(YDROGEOLOGYAND%XTENTOF3ALTWATER)NTRUSION OFTHE'REAT.ECK0ENINSULA 'REAT.ECK ,ONG)SLAND .EW9ORK

53'EOLOGICAL3URVEY 7ATER 2ESOURCES)NVESTIGATIONS2EPORT  0REPAREDINCOOPERATIONWITHTHE .!33!5#/5.49$%0!24-%.4/&05",)#7/2+3 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York By Frederick Stumm ______

U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 99-4280

Prepared in cooperation with the NASSAU COUNTY DEPARTMENT OF PUBLIC WORKS

Coram, New York 2001

Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York i U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, 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 Methods of study...... 2 Seismic-reflection surveys ...... 2 Borehole-geophysical logs...... 4 Water-level data ...... 4 Water-quality data...... 4 Previous investigations...... 6 Acknowledgments...... 6 Hydrogeology ...... 6 Stratigraphy ...... 7 Bedrock...... 7 Lloyd aquifer...... 8 Raritan clay...... 8 Magothy aquifer...... 11 North Shore aquifer ...... 17 North Shore confining unit ...... 17 Upper glacial aquifer ...... 21 Seismic-reflection surveys...... 21 Borehole-geophysical logs ...... 21 Precipitation ...... 23 Ground water...... 26 Pumpage...... 27 Water levels...... 27 Upper glacial aquifer ...... 30 Magothy aquifer ...... 30 Lloyd and North Shore aquifers ...... 30 Water quality...... 31 Chloride ...... 31 Volatile organic compounds ...... 32 Inorganic constituents and metals ...... 32 Extent of saltwater intrusion...... 32 Saltwater wedges A and B ...... 33 Saltwater wedge C...... 35 Saltwater wedge D...... 33 Other saltwater wedges ...... 36 Summary and conclusions...... 38 References cited...... 39

FIGURES 1. Map showing location of Great Neck and Manhasset Neck peninsulas, and of seismic-reflection profiles, Nassau County, N.Y.: A. General location map. B. Location of Great Neck study area...... 3 2. Map showing locations of public-supply wells, observation wells, and golf-course wells within the Great Neck Peninsula, Nassau County, N.Y...... 5 3. Map showing surface altitude of bedrock underlying Great Neck, N.Y...... 9 4. Hydrogeologic sections on Great Neck, N.Y. A. Section A-A«...... 10

Contents iii B. Section B-B« ...... 11 C. Section C-C« ...... 12 D. Section D-D«...... 13 5-10. Maps of Great Neck, N.Y., showing extent and upper surface altitude of selected hydrogeologic units: 5. Lloyd aquifer ...... 14 6. Raritan clay...... 15 7. Magothy aquifer ...... 16 8. North Shore aquifer ...... 18 9. North Shore confining unit ...... 20 10. Upper glacial aquifer ...... 22 11. Section showing interpreted stratigraphy along seismic-reflection profiles E-E´, F´-F, and G«-G off Great Neck, N.Y. A. E-E« in Long Island Sound ...... 23 B. F«-F in Little Neck Bay...... 24 C. G«-G in Manhasset Bay ...... 25 12. Geophysical logs of test well N12581T, Great Neck, N.Y...... 26 13. Hydrograph showing water levels measured at four selected wells during 1992-93 tidal study on Great Neck, N.Y...... 27 14-15. Maps of Great Neck, N.Y., showing water levels in August 1995: 14. Water-table altitude...... 28 15. Potentiometric-surface altitudes in the Lloyd and North Shore aquifers...... 29 16. Graphs showing chloride concentrations and pumpage at selected public-supply wells on the Great Neck, N.Y., peninsula: A. Well N5884, 1957-95. B. Wells N30 and N31, 1952-86. C. Well N1926, 1948-67...... 31 17. Map showing chloride concentrations in Lloyd, North Shore, and upper glacial aquifers, Great Neck, N.Y., 1996...... 34 18. Hydrogeologic section showing extent of saltwater wedge A, Great Neck, N.Y...... 35 19. Graph showing chloride concentrations and pumpage at public-supply wells (A) N687 and (B) N2214 on Great Neck, N.Y., 1950-96...... 36 20. Graph showing chloride breakthrough curve at public-supply wells (A) N1298 (1952-96) and (B) N3443 (1951-96), screened in the Lloyd aquifer, Great Neck, N.Y...... 37

TABLES 1. Generalized description of the hydrogeologic units underlying the Great Neck peninsula, N.Y.,...... 7 2. Total annual pumpage from the Great Neck peninsula, by aquifer, 1992-96 ...... 26

iv Contents

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

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 of the Great Neck Peninsula, Great Neck, Long Island, New York

By Frederick Stumm

Abstract the North Shore and Lloyd aquifers to be hydraulically connected. Great Neck, a peninsula, in the Offshore seismic-reflection surveys in the northwestern part of Nassau County, N.Y., is surrounding embayments indicate at least two underlain by unconsolidated deposits that form a glacially eroded buried valleys with sequence of aquifers and confining units. Seven subhorizontal, parallel reflectors indicative of public-supply wells have been affected by the draped bedding that is interpreted as infilling by intrusion of saltwater from the surrounding silt and clay. The buried valleys (1) truncate the embayments (Little Neck Bay, Long Island Sound, surrounding coarse-grained deposits, (2) are Manhasset Bay). Fifteen observation wells were asymmetrical and steep sided, (3) trend drilled in 1991-96 for the collection of northwest-southeast, (4) are 2-4 miles long and hydrogeologic, geochemical, and geophysical data about 1 mile wide, and (5) extend to more than to delineate the subsurface geology and extent of 200 feet below sea level. saltwater intrusion within the peninsula. Water from six public-supply wells Continuous high-resolution seismic-reflection screened in the Magothy and upper glacial surveys in the embayments surrounding the Great aquifers contained volatile organic compounds in Neck peninsula and the Manhasset Neck peninsula concentrations above the New York State to the east were completed in 1993 and 1994. Department of Health Drinking Water Maximum Two hydrogeologic units are newly Contaminant Levels, as did water from one proposed herein—the North Shore aquifer and the public-supply well screened in the Lloyd aquifer, North Shore confining unit. The new drill-core and from three observation wells screened in the data collected in 1991-96 indicate that the Lloyd upper glacial and Magothy aquifers. aquifer, the Raritan confining unit, and the Four distinct wedge-shaped areas of Magothy aquifer have been completely removed saltwater intrusion have been delineated within from the northern part of the peninsula by the aquifers in Great Neck; three areas extend into extensive glacial erosion. the Lloyd and North Shore aquifers, and the Water levels at selected observation wells fourth area extends into the upper glacial aquifer. were measured quarterly throughout the study. Three other areas of saltwater intrusion also have The results from two studies of the effects of tides been detected. Borehole-geophysical-logging data on ground-water levels in 1992 and 1993 indicate indicate that four of these saltwater wedges range that water levels at wells screened within the from 20 to 125 feet in thickness and have sharp North Shore and Lloyd aquifers respond to tides freshwater-saltwater interfaces, and that and pumping effects, but those in the overlying maximum chloride concentrations in 1996 ranged upper glacial aquifer (where the water table is from 141 to 13,750 milligrams per liter. Seven located) do not. Data from quarterly water-level public-supply wells have either been shut down or measurements and the tidal-effect studies indicate are currently being affected by saltwater intrusion.

Abstract 1

INTRODUCTION Location of Study Area

Increasing stresses on the ground-water system The Great Neck peninsula and its surrounding within the coastal areas of northern Nassau County, embayments encompass an area of approximately N.Y., from pumping of public-supply wells, 19 mi2 in the northwestern part of Nassau County commercial (light industrial) wells, and golf-course (fig. 1A). Great Neck is bounded on the west by Little wells, coupled with increasing chloride concentrations Neck Bay, on the north by Long Island Sound, and on (saltwater intrusion) in water from public-supply the east by Manhasset Bay (fig. 1B). In 1996, the wells, have caused a need for detailed information on population of the peninsula was estimated at 31,400, the hydrogeologic system in the area. Successful or 2 percent of Nassau County’s population (Nassau management of ground-water resources in the County Department of Health, 1996). northern coastal area of Nassau County requires detailed knowledge of the hydrogeologic framework, Methods of Study the extent of saltwater intrusion into the ground-water system, and the effect of pumping on ground-water Data collected during this study included levels. In 1991, the U.S. Geological Survey, in (1) observation-well drilling logs and samples, cooperation with the Nassau County Department of (2) seismic-reflection surveys, (3) borehole-geo- Public Works (NCDPW), began a long-term study to physical logs, (4) quarterly water-level measurements, delineate the hydrogeologic framework and the extent (5) continuous ground-water-level-recorder data, and of saltwater intrusion in Great Neck and other areas (6) water-quality analyses. Additional information was along the northern shore of the county to assist in obtained from NCDPW, Nassau County Department developing management decisions for the protection of Health (NCDH), and New York State Department of and conservation of the ground-water supply. Environmental Conservation, and from previous The study consisted of four major components: studies. The maps and sections are based on all (1) evaluation of the current observation-well network; available geologic data and well drillers’ logs. (2) drilling new observation wells in areas of probable Fifteen boreholes were drilled during 1991-96 saltwater intrusion and in areas where hydrogeologic for the collection of hydrogeologic, geophysical, and data were needed; (3) collection of geologic, water-quality data. Well drilling consisted of mud- hydrologic, geophysical (borehole and seismic- rotary drilling and split-spoon core sampling. Core reflection), and water-quality data; and (4) samples were obtained at regular intervals for geologic compilation of all data to describe the hydrogeologic analysis. Most borings were drilled through the unconsolidated sediments into bedrock. The thickness framework and the extent of saltwater intrusion within of each hydrogeologic unit was determined from the peninsula. geologic core data obtained during the drilling of observation wells and, to a lesser extent, from drillers’ Purpose and Scope logs and interpolation. Color descriptions of core samples in this report are based on the standard This report (1) depicts the hydrogeology Munsell color chart (Kollmorgan Instruments underlying the Great Neck peninsula, (2) describes the Corporation, 1994). ground-water-flow system, and (3) delineates the extent of saltwater intrusion within the major aquifers. Seismic-Reflection Surveys This report includes geologic sections and maps Offshore continuous high-resolution seismic- showing the extent, thickness, and upper surface reflection surveys were used to interpret the depth and altitude of the hydrogeologic units, and water levels continuity of seismic reflectors and lithology (Haeni, and chloride concentrations within these units during 1986, 1988). Applications of this technique for 1992-96. Two new hydrogeologic units—the North hydrogeologic and water-resource studies have been Shore aquifer and the North Shore confining unit are described by Haeni (1986, 1988) and by Reynolds and described in the report. Williams (1988).

