In cooperation with Suffolk County Water Authority Hydrogeologic Framework of the North Fork and Surrounding Areas, Long Island, New York Water-Resources Investigations Report 02-4284 U.S Department of the Interior U.S. Geological Survey Cover. Color shaded-relief map of the North Fork and surrounding areas, Long Island, New York, created from a mosaic of USGS National Elevation Dataset 7.5-minute digital elevation models. Hydrogeologic Framework of the North Fork and Surrounding Areas, Long Island, New York By Christopher E. Schubert, Richard G. Bova, and Paul E. Misut ________________________________________ U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 02-4284 In cooperation with SUFFOLK COUNTY WATER AUTHORITY Coram, New York 2004 U.S. Department of the Interior GALE A. NORTON, Secretary U.S. Geological Survey Charles G. Groat, Director For additional information Copies of this report may be write to: purchased from: U.S. Geological Survey U.S. Geological Survey 2045 Route 112, Bldg. 4 Branch of Information Services Coram, NY 11727 Box 25286 Denver, CO 80225-0286 II CONTENTS Abstract.................................................................................................................................................................................. 1 Introduction ........................................................................................................................................................................... 2 Previous investigations ................................................................................................................................................ 4 Purpose and scope ....................................................................................................................................................... 4 Methods and approach................................................................................................................................................. 4 Acknowledgments ....................................................................................................................................................... 5 Hydrogeologic framework..................................................................................................................................................... 8 Geologic setting........................................................................................................................................................... 8 Bedrock.............................................................................................................................................................. 10 Cretaceous deposits ........................................................................................................................................... 10 Raritan Formation.................................................................................................................................... 10 Matawan Group and Magothy Formation, undifferentiated.................................................................... 11 Post-Cretaceous(?) and Pleistocene deposits..................................................................................................... 12 Post-Cretaceous(?) and early late Pleistocene deposits........................................................................... 12 Wisconsinan deposits............................................................................................................................... 12 Glacial deposits beneath Long Island Sound................................................................................. 12 Lower glacial-lake clay and underlying drift................................................................................. 13 Ronkonkoma Drift......................................................................................................................... 15 Upper glacial-lake clay and Roanoke Point moraine and outwash ............................................... 15 Hydrologic setting ....................................................................................................................................................... 16 Hydraulic properties of water-bearing units ...................................................................................................... 16 Extent of freshwater........................................................................................................................................... 17 Freshwater occurrence and replenishment............................................................................................... 17 Upper glacial aquifer ..................................................................................................................... 17 Magothy aquifer ............................................................................................................................ 18 Effect of confining layers......................................................................................................................... 18 Summary and conclusions..................................................................................................................................................... 19 References cited..................................................................................................................................................................... 21 Figures 1. Map of Long Island, N.Y., showing principal geographic features of North Fork study area in eastern Suffolk County .................................................................................................................................................................. 2 2. Map of study area showing locations of four hydraulically isolated ground-water-flow systems and vertical sections A-A´ through E-E´ on the North Fork, Long Island, N.Y....................................................................... 3 3. Gamma-ray logs, generalized descriptions of geologic cores, and corresponding hydrogeologic units for borings at four wells on the North Fork, Long Island, N.Y............................................................................................... 6 4. Generalized section A-A´ showing geologic and hydrogeologic units on the North Fork, Long Island, N.Y. ................. 8 Plates [Plates are in pocket] 1. Map of study area showing locations of vertical sections and associated boreholes and wells, and vertical sections B-B´ through E-E´ showing hydrogeologic units in the North Fork study area, Long Island, N.Y. 2-4. Maps of North Fork study area showing: 2. Altitude of bedrock surface and of upper surface of Cretaceous hydrogeologic units: (A) bedrock; (B) Lloyd aquifer; (C) Raritan confining unit; and (D) Magothy aquifer. Contents III 3. Pleistocene confining units: (A) thickness of lower confining unit; (B) upper-surface altitude of lower confining unit; (C) thickness of upper confining unit; and (D) upper-surface altitude of upper confining unit. 4. Surficial Pleistocene units and extent of fresh ground water: (A) surficial hydrogeologic units and water-table altitude in March-April 1994; (B) altitude of base of freshwater above lower confining unit; and (C) altitude of freshwater-saltwater interface below upper surface of lower confining unit. Tables 1. Generalized description of geologic and hydrogeologic units in the North Fork study area of eastern Long Island, N.Y................................................................................................................................................... 9 2. Estimated hydraulic values for Pleistocene and uppermost Cretaceous hydrogeologic units on the North Fork, Long Island, N.Y................................................................................................................................. 17 CONVERSION FACTORS, ABBREVIATIONS, and VERTICAL DATUM Multiply By To Obtain Length foot (ft) 0.3048 meter mile (mi) 1.609 kilometer square mile (mi2) 2.59 square kilometer Hydraulic conductivity foot per day (ft/d) 0.3048 meter per day Other abbreviations used in this report milligrams per liter (mg/L) microsiemens per centimeter at 25°C (µS/cm) Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)— a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. IV Contents Hydrogeologic Framework of the North Fork and Surrounding Areas, Long Island, New York By Christopher E. Schubert, Richard G. Bova1, and Paul E. Misut ABSTRACT Sound indicate, however, that the two are correlated at least along the North Fork shore. Ground water on the North Fork of Long The Matawan Group and Magothy Island is the sole source of drinking water, but the Formation, undifferentiated, is the uppermost supply is vulnerable to saltwater intrusion and Cretaceous unit on the North Fork and constitutes upconing in response to heavy pumping. the Magothy aquifer. The upper surface of this Information on the area’s hydrogeologic unit contains a series of prominent erosional framework is needed to analyze the effects of features that can be traced beneath Long Island pumping and drought on ground-water levels and Sound and the North Fork. Northwest-trending the position of the freshwater-saltwater interface. buried ridges extend several miles offshore from This will enable water-resource managers and areas southeast of Rocky Point