Groundwater, water found below the surface of the land.In the United States, many thousands of wells have been closed in the late 20th century becaus of contamination by various toxic substances.Groundwater is of major importance to civilization, because it is the largest reserve of drinkable water in regions where humans can live, Groundwater may appear at the surface in the form of springs, or it may be tapped by wells.During dry periods it can also sustain the flow of surface water, and even where the latter is readily available, groundwater is often preferable because it tends to be less contaminated by wastes and organisms.When heavily populated or highly irrigated arid areas withdraw water from the ground at too rapid rate, the water table in such areas may drop so drastically that it cannot be reached, even by very deep wells.Layers that tend to slow down groundwater flow, such as clays, shales, glacial tills, and silts, are instead called aquitards.Typically, they consist of sands, gravels, limestones, or basalts.