2 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

74° 73° 72° Two continuous high-resolution NY CT seismic-reflection surveys were completed 41° in 1993 and 1994 along the coast of Great LONG ISLAND SOUND NJ Neck to identify the unconsolidated SUFFOLK deposits of the subsurface from Manhasset NEW YORK COUNTY Bay to Little Neck Bay and to correlate ° LOCATION 40 QUEENS MAP these deposits with geologic and 45' CO. NASSAU KINGS CO. 0 5 10 MILES geophysical data obtained from drilling. CO. ATLANTIC OCEAN 0 5 10 KILOMETERS (Locations of three of the survey lines are shown in fig. 1B.) The continuous-seismic- Base from U.S. Geological Survey state base map, 1979 reflection system consisted of a graphic A. General location map recorder, amplifier and filter, power 73û 45« 37´ 30˝ generator, catamaran-mounted sound source, hydrophone array, and digital tape Westchester recorder, all of which were installed in a County shallow-draft 22-ft boat. The sound source ND OU D S and hydrophone array were towed behind G ISLAN LON the slowly moving boat, where the seismic- reflection data were digitally recorded and filtered for real-time graphic display. 40û Hempstead 52« Harbor The seismic signals generated by the 30˝ sound source travel through the water M G« anhasset Neck column and penetrate the deposits F«E« Manhasset Bay underlying the sea floor. Part of the seismic G NASSAU signal is reflected back to the water surface E COUNTY F G from the sea floor and the stratigraphic reat N Little N interfaces at which a change in the acoustic

eck eck Bay impedance (the product of the density and acoustic velocity of each medium) is Study Area (Figure 2) encountered (Haeni, 1986; Robinson and Çoruh, 1988). The reflected signals received by the hydrophone array produce 45« QUEENS an electrical signal that then is amplified, COUNTY recorded on digital tape, filtered, and Mineola plotted. Trapped gas deposits can interfere EXPLANATION with the reflected signal and make the G« geologic data unclear. The resulting SEISMIC-REFLECTION G PROFILE seismic sections (see examples in fig. 11) 0 1 2 345 MILES resemble a vertical geologic section, except PRECIPITATION STATION 0 123 4 5 KILOMETERS that the vertical axis is a function of the time required for the seismic signal to Base from New York State Department of Transportation,1:24,000, 1981 travel from the source to the reflector and B. Location of Great Neck Study Area return (Haeni, 1986). Several seismic- reflection and refraction studies indicate that the average acoustic velocity of Figure 1. Location of Great Neck and Manhasset Neck pen- unconsolidated saturated glacial deposits is insulas, and of seismic-reflection profiles, Nassau County, about 5,000 ft/s (Haeni, 1988; Reynolds N.Y.: A. General location map. B. Location of Great Neck and Williams, 1988); this value was used as study area. an average velocity in this study.

Introduction 3

Borehole-Geophysical Logs electrode spacing (Serra, 1984; Keys, 1990). Only short-normal resistivity logs were used in this study. Borehole-geophysical logs from observation wells supplemented information that could not be Focused electromagnetic-induction (EM) logs obtained by drill cores and water-quality sampling. use an electromagnetic emitter coil that induces The geophysical-logging systems used in this study current loops within the surrounding formation to provided continuous digital records that reflect the generate a secondary electromagnetic field. The physical properties of the sediment, the rock matrix, intensity of the secondary field received by the and the interstitial fluids. At several sites, natural- receiver coil is proportional to the formation gamma radiation (gamma) logs, spontaneous potential conductivity (Keys, 1990; Serra, 1984; Keys and (SP), single-point-resistance (SPR), and short- and MacCary, 1971). EM logs are measured in units of long-normal resistivity (R) logs were collected in millisiemens per meter (mS/m) and are inversely mud-filled open boreholes prior to casing (see related to the ohm-meter value of normal-resistivity examples in fig. 12). At each of the sites, focused logs (Keys, 1990; Serra, 1984; Keys and MacCary, electromagnetic-induction (EM) logs were obtained in 1971). The normally low conductivity of Long Island’s polyvinyl chloride (PVC) cased wells. The five types hydrogeologic deposits is favorable for induction of logs are described below. logging to delineate highly conductive fluids such as saltwater or leachate. For this reason, EM logging has Natural-gamma radiation (gamma) logs are a been used on Long Island to delineate the saltwater- record of the total gamma radiation detected in a freshwater interface (Stumm, 1993a, 1993b, 1994; borehole in counts per second (Keys, 1990). Clays and Stumm and Lange, 1994). Gamma logs used in fine-grained sediments tend to be more radioactive conjunction with EM logs are useful for distinguishing than the quartz-rich sand that forms the bulk of the between conductive fluids and conductive clays. deposits on Long Island. Gamma logs are most commonly used for lithologic and stratigraphic correlation in the northeast. Water-Level Data Spontaneous-potential (SP) logs provide a Water levels were measured quarterly at record of the potential, or voltage, that develops at the selected observation wells throughout the study (1991- contact between clay beds and sand aquifers within a 96). All wells typically were measured on the same borehole (Keys, 1990). SP is measured in millivolts day to minimize discrepancies resulting from nearby and is a function of the chemical activity of the sporadic pumping. All water-level data were checked borehole fluid, the water in the adjacent sediments, and entered into the U.S. Geological Survey (USGS) the water temperature, and the type and quantity of data base in Coram, N.Y. A total of 57 observation clay (Serra, 1984). SP logs are used to determine wells within the Great Neck peninsula and part of the lithology, bed thickness, and salinity of formation Manhasset Neck peninsula were measured (fig. 2). water (Keys, 1990). Continuous water-level recorders were installed Single-point-resistance (SPR) logs provide a at six selected wells, each screened in a different measure of the resistance, in ohms, between an aquifer. Four of the recorders were digital-pressure electrode in the borehole and an electrode at land transducer data loggers that recorded at 1-hour surface. The volume of surrounding material to which intervals; the other two were continuous analog the SPR probe is sensitive is spherical and only 5 to 10 recorders. Hydrographs were plotted and used to times the electrode diameter, and is affected by the identify possible aquifer interconnections and to borehole fluid (Keys, 1990). SPR logs are used to assess the effect of pumping by nearby public-supply obtain high-resolution lithologic information. wells. In addition, a continuous water-level recorder Normal-resistivity (R) logs measure apparent was used at well Q577 (not shown in fig. 2) to provide resistivity in ohm-meters and are used to determine historical data for delineation of long-term water- lithology and water salinity (Keys, 1990). This level trends. technique uses two electrodes typically spaced 16 to 64 in. apart in the borehole, called short- and long- Water-Quality Data normal logs, respectively. The volume of surrounding material to which normal resistivity probes are The USGS collected water samples from 35 sensitive is spherical, with a diameter about twice the observation wells from August through September

4 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

73û46« 44« 42« A 35 B' 12232

40û 50« 12050

Manhasset Bay D 8046 N 8052 D' U O 12102 S 12508 D 9477 N 5357 6012 LA 12079 IS G N 9895 LO 12112 C' 9840 9208 9909 GREAT NECK 9908 30 10100 12013 31 8879 12076 10101 12464 9776 12134 12465 9896 1926 9271 2214 9899 D 9098 10192 12153 3443 7613 9893 9820

