Areas Contributing Ground Water to the Peconic Estuary, and Ground Water Budgets for the North and South Forks and Shelter Island, Eastern Suffolk County, New York U.S. GEOLOGICAL SURVEY Suffolk County Water-Resources Investigations Report 97-4136 Prepared in pooj>eratlon with the Pepohiq Estuary Program and Suffolk County Department of Health Services science for a changing world Areas Contributing Ground Water to the Peconic Estuary, and Ground-water Budgets for the North and South Forks and Shelter Island, Eastern Suffolk County, New York by Christopher E. Schubert U.S. GEOLOGICAL SURVEY Water Resources Investigations Report 97-4136 Prepared in cooperation with the PECONIC ESTUARY PROGRAM and SUFFOLK COUNTY DEPARTMENT OF HEALTH SERVICES Co ram, New York 1998 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. Geological Survey Mark Schaefer, Acting 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 P.O. Box 25286 Denver Federal Center Denver, CO 80225-0286 CONTENTS Abstract..................................................................................................................._^ 1 Introduction......................................................................................................................................................................... 2 Purpose and scope....................................................................................................................................................... 4 Previous investigations................................................................................................................................................ 4 Acknowledgments....................................................................................................................................................... 5 Description of study area .................................................................................................................................................... 5 Population and land use............................................................................................................................................... 5 Water use..................................................................................................................................................................... 5 Hydrology.......................................................^ 13 Precipitation and recharge ................................................................................................................................... 13 Hydrologic boundaries ........................................................................................................................................ 13 Directions of ground-water flow.......................................................................................................................... 16 Areas contributing ground water to the Peconic Estuary, and ground-water budgets for the North and South Forks and Shelter Island ................................................... 17 Delineation of contributing areas................................................................................................................................ 17 Development of ground-water budgets....................................................................................................................... 19 Analysis of Fluctuations in Ground-Water Discharge................................................................................................. 27 Summary and conclusions .................................................................................................................................................. 34 References cited.................................................................................................................................................................. 35 PLATES [Plate is in pocket] 1. Map of eastern Suffolk County, N.Y., showing water-table altitude in March-April 1994, observation-well numbers and water levels, and approximate boundaries of areas contributing ground water to selected ernbayrnents within the Peconic Estuary on the North and South Forks and Shelter Island FIGURES 1. Map of Long Island, N.Y., showing location of the Peconic Estuary and study area in eastern Suffolk County....... 2 2. Map of study area showing locations of the North and South Forks and Shelter Island, local study areas (Meetinghouse Creek, Sag Harbor Cove, and West Neck Bay), selected precipitation-measurement stations, and Peconic River streamflow-gaging station in eastern Suffolk County, N.Y................................................................. 3 3. Maps of local study areas showing observation-well locations and numbers, eastern Suffolk County, N.Y.: A. Meetinghouse Creek study area............................................................................................................................. 6 B. Sag Harbor Cove study area................................................................................................................................... X-9 C. West Neck Bay study area...................................................................................................................................... 10 4. Map of study area showing location of agricultural land in 1994 in East Hampton, Riverhead, Shelter Island, Southampton, and Southold Towns, eastern Suffolk County, N.Y............................................................................. 11 5. Map of study area showing location of sewage-treatment districts and regional public water-supply districts and selected well fields, eastern Suffolk County, N.Y....................................................................................................... 12 6. Maps of local study areas showing water-table altitude in March 1995, observation-well water levels, and approximate boundaries of areas contributing ground water to selected embayments, eastern Suffolk County, N.Y: A. Meetinghouse Creek study area........................................................................................................................... IX B. Sag Harbor Cove study area.................................................................................................................................20-21 C. West Neck Bay study area.................................................................................................................................... 22 7. Map of study area showing locations of areas contributing ground water to selected embayments within the Peconic Estuary on the North and South Forks and Shelter Island, eastern Suffolk County, N.Y............................. 23 8. Graphs showing annual total precipitation at Greenport and water-table altitudes in selected observation wells on the North Fork, calendar years 1976-95, eastern Suffolk County, N.Y................................................................. 28 9. Graphs showing annual total precipitation at Greenport and water-table altitudes in selected observation wells on Shelter Island, calendar years 1976-95, eastern Suffolk County, N.Y. ................................................................. 29 Contents iii 10. Graphs showing annual total precipitation at Bridgehampton and water-table altitudes in selected observation wells on the South Fork, calendar years 1976-95, eastern Suffolk County, N.Y. ........................................................ 30 11. Graphs showing annual total precipitation at Riverhead, water-table altitudes in selected observation wells on the main body of Long Island, and annual mean discharge of the Peconic River at Riverhead, calendar years 1976-95, eastern Suffolk County, N.Y. ........................................................................................................................ 32 TABLES 1. Monthly pumpage in 1994 for selected regional public water-supply districts, eastern Suffolk County, N.Y................. 14 2. Cultivation characteristics of selected major crops grown on Long Island, N.Y.............................................................. 16 3. Long-term mean precipitation amounts at Bridgehampton, Greenport, and Riverhead, eastern Suffolk County, N.Y.... 16 4. Ground-water budgets for contributing areas on the North and South Forks and Shelter Island, eastern Suffolk County, N.Y...................................................................................................................................................................... 25 5. Departures of annual mean water-table altitudes from long-term mean in selected observation wells on the North and South Forks and Shelter Island, and Brown Tide occurrences in the Peconic Estuary, calendar years 1976-95, eastern Suffolk County, N.Y ........................................................................................................................................................ 33 CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM Multiply By To Obtain Length inch (in.) 25.4 millimeter foot (ft) 0.3048 meter Area acre 0.4047 hectare square mile (mi2 ) 2.590 square kilometer Volume gallon 3.785 liter Flow gallon per minute (gal/min) 3.785 liter per minute cubic foot per day (ft3/d) 0.02832 cubic meter per day inch per year (in/yr) 25.4 millimeter per year Sea level: In this report, "sea level" refers