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 ...... 20 11. Section showing interpreted stratigraphy along seismic-reflection profiles D-D´, E-E´, F-F´, and G-G´ off Manhasset Neck. A. D-D´ in Long Island Sound...... 22 B. E-E´ in Hempstead Harbor ...... 23 C. F-F´ in Hempstead Harbor...... 24 D. G-G´ in Long Island Sound...... 25 12. Geophysical logs of well N12508, Manhasset Neck...... 26 13. Map showing water-table altitude on Manhasset Neck, September 1996...... 28 14. Map showing potentiometric-surface altitudes in the Lloyd and North Shore aquifers on Manhasset Neck, September 1996...... 29 15. Graphs showing water-levels in the upper glacial aquifer, Manhasset Neck, 1997: A. Well N9904. B. Well N12240. C. Well N2269. D. Well N12262 ...... 30 16. Graphs showing water levels in the Lloyd and North Shore aquifers, Manhasset Neck, 1997: A. Well N12319 (Lloyd aquifer). B. Well N12321 (North Shore aquifer). C. N12508 (Lloyd aquifer). D. Well N12318 (North Shore aquifer)...... 31 17. Graphs showing chloride concentration and pumpage at selected public-supply wells on Manhasset Neck: A. Well N5209, 1956-95. B. Well N36, 1938-95...... 33 18. Map showing chloride concentrations in upper glacial, Magothy, North Shore, and Lloyd aquifers, Manhasset Neck, 1997...... 34 19. Chloride-breakthrough curves of public-supply wells (A) N1715 and (B) N35, Manhasset Neck ...... 36 20. Geophysical logs of observation wells (A) N12318 and (B) N12506, Manhasset Neck ...... 37 21. Chloride-breakthrough curves of public-supply wells (A) N662 and (B) N657, Manhasset Neck ...... 38

TABLES 1. Generalized description of the hydrogeologic units underlying Manhasset Neck, Nassau County, N.Y...... 7 2. Pumpage on Manhasset Neck, by aquifer, 1992-96...... 27

iv Contents

CONVERSION FACTORS, ABBREVIATIONS. AND VERTICAL DATUM

Multiply By To Obtain

Length inch (in.) 2.54 centimeter foot (ft) 0.3048 meter mile (mi) 1.609 kilometer Area square mile (mi2) 2.590 square kilometer Flow million gallons per day (Mgal/d) 0.0438 cubic meter per second Hydraulic conductivity foot per day (ft/d) 0.3048 meter per day Volume ounce, fluid (fl oz) 29.57 milliliter Acoustic velocity foot per second (ft/s) 0.3048 meter per second

Other abbreviations used in this report hour (h) milligrams per liter (mg/L) million gallons (Mgal) millisiemens per meter (mS/m) minute (min)

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.

Contents v

Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

By Frederick Stumm, Andrew D. Lange, and Jennifer L. Candela

ABSTRACT Offshore seismic-reflection surveys in the surrounding embayments, and drill-core samples, Manhasset Neck, a peninsula on the northern indicate at least four glacially eroded buried shore of Long Island, N.Y., is underlain by valleys with subhorizontal, parallel reflectors unconsolidated deposits that form a sequence of indicative of draped bedding that is interpreted as aquifers and confining units. Ground water at infilling by silt and clay. The buried valleys several public-supply wells has been affected by (1) truncate the surrounding coarse-grained the intrusion of saltwater from the surrounding deposits, (2) are asymmetrical and steep sided, embayments (Manhasset Bay, Long Island Sound, (3) trend northwest-southeast, (4) are 2 to 4 miles Hempstead Harbor). Twenty-two boreholes were long and about 1 mile wide, and (5) extend to drilled during 1992-96 for the collection of more than 400 feet below sea level. hydrogeologic, geochemical, and geophysical Water from 12 public-supply wells screened data to delineate the subsurface geology and the in the Magothy and upper glacial aquifers extent of saltwater intrusion within the peninsula. contained volatile organic compounds in A series of continuous high-resolution seismic- concentrations above the New York State reflection surveys was completed in 1993 and Department of Health Drinking Water maximum 1994 to delineate the character and extent of the contaminant levels, as did water from one public- hydrogeologic deposits beneath the embayments supply well screened in the Lloyd aquifer and surrounding Manhasset Neck. from two observation wells screened in the upper The new drill-core data indicate two glacial aquifer. hydrogeologic units—the North Shore aquifer Five distinct areas of saltwater intrusion have and the North Shore confining unit—where the been delineated in Manhasset Neck; three extend Lloyd aquifer, Raritan confining unit, and the into the Lloyd and North Shore aquifers, and two Magothy aquifer have been completely removed extend into the upper glacial and Magothy by glacial erosion. aquifers. Borehole-geophysical-logging data Water levels at selected observation wells indicate that several of these saltwater wedges were measured quarterly throughout the study. range from a few feet to more than 125 feet in These data, and continuous water-level records, thickness and have sharp freshwater-saltwater indicate that (1) the upper glacial (water-table) interfaces, and that chloride concentrations and Magothy aquifers are hydraulically within these wedges in 1997 ranged from 102 to connected and that their water levels do not 9,750 milligrams per liter. Several public-supply respond to tidal fluctuations, and (2) the Lloyd wells have either been shut down or are currently and North Shore aquifers also are hydraulically being affected by these saltwater wedges. Data connected, but their water levels do respond to show active saltwater intrusion in at least two of pumping and tidal fluctuations. the wedges.

Abstract 1

INTRODUCTION on the east by Hempstead Harbor, and on the south generally by the Long Island Expressway (fig. 1B). The ground-water system in the coastal areas of The population of Manhasset Neck in 1996 was northern Nassau County, N.Y., has been under estimated to be 70,830, or about 5 percent of Nassau increasing stress from pumping of public-supply County’s population (Nassau County Department of wells, commercial wells, and golf-course-irrigation Health, 1997). wells, and the chloride concentrations at many public- supply wells has increased as a result. Successful management of the area’s ground-water resources will Previous Investigations require detailed knowledge of the hydrogeologic framework, the extent of saltwater intrusion into the The ground-water resources of Long Island were ground-water system, and the effect of pumping on first studied by Veatch and others (1906), who described ground-water levels. In 1991, the U.S. Geological the geologic deposits and occurrence of ground water. Survey (USGS), in cooperation with the Nassau Fuller (1914) revised and supplemented much of the County Department of Public Works (NCDPW), earlier geologic work. Suter and others (1949) began a 6-year study to delineate the hydrogeologic contoured the upper surface of the major geologic units. framework and the extent of saltwater intrusion in Swarzenski (1963) described the hydrogeology and northern Nassau County for the protection and hydrologic conditions in north-western Nassau County conservation of the local ground-water supply. during 1955-57; many of his interpretations, and his The study consisted of four components: interpretation of the general hydrogeologic framework, (1) evaluation of the current observation-well network; were verified by the drill-core and seismic-reflection (2) drilling of new observation wells in areas of data obtained in this study. Mills and Wells (1974) probable saltwater intrusion and where hydrogeologic described ice-shove deformation of Manhssset Neck. data were needed; (3) collection of geologic, Kilburn (1979) reinterpreted and renamed parts of the hydrologic, geophysical (borehole and seismic- hydrogeologic framework in Manhasset Neck, largely reflection) data, and water-quality data within the on the basis of drillers’ logs. Much of the drill-core, study area; and (4) compilation of all data to define hydrologic, and seismic-reflection data obtained in the hydrogeologic framework and the extent of saltwater present study, however, contradict many of Kilburn’s intrusion within northern Nassau County. interpretations. Casson (1988) also described the hydrogeology and ground-water levels on Manhasset Neck, largely on the basis of Kilburn’s interpretations. Purpose and Scope Stumm (2000) redefined the hydrogeologic framework of Great Neck and identified the North This report (1) defines the hydrogeologic Shore aquifer and North Shore confining unit. Similar framework underlying the Manhasset Neck peninsula, information on the hydrogeology and extent of (2) describes the ground-water flow system, and saltwater intrusion in northern Nassau County is (3) delineates the extent of saltwater instrusion within included in Stumm and Lange (1994, 1996) and in the major aquifers. It also presents maps and Stumm (1993a, 1993b, and 1994). hydrogeologic sections that show the extent and Smith (1958) described the first use of high- thickness of the units, water-table and potentiometric- power, low-frequency seismic-reflection equipment to surface maps of the major aquifers, and maps and measure the depth to bedrock beneath selected areas of graphs showing the chloride concentrations within Long Island Sound. Oliver and Drake (1951) used these units in 1992-96. seismic-reflection techniques to measure the depth to bedrock in Long Island Sound; and Williams (1981) used high-resolution seismic-reflection surveys and Location of Study Area core samples to describe the sediments beneath Long Island Sound. Grim and others (1970) described the The Manhasset Neck study area (fig. 1A) geology and subsurface morphology of Long Island encompasses about 18 mi2 in the northwestern part of and the Manhasset Bay embayment. Lewis and Stone Nassau County. The study area is bounded on the west (1991) conducted a systematic seismic-reflection by Manhasset Bay, on the north by Long Island Sound, survey to map the deposits beneath most of Long

2 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

Island Sound, except in the immediate shallow coastal 74° 73° 72° areas and embayments of western Long Island’s NY CT northern shore. Tagg and Uchupi (1967) used 41° LONG ISLAND SOUND continuous seismic profiles to define the subsurface NJ morphology of Long Island Sound. SUFFOLK COUNTY NEW YORK LOCATION 40° QUEENS 45' MAP Acknowledgments CO. NASSAU CEAN IC O KINGS CO. T 0510 MILES ATLAN The authors thank James Mulligan, Brian CO. 0510 KILOMETERS Schneider, Raymond Mazza, Kenneth Fischgrund and Base from U.S. Geological Survey state base map, 1979 others of the Nassau County Department of Public A. General location map Works for their assistance and technical support throughout this study. Thanks also are extended to 73° 45´ 37´ 30˝ F. Peter Haeni, Chief of the U.S. Geological Survey Branch of Geophysical Applications and Support, for WESTCHESTER COUNTY his technical assistance and use of the seismic- reflection survey equipment, and to the Albertson, Manhasset-Lakeville, Port Washington, and Roslyn LONG ISLAND SOUND Water Districts, the Sands Point Water Department, and the Port Washington Sewer District for providing 40° G G´ pumpage information and access to their public- 52´ E F´ F H D´ e supply well sites. Thanks also are extended to the U.S. 30˝ mp E´H Manhasset Ne a Merchant Marine Academy at Kings Point for dock rbor stea D Manhasset space during the two seismic-reflection surveys, and to d

Delta Well and Pump Co., Inc. and Hydro Group, Inc. NASSAU well drillers, for providing lithologic samples and Bay COUNTY Great Ne ck L access to boreholes for geophysical logging at various itt le N sites in Manhasset Neck. e ck ck Study Area

Bay (Figure 2) SSW P RE AY METHODS OF STUDY EX

QUEENS ND LA Data were collected from (1) observation-well COUNTY IS 45´ drilling logs and samples, (2) marine-seismic- LONG reflection surveys, (3) borehole-geophysical logs, Mineola (4) quarterly water-level measurements, (5) continuous ground-water-level records, and (6) water-quality analyses. Twenty-two boreholes G´ were drilled in 1992-96 by mud-rotary drilling. Most Seismic-reflection profile boreholes were drilled through the unconsolidated G 0 1 2 345 Miles sediments into bedrock. Split-spoon core samples Precipitation station 0 123 4 5 Kilometers were obtained at regular intervals for geologic Base from New York State Department of Transportation,1:24,000, 1981 analysis. The thickness of each hydrogeologic unit B. Location of Manhasset Neck Study Area was determined from the core data obtained during the drilling of observation wells and, to a lesser extent, from drillers’ logs and interpolation. Additional information was obtained from NCDPW, Nassau County Department of Health (NCDH), New York State Department of Environmental Conservation, and Figure 1. Location of Great Neck and Manhasset Neck peninsulas, and of seismic-reflection profiles, Nassau County, N.Y.: A. General from previous studies. The maps and sections are location map. B. Location of Manhasset Neck study area.