48' 9099 12152 4266 5884 6925 12735 687 Little Neck Bay B 12082 9308 12038 24 25 7747-58 8877 7961-71 9897 12555 8933 9892 12154 7846 12469 11730 12483 10390 12470 1298 12580T C 1900 9188 1889 12581T 4388 8342 5099 9687 700 2169 10992 22 EXPLANATION 8964 1618 D D' LINE OF GEOLOGIC SECTION PUBLIC-SUPPLY-WELL NUMBER 1298 AND IDENTIFIER 12455 1102 4266 OBSERVATION-WELL NUMBER 8970 12450 AND IDENTIFIER NASSAU CO. QUEENS CO. 46« 2576 GOLF-COURSE-WELL NUMBER 5710 AND IDENTIFIER A' (Numbers are assigned by New York State Department of 3905 Environmental Conservation. Prefix N for Nassau County 8038 4243 is omitted. Suffix T represents a test boring.) 0 2,000 4,000 FEET 2576 0 500 1,000 METERS Base from New York State Department of Transportation, 1:24,000, 1981

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

Introduction 5

1996 based on established techniques (Hem, 1992), 1994; Stumm and Lange, 1994). Similar information and the NCDPW collected samples from all newly on the hydrogeology of Manhasset Neck and Long installed observation wells by similar techniques. Island Sound is included in Stumm and Lange (1996). Three casing volumes were pumped from the well, and Smith (1958) described the first use of high- stable readings of temperature and specific power, low-frequency seismic-reflection equipment to conductance of the pumped water were obtained measure the depth to bedrock beneath selected areas of before sampling. All water-quality analyses were Long Island Sound. Oliver and Drake (1951) used completed at the NCDPW Cedar Creek Special seismic-reflection techniques to measure the depth to Projects Laboratory. The USGS resampled selected bedrock in Long Island Sound, and Williams (1981) observation wells in August and September 1996 to used high-resolution seismic-reflection surveys and minimize temporal effects on water quality. Public- core samples to describe the sediments beneath Long supply-well data were obtained from the USGS data Island Sound. Grim and others (1970) describe the base and the NCDH. geology and subsurface morphology of Long Island Filter-press samples were obtained at selected and the Manhasset Bay embayment. Lewis and Stone well sites for chloride and specific conductance (1991) conducted a systematic seismic-reflection analyses at the USGS office in Coram, N.Y. A filter- survey to map the deposits beneath most of Long press sample is obtained by using a nitrogen-gas- Island Sound, except the immediate shallow coastal pressurized chamber to force interstitial water from areas and embayments of the northern shore of uncontaminated parts of a split-spoon core sample western Long Island. Tagg and Uchupi (1967) used obtained during drilling (Lusczynski, 1961). The continuous seismic profiles to define the subsurface sample volume is typically about 0.7 fl oz (2 mL); morphology of Long Island Sound. chloride concentration and specific conductance are measured by hand-held probes. The resulting values were used only as a general indicator of brackish or Acknowledgments saltwater within a formation and were correlated with borehole geophysical logs. Thanks are extended to James Mulligan, Brian Schneider, Raymond Mazza, and others of NCDPW for their assistance and technical support throughout Previous Investigations this study. Thanks are extended to F. Peter Haeni, Chief of the U.S. Geological Survey Branch of The ground-water resources of Long Island Geophysical Applications and Support, for his were first studied by Veatch and others (1906), who technical assistance and use of the seismic-reflection described the geologic deposits and occurrence of survey equipment. Thanks also are given to the Great ground water on Long Island. Fuller (1914) revised Neck North Water Authority and the Manhasset- and supplemented much of the earlier geologic work. Lakeville Water District for providing pumpage Suter and others (1949) contoured the upper surface of information and access to their public-supply well the major geologic units. Swarzenski (1963) described sites, and to the U.S. Merchant Marine Academy at the hydrogeology and hydrologic conditions in Kings Point for dockage space during the two seismic- northwestern Nassau County during 1955-57; many of reflection surveys. The author also expresses his interpretations and his general hydrogeologic appreciation to Delta Well and Pump Co., Inc. and framework were verified by the drill-core and seismic- Hydro Group, Inc. well drillers for providing reflection data collected during this study. Kilburn lithologic samples and access to boreholes for (1979) attempted to redelineate and rename parts of geophysical logging at various sites in Great Neck. the hydrogeologic framework in Great Neck, largely from drillers’ logs, some of which were inaccurate. Much of the drill-core, hydrologic, and seismic- HYDROGEOLOGY reflection data obtained in the present study are not consistent with Kilburn (1979). The present study has The Great Neck peninsula is underlain by generated several reports that describe the unconsolidated glacial deposits of Pleistocene age and hydrogeology of the Great Neck peninsula and the coastal-plain deposits of Late Cretaceous age. These freshwater-saltwater interface (Stumm, 1993a, 1993b, deposits consists of gravel, sand, silt, and clay

6 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

Table 1. Generalized description of hydrogeologic units underlying the Great Neck peninsula, 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, and clay and boul- ders. Varied permeability with an average hydraulic con- ductivity of 270 feet per day and an anisotropy of 10:1. Outwash has the highest hydraulic conductivity. North Shore confining unit Pleistocene deposits Marine and post glacial lake deposits. Clay and silt depos- its with minor parts containing shells. The clay is olive brown and olive gray and is poorly permeable. Unit con- tains a minor sand layer 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 com- mon. Moderately permeable with an average hydraulic conductivity of 50 feet per day and an anisotropy 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 Poorly permeable. Confines water in underlying unit. Aver- age 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 is well sorted. Moderately permeable with an average horizontal hydraulic conductivity of 60 feet per day, and anisotropy of 10:1. Bedrock Hartland Formation; crystalline Highly weathered biotite-garnet-schist with low hydraulic 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. Imperme- able to poorly permeable. underlain by bedrock of early Paleozoic age. The top Shore confining unit—which are used and defined in of the bedrock, which is relatively impermeable, this report for the first time. All other geologic and generally forms the base of the ground-water reservoir. hydrologic unit names used in this report are those The hydraulic characteristics and description of currently used by the USGS. the aquifers, and the relations between hydrogeologic and geologic units underlying the peninsula, are described in table 1. The upper and lower boundaries Stratigraphy of the glacially derived hydrogeologic units are The unconsolidated deposits beneath the Great discerned mainly from lithologic differences between Neck peninsula are underlain by crystalline units rather than the age of the deposits, which is the (metamorphic) bedrock. These unconsolidated basis for geologic correlation. deposits are of Cretaceous and Pleistocene age. The geologic and hydrologic units that form Microscopic and mineralogic analysis of drill-core Long Island’s hydrogeologic framework are described samples were used to delineate the stratigraphic by Suter and others (1949); Perlmutter and Geraghty sequence. (1963); Swarzenski (1963); Kilburn (1979); and Smolensky and others (1989). Geologic correlations Bedrock are revised from those of Kilburn (1979), Swarzenski (1963), and Fuller (1914). The new drill-core data The bedrock underlying the Great Neck area has provide a basis for the two newly proposed been mapped as the Hartland Formation of Middle hydrogeologic units within the Pleistocene Ordovician to Lower Cambrian age (Baskerville, deposits—the North Shore aquifer and the North 1992); its upper surface generally slopes

