Definition of aquitard
: a geologic formation or stratum that lies adjacent to an aquifer and that allows only a small amount of liquid to pass If the water level—and thus the pressure—drops enough, water is slowly squeezed out of the large clayey layers—called aquitards—that more or less seal the aquifer, and an aquifer's compression becomes irreversible. — Peter Bacque, Richmond (Virginia) Times Dispatch, 14 Nov. 2002
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