Deep Saline Groundwaters Within the Canadian Shield

Abstract

Groundwaters have been sampled from depths greater than 1000 m within the Canadian Shield. The samples were obtained from boreholes in mines and from test drilling programs carried out as part of the Canadian Nuclear Fuel Waste Management Program [1].

At the depths explored, water is found in fractures, shear zones and similar structural features. The salinity of the water is frequently very high, with total dissolved solids often exceeding 200 g.L^-1. These saline waters can be classified as Ca-Na-Cl brines. Isotopic analyses for ²H and ¹⁸O show that these fluids are not modern, local meteoric waters which dissolve salts in sedimentary, metasedimentary or metamorphic rocks. The chemical and isotopic data suggest that all saline waters analyzed to date have a similar origin. This source is not well-understood but could be highly modified Paleozoic seawater, residual metamorphic fluids or, less likely, groundwaters that have been in contact with the rocks in the Canadian Shield for millions of years. In all cases the fluids appear to have been isolated from the biosphere for geological time periods.

The existence of these highly saline fluids influences the Nuclear Fuel Waste Management Program in several areas. A major concern is the corrosion resistance of the radioactive waste containers. Saline waters may also influence the buffer and backfill and vault sealing materials, the rock mass and the waste form itself, although the effects may not always be deleterious. Corrosion of underground test equipment during the geoscience research phase is also a concern.