Hydrogeology 7

southeastward from 140 ft to 600 ft below sea level. Lloyd Aquifer The offshore seismic-reflection surveys done in this The Lloyd Sand member of the Raritan study indicated that the base of glacially scoured Formation of Late Cretaceous age (Suter and others, valleys have an undulating surface. Drill-core data 1949; Cohen and others, 1968) consists of an upward indicate the bedrock underlying Great Neck to be a fining sequence of white and pale yellow (Munsell biotite-garnet schist. The bedrock is highly weathered descriptive color value 2.5Y 8/2) sand and gravel with beneath most of the peninsula, as indicated by a white clay lenses. The coarse fraction consists almost saprolitic zone that ranges from 50 ft to more than entirely of well-sorted subangular to subrounded 100 ft thick, except in the northernmost part, where quartz and chert fragments. White and grayish silt also glacial erosion has removed most or all of the may be present in the upper sequences. The clay lenses saprolite. The saprolitic zone consists mostly of white, cannot be correlated over great distances. yellow, and gray clay with quartz fragments. This zone Typically, the Lloyd Sand overlies bedrock on had been previously interpreted by drillers to be a clay the Great Neck peninsula. The Lloyd Sand forms the or clayey gravel layer above the bedrock surface. Lloyd aquifer, which is a major source of water supply in this part of Long Island. Its thickness ranges from Observation wells N4266 and N12013 (fig. 2) less than 50 ft to more than 150 ft. The Lloyd Aquifer penetrated 71 ft and 138 ft into weathered bedrock, was not found in the northernmost part of the respectively, without reaching solid rock. peninsula nor within several of the buried valleys Bedrock-surface contours (fig. 3) indicate a penetrated onshore and offshore (fig. 5), nor at wells general dip to the southeast in most areas. Glacial N12050, N12102, N10101, N12112, or N9840 erosion or scouring of the overlying Cretaceous and (figs. 2, 5). The top of the Lloyd generally dips to the possibly earlier Pleistocene deposits and the southeast except where it has been removed by glacial weathered bedrock, formed a series of buried valleys erosion (fig. 4A) (Stumm and Lange, 1994; Stumm that have truncated the surrounding material. The 1993a, 1994). The upper surface of the Lloyd ranges weathered bedrock at the base of these valleys has from 121 ft to almost 400 ft below sea level in Great been partially or totally removed. Drill-core data from Neck (fig. 5). observation wells N12050 and N12102 in the In northern Nassau County, the Lloyd aquifer northernmost part of Great Neck (fig. 2) indicate that has an average horizontal hydraulic conductivity of the bedrock surface is relatively flat and does not about 60 ft/d (McClymonds and Franke, 1972) and an follow the regional southeastward dip (fig. 3). The anisotropy (horizontal to vertical) of 10:1 (Smolensky and others, 1989). time at which most of the weathered bedrock was removed is unknown, but it could have been during Raritan Clay successive glacial advances during the Pleistocene. Drill-core data and continuous seismic-reflection The unnamed clay member of the Raritan profiles indicate a bedrock cuesta or hogback between Formation, referred to as the Raritan clay in this wells N12102 and N12079 (figs. 2, 11) with more than report, underlies most of the Great Neck peninsula and 50 ft of relief (fig. 4A) (Stumm and Lange, 1994). dips to the southeast (fig. 6). It consists of solid, Drill-core data generally indicate that wherever compact multicolored clays that include gray Cretaceous sediments are penetrated in place, the (2.5Y 4/1), white (2.5Y 9/1), red or tan (10R 4/8, 2.5YR 4/8), and variegated. The Raritan clay ranges in bedrock surface beneath them has a saprolitic zone, thickness from 20 ft to more than 150 ft in Great Neck, and the absence of Cretaceous deposits is typically and the upper surface altitude ranges from 96 ft to accompanied by a lack of saprolite. Seismic-reflection more than 250 ft below sea level. The Raritan clay has data collected in this study (1993-94) indicate that the been severely eroded or completely removed by bedrock surface beneath the embayments of Long glacial scouring in many parts of the peninsula (fig. 6) Island Sound is undulating; this indication is (Stumm, 1993a, 1994). A glacially eroded buried consistent with observations of Tagg and Uchupi valley truncates the Raritan clay from the northwest (1967) and Lewis and Stone (1991), who found the corner of Great Neck to the southeast corner (fig. 6). bedrock surface beneath most of Long Island Sound to The Raritan clay was not penetrated at wells N12050, have been severely eroded during the Pleistocene. N12102, N10101, N12112, nor N9840 (figs. 2, 6).

8 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

73û46« 44« 42« -200 -250 -366 -243 40û 50« -150 D N U Manhasset Bay O -200 S -246 D N -327 LA IS -166 -237 G -150 N LO -257

-200 -200 -203 -354

-277 GREAT NECK -225 -250

-300 -235 -350 -244

-340 -264 -250

48« -348 -389 -310 Little Neck Bay -378 -446 -400 -300

-313 -450 -441 -350 -325

-419

-488 EXPLANATION BEDROCK CONTOUR -- Shows altitude of bedrock surface. Dashed where

-400 approximately located. Contour interval -500 50 feet. Datum is sea level. -468 NASSAU CO. QUEENS CO. 46« -310 PUBLIC-SUPPLY WELL -- Number, where present, -550 indicates depth above or below-sea level, in feet.

-441 -600 OBSERVATION WELL -- Number, where present, indicates depth above or below-sea level, in feet. -611 0 2,000 4,000 FEET

0 500 1,000 METERS Base from New York State Department of Transportation, 1:24,000, 1981

Figure 3. Surface altitude of bedrock underlying Great Neck, N.Y.

Hydrogeology 9

A' N12450 2,000 4,000 FEET 0 0 500 1,000 METERS Raritan clay Magothy aquifer Hydrogeology by F. Stumm, 1994 Lloyd aquifer clay aquifer

Upper glacial N12154

Bedrock

N12038 N4266

GEOLOGIC CONTACT- dashed where approximately located WELL BORING AND TRACE OF GAMMA RADIATION LOG- well locations are shown in figure 2.

North Shore confining unit

N4266 N12013 aquifer Upper glacial EXPLANATION Bedrock

aritan clay Lloyd aquifer R

Upper glacial aquifer

N12102 N12050 NORTH SHORE CONFINING UNIT NORTH SHORE AQUIFER NScu NSaq North Shore confining unit Hydrogeologic section A-A« on Great Neck, N.Y. (Trace of section is shown in fig. 2.) in fig. of section is shown (Trace N.Y. Hydrogeologic section A-A« on Great Neck, North Shore aquifer Bedrock

Long Island Sound NScu Vertical exaggeration X 15 A Figure 4A. 200 100 SEA -100 -200 -300 -400 -500 FEET LEVEL

10 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

B B' FEET 100 N12153 N10100 Little Neck Bay N12079 Manhasset Bay SEA Upper glacial LEVEL Upper glacial aquifer aquifer Upper glacial aquifer NScu Upper glacial North Shore -100 aquifer confining unit North Shore North Shore aritan clay confining unit R confining unit Lloyd aquifer Raritan clay -200 Bedrock Lloyd aquifer sand -300 Bedrock

-400 0 2,000 4,000 FEET

0 500 1,000 METERS Vertical exaggeration X 20 Hydrogeology by F. Stumm, 1994 EXPLANATION NScu NORTH SHORE CONFINING UNIT GEOLOGIC CONTACT- dashed where approximately located NSaq NORTH SHORE AQUIFER WELL BORING AND TRACE OF GAMMA RADIATION LOG-

N12079 well locations are shown in figure 2.

Figure 4B. Hydrogeologic section B-B« on Great Neck, N.Y. (Trace of section is shown in fig. 2.)

Offshore seismic-reflection profiles indicate that the Cretaceous age (Suter and others, 1949; Swarzenski, Raritan clay underlies only the southernmost parts of 1963; Kilburn, 1979). The Magothy aquifer is an Little Neck and Manhasset Bays. upward fining sequence of gray, white, or pinkish The Raritan clay is poorly permeable, with an sands and clay of the undifferentiated Matawan Group average vertical hydraulic conductivity of about and Magothy Formation (Kilburn, 1979; Smolensky 0.001 ft/d (Smolensky and others, 1989). It overlies and others, 1989); in some areas it contains lignite and and confines water in the Lloyd aquifer below (Suter iron-oxide concretions. Most of the Magothy aquifer and others, 1949) and is a major confining unit on in Great Neck has been removed by glacial erosion, Long Island (Smolensky and others, 1989). except in the southernmost part and along the southeastern coast of the peninsula. Magothy Aquifer Drill-core samples indicate that the Magothy The Magothy aquifer consists of fine micaceous aquifer consists of subrounded silty gray (5Y 6/1) and sand, silt, and interbedded clay sediments of pale yellow (2.5Y 7/2) quartz with associated lignite

Hydrogeology 11

C' N12134 Manhasset Neck aquifer North Shore confining unit Upper glacial Hydrogeology by F. Stumm, 1994 F. Hydrogeology by 00 2,000 500 4,000 FEET 1,000 METERS aquifer Raritan clay

Lloyd aquifer Lloyd

Manhasset Bay Upper glacial N10192 Bedrock North Shore confining unit Lloyd aquifer Lloyd Great Neck

GEOLOGIC CONTACT- dashed where approximately located WELL BORING AND TRACE OF GAMMA RADIATION LOG- well locations are shown in figure 2.

Upper glacial aquifer

N8933 N12038

NScu N8933 EXPLANATION Bedrock Raritan clay Upper glacial aquifer aquifer

Upper glacial N11730 Hydrogeologic section C-C« on Great Neck, N.Y. (Trace of section is shown in fig. 2.) in fig. of section is shown (Trace N.Y. Hydrogeologic section C-C« on Great Neck, NORTH SHORE CONFINING UNIT NORTH SHORE AQUIFER Raritan clay North Shore Bedrock Lloyd aquifer Lloyd confining unit

Little Neck Bay Little Neck Figure 4C. NScu NSaq Vertical exaggeration X 20 exaggeration Vertical C 100 FEET -100 -200 -300 -400 SEA LEVEL

12 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

D'

NSaq N12508 Manhasset Neck 2,000 4,000 FEET 500 1,000 METERS 0 0 Hydrogeology by F. Stumm, 1994 F. Hydrogeology by North Shore confining unit Bedrock Lloyd aquifer Lloyd

Manhasset Bay

North Shore confining unit N12112 Upper glacial aquifer GEOLOGIC CONTACT- dashed where approximately located WELL BORING AND TRACE OF GEOPHYSICAL (GAMMA RAY) LOG- well locations are shown in figure 2.

Bedrock N1926 aquifer North Shore Lloyd aquifer

NScu N12013 Raritan clay aquifer

Upper glacial

EXPLANATION N10100 sand Great Neck North Shore confining unit

NORTH SHORE CONFINING UNIT NORTH SHORE AQUIFER N1926 NScu NSaq Hydrogeologic section D-D« on Great Neck, N.Y. (Trace of section is shown in fig. 2.) in fig. of section is shown (Trace N.Y. Hydrogeologic section D-D« on Great Neck, Bedrock

Bay 4D.