Methods of Study 3

based on all available geologic data and some well this study. The seismic results were then correlated drillers’ logs. The standard Munsell color chart was with geologic logs of nearshore observation wells. the basis for the color descriptions of core samples (Kollmorgan Instruments Corporation, 1994). Borehole-Geophysical Logs

Seismic-Reflection Surveys The geophysical-logging systems used in this study provided continuous digital records that reflect Offshore continuous high-resolution seismic- the physical properties of the sediment, the rock reflection surveys were used to interpret the depth and matrix, and the interstitial fluids. The borehole- continuity of seismic reflectors and lithology (Haeni, geophysical logs of the observation wells provided 1986; 1988). Applications of this technique for information that could not be obtained by drill cores hydrogeologic and water-resource studies have been and water-quality sampling. At several sites, natural- described by Haeni (1986, 1988) and by Reynolds and gamma radiation (gamma), spontaneous potential Williams (1988). (SP), single-point-resistance (SPR), and short- and Several continuous high-resolution seismic- long-normal resistivity (R) logs were collected in reflection surveys were conducted along the coast of mud-filled open boreholes prior to casing. At each of Manhasset Neck from Manhasset Bay to Hempstead the sites, focused electromagnetic-induction (EM) logs Harbor in 1993 and 1994 to identify the were obtained in polyvinyl chloride (PVC) cased unconsolidated deposits of the subsurface and to wells. The five types of logs are described below. correlate these deposits with geologic and geophysical Natural-gamma radiation (gamma) data obtained from drilling. (Locations of selected logs—Gamma logs are a record of the total gamma seismic-reflection survey lines are shown in fig. 1B). radiation detected in a borehole (Keys, 1990). Clays The continuous-seismic-reflection system was and fine-grained sediments tend to be more radioactive installed in a shallow-draft, 22-ft boat and consisted of than the quartz sand that forms the bulk of the deposits a graphic recorder, an amplifier and filter, power on Long Island. In the northeastern United States, generator, a catamaran-mounted sound source, a gamma logs are most commonly used for lithologic hydrophone array, and a digital tape recorder. The and stratigraphic correlation. sound source and hydrophone array were towed Spontaneous-potential (SP) logs —These logs behind the slowly moving boat, where the seismic- provide a record of the potential, or voltage, that reflection data were digitally recorded and filtered for develops at the contact between clay beds and sand real-time graphic display. aquifers within a borehole (Keys, 1990). SP differs The seismic signals generated by the sound from one formation to the next and is measured in source travel through the water column and penetrate millivolts (Serra, 1984). SP is a function of the the deposits underlying the sea floor. Part of the chemical activity of the borehole fluid, the water in the seismic signal is reflected back to the water surface adjacent sediments, the water temperature, and the from the sea floor. Changes in the acoustic impedance type and quantity of clay. SP logs are used to (product of the density and acoustic velocity of each determine lithology, bed thickness, and salinity of medium) indicate stratigraphic interfaces (Haeni, formation water (Keys, 1990). 1986; Robinson and Çoruh, 1988). The reflected Single-point-resistance (SPR) logs —These logs signals received by the hydrophone array produce an provide a measure of the resistance, in ohms, electrical signal that is in turn amplified, recorded on between an electrode in the borehole fluid and an digital tape, filtered, and plotted. The resulting seismic electrode at land surface. The volume of surrounding sections resemble a vertical geologic section. The material to which the SPR probe is sensitive is vertical axis is a function of the time required for the spherical and is only 5 to 10 times the electrode seismic signal to travel from the source to the reflector diameter; it also is affected by the borehole fluid and back (Haeni, 1986). Several seismic-reflection and (Keys, 1990). SPR logs are used to obtain high- refraction studies indicate that the average acoustic resolution lithologic information. velocity of unconsolidated saturated glacial deposits is Normal-resistivity (R) logs—This technique uses about 5,000 ft/s (Haeni, 1988; Reynolds and Williams, two electrodes typically spaced 16 to 64 in. apart in 1988); this value was used as an average velocity in the borehole; the logs are called short- and long-

4 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

normal logs, respectively. Normal-resistivity logs resulting hydrographs were used to detect tidal effects, measure apparent resistivity in ohm-meters and are hydraulic connections between aquifers, and the used to determine lithology and water salinity (Keys, effects of pumping by nearby public-supply and (or) 1990). The volume of surrounding material to which golf-course-irrigation wells. normal resistivity probes are sensitive is spherical, with a diameter about twice the electrode spacing (Serra, 1984; Keys, 1990). Only short-normal Water-Sample Collection resistivity logs were used in this study. Focused electromagnetic-induction (EM) The USGS collected 23 water samples from logs—This technique uses an electromagnetic emitter observation wells in March 1997 by established coil that induces current loops within the surrounding techniques (Hem, 1992), and the NCDPW collected formation to generate a secondary electromagnetic samples from all newly installed observation wells by field. The intensity of the secondary field received by similar techniques. Three casing volumes were the receiver coil is proportional to the formation pumped from the well, and stable readings of conductivity (Keys, 1990; Serra, 1984; Keys and temperature and specific conductance of the pumped MacCary, 1971). EM-log values are in units of water were obtained, before sampling. The samples millisiemens per meter (mS/m) and are inversely were analyzed for chloride concentration and specific related to the ohm-meter values of normal-resistivity conductance by the Nassau County Department of logs (Keys, 1990; Serra, 1984; Keys and MacCary, Public Works (NCDPW) Cedar Creek Special Projects 1971). The normally low conductivity of Long Island’s Laboratory, and the results were compared with the hydrogeologic deposits is favorable for induction 1996 New York State Department of Health (NYSDH) logging to delineate highly conductive fluids such as maximum contaminant level (MCL) for drinking saltwater or leachate. For this reason, EM logging has water. Samples were collected from observation wells been used on Long Island to delineate the saltwater- to delineate the freshwater-saltwater interface. freshwater interface (Stumm, 1993a, 1993b, 1994; Additional ground-water-quality data from Manhasset Stumm and Lange, 1994). Data from many wells in Neck are available from the Nassau County this and previous studies indicate that EM log values, Department of Health (NCDH) and the NCDPW. in mS/m, correspond to 10 to 15 times the dissolved Filter-press samples also were obtained at chloride concentration, in milligrams per liter, of filter- selected well sites for chloride and specific press samples from the same interval. Gamma logs conductance analyses at the USGS office in Coram. A used in conjunction with EM logs are useful for distinguishing between conductive fluids and filter-press sample is obtained by using a nitrogen-gas- conductive clays. pressurized chamber to force interstitial water from uncontaminated (by drilling fluid) parts of a split- spoon core sample obtained during drilling Water-Level Measurements (Lusczynski, 1961). The sample volume is typically about 0.7 fl oz (2 mL); chloride concentration was Water levels in observation wells were measured measured by a hand-held ion selective electrode. The quarterly during the study. All observation wells resulting values were used only as a general indicator within the peninsula were measured on the same day of brackish water or saltwater within a formation and to minimize the effect of nearby pumping wells with were correlated with borehole geophysical logs. sporadic pumping intervals. All water-level data were checked and entered into the USGS data base in Coram, N.Y. Water levels at a total of 69 observation HYDROGEOLOGY wells within Manhasset Neck and surrounding areas were measured (fig. 2). Manhasset Neck is underlain by unconsolidated Continuous water-level recorders were installed glacial deposits of Pleistocene age and coastal-plain at seven wells, each screened in a specific aquifer. Five deposits of Late Cretaceous age. These deposits continuous water-level recorders were digital-pressure consist of gravel, sand, silt, and clay and rest upon transducer data loggers that record at 1-hour intervals; crystalline-metamorphic bedrock of early Paleozoic the other two were continuous analog recorders. The age. The top of the bedrock, which is relatively

Hydrogeology 5

73°44´ 73°42´ 73°40´

nd ou d S A lan 40°52´ Is ng Lo 8994 B 9118 12151 12667 6342 6282 9446 9116 C' 12318 8766 8996 12343 H C 8246 12319 e 37 9356 mp 6031 36 stea 12639 8313 7157 4389 12190 1118 d H a 8891 12240 rbor 12241 1484 12209 1483 7244 1482 6095 819-26 11929 12264 4859-60 11928 12263 6087 9609 12262 35 11931 12232 11930 40°50´ 1995 675 MN-25 12506 Manhasset Bay 12320 12522 2635 5209 5530 6089T 5210 9608 657 12508 662 9477 9694 110 12079 12451 8608 12793 12321 2269 9895 1715 12112 1716 2101 9908 9840 2030 9480 12013 30 4223 12134 5918 9019 8790 5876 12927 10100 31 8879 10740 3742 9776 9900 10744 8761 9809 9607 28 1120 5228 9098 29 12507 2052 10192 9899 3540 3521 9902 12511 9820 3443 3523 5883 1870 9901 1871-77 12191 9906 40°48´ 9099 12152 5884 6395 12735 9904 2420 9313 4266 8010 687 12082 9308 8309 9610 24 11732 B' 12038 25 11574 8877 Northern Blvd 2028 8933 7747-58 5528 4623 7961-71 9897 9892 1328 12483 12154 10557 7554 9711 12469 12470 12523 A' 11509 1298 LINE10390 OF GEOLOGIC SECTION12580T 7126 7551 A 1900 12581T 7553 16181889PUBLIC-SUPPLY4388 WELL 7892 7552 8558 5099 5947 11028342OBSERVATION700 WELL 22 10889 553510992GOLF-COURSE WELL 12730 8551 1618 3733 8964 12731 7053 12163 A' 3732 (Numbers are assigned by New York State 5535 Department of Environmental Conservation. 4327 Prefix 'N' for Nassau County omitted on wells. 0 1 2 MN represents well8970 without12450 NYSDEC number. 7651 MILES Suffix 'T' represents test boring.)12455 0 12KILOMETERS 40°46´ 5710 Base from New York State Department of Transportation, 1:24,000, 1981

Figure 2. Locations of public-supply wells, observation wells, and golf-course-irrigation wells within Manhasset Neck, Nassau County, N.Y. (Location is shown in fig. 1B.)

6 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

impermeable, generally forms the base of the ground- hydrologic unit names used in this report are those water reservoir. currently used by the USGS. The water-bearing properties and characteristics of the aquifer, and the relations between hydrogeologic and geologic units, are shown in table 1. The upper and Stratigraphy lower boundaries of the glacier-derived hydrogeologic The unconsolidated deposits of Manhasset Neck units are determined mainly from lithologic overlie southeastward dipping crystalline- differences, not by the age of the deposits, which metamorphic bedrock. These unconsolidated deposits would be the basis for geologic correlation. are of Cretaceous and Pleistocene age. Microscopic The geologic and hydrologic units that form Long and mineralogic analysis of drill-core samples were Island’s hydrogeologic framework are described by used to delineate the stratigraphic sequence. Suter and others (1949); Perlmutter and Geraghty (1963); Swarzenski (1963); Kilburn (1979); and Bedrock Smolensky and others (1989). Geologic correlations The bedrock underlying the Manhasset Neck area are revised from those of Kilburn (1979), Swarzenski has been mapped as the Hartland Formation of Middle (1963), and Fuller (1914). The drill-core data obtained Ordovician to Lower Cambrian Age (Baskerville, in this study and a study in Great Neck (Stumm, 2000) 1992). The upper surface of the bedrock slopes provide a basis for the naming of two hydrogeologic southeastward from 150 to 600 ft below sea level units—the North Shore confining unit, and the North (fig. 3). Drill-core data indicate the bedrock to be a Shore aquifer (Stumm, 2000). All other geologic and biotite-garnet schist. A 50- to 100-ft thick zone of

Table 1. Generalized description of hydrogeologic units underlying Manhasset Neck, Nassau County, N.Y. [Modified from Smolensky and others, 1989, sheet 1]

Hydrogeologic unit Geologic unit Description and hydraulic characteristics Upper glacial aquifer Upper Pleistocene deposits Till and outwash deposits of sand, silt, clay and boulders. Varied permeability with an average hydraulic conduc- tivity of 270 feet per day and an anisotropy of 10:1. Out- wash has the highest hydraulic conductivity. North Shore confining Pleistocene deposits Marine and stratified glacial lake deposits. Clay and silt unit deposits with minor parts containing shells. The clay is olive brown and olive gray and is poorly permeable. Unit contains a minor sand unit that is moderately permeable. North Shore aquifer Pleistocene deposits Sand, silt, and gravel; brown and olive gray, poor to moder- ate sorting. Moderately permeable. Magothy aquifer Matawan Group-Magothy Fine sand with silt and interbedded clay. Gray and pale yel- Formation, undifferentiated low quartz sand. Lignite and iron-oxide concretions common. Moderately permeable with an average hydraulic conductivity of 50 feet per day and an anisot- ropy of 100:1. Raritan confining unit Unnamed clay member of the Clay; solid with multicolors such as gray, white, red, or tan. (Raritan clay) Raritan Formation Very poorly permeable. Confines water in underlying unit. Average hydraulic conductivity of 0.001 foot per day. Lloyd aquifer Lloyd Sand Member of the Fine to coarse sand and gravel with clay lenses. White and Raritan Formation pale-yellow sand well sorted. Moderately permeable with an average horizontal hydraulic conductivity of 60 feet per day, and anisotropy of 10:1. Bedrock Hartland Formation; Highly weathered biotite-garnet-schist with low hydraulic crystalline bedrock conductivity. A thick saprolitic zone 50 to 100 feet thick, consisting of white, yellow, and gray clay, underlies most of the peninsula except in the northernmost part. Impermeable to poorly permeable.

