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

Abstract

Great Neck is a peninsula about 1.5 miles wide and 3.0 miles long on the northern shore of Long Island. It is surrounded by saltwater embayments and is underlain by 250 to 600 feet of unconsolidated deposits that form a sequence of aquifers and confining units. Ground water at several public-supply wells screened within the primary aquifers (Lloyd and Port Washington]?]) has become contaminated by saltwater, and two wells have been abandoned. Ten wells were drilled in 1992 for collection of geologic, geochemical, and geophysical (focused-induction and gamma) data to delineate the degree and vertical extent of saltwater intrusion through focusedinduction electromagnetic (EM) logging. Two areas of saltwater intrusion were identified in the northern part of the peninsula: One is 125 to 50 feet thick in the Port Washington(?) aquifer with a maximum chloride concentration of 15,300 mgJL (milligrams per liter); the other is 20 feet thick in the Lloyd aquifer with a maximum chloride concentration of 1,900 mg/L. Chloride-concentration data from filter-press samples and ground-water (screen-zone) samples correlated closely with focused-induction EM logs’ responses, and the geologic samples correlate well with natural gamma logs’ responses. Thus, focused electromagnetic induction EM logs provide an accurate indication of the vertical extent and degree of saltwater intrusion and can be used to delineate the saltwater-freshwater interface. INTRODUCTION Great Neck is a heavily populated peninsula about 1.5 mi wide and 3.0 mi long in the northwestern comer of Nassau County, Long Island, N.Y. (fig. 1). The Lloyd and Port Washington(?) aquifers are the major source of public water supply in Great Neck. Ground water at several public-supply wells has become contaminated by the intrusion of saltwater from the surrounding saltwater embayments as a result of nearshore pumping. Concern over the potential for further intrusion of saltwater and the resulting effect on public-supply wells on Long Island’s northern shore has fostered the investigation of the area’s subsurface geology and hydrology to delineate the extent of saltwater intrusion and the depth of the saltwater interface. Ten observation wells were drilled on the peninsula in 1992 (fig. 2), in cooperation with the Nassau County Department of Public Works, in an effort to define and delineate the degree of