A

Redox conditions

Source of organics

Advection/diffusion groundwater mixing

Regional groundwater flow

Advection/diffusion groundwater mixing

Regional groundwater flow

Redox conditions Dissolution/precipitation groundwater mixing

Redox conditions Dissolution/precipitation groundwater mixing

Regional groundwater flow

Regional groundwater flow

Redox conditions Till/marine sediments

Ice-melt /T\ migration Dissolution/precipitation

Redox conditions Till/marine sediments

Figure 9.5 Hydrogeochemical conditions related to pre-glacial (a), glacial (b) and post-glacial (c) periods in fractured bedrock areas. Circled are aspects sensitive to surface conditions that may provide signatures of glaciations or interglacials. (Adapted from Smellie & Frape, 1997.)

Regional groundwater

Figure 9.5 Hydrogeochemical conditions related to pre-glacial (a), glacial (b) and post-glacial (c) periods in fractured bedrock areas. Circled are aspects sensitive to surface conditions that may provide signatures of glaciations or interglacials. (Adapted from Smellie & Frape, 1997.)

atmosphere at 0°C. This can lead to the formation of nonhydrothermal iron oxyhydroxides such as those coating fractures in bedrock at a depth of several hundred metres in Sweden (e.g. at 900 m at Aspo in southeast Sweden; Glynn et al., 1999). Up to a depth of 120m, occurrence of iron oxyhy-droxides coincides with the absence of pyrite and calcite (Tull-borg, 1997b). Meltwater rich in oxygen would also lead to sulphide oxidation and characteristic S13C values (Wallin, 1992).

3 Lack of pyrite along the conductive fractures. Borehole cores from crystalline bedrock around Aspo reveal absence of pyrite in hydraulically active fractures, whereas this mineral occurs in closed fractures and in the rock matrix (Tullborg, 1989; Smellie & Laaksoharju, 1992). This possibly is due to the penetration of oxidizing glacial groundwater during the last glaciation, which did not affect the low-conductivity matrix and fractures outside the active groundwater system.

4 High values of S13C in calcites. Values around -5%o and higher (together with low S18O contents) commonly occur in fracture fills in the Fennoscandian shield, e.g. at 600 m depth at Laxemar, possibly indicating glacial meltwater as the precipitant source (Wallin, 1995).

5 Compositional zoning of fracture minerals. Smellie & Frape (1997) pointed out that minerals may reflect more than one glacial cycle as discrete zones, whereas water composition will be a hybrid resulting from mixing, dominated by the most recent events. They give examples of zoned calcites from Sel-lafield, UK with depleted, low temperature fluid inclusions often interpreted as a glacial signature, dated to around the end of the last glaciation. Also at several sites in Finland and Sweden multiple generations of mineral precipitation were documented, interpreted as reflection of repeated cold and warm climatic events (Smellie & Frape, 1997).

6 Low dissolved organic carbon concentration. Water released from the melting glacier base would typically infiltrate the bed without passing through the organic-rich soil zone, and thus have bicarbonic acid concentrations two orders of magnitude lower than found in interglacial meteoric water. A low 14C content (<12ppm C) is a strong indication of glacially derived groundwater (Tullborg, 1997a).

7 Low total dissolved solids contents. Studies in Iceland and Spitsbergen show that modern glacial groundwater is characterized by a non-equilibrium (undersaturation) with respect to most constituents otherwise derived from host rocks, indicating efficient flushing, short residence time and low degree of water-rock interactions (Sigurdsson, 1990; Haldorsen et al., 1996). Dissolved solids are expected to rise after ice retreat when the groundwater flow slows significantly.

8 Uranium-series disequilibrium (USD). Penetration of oxic meltwater should affect the U/Th ratio in minerals along groundwater flow passages because uranium is much more mobile than thorium under oxidizing conditions. This can be useful in assessing the role of glacial meltwater in the substratum during the late Pleistocene glaciations. Indeed, USD determined in bedrock in Palmottu, southeastern Finland indicates clear periodicity in uranium mobilization several times during the past 300,000 yr, correlated with major glaciation phases (Suksi et al., 2001; Rasilainen et al., 2003). At

Kamlunge, northern Sweden the USD indicating depletion of uranium in the past 500,000 yr was determined to a depth of about 500 m in high-conductivity zones of bedrock (Smellie, 1985).

Remnants of glacial meltwater with these characteristics are found in numerous localities in crystalline shield of Scandinavia and North America as well as in some places in soft sediments south of the crystalline basement. Documentation of such remnants at depths of at least 400 m is convincing. Bearing in mind the likelihood of the next glacial maxima at ca. 5, 20, 60 and 100 ka from now as predicted by the Central Climate Change Scenario (King-Clayton et al., 1997), the old glacial groundwater found at such depths and numerical models suggesting penetration of subglacial water down to several kilometres must be of concern in designing radioactive waste disposal strategies (Talbot, 1999). Repositories are considered, for example, in Sweden and northern Germany, well within the range of prospective glaciations. The major problem is the high dissolved oxygen content of glacial groundwater, which would increase the solubility and mobility of many radionuclides (U, Pu, Tc, Np) by several orders of magnitude and may put the waste canisters stability at risk due to oxidization (Glynn et al., 1999). A special hard rock laboratory on the island of Aspo, southeast Sweden serves as a test and research facility to study the suitability of crystalline bedrock for hosting radioactive waste repositories (SKI, 1997). The laboratory, situated 450munder the ground surface has served, among others, to evaluate the impact of groundwater flow under a perspective ice sheet on the repository's safety. The majority of the results show that the site would be affected by enhanced, oxidizing groundwa-ter penetration under a future ice sheet. Because the stability of nuclear waste repositories must be ensured for tens to hundreds of thousands of years (e.g. in USA for 10,000yr, in Switzerland and France for 1,000,000yr), groundwater stress periods imposed by glaciations must be taken into account.

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