Hydrogeology 7

73°44´ 73°42´ 73°40´

SOUND Hempstead Harbor ° 40 52´ ISLAND LONG -147 MANHASSET-272 NECK -415

-321 -150 -309

-179 -200

-250

-173 -300 -307

-288 -350 -366 ° 40 50´ Manha -397 sset Bay -467 -327 -237 -396 -400 -166

-350 -410 -450

-200 -257 -203

-277 -250 -354 -450 -468 -300 -541 -340

-500 -350 -334 40°48´

-348 -389 -400 -550 -310 -378 -446

-561 -600 BEDROCK CONTOUR – Shows altitude of bedrock surface. Dashed where -597 1298 10390 12580T -400 approximately1900 located. Contour 12581Tinterval 509188 feet. Datum1889 is sea level. -577 9687 8342 700 5099 WELL TYPES22 – Number, where present, 2169 10992indicates depth above or below sea level, in feet. 8964 1618 PUBLIC-SUPPLY WELL -561 OBSERVATION WELL 0 1 2 MILES -541 8970 12450 1102 GOLF COURSE WELL12455 0 1 2 KILOMETERS 40°46´

Base from New York State Department of Transportation, 1:24,000, 1981

Figure 3. Surface altitude of bedrock underlying Manhasset Neck, Nassau County, N.Y. (Location is shown in fig. 1B)

8 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

highly weathered bedrock, or saprolite, is present, 1996). The upper surface ranges from 194 ft to 420 ft except in the northern and western parts of the below sea level (figs. 4, 5A, 5B, 5C). Where the Lloyd peninsula, where some or all of it was removed by Sand is present, its thickness ranges from 50 to 100 ft. glacial scouring. The bedrock surface forms a relatively The Lloyd Sand was not present in either the impermeable boundary for Long Island’s ground-water northwestern or the central parts of Manhasset Neck system (McClymonds and Franke, 1972). because it was removed by glacial erosion (Stumm and The saprolitic zone consists mostly of white, Lange, 1996). It was not penetrated within the buried yellow, and gray clay with quartz fragments. This zone valleys onshore and offshore (fig. 4, 5C), nor at wells had been previously interpreted by drillers to be a clay N12151, N12209, N12232, N12320, N12321, or clayey gravel layer above the bedrock surface. N12318, or N12639 (figs. 2, 4, 5A, 5B, 5C). Observation wells N12523 and N12507 (fig. 2) The Lloyd Sand forms the Lloyd aquifer, which is penetrated into weathered bedrock. a major source of water supply in this part of Long Glacial erosion or scouring of the overlying Island. In northern Nassau County, the Lloyd aquifer Cretaceous and possibly earlier Pleistocene deposits has an average horizontal hydraulic conductivity of and the weathered bedrock, formed several buried about 60 ft/d (McClymonds and Franke, 1972) and a valleys that have truncated the surrounding material. horizontal to vertical anisotropy of 10:1 (Smolensky The weathered bedrock at the base of these valleys has and others, 1989). been partly or totally removed. Drill-core data from Raritan clay.—The unnamed clay member of the observation wells N12151 and N12209 in the Raritan Formation, referred to as the Raritan clay in northernmost part of Manhasset Neck (fig. 2) indicate this report, overlies and confines ground water in the that the bedrock surface is relatively flat and does not Lloyd aquifer (Suter and others, 1949; Stumm, 2000) follow the regional southeastward dip (fig. 3). The and is a major confining unit on Long Island time during which most of the weathered bedrock was (Smolensky and others, 1989). The Raritan clay is removed is unknown, but it could have occurred present only in the southernmost parts and along the during successive glacial advances during the northeastern part of Manhasset Neck (fig. 6). The Pleistocene. Drill-core data generally indicate that Raritan clay is a solid, compact clay that is wherever Cretaceous sediments are penetrated in multicolored and includes gray (2.5Y 4/1), white place, the bedrock surface beneath them has a (2.5Y 9/1), red (10R 4/8), or tan (2.5Y 4/8), and saprolitic zone, and the absence of Cretaceous deposits variegated colors. is typically accompanied by a lack of saprolite. The upper surface altitude of the Raritan clay Seismic-reflection data collected in this study (1993- ranges from 99 ft to more than 300 ft below sea level; 94) indicate that the bedrock surface beneath the its thickness in the study area ranges from 50 to 225 ft. embayments of Long Island Sound is undulating; this The Raritan clay was removed by extensive glacial is consistent with observations of Tagg and Uchupi erosion in the northern and western part of the (1967), who found the bedrock surface beneath most peninsula (fig. 6) (Stumm and Lange, 1996). The of Long Island Sound to have been severely eroded Raritan clay was not penetrated at wells N12151, during the Pleistocene. N12209, N12232, N12320, N12321, N12318, or N12639 (figs 2, 5A, 5B, 5C, 6). Offshore seismic- Aquifers and Confining Units reflection profiles indicate the Raritan clay underlies Lloyd aquifer.—The Lloyd Sand Member of the only the southernmost parts of the Manhasset Bay and Raritan Formation of Late Cretaceous age (Suter and Hempstead Harbor. The Raritan clay is poorly others, 1949; Cohen and others, 1968) consists of an permeable, with an average vertical hydraulic upward fining sequence of white and pale-yellow conductivity of about 0.001 ft/d (Smolensky and (Munsell descriptive color value 2.5Y 7/3) sand and others, 1989). gravel with white clay lenses. White and grayish silt Magothy aquifer.—The Magothy aquifer consists may be present in the upper sections of the unit. of fine micaceous sand, silt, and interbedded clay The Lloyd Sand overlies bedrock and generally is sediments of Cretaceous age (Suter and others, 1949; overlain by Raritan clay. Generally, the Lloyd Sand Swarzenski, 1963; Kilburn, 1979). The Magothy dips to the southeast, except where it has been aquifer is an upward fining sequence of gray, white, or removed by glacial erosion (fig. 4) (Stumm and Lange, pinkish sands and clays of the undifferentiated

Hydrogeology 9

73°44´ 73°42´ 73°40´

H e -200 SOUND mp stea 40°52´ d H a ISLAND rbor MANHASSET NECK -240 LONG -250 -194

-200

-148

00 -250 -263 -3

40°50´ Manhasset

-317 B -255 50 ay -3 -121

-240 -255 -132 -322 -200 -221 -264 -180 00 -425 -4 -250 -400 -428 -274 -350 -401 -186 -222 -260 -334 00 40°48´ 3 - 350 -299 -

-263

00 -4 STRUCTURE CONTOUR – Shows altitude -401 00 of hydrologic unit surface. Dashed where -424 -4 approximately located. Contour interval 50 feet. Datum is sea level. 1900 EXTENT1889 4388 OF HYDROGEOLOGIC UNIT 8342 700 5099 10992WELL TYPES22 – Number, where present, indicates8964 depth above or below sea level, in feet. -401 PUBLIC-SUPPLY WELL OBSERVATION WELL 0 1 2 MILES -424 8970 12450 GOLF COURSE WELL12455 0 1 2 KILOMETERS 40°46´

Base from New York State Department of Transportation, 1:24,000, 1981

Figure 4. Extent and altitude of the upper surface of the Lloyd aquifer, Manhasset Neck, Nassau County, N.Y. (Location is shown in fig. 1B)

10 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

A'

Clay N12523 Hydrogeology by F. Stumm Aquifer Magothy Raritan Clay UG Lloyd Aquifer

Bedrock

N12507 N10744 NSaq UG NScu

(ENE) N12321 UG

NScu N12320 EXPLANATION WELL BORING AND TRACE OF GEOPHYSICAL (GAMMA RAY) LOG – Well locations are shown in figure 2. Geologic Contact – dashed where approximately located UG

NScu N12523 UG Upper glacial aquifer

NScu North Shore confining unit NSaq North Shore aquifer N12209 Bedrock UG NScu NSaq

4000 FEET N12151 1000 METERS 2000 Hydrogeologic section A-A´ on Manhasset Neck, Nassau County, N.Y. (Trace of section is shown in fig. 2.) Hydrogeologic section A-A´ on Manhasset Neck, Nassau County, N.Y. (Trace of is shown in fig. 500 Vertical Exaggeration x13 Long Island Sound 0 0 A 0 300 150 Sea Figure 5A. -150 -300 -450 -600 -750 Level

Hydrogeology 11

B' N11574 UG Lloyd aquifer aquifer Magothy

Hydrogeology by F. Stumm N12511 Harbor Hempstead aquifer clay Magothy Raritan Lloyd aquifer

upper glacial aquifer N9019 UG UG NScu Bedrock

NSaq

N12321 N12320 (NE) UG NSaq NScu

EXPLANATION

N12232 WELL BORING AND TRACE OF GEOPHYSICAL (GAMMA RAY) LOG – Well locations are shown in figure 2. Geologic Contact – dashed where approximately located NSaq N9019 UG Upper glacial aquifer NScu NScu North Shore confining unit NSaq North Shore aquifer Bedrock

North Shore UG aquifer

N12639 N12318 UG 4000 FEET 1000 METERS NSaq Bedrock 2000 500 Hydrogeologic section B-B´ on Manhasset Neck, Nassau County, N.Y. (Trace of section is shown in fig. 2.) Hydrogeologic section B-B´ on Manhasset Neck, Nassau County, N.Y. (Trace of is shown in fig. Sound Long Island Vertical Exaggeration x15 0 0 B 0 FEET 150 Sea -150 -300 -450 -600 Level Figure 5B.

12 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

C' Hydrogeology by F. Stumm North Shore confining unit Harbor Hempstead

Bedrock N12319 clay Raritan upper glacial aquifer Lloyd aquifer

EXPLANATION

North Shore confining unit Geologic Contact – dashed where approximately located WELL BORING AND TRACE OF GEOPHYSICAL (GAMMA RAY) LOG – Well locations are shown in figure 2. N6282

UG Upper glacial aquifer N6282 NScu NSaq North Shore aquifer NSaq North Shore UG confining unit NSaq

Bedrock N12318 4000 FEET UG 1000 METERS Hydrogeologic section C-C´ on Manhasset Neck, Nassau County, N.Y. (Trace of section is shown in fig. 2.) Hydrogeologic section C-C´ on Manhasset Neck, Nassau County, N.Y. (Trace of is shown in fig. 2000 500 NScu Long Island Sound Vertical Exaggeration x27 0 0 C FEET (N) 0 Figure 5C. 150 Sea -150 -300 -450 level

Hydrogeology 13

73°44´ 73°42´ 73°40´

H e SOUND mp stea 40°52´ d H ISLAND MANHASSET NE arbor 00 -65 -1 LONG -99 -100

0

-15 -200

-200

- 15

0

-128 0 5 40°50´ Manhasset Bay -2

-237 -294

-145 -163

-165 -229 0 -186 15 00 - 3 -232 -

-197 -301

-209 -200 40°48´ -143 CK

-240 -206

STRUCTURE CONTOUR – Shows altitude -229 00 of hydrogeologic unit surface. Dashed where -312 -4 approximately located. Contour interval 50 feet. Datum10390 is sea level. 12580T 1900 12581T 9188 1889EXTENT4388 OF HYDROGEOLOGIC UNIT 8342 5099 9687 -335 700 2169 10992WELL 22TYPES – Number, where present, -294 indicates8964 depth above or below sea1618 level, in feet.

-335 PUBLIC-SUPPLY WELL OBSERVATION WELL12450 1102 0 1 2 MILES -186 8970 -247 12455 0 1 2 KILOMETERS 40°46´ GOLF COURSE WELL

Base from New York State Department of Transportation, 1:24,000, 1981

Figure 6. Extent and altitude of the upper surface of the Raritan clay, Manhasset Neck, Nassau County, N.Y. (Location is shown in fig. 1B.)