Raritan clay North Shore Lloyd aquifer Lloyd confining unit

Little Neck Little Neck Upper glacial aquifer Vertical exaggeration X 18 exaggeration Vertical D Figure 100 SEA -100 -200 -300 -400 FEET LEVEL

Hydrogeology 13

73û46« 44« 42«

Manhasset Bay 40û 50« D N U O S -150 D N -185 LA IS -121 G N LO

-132 -200 GREAT NECK -221 -150 -145 -180 -150 -160

-175 -222 -250

-185 -186 -260

48«

-243 -299 -187 -300 -251 -263 -200

Little Neck Bay -233

-250 -276 -243

-273 -363 EXPLANATION -300 -350 STRUCTURE CONTOUR -- Shows altitude of hydrogeologic unit surface. -400 Dashed where approximately located. Contour interval 50 feet. Datum is sea level. -360 46« EXTENT OF HYDROGEOLOGIC UNIT

-243 NASSAU CO. -400 PUBLIC-SUPPLY WELL QUEENS CO.

-276 OBSERVATION WELL 0 2,000 4,000 FEET (Number indicates depth above or below sea level, in feet.) 0 500 1,000 METERS Base from New York State Department of Transportation, 1:24,000, 1981

Figure 5. Extent and upper surface altitude of the Lloyd aquifer, Great Neck, N.Y. (Location is shown in fig. 1B.)

14 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

73û46« 44« 42«

40û50« Manhasset Bay D N U O S D N LA -100 IS G -96 N LO

-150 -112 GREAT NECK -161

-115 -144

-129

-130

48« -143 -173 -212 -200 -225 -201 -206

Little Neck Bay

-153 -150 -191 -200

-222 -241 EXPLANATION STRUCTURE CONTOUR -- Shows altitude of hydrogeologic unit surface. Dashed where -200 approximately located. Contour interval 50 feet. -242 Datum is sea level. NASSAU CO. 46« QUEENS CO. EXTENT OF HYDROGEOLOGIC UNIT

-191 PUBLIC-SUPPLY WELL -250 -254

-242 OBSERVATION WELL (Number, where present, indicates depth above or 0 2,000 4,000 FEET below sea level, in feet.) 0 500 1,000 METERS Base from New York State Department of Transportation, 1:24,000, 1981

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

Hydrogeology 15

73û46« 44« 42«

Manhasset Bay D 40û50« N U O S D N LA IS G N LO

GREAT NECK

48« 19 0

0

Little Neck Bay -55 -50

EXPLANATION -80 STRUCTURE CONTOUR -- Shows -50 0 100 altitude of hydrogeologic unit surface. Dashed where approximately located. 50 Contour interval 50 feet. Datum is sea level. 100 114 100 EXTENT OF HYDROGEOLOGIC UNIT NASSAU CO. 46« 59 QUEENS CO. PUBLIC-SUPPLY WELL 59 104 OBSERVATION WELL 104

114 GOLF COURSE WELL 41 (Number, where present, indicates depth above 0 2,000 4,000 FEET or below sea level, in feet.) 0 500 1,000 METERS Base from New York State Department of Transportation, 1:24,000, 1981

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

16 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

and chemically stable heavy opaque minerals. The top aquifer was then capped by clay and silt of the North of the Magothy aquifer ranges from 50 ft below sea Shore confining unit. Subsequent glacial advances in level to more than 100 ft above sea level (fig. 7). nearby areas eroded northwest-southeast trending Glacial erosion has produced a steep, undulating, valleys that reached the bedrock surface and removed north-facing scarp along the southeastern part of the previously deposited sediments, which may have peninsula (fig. 7). Where present, the Magothy aquifer included parts of the North Shore aquifer. These overlies the Raritan clay and has an anisotropy of valleys became filled with hundreds of feet of clay and 100:1 (Smolensky and others, 1989). silt (fig. 4). The buried-valley deposits probably represent the most recent glacial advance in the area, North Shore Aquifer however, and may not be of the same age as the North Shore aquifer in northern Great Neck; therefore, they The name North Shore aquifer is introduced are grouped with undifferentiated Pleistocene-age here to represent a sequence of Pleistocene-age deposits of the North Shore confining unit. Swarzenski sediments that are confined by a Pleistocene-aged clay, (1963) described the deposits in northernmost Great are in contact with bedrock, and are hydraulically Neck as pre-Sangamon and possibly Illinoian age and, interconnected laterally with the Lloyd aquifer. The thus, as the oldest glacial deposits in the area. North Shore aquifer was penetrated during drilling in The North Shore aquifer is a sequence of poorly the northernmost part of Great Neck in 1991-96. Drill- to moderately sorted, dark olive brown (2.5Y 3/3) and core samples obtained during this study indicated a olive-gray (5Y 6/2) gravel, sand, and silt layers. The complex hydrogeologic framework that does not sediments consist of subangular to subrounded quartz, resemble that indicated in previous work. Manhasset rock fragments, unstable opaque minerals, and a large Neck, to the east, contains similar deposits and a percentage of biotite and muscovite micas. The top of similar hydrogeologic framework (Stumm and Lange, the North Shore aquifer ranges from 70 to about 90 ft 1996), as does northwestern Queens County, to the below sea level in the northernmost part of the west (Chu and Stumm, 1995); therefore, a unifying peninsula (N12050, N6012, N12102) to more than nomenclature was needed for the northern shore of 250 ft below sea level within buried valleys in the Long Island. These deposits were called the Jameco northeast-coastal (N12112, N9840, N5357) parts (well Gravel by Swarzenski (1963) and the Port Washington locations shown in figs. 2, 8). The upper surface of aquifer by Kilburn (1979). The name North Shore these deposits appears to be relatively flat. The North aquifer is used in this report in preference to the names Shore aquifer is more than 150 ft thick in the Jameco Gravel or Jameco aquifer because the previous northernmost part of Great Neck but may be only 5 to names imply correlation with Jameco deposits in 10 ft thick in the northeast coastal area. southern Queens County; such a correlation is The North Shore aquifer appears to be questionable. The Port Washington aquifer, as moderately to highly permeable, as inferred from the described by Kilburn (1979), included a large amount rapid response of water levels to tidal fluctuations of material that was found in this study to be of similar to those in the Lloyd aquifer. No hydraulic Cretaceous age (Lloyd aquifer and Raritan clay). conductivity values for this unit have been calculated. Placement of part of the Port Washington aquifer above the Raritan clay is rejected on the basis of drill- core samples and hydrogeologic data (Stumm, 1993a; North Shore Confining Unit Stumm and Lange, 1994). The name North Shore confining unit is given The North Shore aquifer is a distinct here to a sequence of Pleistocene-age clay and silt hydrogeologic unit directly above bedrock and is deposits that occur locally along the northern shore of overlain by a thick sequence of clays and silt, and Queens (Chu and Stumm, 1995), Nassau (Stumm and named the North Shore confining unit in this report Lange, 1994, 1996) and Suffolk (Soren, 1971) (figs. 4, 9). The North Shore aquifer lies in the Counties. Grim and others (1970), Lewis and Stone northernmost part of the Great Neck peninsula (fig. 8) (1991), and Williams (1981) describe these deposits as and consists of moderately sorted stratified drift and part of the stratified glacial-lake clay and silt that outwash deposits that infilled the low-lying areas after underlie Long Island Sound and Manhasset Bay. the removal of the Cretaceous deposits and parts of the Stumm and Lange (1994, 1996) used offshore seismic- bedrock (saprolitic zone) by ice-contact erosion. The reflection surveys, borehole-geophysical logs, and

Hydrogeology 17

73û46« 44« 42«

-100 -87 40û50« Manhasset Bay D N ? U O -69 ? S -50 D N -75 -182 LA -200 ? IS G N LO -247 -250

GREAT NECK

48«

Little Neck Bay

EXPLANATION STRUCTURE CONTOUR -- Shows altitude of hydrogeologic unit surface. Dashed where

-200 approximately located. Contour interval 50 feet. Datum is sea level. NASSAU CO. 46« EXTENT OF HYDROGEOLOGIC UNIT QUEENS CO.

PUBLIC-SUPPLY WELL

-247 OBSERVATION WELL (Number, where present, indicates depth above 0 2,000 4,000 FEET or below sea level, in feet.) 0 500 1,000 METERS Base from New York State Department of Transportation, 1:24,000, 1981

Figure 8. Extent and upper surface altitude of the North Shore aquifer, Great Neck, N.Y. (not shown on Manhasset Neck). (Location is shown in fig. 1B.)

