Preliminary delineation of salty ground water in the northern Atlantic Coastal Plain

Abstract

Salty ground water underlies freshwater in the eastern part of the northern Atlantic Coastal Plain. The transition zone between freshwater and saltwater is represented in this report by a series of maps showing the depths to chloride concentrations of 250, 1,000, 10,000, and 18,000 milligrams per liter. The maps are based on chloride concentrations obtained from self-potential logs as well as from water-quality analyses. Depths to the designated chloride concentrations generally increase inland from the coast except in New Jersey where they are greatest along the coast and in North Carolina where depths to the 10,000 and 18,000 milligrams per liter concentrations are greatest beneath Pamlico Sound. The transition zone between 250 and 18,000 milligrams per liter of chloride is generally 1,500 to 2,300 feet thick except in part of North Carolina, where it is less than 1,000 feet. Depths to 250 and 1,000 milligrams per liter of chloride are probably controlled by the natural flow pattern of fresh ground water. Areas where these concentrations are relatively shallow generally coincide with areas of natural ground-water discharge. Depths to 10,000 and 18,000 milligrams per liter of chloride, and the occurrence offshore of ground water that is fresher than seawater, is attributed to long-term hydrologic conditions during which sea level fluctuations of a few hundred feet recurred several times. The origin of ground water that is saltier than seawater is attributed to the leaching of evaporitic strata beneath the Continental Shelf and Slope followed by westward movement of the brines during periods of sea-level rise.