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Origin of groundwater salinity in the Lac du Bonnet granite, southeastern Manitoba, from 36Cl measurements
Nuclear Instruments and Methods in Physics Research B 92 (1994) 389-392 North-Holland
NOMB
Beam Interactions with Materials&Atoms
Origin of groundwater salinity in the Lac du Bonnet granite, southeastern Manitoba, from 36Cl measurements Mel Gascoyne * Applied Geoscience Branch, AECL
Research, Pinawa, Man. ROE lL0, Canada
Pankaj Sharma Physics Department,
Purdue Universi@, West Lafayette,
IN 47907, USA
Peter W. Kubik Znstitut fiir Mittelenergiephysik,
ETH-Hiinggerberg,
CH-8093 Ziirich, Switzerland
The results of analyses of the 36Cl content of groundwaters in the Lac du Bonnet granite batholith are described and compared with previous modelling calculations and 36Cl content of rock matrix solutions obtained from a borehole leaching experiment. The results are used to provide support for the proposal that groundwater salinity is largely derived from dissolution of soluble salts in the rock matrix.
1. Introduction Groundwaters from depths of up to 1 km in permeable fractures and fault zones in the Lac du Bonnet
granite batholith, southeastern Manitoba (Fig. 11, are being sampled as part of the geoscience studies done by AECL Research for the Canadian Nuclear Fuel Waste Management Program [l]. The groundwaters have previously been analysed for ionic and isotopic composition to determine the source of dissolved salts, groundwater residence time and principal rock-water interactions. One of the major objectives of this work is to determine the source of dissolved salts, particularly for the deeper, more saline groundwaters in the batholith. In previous work [2,3] two main sources of salts have been proposed: 1) intrusion of basinal brines into the granite from the sedimentary rocks of the Manitoba basin, and 2) leaching of soluble salts present in grain boundaries and fluid inclusions in the granite minerals. This paper describes the results of analyses of the 36Cl content of groundwaters in the batholith and compares the results with previous modelling calculations and 36Cl content of rock matrix solutions ob-
* Corresponding 753 2455.
author. Tel. + 1 204 753 2311, fax + 1 204
Elsevier Science B.V. SSDI 0168-583X(93)E1135-9
tained from a borehole leaching experiment. The results are considered in terms of possible origins of groundwater salinity and groundwater residence times.
2. Methods Groundwaters in permeable fractures in the Lac du Bonnet granite have been obtained from over 100 boreholes drilled to depths of up to 1000 m in the batholith. Various techniques of pumping and sampling the groundwaters have been used. These have been described elsewhere [4]. Groundwaters were selected for analysis for 36Cl so that a wide range of salinities (mainly caused by the presence of Na, Ca and Cl) and low 3H levels (indicating minimal drillwater contamination) were represented. Some of these groundwaters were of interest because of their location in the groundwater flow system (i.e., in recharge and discharge areas). Samples of water were also obtained for analysis from two unfractured, packer-isolated sections of boreholes that had been previously filled with deionized water at atmospheric pressure. These boreholes are located at a depth of 420 m in AECL’s Underground Research Laboratory (Fig. 1). The salinity of these waters had increased over a one-year period, presumably because of mixing with rock matrix fluids migrating into the borehole under diffusion and imposed hydraulic gradients. VI. HYDROLOGY
390
M. Gascoyne et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 389-392
Study
Area4
7/
/
Whiteshell Laboratories m
~~~m;d
Research
m
Ddillng Permits (A-l)
I 5
0
I 5km. A
Fig.
1. Location
map
of the study area Underground
showing the outcrop Research Laboratory
Both groundwaters and borehole leaching waters were analysed for their ionic composition by AECL Research using standard analytical techniques. The 3H content of the groundwater was determined under contract by the University of Waterloo Isotope Laboratory using 3H enrichment by electrolysis. The atomic abundance ratio, 36Cl/total Cl, was determined by accelerator mass spectrometry at the University of Rochester, New York, and PRIME Lab, Purdue University, Indiana.
of the Lx du Bonnet granite batholith lease area and borehole drilling sites.
