott and BARKER(1962) COWART(1981) ZIELINSKIand ROSHOLT (1978) WATHEN(1987) SZAaO and ZAVECZA(1987) GII KESONand COWART (1987) ZIEI,INSKIet al. (1987) LIVELYand MOREY(1982) KING et al. (1982) LAROCQUEand GASCOYNE (1986) MINISTRYOF ENERGY, MINES and PETROLEUMRESOURCES (1981) MILTONand BROWN (1983) CULBERTand LEIGHTON (1978) DYCK (1980) BOYLE (1982) DYCKet al. 0976) BECK and BROWN(1985) this study

compares the data with those from private water supply wells located within the same area. The results are interpreted in terms of groundwater and rock geochemistries and local hydrogeological characteristics. This work forms part of the hydrogeochemical characterization of the U R L lease area prior to, and during the construction of the underground facilities. The data were also obtained as part of an extensive monitoring program to determine U mobility in groundwaters in the W R A and to determine whether a specially developed filter (GAscOVNE, 1986) might have to be employed to remove U from U R L shaft drainage water prior to discharge to a local river system. GEOCHEMISTRY OF RADIUM AND URANIUM

Four Ra isotopes occur naturally in groundwaters and of these, 226Ra and 228Ra have relatively long halfqives. The isotope 228Ra occurs in the 232Th

579

decay series and has a highly insoluble parent (232Th) and a fairly short half-life (5.8 a). Therefore, the mobility of 22SRa is more limited than that of 226Ra, which has a more mobile parent (23°Th) and a longer half-life (1600 a). The difficulty in direct determination of 228Ra (a low-energy beta emitter) and its lower radiological toxicity compared to 226Ra (a high-energy alpha emitter) combine to make it a lessstudied radionuclide than 226Ra. In natural waters, Ra exists mainly as the free ion, Ra 2+, or the uncharged complex, RaSO ° (LANcMUIR and RIESE, 1985). Radium is readily removed from solution by adsorption on clays and rock silicates and coprecipitation with insoluble sulphates, but appears to be stabilized in solution by high concentrations of Ca 2+, Mg2+ and C1- (LANGMUIR and MELCHIOR, 1985). In terms of radioactivity levels, U occurs in groundwater mainly as the two isotopes in the 23SU decay series, Z3SU and 234U, with minor activities of the isotope 235U, from the 235U decay series. The mass ratio of 235U to 23SU is a global constant, 1 : 137. However, 234U is not of constant abundance relative to 238U, especially in continental waters, and its activity often exceeds that of 238U because 234U is more easily leached from mineral surfaces. This is largely because 234U is a product of alpha decay and may be injected directly into solution during, or readily leached following, decay of parent 23SU; therefore, 234U is more easily removed by penetrating fluids. Although radioactivity ratios of 234U t o 23Su in natural waters are generally between 1 and 3, values of up to 40 have been observed in some locations (GIL~ESON and COWARX, 1987). Uranium is slightly soluble in most natural waters, occurring mainly as the carbonate ion complexes UO2(CO3) 2- and UO2(CO3) 4-, between pH 7 and 10. At lower pH, phosphate complexes, UOz(HPO4) 2-, dominate to pH 4.5, and below this, various cationic and uncharged complexes are the most stable ones (LANGMUm, 1978). For a typical granite groundwater under CO2-rich, oxidizing conditions, the maximum solubility of U is about 1 g/1 (PAQUETrE and LEMIRE, 1981). Literature values for U concentrations in natural groundwaters (Table 1) seldom attain this limit partly because many waters are reducing and/or are deficient in carbonate complexing species. In addition, diffusion of U from mineral surfaces to some extent controls the rate of accumulation of U in solution, and this process is particularly slow if the main source of U is resistate minerals such as zircon, sphene, and uraninite. SAMPLING AND ANALYTICAL TECHNIQUES Groundwater from exploratory boreholes and private water supply wells in the region of the Underground Research Laboratory and within the surrounding Whiteshell Research Area were sampled for analysis of major ion