18 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

drill-core samples, to correlate these deposits with brackish interglacial environment, they are placed, in buried valleys that extend across the Great Neck this report, with the younger glacial-lake deposits as peninsula and the surrounding embayments of Long an undifferentiated Pleistocene-aged unit of clay and Island Sound, Little Neck Bay, and Manhasset Bay. silt that constitute the North Shore confining unit. The Swarzenski (1963) called these deposits the offshore seismic-reflection profiles in Little Neck Bay, Gardiners Clay, and Kilburn (1979) called them the Long Island Sound, and Manhasset Bay indicate Port Washington confining unit. The name North subhorizontal, parallel reflectors that are indicative of Shore confining unit is used here in preference to the draped bedding (varved clays) and imply deposits of name Gardiners Clay because correlation with the silt and clay (fig. 11). These deposits fill two sharply Gardiners Clay elsewhere on Long Island is defined buried valleys that truncate the surrounding questionable. The Port Washington confining unit of hydrogeologic deposits (Stumm and Lange, 1994, Kilburn (1979) was delineated from drillers’ logs and 1996). The valleys are asymmetrical, steep sided, includes parts of the Raritan clay of Cretaceous age. 2-4 mi long and about 1 mi wide, and trend northwest- Drill-core data obtained in this study are inconsistent southeast. They extend to bedrock, and are sometimes with many previous interpretations and delineations; deeper than 200 ft below sea level (figs. 4A, 4B, 4C, thus, the Port Washington confining unit is no longer 4D) (Stumm and Lange, 1994, 1996). considered to be a valid unit. Tagg and Uchupi (1967) conducted seismic The North Shore confining unit consists of profiles in Long Island Sound and found the bedrock material from two possibly separate depositional surface to contain deep, flat-bottomed, U-shaped sequences—one is a brackish marine clay, the other a troughs that extend to more than 600 ft below sea proglacial freshwater-lake clay. Drill-core samples level. They interpreted these troughs or valleys to be from N12013 and N12465 (fig. 2) contain olive-gray glacially eroded but were unable to discern whether clay (5Y 4/2) with oyster shells, which suggest any of them extend beneath Long Island because near- shallow, brackish conditions. Amino-acid- shore seismic data were lacking. Reeds (1927) racemization dating of oyster shells from well N10101 described varved clay deposits in the New York City (adjacent to N12465) indicates an age of about area; his interpretations suggest clay deposition in 225,000 years before present (Ricketts, 1986), which postglacial freshwater lakes occupying low-lying suggests a Yarmouth age for the brackish, shallow marine environment (North Shore confining unit) in basins in southern New York. He also proposed a series this part of Great Neck. This conclusion is consistent of interconnected lakes (Glacial Lake Flushing) within with the presence of Cretaceous sediments beneath parts of Long Island Sound and northern Queens wells N12013 and N12465 and indicates that these County (fig. 2) and describes these glacial lakes as areas probably represent an older remnant that is either containing extensive layers of varved clays and silt. in place or was ice shoved into the present location. Lewis and Stone (1991) describe similar silt and clay Cores from several other wells contained gray clay and deposits infilling extensive parts of Long Island Sound. silt sequences without shell material; this indicates The North Shore confining unit underlies almost varved clays deposited in a much more recent all of the Great Neck peninsula and ends along an proglacial lake. Little or no Cretaceous material is undulating northeast-southwest-trending line in the typically found beneath many of these areas. southeastern part of the peninsula (fig. 9). The upper The Gardiners Clay is a greenish-brown or gray surface of the North Shore confining unit is relatively marine clay of Pleistocene age that was probably flat in most parts of the peninsula (fig. 9); its altitude deposited during an interglacial period and contains ranges from 20 ft above sea level to more than 60 ft diatoms, foraminifers, and shell fragments (Suter and below sea level, and the unit extends offshore into the others, 1949). Investigation of the stratigraphy and surrounding embayments. The North Shore confining geologic age of these deposits was beyond the scope unit consists of olive-brown (2.5Y 4/2) varved clay of this study, but the most widely accepted opinion is and minor amounts of silt and dark olive-gray clay that the term Gardiners Clay refers only to this south- (5Y 4/2) with shells. Ice-shove deformation and shore unit (Suter and others, 1949). deposition in parts of Great Neck have resulted in Although the marine clays in central Great Neck localized clay layers at altitudes of more than 20 ft might indeed represent an erosional remnant of a above sea level within the unit (fig. 9).

Hydrogeology 19

73û46« 44« 42« 6 -50

40û50« -12

D Manhasset Bay N U 48 O S D -50 N -24 -32 LA -41 IS G N LO -32

-27 GREAT NECK 23 -3 45 12

0 -39 -23

-31 -80 -36 -50 -40

48« 37 -76 -62 61 50 Little Neck Bay 49 0 -18

-30

-68 -50

-75 EXPLANATION STRUCTURE CONTOUR -- Shows altitude of hydrogeologic unit surface. Dashed -50 where approximately located. Contour interval 50 feet. Datum is sea level.

EXTENT OF HYDROGEOLOGIC UNIT NASSAU CO. 46« QUEENS CO. -31 PUBLICÐSUPPLY WELL -18 OBSERVATION WELL

GOLFÐCOURSE WELL 0 2,000 4,000 FEET (Number, where present, indicates depth above or below sea level, in feet.) 0 500 1,000 METERS Base from New York State Department of Transportation, 1:24,000, 1981

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

20 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

Of interest is the lack of sand and gravel within sea floor is 10 to 30 ft below sea level in this area. The the onshore and offshore buried valleys beneath Great first subsurface reflector (at 30- to 50-ft depth) marks Neck and Manhasset Bay that Swarzenski (1963) and the top of a complex, chaotic unit of coarse-grained Kilburn (1979) describe as possible channels or sand and gravel. A major reflector that was present at valleys infilled mostly with sand and gravel. All buried depths of 120 to 190 ft below this unit is uneven and valleys delineated in this study were found to be filled has a vertical offset of more than 50 ft. This reflector is with 200- to 300-ft thick sequences of clay and silt. interpreted, from nearby wells, to be the bedrock Elsewhere on the peninsula, the North Shore confining surface. It has a smooth surface that dips to the south unit is about 50 ft thick. midway between E-E«, then drops about 50 ft with a slightly undulating horizontal surface from there to E«, Upper Glacial Aquifer much like a cuesta or hogback. Seismic-reflection profile F«-F is about 7,000 ft The upper glacial aquifer consists of till, sand, long and lies about 3,000 ft off the northwestern shore gravel, silt, and clay deposits and is underlain in some of Great Neck, in Little Neck Bay, roughly parallel to areas by the North Shore confining unit. This aquifer E-E´ (figs. 1B, 11B). The sea floor along this section includes Pleistocene-age beds of fine- to coarse- ranges from 50 to 10 ft below sea level. The first grained and stratified sand and gravel, unstratified subsurface reflector marks the top of a complex, boulders, clay, till, and some small, shallow pond layered unit of fine-grained silt and sand 10 to 40 ft deposits (Suter and others, 1949). The upper glacial thick. Below this reflector, a series of subhorizontal, aquifer consists of a mixture of brown sand parallel reflections indicative of draped bedding is (7.5YR 4/6), dark yellowish-brown sand (10YR 4/6), interpreted as silt and clay. These deposits fill a and varying amounts of reworked Cretaceous deposits. sharply defined, glacially eroded valley that truncates The upper glacial aquifer is poorly to moderately the surrounding coarse-grained deposits. The valley is sorted and consists of rock fragments and minerals asymmetrical, trends northwest-southeast, is about dominated by quartz. Biotite and muscovite micas 3 mi long and 1 mi wide, and reaches about 200 ft typically are observed. Offshore seismic-reflection below sea level (Stumm and Lange, 1994). surveys and drill-core data indicate that this aquifer Seismic-reflection profile G«-G, about 4,600 ft does not extend a great distance offshore, nor does it long and crossing the opening of Manhasset Bay, infill the valleys beneath Manhasset Bay and Little indicates another major buried valley (figs. 1B, 11C). Neck Bay, as interpreted by Kilburn (1979). The upper Water depth ranges from 20 to 60 ft. The first glacial aquifer overlies the Magothy aquifer in the subsurface reflector represents the top of a complex, southernmost part of Great Neck and overlies most of layered unit of sand and silt 35 ft thick, beneath which the North Shore confining unit in the rest of the is a fine-grained deposit of draped, parallel reflections peninsula (fig. 10). The present land surface includes interpreted as silt and clay. This extensive deposit fills recent deposits and the unsaturated upper part of the an asymmetrical, glacially eroded valley about 1 mi upper glacial aquifer. An average horizontal hydraulic wide that extends to about 235 ft below sea level. Data conductivity of 270 ft/d was determined for the aquifer from the two seismic-reflection surveys in 1993 and (Smolensky and others, 1989). 1994 within Manhasset Bay indicate that the valley is about 2.5 mi long and trends northwest-southeast (Stumm and Lange, 1994; 1996). The bedrock surface Seismic-Reflection Surveys appears to undulate within the valley. Geologic data from well N12050 (fig. 2) correlate with the seismic- The seismic-reflection survey results were reflection data. correlated with geologic logs of nearshore observation wells. Three seismic-reflection profiles—E-E´, F´-F, and G´-G (figs. 11A, 11B, 11C)—were selected to Borehole-Geophysical Logs depict the major subsurface features encountered during seismic-reflection surveys. The borehole geophysical logs shown in figure Seismic-reflection profile E-E´ is about 4,250 ft 12 are gamma, SP, SPR, R, and EM logs of test well long and lies about 1,000 ft off the northwestern shore N12581T and are typical of those used in the study of Great Neck in Little Neck Bay (figs. 1B, 11A). The (fig. 2). The gamma log (fig. 12A) indicates two

