MATERIALS AND METHODS Site of sampling. To have access to deep-subsurface samples, we used the Underground Research Facility, a mine gallery located at a depth of 224 m in the Boom clay formation near Mol, Belgium. The geological cross section is as follows: 0 to 188 m deep, Neogene sands; 188 to 280 m, Boom clay; and 280 to 450 m, alternating sand and clay formations. The Boom clay, dating from the Rupelian period (30 to 35 million years ago), is dominated by illite-smectite of marine origin; 50 to 60% of the material is smaller than 2 m in diameter, and 40 to 45% is between 2 and 60 m; water content is ϳ20% (wet weight); the median porosity is between 10 and 20 nm, and the largest pore size does not exceed 0.1 to 0.2 m in diameter (11a); total organic content is ϳ3% (dry weight); the pH is 8.2 to 8.8; and the sediment temperature is approximately 20ЊC. The interstitial clay water is more than ϳ35,000 years old, consistent with its very low hydraulic conductivity (, ϳ10 Ϫ12 m s Ϫ1) (5). Sampling conditions. Clay samples have been collected at 0

Vol. 61, no. 9, p. 3406: the Acknowledgments should read as follows. "This work was supported by the CNRS and by grants of the European Community and ANDRA as part of the project Archimede-Argiles and of the Direction de la Recherche et des Etudes Doctorales. V.B. was a recipient of a CIFRE fellowship." [This corrects the article on p. 3400 in vol. 61.].