14 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

Matawan Group and Magothy Formation (Kilburn, sorted dark olive brown (2.5Y 3/3) and olive gray (5Y 1979; Smolensky and others, 1989; Stumm, 2000). 6/2) gravel, sand, and silt. The aquifer contains Drill-core samples indicate that the Magothy aquifer subangular to subrounded quartz grains, rock consists of subrounded silty gray quartz sand (5Y 6/1) fragments, unstable opaque minerals, and a large and pale yellow quartz sand (2.5Y 7/2) with associated percentage of biotite and muscovite micas. The aquifer lignite and chemically stable heavy opaque minerals. consists of moderately sorted stratified drift and The Magothy aquifer has been removed by glacial outwash deposits that infilled the low-lying areas after erosion in most of the study area and is present only in the removal of the Cretaceous deposits and parts of the the southern part of the Manhasset Neck peninsula bedrock (saprolitic zone) by ice-contact erosion. It was overlying the Raritan clay (figs. 5A, 5B, 5C, 7). The then capped by clay and silt of the North Shore top of the Magothy aquifer ranges from 208 ft below confining unit. In other nearby areas, subsequent sea level to 41 ft above sea level (fig. 7). Glacial glacial advances eroded northwest-southeastward erosion has produced a steep, undulating, north-facing trending valleys that reached the bedrock surface and scarp along the southeastern part of the peninsula (fig. removed previously deposited sediments that may 7). Where present, the Magothy aquifer overlies the have included parts of the North Shore aquifer. These Raritan clay. It has an average hydraulic conductivity valleys later became filled with hundreds of feet of of 50 ft/d and an anisotropy of 100:1 (Smolensky and clay and silt (figs. 5A, 5B, 5C). The buried-valley others, 1989). deposits probably represent the most recent glacial North Shore aquifer.—The North Shore aquifer is advance in the area, however, and may not be of the a sequence of Pleistocene-age sediments penetrated in same age as the North Shore aquifer in northern the northernmost and central part of Manhasset Neck Manhasset Neck; therefore, they are grouped with (fig. 8) and is described in detail by Stumm (2000). undifferentiated Pleistocene-age deposits of the North Similar deposits, and a similar hydrogeologic Shore confining unit. The North Shore aquifer is about framework, are present to the west on Great Neck 250 ft thick in the northern part of the peninsula and (Stumm, 1994, 2000; Stumm and Lange, 1994, 1996) about 50 thick in the central part (figs. 5A, 5B, 5C, 8). and in northwestern Queens County (Chu and Stumm, The top of the North Shore aquifer ranges from 70 ft 1995); therefore, a unifying nomenclature was needed to almost 300 ft below sea level. The rapid response of for the northern shore of Long Island. North Shore water levels to tides and pumping indicates that this aquifer deposits were called the Jameco Gravel by unit is moderately to highly permeable. No hydraulic Swarzenski (1963) and the Port Washington aquifer by conductivity values for this unit were calculated. Kilburn (1979). The name North Shore aquifer is used in this report in preference to Jameco Gravel or North Shore confining unit.—The name North Jameco aquifer because these imply correlation with Shore confining unit is given to a sequence of Jameco deposits in southern Queens County; such a Pleistocene-age clay and silt deposits that are locally correlation is questionable. The Port Washington present along the northern shore of Queens (Chu and aquifer, as described by Kilburn (1979), included a Stumm, 1995), Nassau (Stumm and Lange, 1994, large amount of material that was found in this study 1996), and Suffolk (Soren, 1971) Counties. The term to be of Cretaceous age (Lloyd aquifer and Raritan “North Shore confining unit” was used for the first clay). The placement of the Port Washington aquifer time in reference to Manhasset Neck and is above the Raritan clay is rejected on the basis of extensively described in Stumm (2000). These drill-core samples and hydrologic data (Stumm and deposits are described by Grim and others (1970), Lange, 1996). Lewis and Stone (1991), and Williams (1981) as part The North Shore aquifer is a distinct of the stratified glacial-lake clay and silt that infill hydrogeologic unit that rests upon bedrock and is Long Island Sound and Manhasset Bay. Stumm and overlain by a thick sequence of clay and silt (the North Lange (1994, 1996) used offshore seismic-reflection Shore confining unit) (figs. 5A, 5B, 5C, 8), except in surveys, borehole-geophysical logs, and drill-core one small, isolated area in the northwestern part of the samples to correlate these deposits with buried study area, where it is overlain by a thin silt layer and valleys that extend across the Manhasset Neck appears to be semiconfined. The North Shore aquifer Peninsula. Stumm (2000) correlated offshore seismic- is described as a sequence of poorly to moderately reflection surveys, geophysical logs, and drill-core

Hydrogeology 15

73°44´ 73°42´ 73°40´

H e SOUND mp stea 40°52´ d H a ISLAND rbor

MANHASSET NECK LONG

40°50´ Manhasset -87 ? B ay -41 -85 ?

- 2 -154 00

- 1 50

-208 ?

-39 40°48´ -50 19 -100 0 -50 -42 -55

0 0 -145 -50 -100 -11 STRUCTURE CONTOUR – Shows altitude 0 -87 -400 of hydrogeologic unit surface. Dashed where approximately10390 located. Contour interval 50 feet. 12580T -50 +31 1900Datum is sea level. 12581T 1889 4388 8342EXTENT700 OF HYDROGEOLOGIC5099 UNIT 22 10992 +41 WELL 8964TYPES – Number, where present,1618 indicates depth above or below sea level, in feet.

-11 PUBLIC-SUPPLY WELL 12450 1102 8970 0 1 2 MILES -87 OBSERVATION WELL12455 40°46´ 0 1 2 KILOMETERS

Base from New York State Department of Transportation, 1:24,000, 1981

Figure 7. Extent and altitude of the upper surface of the Magothy aquifer, Manhasset Neck, Nassau County, N.Y. (Location is shown in fig. 1B)

16 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

73°44´ 73°42´ 73°40´

H e SOUND mp stea 40°52´ d H ISLAND arbor MANHASSET-237 NE

LONG -205 -109 -2 - 1 5 -200 5 0 0

-90 - 1

00

-73 -287

-267

Manhasset Bay -174 40°50´ -259 -180

-182 -200 -231

-247 -250

-207 -183

40°48´ CK

STRUCTURE CONTOUR – Shows altitude -400 of hydrogeologic unit surface. Dashed where approximately10390 located. Contour interval 50 feet. Datum is sea level. 12580T 1900 12581T 1889EXTENT4388 OF HYDROGEOLOGIC UNIT 8342 700 5099 10992WELL22 TYPES – Number, where present, indicates8964 depth above or below sea1618 level, in feet. PUBLIC-SUPPLY WELL 0 1 2 MILES -267 OBSERVATION8970 WELL12450 0 1 2 KILOMETERS 40°46´ GOLF COURSE WELL

Base from New York State Department of Transportation, 1:24,000, 1981

Figure 8. Extent and altitude of the upper surface of the North Shore aquifer, Manhasset Neck, Nassau County, N.Y. (Location is shown in fig. 1B)

Hydrogeology 17

samples with buried valleys that extend across the the North Shore aquifer below, but this interpretation Great Neck peninsula. is not conclusive; therefore, the North Shore confining Swarzenski (1963) called these deposits the unit is considered to be missing in this area. The unit Gardiners Clay, and Kilburn (1979) referred to them as consists of olive-brown (2.5Y 4/2) varved clay and the Port Washington confining unit. The name North minor amounts of silt and dark-olive gray clay (5Y 4/ Shore confining unit is used here in preference to the 2) with shells. The upper surface of the unit ranges name Gardiners Clay because correlation with the from 175 ft below sea level to 30 ft above sea level Gardiners Clay elsewhere on Long Island is (fig. 9). questionable. The Port Washington confining unit of Of interest is the lack of sand and gravel within Kilburn (1979) was delineated from drillers’ logs and includes parts of the Raritan clay of Cretaceous age. the onshore and offshore buried valleys beneath Drill-core data obtained in this study are inconsistent Manhasset Bay and Hempstead Harbor that with many previous interpretations and delineations; Swarzenski (1963) and Kilburn (1979) describe as therefore, the Port Washington confining unit is no possible channels or valleys infilled mostly with sand longer considered a valid unit in this area. and gravel. Most of the buried valleys delineated in The Gardiners Clay is a greenish-brown or gray this study were filled with 200- to 300-ft thick marine clay containing diatoms, foraminifers, and sequences of clay and silt. Elsewhere on the peninsula, shell fragments (Suter and others, 1949). Its the North Shore confining unit ranges from 0 to more stratigraphy and geologic age were not investigated in than 50 ft in thickness. this project. The current widely accepted interpretation Upper glacial aquifer.—The upper glacial aquifer is that the term Gardiners Clay should be applied to a includes the saturated parts of the upper Pleistocene Pleistocene aged shallow marine clay unit present deposits. It consists of Pleistocene-age beds of fine- to along the southern shore of Long Island and probably coarse-grained and stratified sand and gravel, deposited during an interglacial period. unstratified boulders, clay, till, and some small, Tagg and Uchupi (1967) conducted seismic shallow pond deposits (Suter and others, 1949) and, on profiles in Long Island Sound and describe the Manhasset Neck, is underlain in some areas by the bedrock surface to be deeply cut by flat-bottomed, U- North Shore confining unit. In some areas, Cretaceous shaped troughs that extend to more than 600 ft below sea level. They interpreted these troughs or valleys to deposits have been thrusted upward by ice contact and be glacially eroded but were unable to determine are incorporated into the upper glacial aquifer. The whether any of them extend beneath Long Island upper glacial aquifer is poorly to moderately sorted because nearshore seismic data were lacking. Reeds and contains mostly quartz minerals, rock fragments, (1927) described varved clay deposits in the New York and biotite and muscovite micas; it also contains City area. His interpretations suggest clay deposition brown sand (7.5Y 4/6), dark yellowish-brown sand in postglacial freshwater lakes occupying low-lying (10YR 4/6), and varying amounts of reworked basins in southern New York. Reeds (1927) proposed Cretaceous deposits. Offshore seismic-reflection several interconnected lakes (Glacial Lake Flushing) surveys and drill-core data indicate that this aquifer within parts of Long Island Sound and northern does not extend offshore, nor does it infill the valleys Queens County (fig. 1) and describes these glacial beneath Manhasset Bay and Hempstead Harbor, as lakes as containing extensive layers of varved clays interpreted by Kilburn (1979). The upper glacial and silt. aquifer overlies the Magothy aquifer in the The North Shore confining unit underlies the southernmost part of Manhasset Neck and the North northern parts of Manhasset Neck and extends Shore confining unit, and overlies the Raritan clay in offshore (figs. 5A, 5B, 5C, 9), except in a small area in the northwestern part of the peninsula, where a thin, the rest of the peninsula (figs. 5A, 5B, 5C, 10). The silty unit was penetrated above the North Shore present land surface includes recent deposits and the aquifer, but no significant clay deposits of the North unsaturated upper part of the upper glacial aquifer. The Shore confining unit were sampled. This area may upper glacial aquifer has an average horizontal represent a thin, silt-rich area of the North Shore hydraulic conductivity of 270 ft/d (Smolensky and confining unit that confines water to a small degree in others, 1989).

18 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

73°44´ 73°42´ 73°40´ SOUND ISLAND H LONG emp stea 40°52´ -50 d H a -77 rbor -50 MANHASSET NECK -100 -65 NCU -59 -40 -150

-175 4 -50 -75 0 ? -128 -68 -36 -50 Manhasset Bay 40°50´ -129 -12 30 -128

0 -41 -41 -50 30 -32

-27 -5 -50 23 -132 12 -100 -137 0 -20 -133 -36 -80

-31 0 40°48´ 50 37 -62 61 49

-18 STRUCTURE CONTOUR – Shows altitude -30 -400 of hydrogeologic unit surface. Dashed where approximately located. Contour interval 50 feet. Datum is sea level. EXTENT1900 OF HYDROGEOLOGIC UNIT 1889 4388 NCU 8342NO CONFINING UNIT FOUND5099 700 2169 10992 22 WELL TYPES – Number, where present,1618 indicates8964 depth above or below sea level, in feet. PUBLIC-SUPPLY WELL -132 OBSERVATION WELL12450 0 1 2 MILES -59 8970 0 1 2 KILOMETERS 40°46´ GOLF COURSE WELL

Base from New York State Department of Transportation, 1981, 1:24,000

Figure 9. Extent and altitude of the upper surface of the North Shore confining unit, Manhasset Neck, Nassau County, N.Y. (Location is shown in fig. 1B.)

Hydrogeology 19

73°44´ 73°42´ 73°40´

H e SOUND mp stea 40°52´ 21 d H 51 a ISLAND rbor MANHASSET73 50 NECK 16 100 LONG 9 66

1 00 60 122 53 147 52

1

50 111 11 18

17 40°50´ Manhasset Bay 10 141 201 35 33 ? 11 60 56 154 73 9 18

0 2 5 66 110 00 27 23 102 170 18 46 9 192 28 31 50 51 116 44 43 117 23 29 158 200 15 50 69 124 208 134 201 00 55 168 00 1 204 1 31 50 40°48´ 1 50 00 50 1 57 00 2 1 190 200 60 15 111 166 225 10 223 89 00 12 1 97 32 254

182 188 145 STRUCTURE CONTOUR – Shows altitude of 10390 00 hydrogeologic unit surface. Dashed where -4 1900 12581T 193 153 approximately1889 4388 located. Contour interval 50 feet. Datum is sea level. 5099 129 8342 700 200 10992 22 140 WELL TYPES – Number, where present,1618 209 168 indicates depth8964 above or below sea level, in feet.