Hydrogeology 21

73û46« 44« 42«

16 48 40û50« D N 50 U 50 O Manhasset Bay S 11 60 D N 73 18 LA 9 IS 25 G N LO 23 50

18 GREAT NECK 28 9 65 31 50 106 48

43 45 23 60 69 124

48« 100 12 57 111 15 89 10 Little Neck Bay 100 12 96 32

50 7 32 150

82 200 15 50

22

EXPLANATION 80 100 STRUCTURE CONTOUR -- Shows 50altitude of hydrogeologic unit surface. Dashed where 150 100 approximately located. Contour interval 50 feet. Datum is sea level. 220 184 EXTENT OF HYDROGEOLOGIC UNIT 154 NASSAU CO. 46« 45 QUEENS CO. PUBLICÐSUPPLY WELL 179 82 OBSERVATION WELL 200

GOLFÐCOURSE WELL 0 2,000 4,000 FEET (Number, where present, indicates depth above or below sea level, in feet.) 0 500 1,000 METERS Base from New York State Department of Transportation, 1:24,000, 1981

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

22 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

E E' SEA LEVEL (NW) 0

SEA FLOOR

40

80

120 SAND AND GRAVEL

160 WEATHERED BEDROCK DEPTH BELOW SEA LEVEL, IN FEET DEPTH BELOW

200 BEDROCK

240 Vertical Exaggeration X 10.5 0 500 1,000 FEET Assumed velocity of sound 5,000 feet per second 0 150 300 METERS

Figure 11A. Interpreted stratigraphy along seismic-reflection profile E-E´ in Long Island Sound off Great Neck, N.Y. (Trace of section is shown in fig. 1B.)

horizons of clay at 110 and 170 ft below land surface similar to the SP, SPR, and R logs and shows an (30 and 90 ft below sea level). Drillers’ logs indicate increase in conductivity with depth above and below extensive ice-shove deposition of reworked Cretaceous the clay at 110 ft below land surface (30 ft below sea sediments, including the Raritan clay. The bottom of level). The most conductive water detected was just the well was set at the surface of the Raritan clay. The above the deep clay layer at 170 ft below land surface SP and SPR logs (figs. 12B, 12C) indicate a steady (90 ft below sea level) with a maximum conductivity of deflection to lower values just below the water table at about 75 mS/m. about 30 ft below land surface (50 ft above sea level). Resistance values on this log do not appear to fluctuate at the transitions from sand to clay. The R log Precipitation (fig. 12D) correlates with the SP and SPR logs and indicates moderately conductive ground water above All freshwater in the Great Neck peninsula and below the shallow clay layer at 110 ft below land originates as precipitation. The precipitation infiltrates surface (30 ft below sea level). The EM log (fig. 12E) is the land surface, runs off to storm sewers and to

Hydrogeology 23

F 300 METERS 500 150 in fig. 1B.) in fig. 0 BEDROCK 0 1,000 FEET COARSE-GRAINED DEPOSITS COARSE-GRAINED SILT AND CLAY AND CLAY SILT (SE) SAND AND SILT SAND AND SILT DEPOSITS SEA LEVEL COARSE-GRAINED BEDROCK SILT AND CLAY AND CLAY SILT SEA FLOOR Interpreted stratigraphy along seismic-reflection profile F´-F in Little Neck Bay off Great Neck, N.Y. (Trace of section is shown (Trace N.Y. off Great Neck, Bay along seismic-reflection profile F´-F in Little Neck Interpreted stratigraphy F' Vertical Exaggeration X 10.8 Exaggeration Vertical per second) of sound 5,000 feet (Assumed velocity 0

40 80 240

120 160 200 DEPTH BELOW SEA LEVEL, IN FEET FEET IN LEVEL, SEA BELOW DEPTH Figure 11B. Figure 11B.

24 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York

G 300 METERS BEDROCK SAND AND GRAVEL SAND AND GRAVEL n fig. 1B.) n fig. 500 150 0 0 1,000 FEET GAS DEPOSITS SILT AND CLAY AND CLAY SILT (SE) SILT AND CLAY AND CLAY SILT SEA LEVEL BEDROCK SEA FLOOR SILT AND CLAY AND CLAY SILT Interpreted stratigraphy along seismic-reflection profile G´-G in Manhasset Bay off Great Neck, N.Y. (Trace of section is shown i of section is shown (Trace N.Y. off Great Neck, along seismic-reflection profile G´-G in Manhasset Bay Interpreted stratigraphy G' Vertical Exaggeration X 5.9 Exaggeration Vertical per second) of sound 5,000 feet (Assumed velocity 0

40 80 240

120 160 200 DEPTH BELOW SEA LEVEL, IN FEET FEET IN LEVEL, SEA BELOW DEPTH Figure 11C. Figure 11C.

Hydrogeology 25

A. Gamma Radiation B. Spontaneous C. Single Point D. Short-Normal E. Conductivity (EM) (counts per second) Potential (SP) Resistance (SPR) Resistivity (R) (milliSiemens/meter) (millivolts) (ohms) (ohm-meters) 0 60 120 -100 0 100 250 80 170 260 10 40 70 0 0 40 80

50

100

150

200 DEPTH, IN FEET BELOW LAND SURFACE 250

300

Figure 12. Geophysical logs of test well N12581T, Great Neck, N.Y. (Location is shown in fig. 2.)

tidewater, or returns to the atmosphere through Ground Water evapotranspiration. Some of the water that infiltrates the soil is returned to the atmosphere through The major uses of ground water in the Great evaporation and transpiration; the rest percolates to the Neck peninsula are for residences, businesses and water table, where it becomes shallow ground water. Some water moves laterally into stream channels and Table 2. Total annual pumpage from the Great Neck peninsula, by aquifer, 1992-96 becomes base flow; the remainder moves downward [Data from Great Neck Water Authority, Manhasset-Lakeville into the deeper hydrogeologic units and eventually Water District, and New York State Department of Environmental discharges to the surrounding saltwater bodies. Conservation] The average warm-season and cool-season Total pumpage, in millions of gallons precipitation are almost equal, but more precipitation Upper glacial falls in the spring than at other times. The precipitation Year aquifer Magothy aquifer Lloyd aquifer regime on Long Island for 1951-65 was studied by 1992 359.9 1,149.5 1,012.3 Miller and Frederick (1969), who calculated mean annual precipitation on the Great Neck peninsula to be 1993 539.8 1,054.8 1,034.1 about 42 in. The average annual precipitation recorded 1994 518.9 1,402 1,035.2 by NCDPW at Mineola, in central Nassau County 1995 514.9 1,259.8 829.9 (fig. 1B), during 1937-96 is 44.4 in. 1996 499.7 806.8 1,131 Mean 486.6 1,134.6 1,008.5

26 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York small industry, and for irrigation of lawns and golf public-supply pumpage on water levels in aquifers is courses (Great Neck has no agriculture). Water levels discussed in the following sections. were measured in observation wells screened in all major aquifers within Great Neck. Water Levels Pumping generally lowers water levels in Pumpage aquifers and thereby induces a flow gradient toward The vast majority of water for all uses is derived the pumping wells. A major concern for water from public-supply wells owned and operated by the suppliers and managers is that contaminated water Great Neck North Water Authority and the Manhasset- from adjacent or overlying aquifers may be induced to Lakeville Water District; a small amount is from flow toward the well. private wells. Ground water is pumped from the upper All wells near the coast of Long Island are glacial, Magothy, and Lloyd aquifers. The total annual assumed to be affected by tides; thus, the USGS pumpage from each aquifer during 1992-96 is shown measures water levels in these wells during the high- in table 2. Three golf-course-irrigation wells (N2576, tide period (typically 2 h) in the nearest embayment N9687, and N8038), all screened in the Magothy because ground water can fluctuate in a well aquifer (fig. 2), serve the three golf courses in the throughout a tidal cycle. All tidally influenced wells study area. A private irrigation well (N7846, fig. 2) is are measured by the USGS only at high tide to screened in the upper glacial aquifer. Average annual maintain consistency over long-term records. The pumpage of the four wells (combined) for 1992-96 large number of observation wells in Great Neck was 35.4 Mgal, and that for the pumping season (May- would have required 5-7 days for measurement; October) was 0.2 Mgal/d. therefore, two studies were conducted in 1992 and Pumpage from the Magothy aquifer is greater 1993 to indicate which aquifers are affected by tidal than that from the Lloyd aquifer (table 2); average fluctuations in the surrounding embayments. Results annual pumpage for 1992-96 from the upper glacial, of the tidal influence studies in selected wells are Magothy, and Lloyd aquifers (combined) was shown in figure 13. Water levels at four Great Neck 486 Mgal (1.3 Mgal/d), 1,135 Mgal (3.1 Mgal/d), and wells were measured at 20-min intervals during a 1,009 Mgal (2.8 Mgal/d), respectively. The effect of continuous 14-h tidal sequence from high to low tide.

