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Cover: Thin-section photomicrograph of basalt rock with sedimentary material and chemical
precipitate lining a void in the rock. The sample was taken from the unsaturated zone of the
Snake River Group basalt in the Snake River Plain, Idaho. Layers of clay particles (medium to dark
brown) were deposited on altered basalt (yellow to light brown) by infiltrating sediment-charged
waters during recharge. Crystalline carbonate (continuous white) lining precipitated from waters
of later recharge. The dark area, generally to the left of the white crystalline lining, is the void.

Of special interest are chemical reactions in the unsaturated zone which might affect migration
of waste radionuclides buried at the Department of Energy's Idaho National Engineering
Laboratory. The picture was used in the study of hydrogeologic and geochemical conditions made
by the U.S. Geological Survey at the laboratory. Magnification is approximately 400 times.
(Photograph by Ruth Deike, USGS.)

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