580

M. Gascoyne

chemistry, and concentrations of radioactive elements and their isotopes. The distribution of wells and general location of the borehole sampling sites is shown in Fig. 1. Waters from 13 wells were sampled from household faucets after a brief period of flushing if the well was in use, or using a downhole electric pump if the well was not in use. All well water samples were obtained and provided by the Water Resources Branch, Manitoba Natural Resources, Winnipeg. Borehole groundwaters were sampled by a variety of methods such as gas-lifting, swabbing, downhole electric or air pressure pump, and sealed downhole samplers. All 13 wells were < 150 m deep and 11 were abstracting water from fractures in the granitic bedrock. Two of the shallower wells (up to 30 m deep) drew water from surficial sediments overlying the bedrock. Samples obtained from URL and WRA boreholes were taken from depths ranging from near-surface to > 1 km and, in many cases, came from clearly defined fracture zones in the bedrock. These lowdipping zones have been identified at the URL by a combination of core logging, downhole geophysical methods and direct observation in the U RL access shaft (DAvlSON, 1984). The permeability of the fracture zones varies considerably, and over relatively short distances. In some cases, water has been pumped continuously for several days from the more permeable sections at rates of over 500 l/min (DAvISONand KOZAK, 1989). Pumping tests have also shown that the lowdipping fracture zones at the URL are poorly interconnected but they are crosscut by occasional vertical or subvertical fractures, which serve to provide recharge to the zones (DAvlsoN, 1984). The permeable zones penetrated by many of the wells in the study area are likely to be fracture zones similar to those observed at the URL lease area. All water samples were filtered through 0.45 p,m membranes and acidified as soon as possible with HCI or HNO 3 to pH _<1. Radium-226 was determined by the emanation method and scintillation counting in a zinc sulphide cell (GAscOYNE, 1982). Uranium was determined by coprecipiration with ferric hydroxide, followed by separation and purification on anion exchange resin and analysis by alpha spectrometry (GAscovNE, 1981). This method allows determination of the isotope activity ratio, 23au/23SU, and U concentration. The precision and accuracy of results are demonstrated as follows: (1) Thirty-three analyses of an acidified groundwater standard over a 12-month period gave a mean concentration of 43.6 p~g/l and standard deviation of _+3.3 ~g/1. The latter compares reasonably well with the error determined for each analysis from counting statistics alone, (mean = _+1.8 /~g/l). (2) Agreement within _+10% was found with results of Ra and U concentration obtained by three other laboratories (the Radiation Protection Board, Ottawa; the Saskatchewan Research Council, Saskatoon; and Monenco Analytical Laboratories, Calgary) for 12 well waters that were used for an interlaboratory calibration exercise (PoI,LOCK, 1983). Major ion concentrations were determined for the well waters by the Water Resources Branch, Winnipeg, and for the WRA groundwaters by the Analytical Science Branch, WNRE, Pinawa, Manitoba. RESULTS

A s u m m a r y of analytical results for well and borehole waters is given in Table 2. The distribution of R a and U c o n c e n t r a t i o n s is s h o w n in Fig. 2. Radium-226 c o n c e n t r a t i o n s are low ( < 1 Bq/1) in almost all g r o u n d w a t e r s except for wells W-1 and W-2, which are g r o u p e d in the s a m e location, and b o r e h o l e z o n e s that contain saline w a t e r at d e p t h s > - 3 5 0 m. Concentrations of U range from < 1-570 ~g/1 for the well

20 - ~

[]

Borehole

[]

Well

15-~

z

u. 5- I oY

Am,

0

0.4

,q 0.8

. . . . 1.2

,m

..

1.6

2.0 10

20

,m 30

40

RADIUM - 2 2 6 (Bq/L)

15-

]

Borehole

[]

Well

>" 10"

o~:

> 300 ~,~,~

0

10

20

30

40

50

60

100

200

300

URANIUM (,'*g/L)

Fl6. 2. Frequency distribution of (a) 226Ra and (b) U concentrations in well and borehole waters for the Lac du Bonnet and Whiteshell Research Areas.

waters. Analysis of samples from all b o r e h o l e s in the study area (Table 2) has r e v e a l e d waters with radionuclide c o n c e n t r a t i o n s similar to those found in the wells and, in one case, with a higher U value (837 ~g/1). In all samples in Table 2, the activity ratio 234U/238U is g r e a t e r t h a n unity, indicating recent ( < 1 Ma) and preferential dissolution of 234U, over 238U, from the b e d r o c k or surficial sediments. S o m e w h a t g r e a t e r e n r i c h m e n t of 234U is seen in the d e e p e r samples.

Radium in groundwater For the w a t e r supply wells, 226Ra only occurs in waters with high U c o n c e n t r a t i o n s . This correlation is not o b s e r v e d for the b o r e h o l e g r o u n d w a t e r s w h e r e 226Ra is frequently low or absent w h e n U is high. R a d i u m might be e x p e c t e d to correlate with U concentrations in g r o u n d w a t e r s that have had a chance to equilibrate with the rock because the isotopes 238U, 234U and 226Ra are in the same decay series. This is not found in practice, h o w e v e r , largely because of the different geochemical p r o p e r t i e s of the

U a n d R a in g r o u n d w a t e r , M a n i t o b a , C a n a d a

581

T a b l e 2. C o m p o s i t i o n o f well w a t e r s ( W - ) a n d W h i t e s h e l l R e s e a r c h A r e a b o r e h o l e g r o u n d w a t e r s (all o t h e r s ) Well*/ borehole number

Deptht sampled (m)

W-I W-2 W-3 W-4 W-5 W -6 W-7 W-8 W- l 1 W-12 W-13 W - 14 W-15 URL-1 URL-3 URL-4 URL-5 URL-5 URL-6 URL-7 URL-7 URL-7 URL-8 URL-8 URL-9 URL-9 URL-10 URL-10 URL-10 U R L-2 URL-2 M-IA M- 1A M- 1 A M-1B M-2A M-2B M-3A M-3A M-4A M-4A M-4A M-4A M-5A M-5A M-5B M-6 M-7 M-7 M-8 M-9 M-10 M-10 M-11 M-11 M-I 1 M-12 M-12 M-12 M - 13 M- 14 M-14 B-34 B-37 B-41 B-41 209- D R I P S

0-124 0-91 0-146 0-34 0-53 0-44 0-90 0-28, S 0-39 0-91 0-96 N N, S 10%116 9%142 53-73 83-109 22%267 250-280 55-75 125-143 145-165 62-105 275-314 0-203 137-161 53-121 53-121 269-302 458-502 637-699 150-205 206-240 240-324 50-150 221-270 110-152 251-350 3511-400 150-190 191-235 236-290 290-406 150-310 310-357 105-150 81-163 66-79 385-395 3l 1-400 0-208 0-342 3411--450 0-240 230-330 33{)-400 88-98 154-.164 166-176 226-443 /)-300 300-380 0-33 0-30 0-35 36~0 240