(solid
black
line),
the
borehole before the permeable zones were isolated are the probable explanations for 3H levels above l-2 TU. It can be seen that the Cl concentration generally increases with the depth of sampling, except in areas of groundwater recharge and discharge, where dilute groundwater penetrates to depths of up to 360 m (sample M8-3-7) and brackish water occurs at shallow depths (B34-2-4). The relationship between the 36C1/C1 ratio and Cl concentration is shown in Fig. 2, together with the range of values for the borehole leaching test waters.
3. Results
The 36C1/C1 ratios and Cl and 3H concentrations of groundwaters from 19 permeable zones in boreholes in the Lac du Bonnet granite are shown in Table 1, together with ratios and Cl concentrations of the two borehole leaching waters. Tritium concentrations are low or below detection limits in most groundwaters. Contamination by residual drillwater or flow within the
4. Discussion Groundwaters from shallow depths (< 150 m) and those in recharge areas up to 400 m deep typically have low Cl concentrations (< 50 mg/l). This suggests that they have experienced little interaction with the host
M. Gascoyne et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 389-392 Table 1 Values of 36Cl/Cl ratio, 3H and Cl concentration in groundwaters in the Lac du Bonnet granite and borehole leach test waters Sample #
Depth
Cl
[mg/ll
36C1/C1 (X lo-‘?
3H
[ml
17 18 28 30 37 207 235 430 454 510 1240 3070 3430 3710 6010 8700 11500 19000 29500
107+ 11 203+ 19 85+ 9 120+11 88+ 9 41+_ 4 41+ 8 6Ok 9 49+ 5 45f 8 42+ 5 59+ 6 57+ 8 34* 7 49+ 5 41* 7 43+ 6 57* 7 44+ 8
1.3 7.4 < 0.8
1580 1700
25+ 2 21+3
_ -
Groundwaters 50 M14-1-4 360 M8-3-7 230 URL8-7-7 120 M5B-IN-9 310 M2A-3-12 265 MlA-3-7 250 HC29-10 250 HC7 250 HC17-12 40 B34-2-4 250 M13-2-5 605 URL12-13-13 380 WNl-8-17 410 MlO-3-8 370 M14-4-5 390 M7-4-9 895 WD3-895-10 1000 WNll-17-14 1000 WBl-7-7 Borehole leach waters 420 GC2-2 420 MB2-1
[TUI
ND = not determined. a Determined without enrichment by electrolysis.
rock during their comparatively short residence time. The dilute groundwaters have 36C1/Cl ratios ranging between 85 and 200 ( X 10-15). In these samples, 36C1is probably derived largely from cosmogenic and, possi-
.
‘0 + 140
x
1.
’ 1 G \ G
.Y..
100
nn
I
60
_
.,
,..,.
.___.
I
20
II
1.0
. . . . .
c .
leach VIaten
2.0
y...
. . . .
.I..
n
l*
3.0
4.0
5.0
log Cl - (mg/L) Fig. 2. Variation of 36Cl/C1 ratio with Cl concentration in groundwaters from the Lx du Bonnet granite. The range of ratios and Cl concentrations of the borehole leaching experiment waters is shown for comparison.
