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Fig. 6. Migration of fluorescent microbes (the bright regions in the figure) in a heterogeneous sand. Flow is from left to right, with microbes added only below the water table in the left reservoir. The bright region in the upper right represents microbes being preferentially transported through a coarse sand lens above the water table. The dark region slightly left of center in this image is a zone of trapped air in a coarse sand and demonstrates the inability of microbes to enter the upper portion of this sand lens due to the very low moisture content.
