USE OF TIME DOMAIN ELECTROMAGNETIC SOUNDINGS AND BOREHOLE ELECTROMAGNETIC INDUCTION LOGS TO DELINEATE THE FRESHWATER/SALTWATER INTERFACE ON SOUTHWESTERN LONG ISLAND, NEW YORK

The U.S. Geological Survey (USGS), in cooperation with the New York State Department of Environmental Conservation, used surface and borehole geophysical methods to delineate the freshwater/saltwater interface in coastal plain aquifers along the southwestern part of Long Island, New York. Extensive pump­ing of groundwater in Kings and Queens Counties during the early 20th century, combined with freshwater/saltwater interfaces located near or at the coastline in these areas, created saltwa­ter intrusion in the upper glacial, Jameco, Magothy, and Lloyd aquifers. Since then, several public-supply wells in the southern parts of Nassau, Queens, and Kings Counties have been abandoned due to the adverse effects of saltwater intrusion. This extensive saltwater intrusion of the Lloyd aquifer along the southwestern coast of Long Island, N.Y. was documented for the first time by this study. Due to ongoing groundwater pumping in southern Nassau County, the fresh­water/saltwater interface requires continued delineation and monitoring for any inland movement.

In 2015–17, the USGS collected time domain electromagnetic soundings at 12 locations and bore­hole electromagnetic induction logs at 9 wells within the study area to delineate several saltwater intrusion wedges. The upper glacial, Jameco, and Magothy aquifers were grouped into one aquifer complex to simplify interpretations. Three separate wedges, shallow, intermediate, and deep, of saltwater intrusion were delineated in the upper glacial, Jameco, and Magothy aquifer complex.

Additionally, recent re-analysis of geophysical logs collected in 1989 from a borehole drilled in southern Queens County, revealed the Lloyd aquifer was 75-percent intruded by saltwater at that location and time. The interpretation of these logs was supported by a chloride concentration analysis of 13,600 milligrams per liter on a sample from a new borehole drilled in 2020. The geophysical log suite from this well along with the addition data collected for this study provides, for the first time, definitive proof of saltwater intrusion of the Lloyd aquifer on the south shore of western Long Island. These data challenge the long-held belief, theorized in previ­ous studies, that the freshwater/saltwater interface of the Lloyd was miles offshore prior to the onset of aquifer pumping in Queens County.