Geology and ground-water resources of Plum Island, Suffolk County, New York

Plum Island, which has a total area of about 1.3 square miles, is about 2 miles off the northeast tip of Long Island. It is underlain by Cretaceous and Pleistocene unconsolidated deposits resting on a southeastward-sloping Precambrian bedrock surface. The island's fresh ground-water reservoir is contained in stratified upper Pleistocene glacial deposits and probably assumes the shape of a shallow lens, in accordance with the Ghyben-Herzberg principle. Precipitation is the only source of recharge to this reservoir, but it is more than sufficient to replenish the fresh-water draft.' This draft was about 31 million gallons in 1958. The well field of the Department of Agriculture probably has been contaminated slightly by sea water during the past few years--possibly as a result of the hurricanes of 1954 and 1955, by vertical encroachment of sea water, or both. Recommendations to control and alleviate this contamination include a water-conservation program, artificial recharge, a continuing water-level and chloride-monitoring program including construction of 'outpost' wells, and the establishment of an alternative or auxiliary well field.