The aquifer is the layer of gray in this drawing.
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Aquifer
An aquifer is the name for a layer of rock which is capable of holding a large amount of water. Some layers are better at holding water than others, for example a layer of sandstone can hold a good deal of water but a layer of shale cannot. The ability of the material to hold water depends upon how fine-grained it is.
What sometimes takes place within an aquifer is softening of the water, or alteration of the contents of dissolved minerals in the water by contact of the water with the soil of the aquifer.
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