391
bly, bomb sources, as their 3H content suggests recent recharge. Analyses of 14C and ‘H/ 180 contents (Gascoyne, unpublished results) further indicate that these groundwaters are clearly < lo4 a old. Deeper groundwaters and those in the central part of the flow path and in discharge areas have higher Cl concentrations (200-30 000 mg/l). These concentrations may be derived from Cl introduced into the fractures in the granite by intrusion of saline waters present in sedimentary rocks to the west (or, possibly, deep penetration of overlying seas during Paleozoic time). Alternatively, groundwater salinity may be derived from the leaching of salts from the rock matrix. Previous studies of the content of soluble salts in various phases of the Lac du Bonnet granite [3] have shown that Cl concentrations are about 50 mg/kg in unaltered grey granite, with lesser amounts in pink, altered phases. These salts are believed to exist in micropores and as fluid inclusions and grain-boundary deposits, which are accessible to permeating groundwater. The lack of correlation between the 36Cl/C1 ratio and the groundwater Cl content (Fig. 2) also suggests that Cl is derived from the rock matrix because leaching of matrix salts and fluids by groundwaters from fracture wall-rock will give 36Cl/C1 ratios that are typical of the wall-rock matrix and this ratio will be constant regardless of whether the groundwaters are dilute or saline [5]. Alternatively, the consistency of 36C1/C1 ratios for a range of groundwater salinities could also be explained by intrusion of sedimentary basin waters into the granite > 1 Ma ago. Secular equilibrium with the neutron flux in the granite would be attained by the present time. The hypothesis proposing a matrix salt origin is supported by the observation in the borehole leaching experiment that salts can readily migrate through the granite matrix into water-filled boreholes and, presumably, fractures. However, the 36Cl/C1 ratios of the groundwaters in fracture zones are significantly higher (by a factor of 1.5-2.5) than those of the borehole leaching waters whose salinity is derived only from rock-matrix salts. This can be readily explained by the fact that the altered rock typically associated with fractures in this area has been found to be enriched in U and, to some extent Th, by up to a factor of 5 [6]. Groundwater in fractures therefore experiences a greater neutron flux than the water filling the leaching test boreholes in unaltered granite, and so it achieves a higher 36C1/C1 ratio. In previous work, 36Cl/C1 ratios of the saline groundwaters were compared with ratios obtained by modelling [5]. Neutron production rate and energy spectrum were calculated using the SOURCES code [7]. The neutron flux and, consequently, 36C1 production rates were then calculated using the Monte Carlo VI. HYDROLOGY
392
M. Gascoyne et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 389-392
neutron transport code, MCNP [8], for the average elemental composition of the granite [6,9]. A range of 36C1/CI values was generated by sensitivity analyses with the MCNP code in which key elements controlling the 36CI production rate (H, Sm + Gd, I-J, Th) were varied within reasonable limits. The results showed that 36C1/CI ratios in the range of 40-100 X 10-15, with an average of 65 x lo-l5 ( f 35%), were predicted for soluble Cl in the unaltered granite matrix under conditions of secular equilibrium with the in situ neutron flux. The 36C1/C1 ratios for most groundwaters containing > 50 mg/l Cl lay within the calculated range of values, albeit at the lower end. The results of the borehole leaching experiment indicate that ratios from model calculations for the rock matrix are significantly higher than the observed ratios, although agreement is seen if a 2u range in the error limits is applied to the modelled result. Variables and uncertainties in the model are more likely to be the cause of this discrepancy because the borehole leach waters clearly obtain Cl only from the unfractured rock matrix, and the residence time of Cl in the matrix is almost certainly well in excess of the time required for the 36C1/C1 ratio to attain secular equilibrium (- lo6 a>.
5. Conclusions Previous work has shown that 36C1/C1 ratios cannot be used to date groundwater in the Lac du Bonnet granite because of the high levels of in situ production of 36Cl. However, the ratios are most useful in verifying hydrogeological evidence for groundwater recharge locations and determining the depth and extent of penetration of recent recharge into fracture systems in the granite. In this study, 36C1 has been found to be a useful indicator of the source of dissolved salts in groundwaters from fracture zones in a granitic batholith. Although 36C1/C1 ratios cannot distinguish between a rock matrix origin of dissolved salts and intrusion of sedimentary basin waters > 1 Ma ago, the results of the borehole leaching test have demonstrated the existence and potential of the rock-matrix source
because 36C1/C1 ratios of the leach waters compare favourably with those of the groundwaters. Further consideration of other geochemical and isotopic characteristics of the groundwaters in the granite is needed to resolve more definitively the origin of groundwater salinity.
