Ground water generally is more economical than surface water for cooling purposes because of its lower temperature. Because of favorable conditions on Long Island, many supply wells have been drilled for cooling purposes. The water table in an area of 45 square miles in western Long Island was below sea level in 1933. In order to prevent further overdevelopment, the New York State Water Power and Control Commission has required that ground water pumped for cooling purposes from wells constructed since 1933 be returned to the ground. The amount of recharge during the cooling season has increased from about 1/2 million gallons a day in 1933 to 30 million gallons a day in 1940. The temperature of the water returned to the ground ranges from 2 to 20 degrees higher than the temperature of the water pumped from the ground. The return of warm water causes a rise in ground-water temperature and this decreases the advantage of using ground water for cooling purposes. The U. S. Geological Survey has observed ground-water temperatures periodically at about 350 wells since 1936. The return of warm water has caused a rise of water temperature as much as 20 degrees at some of the pumping wells. The observations show that a gradual rise of ground-water temperature has occurred in a considerable part of western Long Island.