[en] The hydrogeochemistry of the Lac du Bonnet granitic batholith has been determined for the region of the Whiteshell Research Area (WRA) in southeastern Manitoba, Canada. This work forms part of the geosciences studies performed for the Canadian Nuclear Fuel Waste Management Program over the period 1980-1995 by Atomic Energy of Canada Limited (AECL). Knowledge of the variation of groundwater chemistry and its causes is useful in assessing the performance and safety of a nuclear fuel waste vault located at depths of up to 1000 m in a crystalline rock formation of the Canadian Shield. Groundwaters and matrix pore fluids have been obtained by standard sampling methods from shallow piezometers in clay-rich overburden, from packer-isolated borehole zones intersecting fractures or fault zones in the bedrock, and from boreholes in unfractured rock in AECL's Underground Research Laboratory (URL). Eighty-six individual fracture groundwaters have been sampled and analysed from permeable zones in 53 boreholes drilled to depths of up to 1000 m in the Lac du Bonnet batholith. In addition, 28 groundwaters from piezometers in a large wetland area near the URL have been sampled and analysed to determine the influence of clay-rich overburden on the bedrock hydrogeochemistry. Analyses have been made for major and minor ions, pH, Eh, trace metals, and stable and radioactive isotopes, to characterise these groundwaters and relate them to their hydrogeologic regimes. Shallow groundwaters in the fractured bedrock are generally dilute (TDS <0.3 g/l), Ca-Na-HCO₃ waters and show little indication of mixing with Ca-Mg-HCO₃-SO₄ groundwater from overburden sediments. The near-modern levels of ³H and ¹⁴C, and a warm-climate ²H/¹⁸O signature in these groundwaters, indicates that the upper ∼200 m of fractured bedrock contains an active groundwater circulation system with a residence time of tens to hundreds of years. Deeper fracture groundwaters (200-400 m depth) in recharge areas, are more alkaline, Na-Ca-HCO₃ waters and evolve to Na-Ca-HCO₃-Cl-SO₄ waters with increasing distance along the flow path. Isotopic data indicate the presence of a glacial melt-water component suggesting that the residence times of these waters are 10³-10⁵ a. These waters form a transition zone between the upper, advective flow regime and a deeper regime in sparsely fractured rock where groundwater in fractures and fracture zones is largely stagnant. At these depths (> 500 m), Na-Ca-Cl-SO₄ waters of increasing salinity (up to 50 g/l) with depth are found and in some fractures the waters have evolved to a Ca-Na-Cl composition. Isotopic data indicate that these waters are warm-climate and pre-glacial in origin, with residence times of over 1 Ma. Pore fluids observed to drain from the unfractured rock matrix in the URL facility are almost pure Ca-Cl in composition, ∼90 g/l salinity, and have a ²H/¹⁸O composition displaced well to the left of the global meteoric water line, about which all other WRA groundwaters lie. This information indicates that these pore fluids have undergone prolonged water-rock interaction and have residence times of 10¹-10³ Ma. Most of the deeper fracture groundwaters and pore fluids have low Br/Cl ratios and moderate to high δ³⁴S values of dissolved SO₄ which indicates that their salinity could be derived from a marine source such as the basinal sedimentary brines and evaporites to the west of the batholith. These fluids may have entered the batholith during early Paleozoic times when sedimentary rocks were deposited over the granite and were driven by a hydraulic gradient resulting from higher ground in western Canada. The hydrogeochemical data and interpretations show that below ∼500 m in the WRA, fracture-hosted groundwaters are very saline, reducing and old, and are, therefore, indicative of stagnant conditions over the period of concern for nuclear waste disposal (1 Ma). The intact rock matrix at these depths is extremely impermeable as indicated by the presence of pore fluids with unusual geochemical and isotopic characteristics. The pore fluids may represent basinal brines that have evolved geochemically and isotopically to their current composition over periods as long as 10³ Ma

[en] The ²²⁷Th/²³⁰Th dating method is described in detail and its usefulness investigated by comparing ages of sixteen Pleistocene carbonates (mainly cave deposits) with those determined by the ²³¹Pa/²³⁵U and ²³⁰Th/²³⁴U methods. The ²²⁷Th/²³⁰Th ages are found to be critically dependent on corrections for decay of ²²⁷Th prior to alpha counting and ingrowth of daughter isotopes of ²³²Th derived from clastic detritus. Of nineteen sets of ages determined for the sixteen samples, good agreement is found for only seven sets. Differences are attributed to low U content of some samples and the possibility of excess ²²⁷Th in the calcite of samples younger than ∼ 50 ky, possibly due to the coprecipitation of ²³¹Pa during formation. Calculated 'negative' ²²⁷Th/²³⁰Th ages may be a direct result of this process and the fact that, unlike the other methods, the activity ratio is non-zero at zero age. Nevertheless, the ²²⁷Th/²³⁰Th is found to be a useful alternative dating technique for carbonates which are between ∼ 50 and 300 ky, because no spiking is required. It also serves as a check for partial concordancy with ages dated by the other methods. (author)