Acknowledgements
R. Watson (AECL Research) is acknowledged for preparing and purifying AgCl precipitates for 36C1 analysis. J.J. Hawton, J.P.A. Larocque and J.D. Ross are thanked for their assistance in obtaining the groundwater samples. The Canadian Nuclear Fuel Waste Management Program is funded jointly by AECL and Ontario Hydro under the auspices of the CANDU Owners Group.
References
[ll S.H. Whitaker, Radioact. Waste Manage. Nucl. Fuel Cycle 8 (1987) p. 145.
[21 M. Gascoyne, J.D. Ross, A. Purdy, SK. Frape, R.J. Drimmie, P. Fritz and R.N. Betcher, Proc. 6th Int. Symp. on Water-Rock Interaction - WRI-6, Malvern, U.K. (1989) p. 243. 131 M. Gascoyne, J.D. Ross, R.L. Watson and D.C. Kamineni, Proc. 6th Int. Symp. on Water-Rock Interaction - WRI-6, Malvern, U.K. (1989) p. 247. [41 M. Gascoyne, C.C. Davison, J.D. Ross and R. Pearson, in: Saline Water and Gases in Crystalline Rocks, eds. P. Fritz and S.K. Frape (Geol. Assoc. of Can. Spec. Paper No. 33, 1987) p. 53. 151 M. Gascoyne, D.C. Kamineni and J. Fabryka-Martin, Proc. 7th Int. Symp. on Water-Rock Interaction - WRI-7, 2, Park City, Utah (1992) p. 933. 161D.C. Kamineni, CF. Chung, J.J.B. Dugal and R.B. Ejeckam, Chem. Geol. 54 (1986) 97. [71 W.B. Wilson, M. Bozoian and R.T. Perry, Int. Conf. on Nuclear Data for Science and Technology, Mito, Japan, 1988, p. 1193. Bl J. Briesmeister, Los Alamos National Laboratories Report, LA-7396-M, Rev. 2 (1986). 191 D. Stone, D.C. Kamineni, A. Brown and R. Everitt, Can. J. Earth Sci. 26 (1988) 387.
NOMB
Beam Interactions with Materials&Atoms
Origin of groundwater salinity in the Lac du Bonnet granite, southeastern Manitoba, from 36Cl measurements Mel Gascoyne * Applied Geoscience Branch, AECL
Research, Pinawa, Man. ROE lL0, Canada
Pankaj Sharma Physics Department,
Purdue Universi@, West Lafayette,
IN 47907, USA
Peter W. Kubik Znstitut fiir Mittelenergiephysik,
ETH-Hiinggerberg,
CH-8093 Ziirich, Switzerland
The results of analyses of the 36Cl content of groundwaters in the Lac du Bonnet granite batholith are described and compared with previous modelling calculations and 36Cl content of rock matrix solutions obtained from a borehole leaching experiment. The results are used to provide support for the proposal that groundwater salinity is largely derived from dissolution of soluble salts in the rock matrix.
1. Introduction Groundwaters from depths of up to 1 km in permeable fractures and fault zones in the Lac du Bonnet
granite batholith, southeastern Manitoba (Fig. 11, are being sampled as part of the geoscience studies done by AECL Research for the Canadian Nuclear Fuel Waste Management Program [l]. The groundwaters have previously been analysed for ionic and isotopic composition to determine the source of dissolved salts, groundwater residence time and principal rock-water interactions. One of the major objectives of this work is to determine the source of dissolved salts, particularly for the deeper, more saline groundwaters in the batholith. In previous work [2,3] two main sources of salts have been proposed: 1) intrusion of basinal brines into the granite from the sedimentary rocks of the Manitoba basin, and 2) leaching of soluble salts present in grain boundaries and fluid inclusions in the granite minerals. This paper describes the results of analyses of the 36Cl content of groundwaters in the batholith and compares the results with previous modelling calculations and 36Cl content of rock matrix solutions ob-
* Corresponding 753 2455.
author. Tel. + 1 204 753 2311, fax + 1 204
Elsevier Science B.V. SSDI 0168-583X(93)E1135-9
tained from a borehole leaching experiment. The results are considered in terms of possible origins of groundwater salinity and groundwater residence times.