U (~g/l)

234u/Z38u

313 277 255 87 17 41 573 < 1 86 348 85 338 51 11 8 30 114 17 5 73 16 9 37 23 103 100 95 227-837 15 96 12 5 12 0.4 44 7 29 <0.5 0.4 7 0.9 0.7 2.5 <0.5 2 11 36 15 5 50 < 1 44 27 16 85 9 54 26 70 3.9 21 0.8 2 255 35 50 5

4.4 4.5 4.7 4.2 2.6 2.4 1.4 1.4 2.7 1.4 1.4 1.6 1.2 4.8 8.4 2.6 5.2 5.3 5.4 6.5 5.8 6.4 5.6 5.0 4.2 4.8 3.3 2.0-2.8 4.6 3.5 2.8 6.9 4.4 3.3 6.0 5.7 3.6 -3.6 5.0 5.5 -5.5 -6.0 5.2 5.8 4.3 3.7 4.1 4.7 2.4 4.2 3.0 2.9 6.1 4.3 4.7 4.3 4.0 4.5 6.0 5.0 1.8 4.2 3.4 7.0

226Ra (Bq/l)

C1 (mg/l)

0.45 3.52 0.15 0.16 <0.1/2 -0.23 I).02 ------<1/.02 0.03 0.04 -0.08 -----0.06 0.09 0.03 -0.04 0.05 0.29 -----0.05 -0.29 ----0.08 --0.05 0.119 16 <0.02 -0.03 2.14 0.03 ----<0.02 -0.09 2.01 -0.118 --

6 16 17 4 48 16 21 12 28 8 7 11 3 11 97 1.7 3 150 336 1.4 1.2 1.9 10 37 2 2 3 3 161 90 2600 22 29 199 3 6 8 2980 2981/ 28 61 65 527 281 3980 36 1.6 191 8500 24 1 117 3020 9 496 9 5 7 24 12411 116 5800 275 28 3

--

0.96

2

310

Fe (mg/l) 0.6 1.1 0.2 0.6 0.6 0.03 2 14 2 0.2 1 0. 11.1)6 {1.6 <0. I 1.3 <0.1 <0.08 0.2 ---<0.1 <0.1 11.2 11.7 0.2 <0.1-0.7 0.1 0.5 0.3 <0.08 0.1 < 0.08 -
0.06

continued

582

M. Gascoyne Table 2.----continued Well*/ borehole number

Deptht sampled (m)

U (/zg/l)

WN-1 WN-4 WN-8 WN-10 WN-11 WA-1 WB-1

381-402 356-410 305-325 298-370 1099-1201 210-310 210-300

23 77 29 1.1 3 15 4

234U/238U 4. ! 3.0 4.4 4.8 4.2 3.1 4.7

226Ra (Bq/1)

CI (mg/1)

Fe (mg/l)

2.73 7.65 0.11 4.11 37.9 0.12 0.05

3430 3910 284 2910 18950 2.5 294

0.27 0.26 0.02 3.9 0.39 -1.3

*W = water supply well, all others are exploratory boreholes. -tSampling interval in well or borehole, N = depth not known, S = completed in surficial sediments (all others are in bedrock). $Seepage into 240-m level of URL facility. two elements and the fact that they are separated in the decay series by a long-lived, insoluble isotope of thorium, 23°Th. The presence of Ra in a few shallow groundwaters may be due, instead, to localized precipitation of 23°Th coupled with active dissolution of alkaline-earth elements (Ca 2+ , Sr 2+ , etc.). Higher concentrations of Ra are seen in the more saline waters as shown by the relation (Fig. 3) between Ra and total dissolved solids (TDS). This association is c o m m o n l y found in brines in oil-bearing formations and other rock types (LA~MtrlR and MELCHIOR, 1985) and may be explained by cation saturation of exchange sites in contacting solid phases, thereby inhibiting sorption of Ra.

Uranium in groundwater U r a n i u m concentrations observed in many of the well waters and borehole groundwaters are anomalously high c o m p a r e d to groundwaters in other granitic rocks (Table 1). The cause of this enrichment could be a combination of the following factors: (1) High U in source rocks, fracture-filling minerals and overburden; (2) High concentrations of H C O 3 in shallow groundwaters; and (3) Presence of oxidizing conditions in groundwaters to depths of at least 100 m.

101 -

10 0 .

ee

=Z ::)

a

if00

lO-L

• e• •

•B eiD• • • =

boreholes

• =

well8

<2x10_ 2 • ek

lo-~

i

102

i

103 TOTAL

i

104 DISSOLVED

SOLIDS

105

(rag/r;)

FIG. 3. Variation of 226Ra concentration with total dissolved solids (TDS) content of well waters and groundwaters from the Whiteshell Research Area.

