Southern New Castle County has a land area of 190 square miles in northcentral Delaware. It is predominantly a rural area with a population of about 9,500 people who are engaged chiefly in agriculture. By and large, the residents are dependent upon ground water as a source of potable water. This investigation was made to provide knowledge of the availability and quality of .the ground-water supply to aid future development. The climate, surface features, and geology of the area are favorable for the occurrence of ground water. Temperatures are generally mild and precipitation is normally abundant and fairly evenly distributed throughout the year. The topography of the area is relatively fiat and, hence, the streams have low gradients. The surface is underlain to a considerable depth by highly permeable unconsolidated sediments that range in age from Early Cretaceous to Recent. Nearly all the subsurface stratigraphic units yield some water to wells, but only four parts or combinations of these units are sufficiently permeable, to yield large supplies. These are, from oldest to youngest, the nonmarine Cretaceous sediments and the Magothy Formation, the Monmouth Group, the Rancocas Formation, and .the surficial terrace and valley-fill deposits. In the northern part of the area the nonmarine Cretaceous sediments and the Magothy Formation can be reached economically by wells. Yields in excess of 300 gpm (gallons per minute) have been obtained from wells screened in this aquifer, but the maximum productivity of the aquifer has not been .tested. The Monmouth Group is used as a source of water in the central part of the area, where some wells yield as much as 125 gpm. The Rancocas Formation is the principal aquifer in the southern part of the area. Yields of 200-400 gpm can be expected from this aquifer, owing to its uniformly coarse texture, particularly in the upper part of the formation. The terrace deposits compose the shallow watertable aquifer throughout the area. In places the water-table aquifer is connected hydraulically to each of .the other three aquifers. The yields of wells tapping this aquifer are generally small, because the saturated thickness of the aquifer is small. The aquifer does provide a convenient and economical source of water for domestic supplies, .and the quality o# the available water supply is generally satisfactory for most purposes. The use of water in the area was estimated to be about 1.77 million gallons per day in 1959. Rural uses amounted to about 75 percent of the total, and municipal and industrial uses accounted for .the remainder. Water for irrigation of crops constituted about half of the water pumped for rural use. The total use of ground water in the area is a mere fraction of the supply available. Each of the four major aquifers is capable of vastly increased production. Future development, however, will be .limited by the changes in the quality of the water resulting from the future pumping regime-and the expanded pattern of development. Salt-water encroachment will become a problem in the eastern part of the area if steps are not taken to avoid it.