254 PUBLIC-SUPPLY WELL 12450 0 1 2 MILES 145 OBSERVATION8970 WELL 12455 0 1 2 KILOMETERS 40°46´ GOLF COURSE WELL

Base from New York State Department of Transportation, 1:24,000, 1981

Figure 10. Extent and altitude of the upper surface of the upper glacial aquifer, Manhasset Neck, Nassau County, N.Y. (Location is shown in fig. 1B.)

20 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

Interpretation of Geophysical Data interpreted to be silt and clay deposits that fill a large buried valley that extends to bedrock, about 400 ft Seismic-reflection surveys.—The seismic- below sea level. The bedrock surface appears to reflection survey results were correlated with the increase in depth from about 250 ft below sea level on geologic logs of nearshore observation wells. Four the west side of the valley to 400 ft below sea level on seismic-reflection profiles—D-D´, E-E´, F-F´, and G- G´—depict the major subsurface features encountered the east side. The subhorizontal parallel reflections during the seismic-reflection surveys. that are interpreted as silt and clay truncate the surrounding coarse-grained deposits (chaotic Seismic-reflection profile D-D´ is about 3,750 ft reflectors) to the east. long and lies about 1,640 ft off the northwestern coast of the Manhasset Neck peninsula (figs. 1B, 11A). The Borehole geophysical logs.—Examples of sea floor in this section is about 20 ft below sea level. borehole geophysical logs are shown in figure 12. The seismic-reflection survey shows subhorizontal These are gamma, SP, SPR, R, and EM logs of well parallel reflections, which are interpreted to be silt N12508, in the west-central part of Manhasset Neck and clay deposits that fill two buried valleys that (fig. 2). The gamma log indicates a thin layer of sand extend to about 100 ft below sea level and are (upper glacial aquifer) underlain by 225 ft of silt and separated by coarse-grained deposits about 75 ft thick. clay of the North Shore confining unit. Beneath the The bedrock surface appears to range from 100 to clay unit is sand (North Shore and Lloyd aquifers) 125 ft below sea level. about 130 ft thick resting upon weathered bedrock. Seismic-reflection profile E-E´ is about 5,000 ft The SP log shows a slight deflection to the left at long and 1,500 ft off of the northeastern coast of 250 ft below sea level that indicates a change in Manhasset Neck (figs. 1B, 11B). The profile suggests lithology from clay (North Shore confining unit) to a buried valley in the western part of the section that sand (North Shore and Lloyd aquifers). The SPR log extends to a depth of almost 300 ft below sea level and indicates a major deflection to the right (increase in truncates the surrounding deposits. The profile of the resistance) in the upper 25 ft of the North Shore valley is dominated by subhorizontal parallel aquifer and a sudden deflection to the left in the reflections, which are interpreted to be silt and clay. remaining 105 ft of the North Shore and Lloyd The bedrock surface in this valley is shown to be from aquifers. The R log correlates with the SP and SPR 250 to 275 ft below sea level. The seismic reflectors logs and shows a zone of low conductivity (high are interpreted as a buried valley. The material to the resistivity) in the upper 25 ft of the sand and a zone of west correlates with the geologic and geophysical logs high conductivity (low resistivity) at its base. The EM from well N12151 (fig. 2), and the material to the east log shows an increase in conductivity with depth at correlates with the geologic and geophysical logs from 280 ft below land surface (220 ft below sea level). The well N12151 (fig. 2). EM log delineated a 105-ft-thick wedge of saltwater Seismic-reflection profile F-F´ crosses the north with a sharp freshwater-saltwater interface. The water end of Hempstead Harbor and is about 5,300 ft long sample from the base of the sand (Lloyd aquifer) had a (figs. 1B, 11C). The water depth here is about 30 ft. chloride concentration of 600 mg/L, and the The section delineates the buried valley beneath the geophysical logs indicate that saltwater has intruded northern part of Hempstead Harbor. The valley is into all of the Lloyd aquifer and most of the North almost 1 mi wide and extends to 400 ft below sea Shore aquifer in this area. level. The first subsurface reflector is interpreted as the top of a complex layered unit of sand and silt about 30 ft thick, beneath which are draped, parallel Precipitation reflections that are interpreted as silt and clay more than 300 ft thick. All freshwater on Manhasset Neck is derived Seismic-reflection profile G-G´ is about 8,220 ft from precipitation that infiltrates the soil. Some of the (1.6 mi) long and lies about 4,000 ft off the precipitation returns to the atmosphere through northeastern coast of the Manhasset Neck peninsula evaporation and transpiration; the rest percolates to the (figs. 1B, 11D). The sea floor in this section is about water table, where it becomes shallow ground water. 35 ft below sea level. The seismic-reflection survey Some moves laterally into stream channels and shows subhorizontal parallel reflections, which are becomes base flow; the rest moves downward into the

Hydrogeology 21

D s are shown in fig. 1B.) BEDROCK SILT AND CLAY SILT (SE) SEA FLOOR SEA FEET METERS 250 0500 0 rtical Exaggeration = 5x Interpreted stratigraphy along seismic-reflection profile D-D´ in Long Island Sound off Manhasset Neck, N.Y. (Traces of D' Ve 0

50

100 150 200 250 300 350

(Assumed velocity of sound 5000 feet per second) per feet 5000 sound of velocity (Assumed DEPTH BELOW SEA LEVEL, IN FEET IN LEVEL, SEA BELOW DEPTH Figure 11A.

22 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

E' s are shown in fig. 1B.) SAND CLAY BEDROCK (NE) SEA FLOOR SEA FEET METERS 250 SILT AND CLAY SILT Interpreted stratigraphy along seismic-reflection profile E-E´ in Hempstead Harbor off Manhasset Neck, N.Y. (Traces of BEDROCK 0500 0 rtical Exaggeration = 5.2x Ve E Figure 11B. 0

50

350 400 100 150 200 250 300

(Assumed velocity of sound 5000 feet per second) per feet 5000 sound of velocity (Assumed DEPTH BELOW SEA LEVEL, IN FEET IN LEVEL, SEA BELOW DEPTH

Hydrogeology 23

F s are shown in fig. 1B.) BEDROCK (SE) SILT AND CLAY SILT SEA FLOOR SEA FEET METERS 250 0500 0 rtical Exaggeration = 6.2x Interpreted stratigraphy along seismic-reflection profile F-F´ in Hempstead Harbor off Manhasset Neck, N.Y. (Traces of Ve F' 0

50

100 150 200 250 300 350 400 450 500 (Assumed velocity of sound 5000 feet per second) per feet 5000 sound of velocity (Assumed DEPTH BELOW SEA LEVEL, IN FEET IN LEVEL, SEA BELOW DEPTH Figure 11C.

24 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

G' s are shown in fig. 1B.) SEA FLOOR SEA BEDROCK (N) SILT AND CLAY SILT BEDROCK FEET METERS 250 500 DEPOSITS 0 0 COARSE-GRAINED rtical Exaggeration = 5.8x Interpreted stratigraphy along seismic-reflection profile G-G´ in Long Island Sound off Manhasset Neck, N.Y. (Traces of Ve G 0

50

100 150 200 250 300 350 400 450 500

(Assumed velocity of sound 5000 feet per second) per feet 5000 sound of velocity (Assumed DEPTH BELOW SEA LEVEL, IN FEET IN LEVEL, SEA BELOW DEPTH Figure 11D.

Hydrogeology 25

Well N12508 GSPSPR R EM 050100 150 -175 -125 -75 -25 150 200 250 300 0 150 300 450 0 20 40 Sand 0

50

100

North Shore confining unit 150

200

250 North Shore aquifer 300

Lloyd aquifer 350 625

DEPTH BELOW LAND SURFACE, IN FEET LAND SURFACE, DEPTH BELOW 400 Bedrock

EXPLANATION

G Gamma log (counts per second) SP Spontaneous potential log (millivolts) SPR Single-Point-Resistance log (ohms) R Short-Normal-Resistivity log (ohm-meters) EM Electromagnetic induction log (millisiemens per meter) Well screen-zone sample location; number is chloride 625 concentration (milligrams per liter)

Figure 12. Geophysical logs of well N12508, Manhasset Neck, Nassau County, N.Y. (Location is shown in fig. 1B.)

deeper hydrogeologic units and eventually discharges the spring and is least in the fall (Miller and Frederick, to the surrounding saltwater bodies. 1969). Estimates of average annual recharge were The mean annual precipitation recorded at equal to about one-half the average annual Mineola in central Nassau County (fig. 1) during precipitation (Franke and McClymonds, 1972). 1937-96 is 44.35 in (Nassau County Department of Health, 1997). Annual average precipitation for Ground Water Manhasset Neck is about 42 in (Miller and Frederick, 1969). The average warm-season and cold-season Ground water on Manhasset Neck is used precipitation values are almost equal, but the number primarily for residences, businesses, and small of days with precipitation equal to or less than 1 in. industry, and for irrigation of lawns and golf courses. shows a seasonal variation—precipitation is greatest in (Manhasset Neck has no agriculture.) Water levels in

26 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

all major aquifers within Manhasset Neck were October of 1992-96, for the upper glacial, Magothy, measured in observation wells. North Shore and Lloyd aquifers, respectively, was 9.28 Mgal (0.25 Mgal/d), 31.6 Mgal (0.86 Mgal/d), 11.3 Pumpage Mgal (0.31 Mgal/d), 25.9 Mgal (0.71 Mgal/d). Public-supply wells on Manhasset Neck are owned and operated by the Albertson, Manhasset- Water Levels Lakeville, Port Washington, and Roslyn Water Pumping generally lowers water levels in aquifers Districts, and the Sands Point Water Department. and thereby induces a flow gradient toward the Ground-water pumpage from the Magothy aquifer pumped wells. A major concern for water suppliers within Manhasset Neck is much larger than that from and managers is that contaminated water from any other aquifer. The average annual ground-water adjacent or overlying aquifers may be induced to flow pumpage during 1992-96 from the upper glacial, toward the well. Magothy, North Shore, and Lloyd aquifers was 366 All wells near the coast of Long Island are Mgal (1.0 Mgal/d), 3,012 Mgal (8.3 Mgal/d), 333 assumed to be affected by tides; thus, the USGS Mgal (0.9 Mgal/d), and 604 Mgal (1.7 Mgal/d), requires that water levels in these wells be measured respectively (table 2). Combined average annual during the high-tide period (typically 2 hours) in the pumpage for the North Shore and Lloyd aquifers nearest embayment. Water levels in observation wells during 1992-96 was 937 Mgal (2.6 Mgal/d). The were measured quarterly during 1992-97; those in values in table 2 represent the annual combined September 1996 are depicted in figures 13 (water-table industrial, golf-course, and public-supply pumpage aquifers) and 14 (confined aquifers). The following from each aquifer for 1992-96. The effect of public- paragraphs describe water levels in the four aquifers of supply pumping on water levels in the aquifers is Manhasset Neck during 1992-97. discussed in the following sections. Upper glacial and Magothy aquifers (fig. 13)—The water table is within the upper glacial aquifer in most of Manhasset Neck, but is in the Table 2. Pumpage on Manhasset Neck, Nassau County, N.Y., by aquifer, 1992-96 Magothy aquifer in the southernmost part. Water [Data from the Albertson, Manhasset-Lakeville, Port Washington, and levels of September 1996 at wells screened in the Roslyn Water Districts; Sands Point Water Department, and New York water table (upper glacial and Magothy aquifers) (fig. State Department of Environmental Conservation.] 13) ranged from 3 ft to 120 ft above sea level. The Total pumpage, in millions of gallons highest recorded values were in the central part of the Year Upper glacial Magothy North Shore Lloyd peninsula (well N12451) and in the southern part (well aquifer aquifer aquifer aquifer N9906). In general, the water-table configuration tends 1992 284.1 2,990.2 271.4 524.6 to parallel the land surface. Water in the Magothy 1993 316.8 3,580.2 350.6 483.4 aquifer becomes increasingly confined with depth. 1994 367.6 2,940.1 353.1 678.6 Water-level data obtained during quarterly 1995 430.4 3,008.7 376.5 589.6 measurements in the Magothy aquifer, where it is 1996 428.9 2,540.6 315.1 744.8 present in the southern part of the peninsula, represent the water table because the Magothy in this area is Mean 365.6 3,012.0 333.3 604.2 hydraulically connected with the upper glacial. Water-level data from selected wells indicate that Average annual 1992-96 pumpage by one the upper glacial aquifer is not significantly affected industrial well screened in the upper glacial aquifer by tidal fluctuations except where localized clay layers (N9610) was 4.33 Mgal (0.012 Mgal/d). In addition, confine water locally. This can be seen in the six wells supply five golf courses within the study hydrographs for well N9904, in southern Manhasset area. Two are screened in the upper glacial aquifer Neck, and at well N12240 in the northern part (figs. (N3742, N8761), two are screened within the Magothy 15A, 15B). Well N2269, near the center of the aquifer (N5535, N7053), one is screened in the North peninsula (fig. 15C) and screened 95 to 99 ft below sea Shore aquifer (N8246), and one is screened in the level, is affected by pumping of nearby public-supply Lloyd aquifer (N8790). Average annual golf-course well N2030 (fig. 2), which is screened at 88 to 113 ft pumpage for the pumping season of May through below sea level. (Note downward spikes in the