U and Ra in groundwater, Manitoba, Canada These possible causes are considered in greater detail below. Leaching f r o m source materials. KAMINENI et al. (1986) report a mean U concentration of 6.6 t~g/g for unaltered grey granite from the Lac du Bonnet granite batholith, a level that is slightly higher than other granitic bodies of similar age (about 4 /xg/g). Of probably more significance is the enrichment of U in wall rock and mineral coatings of fractures in the granite. Analyses of drill core samples from the U R L lease area (GAsCOYNE and CRAMER,1987) show that U may be enriched up to an order of magnitude over the surrounding rock in the few centimetres of core adjacent to a fracture. Evidence for recent leaching of this U is the strong depletion of 234Uin altered rock immediately contacting fracture zones at depths of up to 160 m. High U levels in hematite-rich wallrock in these zones show decreasing 234U/238U activity ratios (to 0.65) and high 23°Th/234Uratios toward the fracture, indicating the loss of U, especially 234U, to groundwater in the last 1 Ma. Analyses of surface rocks for Th/U abundance ratios show a deficiency of U and high 23°Th/234U ratios, indicating recent and pervasive leaching of U at shallower depths. These observations support the groundwater data and indicate both modern and geologically recent U removal by groundwater with preferential dissolution of 2 3 4 U in, at least, the upper 160 m of granite at the U R L site. Evidence of U migration at greater depths at the U R L site is found in results of analyses of altered cores from 262 m. These have both high and low 234U/238Uratios showing that deposition and leaching of 234U, and possibly 2380, has occurred in the last 1 Ma (GAscOYNEand CRAMER, 1987). In addition to rock-derived U, BETCHER et al. (1989) note that Quaternary overburden materials, particularly fractured clays, may also be a significant source of dissolved U because high U concentrations have been observed in wells completed in these deposits. Although the U content of the clays is relatively low ( - 2 /zg/g), it is believed that this is labile U and is easily dissolved during subaerial weathering by downward percolation of low-pH, high-O 2 waters. Bicarbonate concentrations in groundwater. Uranium solubility is enhanced in the presence of HCO 3 because of the stability of the uranyl carbonate complex in waters of the pH range 7-9. Deeper, more saline groundwaters in the batholith contain less HCO 3 (usually <100 mg/1) and these waters tend to be deficient in U. The significance of HCO 3 in causing high U concentrations in groundwaters is examined in Fig. 4. Although a linear relation is not obvious, there is an indication that waters containing >150 mg/1 HCO3 contain more U.

583

!837

~,

!573

•*

3oo

:L v z

o_

• - URL Boreholes • - Wells

2OO W U Z 0 U

| IO0 200 3 0 0

400

500 600 700 800 900 1000

BICARBONATE CONCENTRATION (mg/I.)

Fla. 4. Variation of U with HCO 3 concentrations for WRA and well groundwaters.

Groundwater redox conditions. Relatively oxidizing conditions appear to exist in groundwaters to depths of at least 100 m in the Whiteshell Research Area as shown by the relation of U concentration to depth and measured Eh potential. The variation of U concentration with depth of well or borehole interval is shown in Fig. 5a. High U concentrations tend to be found in shallow groundwaters (<100 m) and U content decreases with increasing depth, at least to 200 m. This type of correlation has been observed in previous studies of groundwater in crystalline rocks (ANDREWS et al., 1982b; MILTON and BROWN, 1983) and is explained by the more reducing nature of groundwater at depth, causing reduction of U(VI) to the less soluble U(IV). In the W R A such a correlation may be more difficult to perceive because groundwater flow paths tend to be dominated by subhorizontal fracture zones, along which water may move to different depths but without necessarily a change in redox condition. Most groundwaters, however, from below depths of about 200 m contain <30 /~g/1 U, in general agreement with the concept of lower redox potential at depth. The 234U/z38U activity ratios of all groundwaters (Fig. 5b) show that low ratios (<2.0) tend to be found only in shallow groundwaters, and higher ratios occur at greater depths. A trend of decreasing U concentration with increasing isotopic ratio can be seen in Fig. 5c although several groundwaters are exceptions to this. This trend is less distinct than that reported for other areas (OsMoND and COWART, 1976; ANDREWS et al., 1982b; MILTON and BROWN, 1983) possibly because oxidizing conditions extend to varying depths, controlled by the sub-horizontal fracture zones, that either receive recharge from, or discharge through, intersecting vertical fractures. Estimation of the redox potential of groundwaters in this study has been made from measured concentrations of Fe and U and electrode potential measurements. Total Fe concentrations for these waters are shown in Table 2. Most W R A groundwaters contain

584

M. Gascoyne a

URANIUM C O N C E N T R A T I O N ( p g l t , ) 0

100

200

300

i

i

i

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F1G. 5. Variation of (a) U concentration and (b) 234 U/238 U activity ratio with depth of sampling zone and (c) relation between U concentration and 2 3 4 U / 2 3 8 U activity ratio for well and borehole groundwaters. The horizontal lines refer to the multiple samples taken from borehole URL-I 0. very little Fe (generally <0.2 mg/l) compared to groundwater in other granitic terrains (KAY and BATH, 1982; WIKBERG et al., 1983) where concentrations usually lie between 0.5 and 4 mg/1. These higher concentrations apply to waters of near-neutral pH, when Fe z+ is relatively stable. Granite groundwaters at Stripa, Sweden, however, are deficient in Fe (generally <0.06 mg/1) and this has been attributed to the relatively high pH of the water (8-10) coupled with the solubility control of Fe oxyhydroxides (NORDSTROMet al., 1985). Although not listed in Table 2, groundwaters in the W R A lie in a fairly narrow pH range (8.0-8.5), at which Fez+ becomes stable only at low Eh (< -100mV). The low Fe concentrations suggest that W R A groundwaters are more oxidizing than this value. In addition, measure-