2. Methods Groundwaters in permeable fractures in the Lac du Bonnet granite have been obtained from over 100 boreholes drilled to depths of up to 1000 m in the batholith. Various techniques of pumping and sampling the groundwaters have been used. These have been described elsewhere [4]. Groundwaters were selected for analysis for 36Cl so that a wide range of salinities (mainly caused by the presence of Na, Ca and Cl) and low 3H levels (indicating minimal drillwater contamination) were represented. Some of these groundwaters were of interest because of their location in the groundwater flow system (i.e., in recharge and discharge areas). Samples of water were also obtained for analysis from two unfractured, packer-isolated sections of boreholes that had been previously filled with deionized water at atmospheric pressure. These boreholes are located at a depth of 420 m in AECL’s Underground Research Laboratory (Fig. 1). The salinity of these waters had increased over a one-year period, presumably because of mixing with rock matrix fluids migrating into the borehole under diffusion and imposed hydraulic gradients. VI. HYDROLOGY
390
M. Gascoyne et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 389-392
Study
Area4
7/
/
Whiteshell Laboratories m
~~~m;d
Research
m
Ddillng Permits (A-l)
I 5
0
I 5km. A
Fig.
1. Location
map
of the study area Underground
showing the outcrop Research Laboratory
Both groundwaters and borehole leaching waters were analysed for their ionic composition by AECL Research using standard analytical techniques. The 3H content of the groundwater was determined under contract by the University of Waterloo Isotope Laboratory using 3H enrichment by electrolysis. The atomic abundance ratio, 36Cl/total Cl, was determined by accelerator mass spectrometry at the University of Rochester, New York, and PRIME Lab, Purdue University, Indiana.
of the Lx du Bonnet granite batholith lease area and borehole drilling sites.
(solid
black
line),
the
borehole before the permeable zones were isolated are the probable explanations for 3H levels above l-2 TU. It can be seen that the Cl concentration generally increases with the depth of sampling, except in areas of groundwater recharge and discharge, where dilute groundwater penetrates to depths of up to 360 m (sample M8-3-7) and brackish water occurs at shallow depths (B34-2-4). The relationship between the 36C1/C1 ratio and Cl concentration is shown in Fig. 2, together with the range of values for the borehole leaching test waters.
3. Results
The 36C1/C1 ratios and Cl and 3H concentrations of groundwaters from 19 permeable zones in boreholes in the Lac du Bonnet granite are shown in Table 1, together with ratios and Cl concentrations of the two borehole leaching waters. Tritium concentrations are low or below detection limits in most groundwaters. Contamination by residual drillwater or flow within the
4. Discussion Groundwaters from shallow depths (< 150 m) and those in recharge areas up to 400 m deep typically have low Cl concentrations (< 50 mg/l). This suggests that they have experienced little interaction with the host
M. Gascoyne et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 389-392 Table 1 Values of 36Cl/Cl ratio, 3H and Cl concentration in groundwaters in the Lac du Bonnet granite and borehole leach test waters Sample #
Depth
Cl
[mg/ll
36C1/C1 (X lo-‘?
3H
[ml
17 18 28 30 37 207 235 430 454 510 1240 3070 3430 3710 6010 8700 11500 19000 29500
107+ 11 203+ 19 85+ 9 120+11 88+ 9 41+_ 4 41+ 8 6Ok 9 49+ 5 45f 8 42+ 5 59+ 6 57+ 8 34* 7 49+ 5 41* 7 43+ 6 57* 7 44+ 8
1.3 7.4 < 0.8
1580 1700
25+ 2 21+3
_ -
Groundwaters 50 M14-1-4 360 M8-3-7 230 URL8-7-7 120 M5B-IN-9 310 M2A-3-12 265 MlA-3-7 250 HC29-10 250 HC7 250 HC17-12 40 B34-2-4 250 M13-2-5 605 URL12-13-13 380 WNl-8-17 410 MlO-3-8 370 M14-4-5 390 M7-4-9 895 WD3-895-10 1000 WNll-17-14 1000 WBl-7-7 Borehole leach waters 420 GC2-2 420 MB2-1
[TUI
ND = not determined. a Determined without enrichment by electrolysis.
