If the boundary moves far enough, some wells become saline, thus contaminating the water supply. The location and magnitude of the ground-water withdrawals with respect to the location of the saltwater determines how quickly and by how much the saltwater moves. Relation of fresh and saline ground water. In coastal areas, fresh ground water discharges to the surrounding saline surface-water bodies by flowing over the denser saline ground water. In both coastal and inland areas, large drawdowns in an individual well can cause underlying saline water to migrate upward into the well and cause contamination of the water being discharged.
Some prominent examples follow.
In Hawaii, several aquifers susceptible to saltwater intrusion underlie the island of Oahu. In Florida, saltwater intrusion occurs in the Jacksonville, Tampa, and Miami areas. Farther north on the Atlantic Coast, saltwater intrusion is occurring near Brunswick and Savannah, Georgia, and on Hilton Head Island, South Carolina. In New Jersey, aquifers underlying parts of Atlantic, Gloucester, Monmouth, Cape May, Ocean, and Salem Counties are being affected by saltwater intrusion. The threat of saltwater intrusion is always present on Long Island, New York, and Cape Cod, Massachusetts, because saltwater bodies surround both localities.