ments of Eh have been made at the surface (in a sealed flow cell) and downhole (by submerged sensor) which indicate that the Eh of the WRA groundwaters is generally > - 1 0 0 mV (Ross and GASCOYNE, 1989). The U data also indicate that the groundwaters are relatively oxidizing because, at pH 8-8.5, significant dissolved U can only be found if Eh is > - 5 0 mV (LANGMUIR, 1978). Exceptions to these conditions are found in some groundwaters in the study area (e.g. M-12) where pH exceeds 9.0 but U is still present and Fe absent (Table 2), and in deeper, highly saline waters (e.g. M-7) where pH is near neutral and both U and Fe are present. The well waters are similar to the groundwaters encountered in the exploratory boreholes at the W R A except that they may contain more Fe (up to 14

U and Ra in groundwater, Manitoba, Canada mg/l) and have pH values of ~7.5. The lower pH stabilizes the higher Fe 2+ content but because U concentration is at least as large in well waters as in WRA groundwaters, it is likely that the Eh of well water is greater (in fact, most wells are shallower than the sampling zones in the boreholes). In addition, HCO3 concentrations in the well waters are at least twice those of groundwaters from the boreholes (because of mixing of HCO3-rich shallow groundwater and interaction with the overburden) and this further stabilizes dissolved U concentrations.

Effects of pumping Several borehole zones at the URL were repeatedly sampled during groundwater pumping tests which were conducted to determine the hydraulic characteristics of the URL site. The variation of U concentration and 234Uf138U activity ratio during these tests is shown in Fig. 6. In general, U concentrations either remained constant or decreased during pumping. Higher concentrations at the start may be due to withdrawal of water that had been in a relatively higher redox environment, as might be found in a recently drilled borehole that had been incompletely flushed out. Boreholes URL-9, URL-

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234U/238U activity ratios (dots) with volume of water removed during continuous pumping of four boreholes at the URL site over a period of several days. Uncertainties of the isotope activity ratios are shown by vertical lines and of concentrations, generally lie within the symbol limits.

585

10, M-7 and M-12 had all been drilled within the previous three years and, until the pump test, had been pumped only infrequently and for short durations. The data for borehole M-7 (Fig. 6), however, show remarkably constant U concentrations and suggest that the zone being pumped was undiluted by drill or surface water and the groundwater had attained a steady-state, if not equilibrium, composition with respect to the host rock. This interpretation is supported by the absence of 3H in the groundwater, the observed lack of change in chemical composition during pumping, and the greater depth of the zone (390 m) which distances it from surface contact. The data from borehole M-7 are discussed in greater detail in the section on solubility controls.

Long-term variations in U concentration In addition to variations during pump tests, the possibility of long-term variation in U concentration was examined by periodic pumping and sampling of one zone in borehole URL-10, a borehole close to the location of the URL shaft. The zone is between 53 and 121 m deep and it samples water mainly from the upper sub-horizontal fracture zone at a depth of about 110 m at the URL. Results are shown in Fig. 7, together with hydraulic characteristics and other geochemical parameters. It can be seen that, over the period October 1983 to November 1985, there were two occasions of pronounced U enrichment in groundwater from this zone, both occurring during late summer. The 2 3 4 U / 2 3 8 U isotope activity ratios also varied throughout the period, but they correlate inversely with U concentration (Fig. 7). With the possible exception of CI, no similar variation can be seen in dissolved ion concentrations, or total Fe as shown in Fig. 7. These characteristics may be explained as follows. Sinking and excavation of the URL shaft began in June 1984, and the groundwater level in the sample zone dropped by almost 40 m over a four-month period (Fig. 7). This drawdown was maintained to the end of the study. The two periods of U enrichment are unlikely to be due to the period of changing groundwater level in the fracture zone because, during the second year (1985), groundwater levels remained relatively stable and only responded to natural recharge events. The large enrichment is probably a natural seasonal characteristic and is magnified by effects of the drawdown, which has caused a reversal of natural groundwater flow along the fracture zone. Groundwater recharge entering the fracture zone from snow melt or early summer rainfalls dissolves U from surficial deposits or U-rich precipitates and wall-rock in the fracture zone. The inverse correlation of U concentration with 2 3 4 U / 2 3 8 U ratio supports this interpretation because spring and summer recharge has a short residence time in the groundwater flow system which causes less 2 3 4 U t o be

586

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FIG. 7. Variation of U concentration and 234U/238U activity ratio in groundwater from borehole URL- 10, zone 53-121 m, over the period of excavation of the URL shaft (shown as water-level drawdown). Variations in other dissolved ion concentrations are also shown. contributed by preferential leaching of 234U from mineral phases. The lag time between groundwater recharge and the occurrence of the U pulse in URL-10 is estimated to be about one to three months assuming that spring snow melt and early summer rainfall are the main sources of recharge. A short groundwater flow time is also predicted from estimates of flow velocity calculated from Darcy's Law, as follows. The flow rate, Q, is related to the hydraulic conductivity, K, the hydraulic gradient, i, and the cross-sectional area of flow, A, thus Q = K i A , and the groundwater flux q = Q / A = K i . Because flow in crystalline rock occurs mainly in fractures, then for a fracture zone of porosity n, the groundwater velocity is q / n = K i l n . In the case of the fracture zone passing through borehole URL-10, K has been determined from hydraulic measurements to be about 10-7 m/s (DAvISON, pers. comm.), i is about 0.4 and n is about 10-2 . Therefore, flow velocity is calculated to be about 1250 m/a, and the transit time between the recharge area to the borehole sample zone, a distance of about 100 m, is approximately one m o n t h - - a time interval comparable to that estimated above. The exact geochemical cause of the high U pulse

