Use Of Focused Electromagnetic-Induction Borehole Geophysics To Delineate The Saltwater-Freshwater Interface In Great Neck, Long Island, New York

Great Neck is a peninsula about 1.5 miles wide and 3.0 miles long on the northern shore of<br>Long Island. It is surrounded by saltwater embayments and is underlain by 250 to 600 feet of<br>unconsolidated deposits that form a sequence of aquifers and confining units. Ground water at<br>several public-supply wells screened within the primary aquifers (Lloyd and Port Washington]?])<br>has become contaminated by saltwater, and two wells have been abandoned. Ten wells were<br>drilled in 1992 for collection of geologic, geochemical, and geophysical (focused-induction and<br>gamma) data to delineate the degree and vertical extent of saltwater intrusion through focusedinduction<br>electromagnetic (EM) logging.<br>Two areas of saltwater intrusion were identified in the northern part of the peninsula: One<br>is 125 to 50 feet thick in the Port Washington(?) aquifer with a maximum chloride concentration<br>of 15,300 mgJL (milligrams per liter); the other is 20 feet thick in the Lloyd aquifer with a<br>maximum chloride concentration of 1,900 mg/L.<br>Chloride-concentration data from filter-press samples and ground-water (screen-zone)<br>samples correlated closely with focused-induction EM logs’ responses, and the geologic samples<br>correlate well with natural gamma logs’ responses. Thus, focused electromagnetic induction EM<br>logs provide an accurate indication of the vertical extent and degree of saltwater intrusion and can<br>be used to delineate the saltwater-freshwater interface.