rock during their comparatively short residence time. The dilute groundwaters have 36C1/Cl ratios ranging between 85 and 200 ( X 10-15). In these samples, 36C1is probably derived largely from cosmogenic and, possi-
.
‘0 + 140
x
1.
’ 1 G \ G
.Y..
100
nn
I
60
_
.,
,..,.
.___.
I
20
II
1.0
. . . . .
c .
leach VIaten
2.0
y...
. . . .
.I..
n
l*
3.0
4.0
5.0
log Cl - (mg/L) Fig. 2. Variation of 36Cl/C1 ratio with Cl concentration in groundwaters from the Lx du Bonnet granite. The range of ratios and Cl concentrations of the borehole leaching experiment waters is shown for comparison.
391
bly, bomb sources, as their 3H content suggests recent recharge. Analyses of 14C and ‘H/ 180 contents (Gascoyne, unpublished results) further indicate that these groundwaters are clearly < lo4 a old. Deeper groundwaters and those in the central part of the flow path and in discharge areas have higher Cl concentrations (200-30 000 mg/l). These concentrations may be derived from Cl introduced into the fractures in the granite by intrusion of saline waters present in sedimentary rocks to the west (or, possibly, deep penetration of overlying seas during Paleozoic time). Alternatively, groundwater salinity may be derived from the leaching of salts from the rock matrix. Previous studies of the content of soluble salts in various phases of the Lac du Bonnet granite [3] have shown that Cl concentrations are about 50 mg/kg in unaltered grey granite, with lesser amounts in pink, altered phases. These salts are believed to exist in micropores and as fluid inclusions and grain-boundary deposits, which are accessible to permeating groundwater. The lack of correlation between the 36Cl/C1 ratio and the groundwater Cl content (Fig. 2) also suggests that Cl is derived from the rock matrix because leaching of matrix salts and fluids by groundwaters from fracture wall-rock will give 36Cl/C1 ratios that are typical of the wall-rock matrix and this ratio will be constant regardless of whether the groundwaters are dilute or saline [5]. Alternatively, the consistency of 36C1/C1 ratios for a range of groundwater salinities could also be explained by intrusion of sedimentary basin waters into the granite > 1 Ma ago. Secular equilibrium with the neutron flux in the granite would be attained by the present time. The hypothesis proposing a matrix salt origin is supported by the observation in the borehole leaching experiment that salts can readily migrate through the granite matrix into water-filled boreholes and, presumably, fractures. However, the 36Cl/C1 ratios of the groundwaters in fracture zones are significantly higher (by a factor of 1.5-2.5) than those of the borehole leaching waters whose salinity is derived only from rock-matrix salts. This can be readily explained by the fact that the altered rock typically associated with fractures in this area has been found to be enriched in U and, to some extent Th, by up to a factor of 5 [6]. Groundwater in fractures therefore experiences a greater neutron flux than the water filling the leaching test boreholes in unaltered granite, and so it achieves a higher 36C1/C1 ratio. In previous work, 36Cl/C1 ratios of the saline groundwaters were compared with ratios obtained by modelling [5]. Neutron production rate and energy spectrum were calculated using the SOURCES code [7]. The neutron flux and, consequently, 36C1 production rates were then calculated using the Monte Carlo VI. HYDROLOGY
392
M. Gascoyne et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 389-392
neutron transport code, MCNP [8], for the average elemental composition of the granite [6,9]. A range of 36C1/CI values was generated by sensitivity analyses with the MCNP code in which key elements controlling the 36CI production rate (H, Sm + Gd, I-J, Th) were varied within reasonable limits. The results showed that 36C1/CI ratios in the range of 40-100 X 10-15, with an average of 65 x lo-l5 ( f 35%), were predicted for soluble Cl in the unaltered granite matrix under conditions of secular equilibrium with the in situ neutron flux. The 36C1/C1 ratios for most groundwaters containing > 50 mg/l Cl lay within the calculated range of values, albeit at the lower end. The results of the borehole leaching experiment indicate that ratios from model calculations for the rock matrix are significantly higher than the observed ratios, although agreement is seen if a 2u range in the error limits is applied to the modelled result. Variables and uncertainties in the model are more likely to be the cause of this discrepancy because the borehole leach waters clearly obtain Cl only from the unfractured rock matrix, and the residence time of Cl in the matrix is almost certainly well in excess of the time required for the 36C1/C1 ratio to attain secular equilibrium (- lo6 a>.
