Geochemistry

Water in the Basement

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Science  02 May 2003:
Vol. 300, Issue 5620, pp. 705
DOI: 10.1126/science.300.5620.705a

Highly saline subsurface waters, not constrained to specific types or ages of the aquifer rocks in which they reside, are common in deep wells, mines, and boreholes. It is generally thought that these brines are marine in origin and were later concentrated to their present salinity, but how this concentration occurred has been an unresolved question. The answer is of practical relevance because the suitability of some of the proposed sites for nuclear waste storage is based on the assumption that the saline waters are very old, Early Phanerozoic or even Precambrian. Such an old age would imply that the deep waters are practically stagnant and that waste stored there would not be mobilized after dissolution.

Starinsky and Katz argue that the saline brines in crystalline rocks in the Canadian, Fennoscandian, and Bohemian Shields are neither old nor stagnant. They propose that they represent waters of marine origin that were concentrated by seawater freezing during the Pleistocene glacial periods and infiltrated into their present sites under a highly dynamic flow environment. In their model, the cryogenic brines formed intermittently during and between glacial periods, with the repeating advance and retreat of the ice sheets as a major control on the direction and intensity of brine flow. These results should raise concern about the planning and construction of high-grade nuclear waste repositories in such rocks.—HJS

Geochim. Cosmochim. Acta 67, 1475 (2003).

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