The storage and movement of ground water depends on the types of rocks and associated
interconnected spaces in which the water occurs. The Piedmont Province in northernmost
Delaware is underlain by crystalline rocks. Because of the massiveness and hardness of such
rocks, they yield little or no interstitial water to wells. Water is stored in and moves through fractures, cracks, and solution cavities. The amount of water available depends on the number and size of openings, and the degree to which they are interconnected. Wells drilled in the Piedmont range from 100 to 400 feet in depth and yields are highly variable over very short distances.
In the Coastal Plain, the rest of the State, ground water is stored and transmitted in spaces between adjacent rock particles. As much as 30 percent of the rock mass may be saturated. Unconsolidated rocks are analogous to a bathtub filled with sand into which water is poured. The Coastal Plain consists of sandy water-bearing units referred to as aquifers interlayered between non-water-bearing units. Wells constructed for domestic use range in depth from 15 feet to 500 feet. Yields are generally much greater than those obtained from the crystalline rocks of the Piedmont. In general, minimum well yields of 3 to 5 gallons per minute are adequate for most domestic water supply systems.