during the summer is not clear but it may be due to (1) increased biological activity in the soil, producing more CO2 for rock dissolution and hence U leaching, or (2) higher levels of dissolved oxygen in the recharge groundwater (older groundwater is likely to be depleted in oxygen from rock-water interaction over the longer residence time). These effects combine to increase U concentration because groundwater Eh is increased (and, hence, U is oxidized to the more soluble U(VI) form) and the U is stabilized in solution as the uranyl carbonate species. Although a correlation between U and H C O 3 concentrations might, therefore, be expected, none is observed (Fig.

7). As indicated above, the degree of the seasonal U enrichment in borehole URL-10 groundwater may be induced by groundwater level changes caused by drainage to the adjacent U R L shaft. Most boreholes at the U R L are influenced by this disturbance and are therefore unsuitable for determining whether a similar seasonal effect exists in an undisturbed hydraulic situation. Although no regular sampling for U has yet been done in an undisturbed borehole elsewhere in the W R A , routine sampling of a water-supply well in the Lac du Bonnet area shows no seasonal effects

U and Ra in groundwater, Manitoba, Canada over a similar two-year period (Table 3). This well creates its own hydraulic disturbance (although considerably smaller than at the U R L ) because it is pumped daily for household supply (expected usage is between 500 and 1500 l/d, estimated from data given by FUNK et al. (1980)). The results in Table 3 are for samples taken at both anticipated high and low U concentration periods, yet they show no m o r e than a _+5% variation from a m e a n concentration of 200/zg/1 throughout the year. The difference between the well and borehole U R L - 1 0 observations may be due to the differing groundwater recharge and hydrogeological conditions at each location. G r o u n d w a t e r in the fracture zone of borehole U R L - 1 0 is derived from recent recharge as indicated by the previous discussion and by 3H values as high as 45 T . U . These values are close to those of modern precipitation in the area ( - 5 0 T . U . ) . The well water, however, has no detectable 3H and hence, is older than - 3 0 a. This suggests that the well intersects a longer or slower flow path, one which will respond less to recharge events and, therefore, probably have a m o r e stable U concentration. G r o u n d w a t e r in the U R L - 1 0 zone responds more readily to recharge events and has a U concentration that is controlled by the influx of oxygenated fresh water through o v e r b u r d e n and fractured rock containing readily available U.

Table 3. Uranium concentrations and activity ratios of a domestic well water near Lac du Bonnet, Manitoba Sample 28-G 28-G-0 28-G-02 28-G-03 28-G-05 28-G53-0 28-H-0 28-H-7-1)

Date 84~)1q39 84-03-15 84-04-24 84-07-11 84--09-26 85~)6-27 85-10-30 86-04-23

U (/xg/l) 216 210 199 2ll 202 191 195 207

234U/238U

_+ 6 _+ 6 _+ 3 _+ 3 + 4 _+ 4 _+ 15 +_ 5

1.67 1.64 1.81 1.67 1.65 1.71 1.57 1.70

+_ 0.04 _+ 0.04 _+ 0.02 _+ 0.01 _+ (I.03 + 0.03 _+ 0.12 _+ 0.04

587 DISCUSSION

Calculations of the state of saturation of W R A groundwaters with respect to the low-temperature secondary minerals, calcite and gypsum, has been reported by GASCOYNE et al. (1987). Almost all waters are saturated with respect to calcite and undersaturated with respect to gypsum. These results are supported by the observed abundance of calcite and general absence of gypsum in fractures at all depths in the W R A . The pH, cation and silica concentrations of most W R A groundwaters indicate stability with respect to the clay minerals, illite and kaolinite. Only illite is observed in any abundance in fractures in the W R A , possibly because of kinetic control of groundwater composition in the muscovit e-illit e-kaolinite transition.

Speciation o f uranium The compositions of several groundwaters that typify the ranges in pH, Eh and salinity found in the W R A were used as inputs to the geochemical code S O L M N Q (GooDwIN and MUNDAY, 1983), a modified, interactive version of S O L M N E Q , to examine the controls on U speciation. The results are summarized in Table 4. In the more reducing groundwaters where U concentration is low, U (IV) hydroxy species predominate. The U ( V I ) carbonate complexes predominate in oxidizing (Eh > 0) groundwaters. The speciation of U ( V I ) in W R A groundwaters has been independently confirmed by results of testing an A E C L - d e s i g n e d , anionic resin filter for removal of U from drinking water (GAscOYNE, 1986). Tests show that > 9 9 % of dissolved U is r e m o v e d from the first - 3 0 0 0 1 of water passed through the filter (resin volume - 1 1), thereby indicating the predominance of uranyl anion species.

