These sediments act as the recharge sponge for the underlying aquifers. They fill rapidly during periods of rainfall and slowly release water both to streams and to deeper sands. In central and southern Delaware the Columbia is presently a source of irrigation and many public water supplies. Indeed the Columbia contains much of the fresh water in the State and also is the most vulnerable to pollution. The crystalline or hard rocks of northern Delaware, which are an extension of the basement rocks beneath the Coastal Plain, have much lower water-bearing capacities than do the Coastal Plain sediments.
Water is stored in fractures or openings that constitute a small volume of the total rock mass.
Predicting the occurrence of these openings is difficult. The marble valleys of the Hockessin and Pleasant Hill areas are an exception in that high yields are initially obtained. However, the recharge area of the marble is small so that the total yield available is only a few million gallons per day on a long-term basis. The top of this zone of saturation is known as the water table and its elevation or its distance below land surface responds rapidly to rainfall. Obviously the water in the aquifer is prevented from reaching this height by the overlying confining beds.