Nitrogen-15/nitrogen-14 ratios of ground-water nitrate, Long Island, New York

Ground water is the sole source of fresh water for more than 27 million residents of Long Island, N.Y. Knowledge of the water's nitrate concentration is necessary to judge its potability. The procedures adopted are described and the results obtained are discussed. The nitrogen isotope value of nitrate in water from the upper glacial aquifer, the water-table aquifer on Long Island, shows a shift from lighter values in the eastern part of the island where land is used mainly for agriculture, to heavier values toward New York City where land is used for residential purposes with septic tanks or sewers .