[en] The concept for disposal of Canada's nuclear fuel waste in a geologic environment on the Canadian Shield has recently been presented by Atomic Energy of Canada Limited (AECL) to governments, scientists, and the public, for review. An important part of this concept concerns the geochemical environment of a disposal vault and includes consideration of rock and groundwater compositions, geochemical interactions between rocks, groundwaters, and emplaced vault materials, and the influences and significance of anthropogenic and microbiological effects following closure of the vault. This paper summarizes the disposal concept and examines aspects of the geochemical environment. The presence of saline groundwaters and reducing conditions at proposed vault depths (500-1000 m) in the Canadian Shield has an important bearing on the stability of the used nuclear fuel, its container, and buffer and backfill materials. The potential for introduction of anthropogenic contaminants and microbes during site investigations and vault excavation, operation, and sealing is described with examples from AECL's research areas on the Shield and in their underground research laboratory in southeastern Manitoba. (author)

[en] Groundwater composition evolves along flow paths from recharge to discharge in response to interactions with bedrock and fracture-filling minerals, and dissolution of soluble (Cl-rich) salts in the rock matrix. The groundwater redox potential changes from oxidizing to reducing conditions due, initially, to rapid consumption of dissolved oxygen by organics in the upper ∼100 m of bedrock and, subsequently, interaction with Fe (II)-containing minerals. Measured Eh values of groundwaters at depth in the granitic Lac du Bonnet batholith indicate that biotite and chlorite control groundwater redox potential. This is supported by other geochemical characteristics such as absence of CH₄, H₂S, H₂, NO₃, low concentrations of Fe (II), and abundance of SO₄. Further evidence of evolution of redox conditions is given by variations in U concentration ranging from up to 1000 μg/L in dilute near-surface waters to <1 μg/L in some deep, saline groundwaters. Groundwaters at about 400 m depth in a recharge area on the Lac du Bonnet batholith contain significantly more U than groundwaters further along the flow path or near surface in discharge areas. Uranium concentration is found to be a useful and sensitive indicator of redox conditions. (author)

[en] AECL has developed a concept for permanent geological disposal of used nuclear fuel waste in Canada. This concept would involve disposal of the waste in corrosion-resistant metal containers, surrounded by compacted clay-based buffer and backfill materials, in a vault 500-1000 m deep in granitic rock of the Canadian Shield. Such a vault would not be a sterile environment. Microbial activity would be expected in those areas of a vault where effects of heat, moisture content, and radiation would not exclude microbial life and where sufficient nutrients would be present. Although the granitic rock environment is naturally nutrient-poor, a substantial amount of nutrients could be introduced from residues of explosives used in the excavation of a vault. Using standard rock leaching techniques, measurements of the concentrations of such residues were made in excavated rock, tunnel walls and mine service-water supplies at AECL's Underground Research Laboratory. The effects of these residues on the bacterial population size in groundwater were also determined. Results showed that the largest potential nutrient addition (both N and C) to a vault would result from using untreated broken rock as part of the backfill. Nitrate in the residues could increase groundwater bacterial populations by several orders of magnitude. (author)

[en] The generation of gases and possible development and migration of a gas phase at depth in groundwater-saturated, fractured rock has become an important aspect of assessing the performance and safety of radioactive waste disposal sites. To study gas-phase migration in this environment, helium gas was injected at constant pressure through an access borehole into an inclined fracture zone at a depth of about 40 m, in the granitic Lac du Bonnet Batholith, southeastern Manitoba, Canada. The gas flow rate, arrival time and pattern of distribution of gas at the surface were monitored by soil gas surveys. Gas flow rate increased from 5 to 20 1 min^-1 over the 11-day period of the test indicating removal of water from the flow paths. Breakthrough of injected gas at the surface was detected within 2 days. Two areas of high concentrations of gas discharge were observed within 40 m of the injection borehole, indicating gas transport through near-vertical fractures. A larger area of trace He concentrations was detected 200 m away indicating transport along the fracture zone. The field results were compared with predictions of a simple analytical model derived from Braester and Thunvik (1983). Good agreement was found when the influence of fracturing in the bedrock and a low-permeability overburden and 'excess porosity' due to non-uniformity of fracture apertures were included in the model. The model was then used to estimate and map the relative hydraulic conductivities of individual gas flow paths in the fractured rock. (author)