Solubility controls Redox and salinity. Table 4 also shows the maxi-

Table 4. Examples of speciation and calculated U solubility obtained using the geochemical code, SOLMNQ, for WRA groundwaters with in situ pH and Eh measurements Measured

Borehole M3A M5B M7 M12

Sample number -IN-8 -IN-9 -390-16 -156-16

U (txg/1) 0.4 11 5 26

Predicted

(/xg/1)

pH

Eh (mV)

46 187 25 271

6.9 7.6 7.1 8.0

-50 +20 -140 +30

HCO3

*a = UO~(CO3)~- ; b = UO2(CO3) 4- ; C = U(OH)5 ; d = UO22+; e *S1 = log (observed [U]/predicted solubility of UO2 or U409). :~_= but see text.

U* Speciation d,a,b b,a c$,d,e b,a = U(OH)4;

Stable solid phase

Maximum solubility (~g/1)

Saturation+ index

UO2 UO2 UO~ U409

<(/.001 19
>2 -(/.2 >2 0

listed in order of abundance.

588

M. Gascoyne

mum U solubility and the saturation indices calculated by SOLMNQ for each groundwater. In all cases, UO2 or U409 is predicted to be the stable phase. Groundwaters with low Eh (<0) appear to be supersaturated with respect to these minerals and calculations predict that U concentrations should be extremely low in these groundwaters (<0.001/~g/1). Under mildly oxidizing conditions, however, the groundwaters are considerably undersaturated and U concentrations of at least 760 /~g/l might be expected. These levels have, in fact, been seen in the monitoring of borehole URL-10, as described above, but unfortunately no pH and Eh measurements were made at the time of sampling that might support these results. In a recent sampling of borehole URL-10 groundwater, an Eh value of + 145 mV was measured but U concentration was only 170 /~g/l, suggesting that redox is not the only control. Other possible controls are availability of soluble U minerals or of U sorbed onto mineral surfaces, and contact time of these minerals with groundwater. A further anomaly in the expected U-redox relation is the result for the saline groundwater collected from a depth of 360 m in borehole M-7. Uranium concentration remained very constant over a 16-d pumping period ( - 5 p~g/l, see Fig. 6) but all redox probe measurements (values range from 0 to 165 mV) and resulting equilibria calculations indicate a maximum U solubility of about 0.001/~g/1 (see above discussion, Table 4). The redox probe measurements could be unreliable for two reasons: (1) electrodes of the Eh sensor could be too low as a result of poisoning by HzS that could be detected occasionally during pumping; and (2) a single Eh value may not be representative for this groundwater because the main redox-controlling species, Fe 2+ , is of low concentration (0.5 mg/1). The electrodes may be responding to a half-cell reaction that has no bearing on the solution Eh. Nevertheless, both a calomel and a Ag/AgC1 electrode gave comparable Eh values and this measured range is not unreasonably low for a deep groundwater (BAAs-BECKINGet al., 1960). If it can be assumed that residence time of the groundwater collected from 360 m depth in borehole M-7 has been long enough to ensure that equilibrium has been attained (3H data indicated the water to be at least 30 a old), then the predicted speciation or values of equilibrium constants in SOLMNQ may be suspect. The SOLMNQ constants are taken from a review by LEMIREand TREMAINE(1980). Recent data (PARKS and POHL, 1988) indicate that the speciation of U(IV) at pH >7 is dominated by U(OH)4, not U(OH)5, and that U solubility under reducing conditions is higher (up to about 10-9.5 mol/l, or 0.08 /zg/l) and less dependent on pH. The observed U concentration of this sample is still two orders of magnitude greater than this limit. Several other deep, saline groundwaters in the W R A (e.g. M-5A, M-10, M-13, and WN-11) also possess significant (>1

/~g/1) U concentrations (Table 2) indicating that this is a more widespread characteristic. In a recent reexamination of U solubility in natural waters, LEMmE (1988) found that higher U concentrations than previously calculated could exist in saline groundwaters because of formation of complexes with C1, SO 4 and F, and stabilization of charged species. The results obtained in the present study have been compared with Lemire's solubility predictions for 30 of the samples shown in Table 2 that have detailed measurements of pH and Eh obtained using a sealed flow-through cell at the surface (Ross and GASCOYNE, 1989, unpub, data). The results are shown in Fig. 8. Eh is represented in ranges of 50 mV because of measurement uncertainties and indications that Eh may be lower downborehole than at the surface (WIKBERGet al., 1987; Ross and GASCOYNE, 1989). The distribution of data

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Fio. 8. Relation between U concentration, pH and redox potential (Eh), measured for groundwaters of varying salinity in the WRA compared with calculated U solubility relations for dilute (GGW, TDS = 0.1 g/l) and saline (SCSSS, TDS = 55 g/I) synthetic groundwaters.