Hydrogeology 27

73°44´ 73°42´ 73°40´

SOUND 40°52´ H 4 e mp ISLAND stea 20 20 8 d LONG H 18 a rbor 3 40 60 8 79 80 28

7 5 8 9 100 40°50´ Manhasset Bay

112 120 120

16 10

9 ? 20 20 25 23 40 74 44 24 14 60 ? 19 28 75

80 86 40°48´

45 20 37 60 40 10 26 54 12470 STRUCTURE12469 CONTOUR – Shows altitude of 10390 129800 hydrogeologic unit surface. Dashed12580T where -4 1900 12581T approximately1889 4388 located. Contour interval 20 feet. Datum is sea level. 8342 700 5099 22 10992WELL TYPES – Number, where present, indicates8964 depth above or below sea 1618level, in feet. PUBLIC-SUPPLY WELL 254 OBSERVATION WELL12450 0 1 2 MILES 145 8970 12455 0 1 2 KILOMETERS 40°46´ GOLF COURSE WELL

Base from New York State Department of Transportation, 1:24,000, 1981

Figure 13. Water-table altitude on Manhasset Neck, Nassau County, N.Y., September 1996. (Location is shown in fig. 1B.)

28 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

73°44´ 73°42´ 73°40´

SOUND 40°52´ H ISLAND e MANHASSET-3 NE mp 7 LONG stea -6 d 9 H 4 a rbor -10 -10 0 5 10 8 10

0 0 10 -2 -3 40°50´ Manhasset Bay 11 12 0 -12 11 0 14 0 12 -1 -20

-8 -7

8 -13 -10 -20 -20 13 11 -13 11 -10 -11 0

40°48´ CK -2 2 1 2 -20 0 0 -10 -10

POTENTIOMETRIC SURFACE CONTOUR – 0 2 Shows altitude of potentiometric surface. 00 -4 Dashed where approximately located. Hachures indicate depression. Contour interval 10 feet. Datum is sea level. WELL TYPES – Number, where present, indicates depth above or below sea level, in feet. PUBLIC-SUPPLY WELL 0 1 2 MILES 10 OBSERVATION8970 WELL12450 GOLF COURSE WELL12455 0 1 2 KILOMETERS 40°46´

Base from New York State Department of Transportation, 1:24,000, 1981

Figure 14. Potentiometric-surface altitudes in the Lloyd and North Shore aquifers on Manhasset Neck, Nassau County, N.Y., September 1996. (Location is shown in fig. 1B.)

Hydrogeology 29

39.2 30.8 A. N9904 B. N12240 30.7 39.0 30.6 38.8 30.5 38.6 30.4 38.4 30.3 30.2 38.2 30.1 38.0 30.0 37.8 29.9 37.6 29.8 28 5 10 15 20 25 31 5 10 15 20 25 28 5 10 15 20 25 31 5 10 15 20 25 March1997 April March 1997 April

26.5 13.0 C. N2269 12.9 D. N12262 26.0 12.8 25.5 12.7

25.0 12.6 WATER LEVEL, IN FEET ABOVE SEA LEVEL 12.5 24.5 12.4 24.0 12.3 12.2 23.5 12.1 23.0 12.0 28 5 10 15 20 25 31 5 10 15 20 25 28 5 10 15 20 25 31 5 10 15 20 25 March 1997 April March 1997 April

Figure 15. Water levels in the upper glacial aquifers on Manhasset Neck, Nassau County, N.Y., 1997: A. Well N9904. B. Well N12240. C. Well N2269. D. Well N12262. (Locations are shown in fig. 2.)

hydrograph fig. 15C.) Some tidal fluctuations can be from quarterly synoptic measurements made during seen at well N12262 where the upper glacial is 1992-96 indicate that water in the Lloyd and North semiconfined (fig. 15D). Shore aquifers flows northward and develops large Lloyd and North Shore aquifers (fig. 14)—Water- cones of depression in response to local public-supply level data from quarterly measurements and water- pumping. Some wells show tidal fluctuations of as level records indicate that the Lloyd and North Shore much as 4 ft, as seen from data from well N12319, on aquifers are affected by tidal fluctuations along the the northeastern coast of the peninsula (figs. 2, 16A). coast and are hydraulically connected (Stumm, 2000; Lloyd and North Shore aquifer wells in the central part Stumm and Lange, 1996). Both aquifers respond to of the peninsula, such as well N12321, do not respond local public-supply and golf-course pumping and to tidal fluctuations but are affected by nearby show large cones of depression (fig. 14). pumping (fig. 16B). Water levels in this well increase Predevelopment water levels within the Lloyd aquifer in the fall and winter in response to a decrease in in 1900 are inferred to have been above sea level pumping of nearby public-supply wells N1716, throughout the northern part of Nassau County N1715, N9809, and golf-course irrigation well N8790. (Kimmel, 1973). Water levels within the Lloyd aquifer The potentiometric surface of the Lloyd and in the area were below sea level in 1947 (Lusczynski, North Shore aquifers in September 1996 (fig. 14) 1952), in 1971 (Kimmel, 1973), in 1975 (Rich and indicates that water levels ranged from 13 ft above to others, 1975), in 1979 (Donaldson and Koszalka, more than 20 ft below mean sea level. Two large cones 1983), and in 1984 (Doriski, 1987). Water-level data of depression have developed in response to public-

30 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

12 20 A. N12319 (Lloyd Aquifer) 19 B. N12321 (North Shore Aquifer) 11 18 17 10 16 9 15 14 8 13 12 7 11 6 10 17 18 19 20 21 22 23 24 25 26 27 28 29 September October November December January February September 1996 1996 1997

6 8 4 C. N12508 (Lloyd Aquifer) D. N12318 (North Shore Aquifer) 2 0 7 -2 -4 WATER LEVEL, IN FEET ABOVE SEA LEVEL -6 6 -8 -10 -12 5 -14 -16 -18 4 September October November December January February September October November December January February 1996 1997 1996 1997

Figure 16. Water levels in the Lloyd and North Shore aquifers on Manhasset Neck, Nassau County, N.Y., 1997: A. Well N12319. B. Well N12321. C. Well N12508. D. Well N12318. (Locations are shown in fig. 2.) supply and golf-course pumping. These depressions aquifer in this area is a less effective confining unit extend over the northern and western parts of the than the clays associated with the North Shore peninsula and into the surrounding saltwater confining unit; this also is seen in the upper glacial embayments. The potentiometric surface at most wells aquifer. This area also corresponds to the one small in the western and northern parts of the peninsula is area in Manhasset Neck in which no North Shore below sea level. Summer pumping causes ground- confining unit was encountered above the North Shore water flow in the Lloyd and North Shore aquifers to aquifer. The large water-level fluctuations at this well reverse direction from seaward to landward; that is, are due to cyclic pumping of nearby public-supply from the surrounding saltwater embayments inland wells N36, N37, and N8313, or of nearby private toward the pumping wells (areas of lowest hydraulic wells. In contrast is the nearly 2-ft tidal fluctuations head). This reversal of the normal ground-water flow seen at observation well N12508 (fig. 16C) to the path is the primary reason for the past and present south. Drill-core and geophysical-log data indicate saltwater intrusion in the peninsula. that N12508 is screened within the Lloyd aquifer, is The hydrograph of observation well N12318 overlain by parts of the North Shore aquifer, and is (fig. 16D), screened in the North Shore aquifer, shows confined by more than 200 ft of clay of the North the smallest (0.3 ft) tidal fluctuation in that aquifer, yet Shore confining unit. Pumping from public-supply the well is one of the closest to the coast. This suggests wells N1715, N1716, and N9809, and golf-course that the thin silt confining unit above the North Shore irrigation well N8790, appears to have a greater effect

Hydrogeology 31

on water levels at N12508 than at N12321 to the east analysis of the water quality and review of historical (fig. 16B, 16C). A water-level rebound of almost 15 ft data, however, were beyond the scope of this study. in the Lloyd aquifer at N12508 was measured by Chloride—The chloride concentrations at public- continuous recorders near the end of September 1996 supply wells screened in the upper glacial and (0.9 mi northwest of N8790). Pumpage records of the Magothy aquifers on Manhasset Neck ranged from 5 nearby Lloyd aquifer and North Shore aquifer public- to 30 mg/L, except at public-supply well N5209 in the supply and golf-course wells indicate that pumpage eastern part of the study area (fig. 2, 17A). A peak was cut back or stopped at most of these wells at that chloride concentration of 85 mg/L was reported in time. In contrast, the continuous recorder at well 1986, and a peak chloride concentration of 25 mg/L in N12321 (0.7 mi northeast of N8790) indicated a 1995 (fig. 17A). Chloride concentrations in water from water-level rebound of only 3 ft within the North public-supply wells screened in the Lloyd aquifer Shore aquifer near the end of September 1996. Despite ranged from 5 to 10 mg/L, and those in water from the the shorter distance from N8790 to N12321 than to North Shore aquifer (well N36, fig. 2) in northern N12508, the 3-ft water-level rebound at N12321 was Manhasset Neck ranged from 10 to 20 mg/L, but less than 25 percent of that at Lloyd well N12508. In reached 32 mg/L in 1989 (figs. 2, 17B). addition, the mean water level for September 1996 Several areas of elevated chloride concentration through February 1997 was 5.75 ft below sea level at were detected in 1997. The chloride concentration at N12508 and 14.9 ft above sea level at N12321—a observation well N9608 (fig. 18) near Hempstead mean difference of almost 21 ft. Potentiometric- Harbor, and screened in the upper glacial aquifer, was surface maps of the Lloyd and North Shore aquifers 10,000 mg/L; that at observation well N12522 at indicate a steep hydraulic gradient in the central part Hempstead Harbor Park and screened in the Magothy of the peninsula. These data suggest a sharp difference aquifer (fig. 18) was 9,750 mg/L; and that at in either the hydrogeologic framework or a dramatic observation well N12508, on the western side of change in the horizontal hydraulic conductivity of the Manhasset Neck and screened in the Lloyd aquifer Lloyd and North Shore aquifers in the northwest- (fig. 18) was 625 mg/L. Another area of high chloride central part of the peninsula than in the northeast and concentrations is on the western part of Manhasset east-central parts. A possible explanation to these Neck, where the chloride concentrations at wells differences may be a hydraulic opening in the N12263 and N12232 (fig. 18), screened in the North confining units (North Shore and Raritan clay) that Shore aquifer, were 57.5 mg/L and 102 mg/L, overlie the Lloyd and North Shore aquifers. The respectively; and that at well N12262 (fig. 18), hydrogeologic sections in figures 5A and 5B indicate a screened in the upper glacial aquifer, was 90 mg/L. possible gap of more than 50 ft between the bottom of the North Shore confining unit and the top of the Volatile organic compounds—Volatile organic Raritan clay in this part of Manhasset Neck; water compounds (VOC’s) in the ground water on from the overlying upper glacial and Magothy aquifers Manhasset Neck could be derived from domestic therefore may flow downward through this gap into cesspool and drain cleaners and from solvents and the Lloyd and North Shore aquifers. degreasers used by industries. In general, the Lloyd aquifer appears to be protected from the downward movement of synthetic organic compounds in Water Quality overlying aquifers by the Raritan clay and North Shore The USGS collected water samples from 23 confining unit. In 1996, the most frequently detected observation wells in the study area in 1997. The VOC’s in Nassau County public-supply wells (raw objective was to obtain data on chloride concentrations water samples) were tetrachloroethylene (PCE), and specific conductance for delineating the trichloroethlyene (TCE), and 1,1,1-trichloroethane freshwater-saltwater interface. Additional data on the (TCA) (Nassau County Department of Health, 1997). concentrations of inorganic constituents, metals, and Data from NCDH indicate that VOC concentrations in volatile organic compounds (VOC’s) were obtained raw water from public-supply wells N6087, N4860 from Nassau County. These samples were compared (screened in the upper glacial aquifer), and N9809 with the NYSDH drinking water MCL’s (Nassau (screened in the Lloyd aquifer) in the central part of County Department of Health, 1997) to generalize Manhasset Neck (fig. 2) were above the New York water-quality conditions on Manhasset Neck. Detailed State MCL’s and required treatment by Granular

