Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York BY FREDERICK STUMM, ANDREW D. LANGE, AND JENNIFER L. CANDELA U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 00-4193 In cooperation with the NASSAU COUNTY DEPARTMENT OF PUBLIC WORKS Coram, New York 2002 U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey. 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 Purpose and scope....................................................................................................................................................... 2 Location of study area................................................................................................................................................. 2 Previous investigations................................................................................................................................................ 2 Acknowledgments....................................................................................................................................................... 3 Methods of study ................................................................................................................................................................ 3 Seismic-reflection surveys .......................................................................................................................................... 4 Borehole-geophysical logs.......................................................................................................................................... 4 Water-level measurements .......................................................................................................................................... 5 Water-sample collection.............................................................................................................................................. 5 Hydrogeology .................................................................................................................................................................... 5 Stratigraphy................................................................................................................................................................. 7 Bedrock ............................................................................................................................................................... 7 Aquifers and confining units ............................................................................................................................... 9 Lloyd aquifer ............................................................................................................................................... 9 Raritan clay.................................................................................................................................................. 9 Magothy aquifer .......................................................................................................................................... 9 North Shore aquifer ..................................................................................................................................... 15 North Shore confining unit .......................................................................................................................... 15 Upper glacial aquifer ................................................................................................................................... 18 Interpretation of geophysical data....................................................................................................................... 21 Seismic-reflection surveys........................................................................................................................... 21 Borehole geophysical logs........................................................................................................................... 21 Precipitation ................................................................................................................................................................ 21 Ground water .............................................................................................................................................................. 26 Pumpage.............................................................................................................................................................. 27 Water levels ......................................................................................................................................................... 27 Upper glacial and Magothy aquifers ........................................................................................................... 27 Lloyd and North Shore aquifers .................................................................................................................. 30 Water quality ....................................................................................................................................................... 32 Chloride ....................................................................................................................................................... 32 Volatile organic compounds ........................................................................................................................ 32 Inorganic constituents and metals................................................................................................................ 33 Extent of saltwater intrusion............................................................................................................................................... 35 Characteristics of saltwater intrusion.................................................................................................................................. 35 Saltwater wedges A, B, C, D, and E ................................................................................................................................... 35 Summary and conclusions.................................................................................................................................................. 39 References cited.................................................................................................................................................................. 40 FIGURES 1. Map showing location of the Great Neck and Manhasset Neck peninsulas, and of seismic-reflection profiles, Nassau County, N.Y.: A. General location map. B. Location of Manhasset Neck study area .................... 3 2. Map showing locations of public-supply wells, observation wells, and golf-course wells within Manhasset Neck............................................................................................................................................................................ 6 3. Map showing surface altitude of the bedrock underlying Manhasset Neck ............................................................... 8 4. Map showing extent and altitude of the upper surface of the Lloyd aquifer on Manhasset Neck.............................. 10 Contents iii 5. Hydrogeologic sections on Manhasset Neck: A. Section A-A´........................................................................................................................................................ 11 B. Section B-B´. ....................................................................................................................................................... 12 C. Section C-C´. ...................................................................................................................................................... 13 6-10. Maps of Manhasset Neck showing extent and altitude of the upper surface of selected hydrogeologic units: 6. Raritan clay............................................................................................................................................................... 14 7. Magothy aquifer ....................................................................................................................................................... 16 8. North Shore aquifer .................................................................................................................................................. 17 9. North Shore confining unit ....................................................................................................................................... 19 10. Upper glacial aquifer ...............................................................................................................................................