5. Conclusions Previous work has shown that 36C1/C1 ratios cannot be used to date groundwater in the Lac du Bonnet granite because of the high levels of in situ production of 36Cl. However, the ratios are most useful in verifying hydrogeological evidence for groundwater recharge locations and determining the depth and extent of penetration of recent recharge into fracture systems in the granite. In this study, 36C1 has been found to be a useful indicator of the source of dissolved salts in groundwaters from fracture zones in a granitic batholith. Although 36C1/C1 ratios cannot distinguish between a rock matrix origin of dissolved salts and intrusion of sedimentary basin waters > 1 Ma ago, the results of the borehole leaching test have demonstrated the existence and potential of the rock-matrix source
because 36C1/C1 ratios of the leach waters compare favourably with those of the groundwaters. Further consideration of other geochemical and isotopic characteristics of the groundwaters in the granite is needed to resolve more definitively the origin of groundwater salinity.
Acknowledgements
R. Watson (AECL Research) is acknowledged for preparing and purifying AgCl precipitates for 36C1 analysis. J.J. Hawton, J.P.A. Larocque and J.D. Ross are thanked for their assistance in obtaining the groundwater samples. The Canadian Nuclear Fuel Waste Management Program is funded jointly by AECL and Ontario Hydro under the auspices of the CANDU Owners Group.
References
[ll S.H. Whitaker, Radioact. Waste Manage. Nucl. Fuel Cycle 8 (1987) p. 145.
[21 M. Gascoyne, J.D. Ross, A. Purdy, SK. Frape, R.J. Drimmie, P. Fritz and R.N. Betcher, Proc. 6th Int. Symp. on Water-Rock Interaction - WRI-6, Malvern, U.K. (1989) p. 243. 131 M. Gascoyne, J.D. Ross, R.L. Watson and D.C. Kamineni, Proc. 6th Int. Symp. on Water-Rock Interaction - WRI-6, Malvern, U.K. (1989) p. 247. [41 M. Gascoyne, C.C. Davison, J.D. Ross and R. Pearson, in: Saline Water and Gases in Crystalline Rocks, eds. P. Fritz and S.K. Frape (Geol. Assoc. of Can. Spec. Paper No. 33, 1987) p. 53. 151 M. Gascoyne, D.C. Kamineni and J. Fabryka-Martin, Proc. 7th Int. Symp. on Water-Rock Interaction - WRI-7, 2, Park City, Utah (1992) p. 933. 161D.C. Kamineni, CF. Chung, J.J.B. Dugal and R.B. Ejeckam, Chem. Geol. 54 (1986) 97. [71 W.B. Wilson, M. Bozoian and R.T. Perry, Int. Conf. on Nuclear Data for Science and Technology, Mito, Japan, 1988, p. 1193. Bl J. Briesmeister, Los Alamos National Laboratories Report, LA-7396-M, Rev. 2 (1986). 191 D. Stone, D.C. Kamineni, A. Brown and R. Everitt, Can. J. Earth Sci. 26 (1988) 387.