32 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York

180 90 Activated Carbon (GAC). Water from 10 public- supply wells screened in the Magothy aquifer also 160 80 exceeded the State MCL’s. Four of these wells

140 70 (N3523, N2052, N7551, N7552) were treated by GAC, and six (N5528, N2028, N5947, N4327, 120 60 N3733, and N3732) were treated with airstrippers. Two observation wells screened in the upper 100 50 glacial aquifer had VOC concentrations that exceeded the State MCL’s for 1,1,1- 80 40 trichloroethane (TCA) (well N12262) and tetrachloroethylene (PCE) (well N12264). 60 A. N 5209 30 PUMPAGE The presence of VOC’s in raw water from the CHLORIDE 40 20 only Lloyd public-supply well in Manhasset Neck (N9809) suggests a possible hydraulic 20 10 interconnection between the upper glacial and the Lloyd and North Shore aquifer. This public- 0 0 19501960 1970 1980 1990 2000 supply well is screened within the buried valley in the central part of the peninsula. A large municipal 90 35 landfill that lies 3,500 ft. east of N9809, is a B. N 36 potential source of future contamination. The 80 PUMPAGE proposed gap in the confining units (North Shore CHLORIDE 30 and Raritan clay) is between the public-supply 70 well and the landfill. Further study would be 60 25 needed to determine the extent of hydraulic

PUMPAGE, IN MILLIONS OF GALLONS PER YEAR interconnection and the hydraulic gradients 50 between these aquifers. 20 Inorganic constituents and metals—No 40 CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER samples from public-supply wells on Manhasset Neck that are currently in use contained inorganic 30 15 constituents or metals in concentrations above the

20 State MCL’s. Public-supply wells N4859 and 10 N9446 (fig. 2) were taken out of use in response to 10 high iron concentrations in the raw water. Water from one observation well screened in the upper 0 5 glacial aquifer (N9608, fig. 2), and from two 1960 19201940 1980 2000 observation wells screened in the Lloyd aquifer YEAR (N12134, N12319) and four wells screened in the North Shore aquifer (N12321, N12151, N12241, N12263) slightly exceeded the State MCL for iron. Water from two observation wells—one screened in the upper glacial aquifer (N12264) Figure 17. Chloride concentration and pumpage at and one in the North Shore aquifer selected public-supply wells on Manhasset Neck, (N12318)—exceeded the State MCL’s for iron Nassau County, N.Y. A. Well N5209, 1956-95. B. Well and manganese. No samples from the selected N36, 1938-95. (Data from Nassau County Department of Health and New York State Department of Magothy wells were found to contain any Environmental Conservation. Locations are shown in inorganic constituents or metals in concentrations fig. 2.) above the State MCL’s.

Hydrogeology 33 73°44´ 73°42´ 73°40´

SOUND H emp 40°52´ ISLAND stea 12151(NS) d LONG 8 H MANHASSET NE a 50 rbor

2 C 00 9446(NS) 1 24 12318(NS) 12319(L) 180 8246(NS) 5 <3 12639(NS) 11

12190(L) <3 B

100 1 250 0 12232(NS) , 102 000 40°50´ Manhasset Bay 9608(UG) 10,000 5209(UG) MN-25(UG) 85 1000 1400 12522(M) 9750 A 12508(L) 250 625 12321(NS)1 1 00 00 5 D 12793(L) 250 1716(L) 1 18 10 000 1715(L) 6 12134(L) 8 8790(L) 20 9809(L) 24 12507(L) 5

1 E 00

40°48´ CK

LINE OF EQUAL CHLORIDE CONCENTRATION – -400 Dashed where inferred. Concentration is in milligrams per liter. A SALTWATER WEDGE 12469 12470 WELL TYPES – Number, where present, 1298 10390 12580T 1900indicates depth above or below12581T sea level, in feet. 1716(L)1889 4388 10 8342PUBLIC-SUPPLY700 WELL 5099 10992 22 8790(L) GOLF8964 COURSE WELL 1618 20 OBSERVATION WELL – Top number is well number; 12232(NS) letter in parentheses is aquifer designation(L=Lloyd, 0 1 2 MILES 102 NS=North8970 Shore, 12450M=Magothy, UG=upper glacial). Bottom number indicates12455 chloride concentration in 0 1 2 KILOMETERS 40°46´ milligrams per liter.

Base from New York State Department of Transportation, 1981, 1:24,000

Figure 18. Chloride concentrations in upper glacial, Magothy, North Shore, and Lloyd aquifers, Manhasset Neck, Nassau County, N.Y., 1996. (Location is shown in fig. 1B.)

34 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York EXTENT OF SALTWATER INTRUSION The 50 years of chloride-concentration and pumpage data from public-supply wells on Great Neck Saltwater intrusion is the most common type of indicate that, once a concentration of 50 mg/L is water-quality degradation in coastal-plain aquifers exceeded at a public-supply well, the concentrations (Fetter, 1994). In coastal areas, the hydraulic head will remain above 50 mg/L even if pumpage is under predevelopment (nonpumping) conditions is decreased (Stumm, 2000). The technique of pumping higher on land than in the surrounding saltwater only in alternate years at wells with elevated chloride embayments; thus, ground water flows from areas of concentrations only slightly delays an inevitable rapid high potential to areas of lower potential, and increase in chloride concentrations. This is because freshwater and saltwater meet at an equilibrium point once the toe, or leading edge, of the saltwater wedge that lies offshore. When the natural hydraulic gradient reaches the public-supply well, it is “upconed” into the is reversed by pumping, however, fresh ground water screen zone of the well and responds to decreased flows toward the pumping well instead of seaward pumping for only a short time before stabilizing into a toward the interface, and the interface itself rapid, continuous increase. (equilibrium point between freshwater and saltwater) Filter-press samples were obtained from the 15 moves landward. Saltwater intrusion occurs if this boreholes installed for this study and were analyzed interface moves inland. for chloride concentration. The values were then correlated with those for water samples from the well- screen zone, and with geologic and geophysical logs, Characteristics of Saltwater Intrusion to delineate the extent of the saltwater intrusion. Water samples from public-supply wells were also analyzed Seawater has an average chloride concentration of for chloride concentration. 19,350 mg/L (Drever, 1988). The predevelopment chloride concentration of fresh ground water on Long Island was 10 mg/L or less (Lusczynski and Saltwater Wedges A, B, C, D, and E Swarzenski, 1966), and the shallow (upper glacial) ground water in urbanized areas of Long Island Five wedge-shaped zones of saltwater intrusion generally has an ambient chloride concentration of were identified on Manhasset Neck (fig. 18)—three less than 40 mg/L (Buxton and others, 1981; Heisig (A, B, C) in the Lloyd and North Shore aquifers, and and Prince, 1993). The increase since the two (D, E) in the upper glacial and Magothy aquifers. predevelopment period is associated with Typically, the saltwater wedges form on an contamination from land-surface sources. In this impermeable layer at the base of an aquifer because report, “ambient” water is defined as ground water the density of saltwater is greater than that of with a chloride concentration less than 40 mg/L; freshwater. The impermeable layer can be bedrock or a “brackish” water as ground water with a chloride confining unit. Saltwater wedges decrease in thickness concentration of 40 to 250 mg/L, and “salt- landward and have relatively sharp (about 10 ft thick) water” as water with a chloride concentration greater saltwater-freshwater interfaces (Stumm, 1993a). than 250 mg/L (Lusczynski and Swarzenski, 1966, Saltwater wedge A (fig. 18) is at the base of the Chu and Stumm, 1995). The following section Lloyd aquifer and North Shore aquifer in the western describes the extent of the saltwater intrusion into part of Manhasset Neck. Water from well N12508, Manhasset Neck. screened at 295 to 315 ft below sea level, had a An increase in chloride concentrations above chloride concentration of 625 mg/L in 1997. predevelopment levels in the deep, confined aquifers Geophysical logs obtained at that site indicate that a of Manhasset Neck are indicative of saltwater saltwater wedge about 100 ft thick has intruded into intrusion from the surrounding embayments in part of the North Shore aquifer and all of the Lloyd response to pumping. Ground water at several public- aquifer (fig. 12). The sharp freshwater-saltwater supply wells has been affected by saltwater intrusion interface indicates active saltwater intrusion due to (Stumm and Lange, 1996). Historical chloride overpumping of the Lloyd and North Shore aquifers at concentrations at public-supply wells in Manhasset a cluster of public-supply and golf-course-irrigation Neck indicate that the background chloride wells southeast of N12508. The combined pumpage concentration for the Lloyd aquifer is 5 to 10 mg/L. from public-supply wells N1715, N1716, and N9809,

Extent of Saltwater Intrusion 35 and golf-course well N8790, are creating a large cone 22 of depression that extends westward and 20 A. N1715 northwestward toward Manhasset Bay. Observation 18 well N12508 could be considered an outpost well to 16 indicate the imminent migration of the saltwater toe 14 toward the pumping cluster of wells about 1 mi to the 12 southeast. The large drawdowns seen at N12508, and 10 the thickness and concentration of the saltwater 8 wedge, indicate that intrusion is currently taking place. Chloride-breakthrough curves for public- 6 supply well N1715 indicate that peak chloride 4 concentrations are showing signs of an increase 2 0 similar to those seen at other wells that were 19501960 1970 1980 1990 2000 ultimately affected by saltwater (fig. 19A). The toe of the freshwater-saltwater interface in wedge A may be 1,400 approaching the cluster of public-supply and golf- B. N35 AND N12232 course-irrigation wells N1715, N1716, N9809, and 1,200 N8790 (fig. 2). 1,000 Saltwater wedge B (fig. 18), which is about 0.5 mi northwest of wedge A, caused the shutdown and 800 abandonment of public-supply well N35 in 1944 600 (maximum historical chloride concentration 1,200 mg/L) (figs. 18, 19B). Chloride-breakthrough 400 N12232 Maximum contaminant level, CHLORIDE CONCENTRATOIN, IN MILLIGRAMS PER LITER curves for well N35 indicate a rapid intrusion of 200 250mg/L saltwater into the North Shore aquifer during the 0 1940’s as a result of overpumping (fig. 19B). The 1930 1940 1950 1960 1970 1980 1990 2000 saltwater wedge at well N12232 had a maximum YEAR chloride concentration of 325 mg/L in 1993 and of 102 mg/L in 1997. EM-log data from observation well Figure 19. Chloride-breakthrough curves for public-supply wells N12232 indicate that brackish water and saltwater on Manhasset Neck, Nassau County, N.Y.; A. Well N1715. have intruded into most of the North Shore aquifer B. Well N35. (Locations are shown in fig. 2.) here. Well N12232 was drilled about 100 ft east (landward) of the abandoned public-supply well N35. N12318, is 200 mg/L in the North Shore aquifer Pumpage from N35 during the 1940’s probably (fig. 20A) (Stumm and Lange, 1996). induced the saltwater intrusion that is present today. EM-log data indicate that brackish ground water Chloride concentrations within wedge A are has intruded most of the upper glacial and North Shore increasing, and those within wedge B are decreasing; aquifers here. Pumpage from the nearby cluster of therefore, wedges A and B probably are not connected. public-supply wells N36, N37, and N8313, and golf- The upper glacial aquifer also appears to have been course-irrigation well N8246, are creating a large cone intruded by brackish ground water in the area within of depression that may be inducing saltwater intrusion wedge B (fig. 18); observation well N12264 (fig. 2) at wedge C. Chloride-breakthrough curves for public- had a chloride concentration of 90 mg/L in 1997. supply well N36 indicate a substantial increase in Saltwater wedge C (fig. 18) is probably at the chloride concentrations from about 10 mg/L during base of the North Shore aquifer and in most of the the 1970’s to between 20 and 35 mg/L at present upper glacial aquifer in the northern part of Manhasset (fig. 17B). This pattern of gradual increases to Neck. Here well N12318 is screened at 137 to 157 ft 50 mg/L in Lloyd aquifer public-supply wells has been below sea level. Chloride concentration at this well in noted in Great Neck (Stumm, 2000). Typically, 1997 was 180 mg/L, an increase from 103 mg/L in chloride concentrations of 50 mg/L are ignored by 1994. The estimated peak chloride concentration in the public-supply companies and State agencies because wedge, inferred from EM-log responses at well this is far below the 250-mg/L MCL. Data from wells

36 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York GEM 015050100 0 2040 60 80 0

50

100

Sand, silt,

and clay IN FEET SURFACE, 150 DEPTH BELOW LAND 103 Bedrock

200 A. N12318

GSP SPR R EM 0 50 100 150 0 30 60 90 120 145 170 195 0 200 400 600 0 150 300 450 600 0

50 Gray clay 100

150 Sand 200 6,650 250

Raritan clay 300

350

400 Lloyd aquifer 450 DEPTH BELOW LAND SURFACE, IN FEET DEPTH BELOW LAND SURFACE, 500 12 Bedrock 550 B. N12506

EXPLANATION

G Gamma log (counts per second) SP Spontaneous potential log (millivolts) SPR Single-Point-Resistance log (ohms) R Short-Normal-Resistivity log (ohm-meters) EM Electromagnetic induction log (millisiemens per meter) Well screen zone sample location; number is 103 chloride concentration (milligrams per liter)

Figure 20. Geophysical logs from two wells on Manhasset Neck, Nassau County, N.Y., A. Well N12318. B. Well N12506. (Locations are shown in fig. 2.)