U and Ra in groundwater, Manitoba, Canada points in Fig. 8 shows that U concentration ranges over almost three orders of magnitude for a variation of three pH units, and that there is no significant U pH relation. There is, however, some clustering of "saline" (>5 g/1TDS), relative to non-saline, groundwater data points with the saline samples tending to have lower U content. Similarly, oxidizing groundwaters (open symbols, Eh > 0 mV) tend to contain more U than reducing groundwaters (solid symbols, Eh < 0 mV). Also shown on Fig. 8 are the modelled solubility limits for fresh and saline groundwaters for three different redox conditions. These relations are determined from the revised thermodynamic database, with allowance for salinity effects, reported by LEMIRE (1988). The solubility of U has been determined for groundwater in equilibrium with common U-minerals for the redox conditions: (1) oxidizing (Eh = 0.8~).0592 pH) (2) mildly reducing (Eh = 0.5~).0592 pH) (3) reducing (Eh = 0.2~).0592 pH) For groundwater of pH - 8 , these correspond to Eh values of +300, 0 and - 3 0 0 mV, respectively, limits which generously cover the range of values measured for W R A groundwaters. Predicted variations in U content of groundwater of pH - 8 therefore range from 10-3 to 10-10 mol/1 (240 mg/l~).024/~g/l) depending on redox state (Fig. 8). The relations also show U to be more soluble in a saline than a fresh groundwater especially in the pH range 7-10. It can be seen from the distribution of data points in Fig. 8 that most WRA groundwaters fall close to, or below, the calculated solubilities for the "mildly reducing" redox state. However, higher salinity groundwaters tend to have lower U contents and this may be because their lower Eh values tend to override any effects of increased solubility due to higher salinity. The general location of data points on Fig. 8 indicates that the U concentrations are in reasonable agreement with the redox potentials of the groundwaters measured in a flow cell at the surface. However, if lower values of Eh are more representative of the true in situ redox potential, then the observed U concentrations are higher than predicted solubilities. Colloids and dissolved organics. Other possible influences on the U contents of the well and borehole groundwaters include the presence of U in colloidal form (i.e. U not truly in solution but which can pass through a 0.45/~m filter and be dissolved during the analytical procedure), or the complexation of U by dissolved organic compounds. Analysis of the fraction filtered to remove all particles >1 nm in several saline groundwaters in the W R A (P. VILKS, pers. comm.) has shown that <10% of total U is colloidal. Dissolved organic C concentrations in W R A saline groundwaters appear to be generally low ( - 2 mg/1). No attempt has been made so far to determine if U is associated with these organics.

589

Reaction kinetics. The previous discussion assumes that chemical equilibrium exists between dissolved U, other dissolved species and the solution redox conditions. Alternatively, the U system may not be coupled kinetically to electroactive species in solution at these temperatures and, therefore, the rate of equilibration of U-species with rock materials at a given Eh is extremely slow. In addition, disturbances due to borehole drilling, drillwater injection and groundwater pumping may cause persistence of nonequilibrium conditions, which may effect pH, Eh and U content for periods of months to years. However, the general agreement between U content, Eh, pH and groundwater salinity, shown in Fig. 8, tends to argue against a kinetic control and indicates instead that U content may be a useful qualitative indicator of groundwater redox conditions. The observation of high U concentrations in groundwaters of a relatively low U granite (i.e. up to 1 mg/l U for a granite containing 1-30 mg/kg U) contrast markedly with data obtained for groundwaters in contact with a U ore body in northern Saskatchewan (CRAMER, 1986). In the latter area, groundwaters contain very little U (up to 12 p~g/1)yet are in contact with rocks containing up to 65% U3Os. These groundwaters are compositionally very similar to shallow W R A groundwaters except for a generally low redox potential and lower HCO 3 and higher Fe2+ contents (both are likely a result of the low Eh). These differences clearly underline the significance of redox potential in controlling U concentration in groundwater.

SUMMARY AND CONCLUSIONS Uranium concentrations up to 840/zg/1 and 220Ra concentrations up to 38 Bq/l have been found in groundwaters of the Lac du Bonnet granite batholith, Manitoba. High Ra levels are mainly confined to a few shallow freshwater wells and to deep saline groundwaters in the Whiteshell Research Area. High U concentrations are more common and occur mainly in borehole and domestic well waters in the upper 100 m of the batholith. The U contents are considerably higher than many of those previously reported for Canadian and global groundwaters, yet there is no obvious concentration of U in the area (such as a U ore body or highly mineralized veins) other than some enrichment in the altered rock of permeable fracture zones. The key control for U concentration in groundwater appears to be redox potential, provided that the water has adequate residence time (of the order of a few months) in the U source zone (overburden and fracture wall rock) and has significant dissolved HCO 3 concentration (>150 mg/1) to stabilize the soluble uranyl carbonate complexes. Seasonal variations of U concentration may also be explained by redox control, whereby peak concentrations derive

590

M. Gascoyne

f r o m o x y g e n a t e d recharge waters during early summ e r m o n t h s . The p r e d i c t e d speciation of U in t h e s e g r o u n d w a t e r s is consistent with the success o f an anionic filter system for r e m o v i n g U f r o m local drinking waters. A l t h o u g h not p r e s e n t in sufficient c o n c e n t r a t i o n to control solution r e d o x potential, U could be a useful indicator of r e d o x potential because of the b r o a d d e p t h - - c o n c e n t r a t i o n relation, and the general agreem e n t b e t w e e n m e a s u r e d E h and that calculated using r e c e n t t h e r m o d y n a m i c equilibria. Acknowledgements--This paper is a culmination of work begun in 1982. During this period, many people have been involved in the sampling and analysis of groundwaters, and particular thanks go to Alain Larocque, Alice Wehlau, Jim Ross and Don Daymond. Dennis Brown, Head of Water Standards and Studies and Bob Betcher of the Groundwater Resources Branch, of the Province of Manitoba, are gratefully acknowledged for providing samples and water chemistry data for well waters near to the Underground Research Laboratory. Most of the in situ pH and Eh data referred to in this study has been obtained by the efforts of Jim Ross. This manuscript has benefited by comments received from R. N. Betcher, N. C. Garisto, B. W. Goodwin and R. J. Lemire, C. J. Bland and J. N. Andrews. Editorial handling: Brian Hitchon

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