[en] Radon and helium in soil gases have been used to identify locations of groundwater discharge and the presence of fractures outcropping beneath overburden in two areas near the Underground Research Laboratory (URL), Lac du Bonnet, Manitoba, Canada. In particular, groundwater discharge from a known, inclined fracture zone at the URL was clearly identified by a helium excess in overlying soil gases. A model was developed to describe gas phase flow in bedrock and overburden at this location, from gas injection in an adjacent borehole. Predictions were made of gas transport pathway and breakthrough time at the surface, in preparation for a gas injection test

[en] The lithium metaborate fusion technique for analysis of rock samples has been adapted for the alpha spectrometric determination of uranium and thorium isotope abundances in granite. Powdered granite is spiked with a solution of a uranium-thorium isotope tracer, mixed with LiBO₂ in a 1: 3 ratio and fused at 950^oC in a graphite crucible. The mixture is poured into 1 M HNO₃ and stirred until dissolved. Uranium and thorium are simultaneously extracted with 10% tributylphosphate(TBP) in amyl acetate using AI(NO₃)₃ as the salting agent, and then back-extracted into 1 M H₂SO₄. Uranium is separated from thorium using anion exchange resin and, after further purification, each is plated onto steel discs for alpha counting. Overall chemical yields are adequate at present (generally 20 to 60%). Preliminary tests show the TBP extraction step to be almost quantitative for both elements, in spite of the presence of silicon and high concentrations of aluminium. This procedure is much faster than the usual acid digestion technique, and uranium and thorium discs for counting can be prepared in approximately eight hours, starting from rock powder. (author)

[en] This report examines and summarizes all work that has been done from 1980 to the present in determining the age of rock crystallization, fracture initiation, fracture reactivation and rates of fracture movement in the Lac du Bonnet Batholith to provide information for Atomic Energy of Canada Limited's (AECL) Canadian Nuclear Fuel Waste Management Program. Geological and petrographical indicators of relative age (e.g. cross-cutting relationships, sequences of fracture infilling minerals, P-T characteristics of primary and secondary minerals) are calibrated with radiometric age determinations on minerals and whole rock samples, using ⁸⁷Rb-⁸⁷Sr, ⁴⁰K-³⁹Ar, ⁴⁰Ar-³⁹Ar and fission track methods. Most fractures and fracture zones inclined at low angles are found to be ancient features, first formed in the Early Proterozoic under conditions of deuteric alteration. Following some movement on fractures in the Late Proterozoic and Early Paleozoic, reactivation of fractures during the Pleistocene is established from uranium-series dating methods and use of stable isotopic contents of fracture infilling minerals (mainly calcite). Some indication of movement on fracture zones during the Pleistocene is given by electron spin resonance dating techniques on fault gouge. The slow rate of propagation of fractures is indicated by mineral infillings, their P-T characteristics and U-series calcite ages in a fracture in sparsely fractured rock, accessible from AECL's Underground Research Laboratory. These results collectively indicate that deep fractures observed in the batholith are ancient features and the fracturing and jointing in the upper 200 m is relatively recent (< 1 Ma) and largely a result of stress release. (author)

[en] The chlorine isotopes ³⁶Cl and ³⁷Cl have been shown to be useful tracers of groundwater, and for investigations of sources of dissolved Cl, mixing of fluids, water-rock interactions in sedimentary environments and in identifying solute sources and transport mechanisms. In addition, the radioactive isotope, ³⁶Cl, is a useful tracer for determining the residence time of groundwater. This report examines the results of Cl isotopic analysis of groundwaters from as deep as 1000 m at the Olkiluoto site in southwest Finland. Thirty-four samples were analysed for ³⁶Cl/Cl and 29 were analysed for ³⁷Cl (expressed as δ ³⁷Cl). The value δ³⁷Cl was found to stabilize at higher salinities and the maximum range of δ³⁷Cl was from about - 0.6 to +0.6 per mille. Because of this limited range and the relatively large error margins associated with the δ³⁷Cl measurement, the usefulness of this ratio appears to be limited. Therefore, the main part of this report is largely focused on ³⁶Cl. Estimation of residence time of ³⁶Cl gives results that support the presence of at least five groundwater types at Olkiluoto. The consistency of ³⁶Cl/Cl ratios in groundwaters of several widely separated, deep locations and different rock compositions, suggests that these deeper groundwaters are in secular equilibrium and, therefore, likely to be older than 1.5 million years. (orig.)