Extent of Saltwater Intrusion 37 in Great Neck suggest, however, that increases to glacial aquifer has high chloride concentrations, concentrations of 50 mg/L tend to be followed by whereas interstitial water within the Lloyd has low complete intrusion of saltwater at these wells, with chloride concentrations. The EM log correlates with little chance of recovery (Stumm, 2000). the electric logs and indicates the presence of saltwater Saltwater wedge D (fig. 18) is in the upper glacial throughout the entire 100-ft thickness of the upper and Magothy aquifers on the eastern shore of glacial and Magothy aquifers and a maximum chloride Manhasset Neck, in parts of a former sand-mining concentration of 9,750 mg/L in 1997 (fig. 20B). operation. Chloride contamination of this aquifer may Chloride-breakthrough-curve data from defunct have been due to the mining practice of washing and public-supply well N662 screened in the Lloyd processing the sand and gravel with saltwater pumped aquifer, and a defunct industrial well N657 also from Hempstead Harbor into settling ponds, or by screened in the Lloyd aquifer, suggests that saltwater pumping industrial wells affected by saltwater had intruded the Lloyd aquifer in this area during the intrusion at the sand pit (E. L. Vopelak, New York 1940’s (figs. 21A and B). State Department of Environmental Conservation, Geophysical, filter-press, and water quality data written commun., March 19, 1979). The gamma log from observation well N12506 do not indicate salt- and drill-core samples from well N12506, near water in the Lloyd just north of N657 and N662 Hempstead Harbor (figs. 2, 20B), indicate three major (fig. 2). Chloride concentrations ranging from 1,400 to aquifers—the upper glacial, Magothy, and Lloyd, 10,000 mg/L at two observation wells in the western separated by the Raritan clay. The SP and SPR logs and eastern parts of the wedge, respectively, indicate a show no change in slope between the confining units gradual westward (landward) decrease in chloride and the upper glacial and Magothy aquifers; this concentrations in wedge D. Chloride-breakthrough indicates that the aquifer contains conductive ground curves for public-supply well N5209, screened in the water (saltwater). A deflection to the right in the SP upper glacial aquifer, suggest intrusion of brackish and SPR logs within the Lloyd aquifer indicates that water from saltwater wedge D toward the well the Lloyd contains freshwater (fig. 20B). Filter-press (fig. 17A). Chloride concentrations at public-supply data indicate that the interstitial water within the upper well N5209 were at ambient levels during the 1960’s

450 2,800 A. N662 B. N657 400 2,600

2,400 350

2,200 300

Maximum contaminant level, 2,000 250 250 mg/L 1,800 200 1,600 150 1,400 IN MILLIGRAMS PER LITER

CHLORIDE CONCENTRATION, 100 1,200

50 1,000

0 800 1935 1940 1950 1960 1970 1980 1935 1940 1950 1960 YEAR YEAR

Figure 21. Chloride-breakthrough curves for two public-supply wells on Manhasset Neck, Nassau County, N.Y., A. Well N662. B. Well N657. (Locations are shown in fig. 2.)

38 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York but increased rapidly during the 1970’s to more than nearby landfill) from the upper glacial and Magothy 100 mg/L, probably in response to the lowering of aquifers to flow downward into the Lloyd and North hydraulic heads at the public-supply well. Shore aquifers. The seismic- Saltwater wedge E (fig. 18) is south of wedge D, reflection surveys helped to delineate the extent of the near observation well N12511 (fig. 2). The Raritan clay and Lloyd sand offshore. geophysical logs of this well indicate two major Most of the Magothy aquifer has been completely aquifers—the upper glacial and Lloyd separated by removed in Manhasset Neck by glacial erosion, except the Raritan clay. The resistivity logs show an increase in the southernmost part and along the southeastern in conductivity (decrease in resistivity) with depth in coast of the peninsula. In the northern part, it has been the upper glacial aquifer, which suggests that a 45-ft- replaced by the upper glacial aquifer. thick zone of conductive brackish or salty ground Offshore seismic-reflection surveys, borehole- water has intruded into the base of the aquifer. geophysical logs, and drill-core samples were used to Whether this wedge is connected with the saltwater of correlate offshore glacial-lake deposits with buried wedge D, is uncertain. valleys that extend across Manhasset Neck. Offshore seismic-reflection-profiles in Long Island Sound, Manhasset Bay, and Hempstead Harbor indicate silt SUMMARY AND CONCLUSIONS and clay deposits that infill four sharply defined buried valleys that truncate the surrounding coarse- Saltwater encroachment at several public-supply grained deposits. The valleys are asymmetrical, steep wells within Manhasset Neck prompted a study sided, northwest-southeast trending, and are several (1992-96) to collect hydrogeologic, geochemical, and miles long and about 1 mi wide. The floors of these geophysical data to delineate the subsurface geology valleys extend to bedrock and are more than 400 ft and the extent of saltwater intrusion. The results of a below sea level. series of continuous high-resolution seismic-reflection surveys were used to delineate the character and extent The upper glacial aquifer consists of till, sand, of the deposits beneath Manhasset Bay and gravel, silt, and clay deposits underlain in some areas Hempstead Harbor. Borehole-geophysical logs by the North Shore confining unit. The offshore (gamma, electric, and EM induction) were used to seismic-reflection surveys and drill-core data indicate delineate hydrogeologic units and the extent of that the upper glacial aquifer does not extend offshore saltwater intrusion at selected observation wells. and does not infill the valleys beneath Manhasset Bay New drill-core data led to the identification and and Hempstead Harbor, as proposed in previous naming of two hydrogeologic units—the North Shore studies. The water table is in the upper glacial aquifer aquifer and the North Shore confining unit. The North in all but the southernmost part of the peninsula, where Shore aquifer is a sequence of Pleistocene-age it is in the Magothy aquifer. sediments in the northern and central parts of Water-level data from quarterly measurements Manhasset Neck. The North Shore confining unit is a and continuous water-level records indicate that the sequence of Pleistocene-age clay and silt deposits that upper glacial aquifer is hydraulically connected to the occur locally along the northern parts of Queens, Magothy aquifer and that water levels in these Nassau, and Suffolk Counties. The Lloyd aquifer, aquifers do not respond to tidal fluctuations, whereas Raritan clay, and Magothy aquifer were severely the Lloyd and North Shore aquifers are hydraulically eroded or completely removed from many parts of the connected and are affected by public-supply pumping peninsula by extensive glacial scouring and were not and by tides. found in the northern or central parts. The potentiometric surface of the Lloyd and A glacially eroded buried valley appears to have North Shore aquifers measured in September 1996 incised the Raritan clay from the northwestern part to indicate the presence of several large cones of the central part of Manhasset Neck. Drill-core depression that have formed as a consequence of samples, geophysical logs, and driller’s logs indicate a pumping from public-supply and golf-course wells. possible gap between the confining units (North Shore The potentiometric surfaces at most of the wells and Raritan) overlying the Lloyd and North Shore screened in these aquifers were below sea level and aquifers in the east-central part of the peninsula; this extended over large parts of the peninsula and into the gap may allow contaminated water (potentially from a surrounding saltwater embayments. Water levels

Summary and Conclusions 39 ranged from 13 ft above sea level to more than 20 ft logs from that site indicate a 92-ft-thick wedge of below sea level. Increased pumping in the summer saltwater at the base of the Lloyd. This saltwater causes ground-water flow in the Lloyd and North wedge is interpreted to be moving toward a major Shore aquifers to reverse direction—that is, to flow cluster of public-supply and golf-course supply wells inland from the saltwater embayments toward the N1715, N1716, N9809, and N8790. pumping wells. This reversal of normal seaward flow Saltwater wedge B is in the North Shore aquifer, is the primary cause of past and present saltwater about 0.5 mi northwest of wedge A. Increasing intrusion on Manhasset Neck. chloride concentrations in this area caused the Water from 13 public-supply wells and two abandonment of public-supply well N35. The wedge observation wells screened in the upper glacial, at adjacent well N12232 had a maximum chloride Magothy, and Lloyd aquifers contained several VOC’s concentration of 325 mg/L in 1993; geophysical logs in concentrations greater than the New York State indicate intrusion of brackish ground water throughout Department of Health maximum contaminant levels the entire North Shore aquifer here. The decreasing (MCL’s). Two public-supply wells were taken out of concentration at wedge B since 1946 suggests that it is use as a result of high iron concentrations and eight not hydraulically connected with wedge A, at which observation wells in the upper glacial, Lloyd, and chloride concentrations are increasing. North Shore aquifers had iron concentrations in excess Saltwater wedge C is at the base of the North of State MCL’s. Shore aquifer and parts of the upper glacial aquifer, in In general, the Lloyd aquifer appears to be northern Manhasset Neck. Water from observation protected from the downward movement of synthetic well N12318 had a chloride concentration of organic compounds from the overlying aquifers by the 180 mg/L in 1997. The estimated peak concentration Raritan clay and North Shore confining unit, except in in the wedge, inferred from induction-log responses at the east-central part of the peninsula, where a gap in well N12318, is 200 mg/L. Increasing chloride the confining units may be present. concentrations at public-supply well N36 and The chloride concentration at public-supply wells observation well N12318 indicate that saltwater screened in the upper glacial and Magothy aquifers intrusion may be occurring here. ranged from 5 to 30 mg/L except at public-supply well Saltwater wedge D is in the upper glacial and N5209, on the eastern side of Manhasset Neck, where Magothy aquifers on the eastern shore of Manhasset the maximum chloride concentration in 1986 was Neck. Geophysical logs indicate that saltwater has 85 mg/L. Chloride concentrations at public-supply intruded the entire 100-ft thickness of the upper wells screened in the Lloyd aquifer ranged from 5 to glacial and Magothy aquifers; the maximum chloride 10 mg/L, and those at well N36, screened in the North concentration was 9,200 mg/L. Chloride-breakthrough Shore aquifer, ranged from 10 to 20 mg/L but peaked curves for public-supply well N5209 indicate at 32 mg/L in 1989. Public-supply well N35 had a intrusion of brackish ground water toward the well historical peak concentration of 1,200 mg/L in 1946 from this wedge. and was shut down. Chloride concentration at three Saltwater wedge E is in the upper glacial aquifer observation wells (N9608, N12522, and N12508) south of wedge D. The resistivity logs from well exceeded the State’s MCL. N12511 show an increase in conductivity with depth in the aquifer, this indicates that a 45-ft-thick zone of Five areas of saltwater intrusion were conductive ground water (saltwater) has intruded into delineated—three in the Lloyd and North Shore the base of the aquifer. aquifers, two in the upper glacial and Magothy aquifers. The wedges decrease in thickness landward and have relatively sharp saltwater-freshwater REFERENCES CITED interfaces. The thickness of these zones ranges from a few feet to 125 feet, and the maximum chloride Baskerville, C.A., 1992, Bedrock and engineering geologic concentration ranges from 102 to 9,750 mg/L. maps of Bronx County and parts of New York and Saltwater wedge A is at the base of the Lloyd and Queens Counties, New York: U.S. Geological Survey North Shore aquifers in the western part of Manhasset Miscellaneous Map I-2003, 2 sheets, scale 1:24,000. Neck. The water sample from well N12508 had a Buxton, H.T., Soren, Julian, Posner, Alex, and Shernoff, chloride concentration of 625 mg/L, and geophysical P.K., 1981, Reconnaissance of the ground-water

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42 Hydrogeology and Extent of Saltwater Intrusion on Manhasset Neck, Nassau County, New York