8

6 N10100 (North Shore aquifer) 4 LITTLE NECK 2 BAY TIDE

0 N12050 (North Shore aquifer) -2

-4 N12076 -6 (Lloyd aquifer) BELOW MEAN SEA LEVEL -8 N10101

WATER LEVEL, IN FEET ABOVE OR (Lloyd aquifer) -10 6 8 10 12 14 16 18 20 TIME SINCE MIDNIGHT, IN HOURS

Figure 13. Water levels measured at four selected wells during 1992-93 tidal study on Great Neck, N.Y. (Locations are shown in fig. 2.)

Hydrogeology 27 73û46' 44' 42' 9

40û50' D N U O Manhasset Bay S D N LA 11 IS G N LO 16 10

9 GREAT NECK 20 10 25 18 74 70 19 14 60 50

40 -15

30 48' 0

Little Neck Bay 8 25

10 9 14 20 11 26

10 EXPLANATION 20 30 WATER-TABLE CONTOUR -- Shows altitude of water-table surface. Dashed where approximately 36 20 located. Hachures indicate depression. 28 Contour interval 10 feet. Datum is sea level. NASSAU CO. 46' QUEENS CO. PUBLICÐSUPPLY WELL 40

25 OBSERVATION WELL -- Number, where present, indicates depth above or below sea level, in feet. 0 2,000 4,000 FEET GOLFÐCOURSE WELL 0 500 1,000 METERS Base from New York State Department of Transportation, 1:24,000, 1981

Figure 14. Water-table altitude in Great Neck, N.Y., August 1995.

28 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York 73û46' 44' 42'

0

40û50' D N U O Manhasset Bay S 4 -7 D N LA 0 -1 IS G N LO -8 7 -8 -10 -9 -3 -10 -11 -20 GREAT NECK

-10 -12

-10 0

48'

-2 0 -10

-20 -1 -39 Little Neck Bay

0 -19 -20 -10 -2 0

-20

EXPLANATION POTENTIOMETRIC CONTOUR -- Shows

20 altitude of potentiometric surface. Dashed50 where approximately located. Hachures indicate depression. Contour interval 10 feet. Datum is sea level. -4 EXTENT OF HYDROGEOLOGIC UNIT NASSAU CO. 46' HYDRAULICALLY ISOLATED AREA QUEENS CO.

PUBLICÐSUPPLY WELL

-2 OBSERVATION WELL -- Number, where present, indicates depth above or below sea level, in feet. 0 2,000 4,000 FEET GOLFÐCOURSE WELL 0 500 1,000 METERS Base from New York State Department of Transportation, 1:24,000, 1981

Figure 15. Potentiometric-surface altitudes in the Lloyd and North Shore aquifers in Great Neck, N.Y., August 1995.

Hydrogeology 29 A tide-measurement station was established on a dock sea level; here the water levels respond as in a confined in Little Neck Bay, on the west coast of Great Neck aquifer. This well indicated about 18 ft of drawdown (fig. 2), for use during the tidal influence studies. from pumping at public-supply well N5884 (fig. 2). Tidally influenced wells (wells requiring measurement Lloyd and North Shore aquifers (fig.15).— during high tide) were selected on the basis of results These two aquifers are in stratigraphic contact from the 1992 and 1993 tidal influence studies. laterally and are hydraulically connected. Water in the Water levels in observation wells were measured Lloyd aquifer is confined beneath the Raritan clay, and quarterly during 1992-96. Water levels during August water in the North Shore aquifer is confined beneath 1995 were at their lowest levels and are plotted in the North Shore confining unit. figures 14 and 15. The following paragraphs describe Predevelopment (1900) water levels within the water levels in the four aquifers underlying Great Lloyd aquifer are inferred to have been above sea level Neck during 1992-96. throughout the Great Neck peninsula (Kimmel, 1973). Upper glacial aquifer (fig. 14).—The water Water levels within the Lloyd aquifer in Great Neck table is within the upper glacial aquifer throughout the have been measured below sea level in 1947 Great Neck peninsula, except in the southernmost part, (Lusczynski, 1952), in 1971 (Kimmel, 1973), in 1975 where it is in the unconfined (upper) part of the (Rich and others, 1975), in 1979 (Donaldson and Magothy aquifer. Farther north, the Magothy is absent Koszalka, 1983), and in 1984 (Doriski, 1987). Water- (fig. 7). In isolated areas of Great Neck, parts of the level data from quarterly synoptic measurements upper glacial aquifer are semiconfined by the North during 1992-96 indicate that water in the Lloyd and Shore confining unit. Water levels during August 1995 North Shore aquifers flows northward and develops in wells screened near the water table (upper glacial large cones of depression in response to local public- and Magothy aquifers) are depicted in figure 14. supply pumping. Water-level data from the studies of Generally, the highest water levels are in the southern tidally affected wells in 1992 and 1993 indicate that part of Great Neck, except at well N9896 in the both aquifers are strongly affected by tides along the northeast part, where the water level was 74 ft above coast and are hydraulically connected (Stumm, 1993a; sea level. Water levels in the remaining wells screened Stumm and Lange, 1994). Tidal fluctuations at some in the upper glacial aquifer wells ranged from 8 ft to wells (N12076 for example) can be as much as 4 ft 36 ft above sea level. (fig. 13). Water levels in the central part of the Data from the two studies of selected tidally peninsula do not seem to be affected by tides but are affected wells in the Great Neck peninsula in 1992 and strongly affected by pumping from public-supply 1993 indicate that, on most of the peninsula, water wells. One well, which was previously interpreted to levels in the upper glacial aquifer and the unconfined be screened in the Lloyd aquifer (well N10100 in the part of the Magothy aquifer do not respond to tides. north-central part of the peninsula, fig. 2), was found An exception was at well N12152 (screened in the to be screened beneath a mostly clay-filled buried Magothy), near the south end of Manhasset Bay valley. Water levels in this well did not fluctuate and (fig. 2), where the Magothy aquifer is affected by were not affected by tides or by pumping, whereas pumping from public-supply well N5884 (screened in wells screened in the Lloyd aquifer beyond this buried both aquifers) (fig. 2), and water levels during valley were affected by either or both. Drillers’ logs nonpumping periods fluctuate with the tide. and gamma logs indicate that this well is screened not Magothy aquifer (fig. 14).—Ground water in the within the Lloyd, but within a thin, Pleistocene-age Magothy aquifer becomes increasingly confined with sand unit (sandy part of the North Shore confining depth. Water-level data obtained during quarterly unit) beneath the base of the clay-filled buried valley. measurements in the aquifer, where present in the Water levels within the buried valley (near well southern part of the peninsula, represent the water N10100) in central Great Neck were consistently table because the Magothy in this area is in hydraulic about 5-10 ft above sea level during 1992-96, in connection with the upper glacial aquifer (fig. 14). contrast to those on the rest of the peninsula in the Water levels at wells screened in the upper parts Lloyd and North Shore aquifers, which are affected by of the Magothy aquifer do not appear to be affected by pumping and tides (fig. 15). Data from 1992-96 also nearby pumping. An exception is at well N12152, near indicate that the sandy unit (N10100) at the base of the the south end of Manhasset Bay and screened below buried valley in this part of the peninsula is not

30 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York hydraulically connected with the Lloyd aquifer or with 450 180 other surrounding units, and is thus considered part of PUMPAGE the North Shore confining unit. 400 CHLORIDE 160 The potentiometric surface of the Lloyd and North 350 140 Shore aquifers, as measured in August 1995 (fig. 15), has 300 120 about four or five large cones of depression throughout the peninsula that are caused by public-supply pumping. 250 100 The potentiometric surface at most wells was below sea 200 80 level, and these depressions extended over large parts of 150 60 the peninsula and into the surrounding saltwater embayments. Water levels in 1995 ranged from 4 ft 100 40 above to almost 40 ft below sea level. The increased 50 20 pumping in summer causes ground-water flow in the 0 0 Lloyd and North Shore aquifers to reverse direction; that 1950 1960 1970 1980 1990 2000 is, to flow inland from the surrounding saltwater A. WELL N 5884 300 1,800 embayments toward the pumping wells (the areas of PUMPAGE FOR N30 & N31 lowest hydraulic head). This reversal of the normal CHLORIDE N30 1,600 250 seaward flow is the primary cause of past and present 1,400 saltwater intrusion in the peninsula. Analysis of long- term hydrographs of wells screened in the Lloyd and 200 1,200 North Shore aquifers indicates that local pumping has a 1,000 150 much greater effect than regional trends on water-level 800 fluctuations. 100 600

Water Quality 400 50 MGL (250 mg/L) Data on ground-water quality on the Great Neck 200 peninsula are available from analyses made by the 0 0 19501960 1970 1980 1990 NCDH and the NCDPW. In addition, 36 samples from B. WELLS N 30, N 31 PUMPAGE, IN MILLIONS OF GALLONS PER YEAR observation wells and 1 sample from Long Island Sound 50 900 were collected by the USGS during August and PUMPAGE CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER September of 1996 and analyzed by the NCDPW Cedar CHLORIDE 800 40 Creek laboratory. The objective was to obtain chloride 700 concentrations and specific-conductance data for 600 delineating the freshwater-saltwater interface. Detailed 30 analysis of the water quality and review of historical data 500 were beyond the scope of this study. 400 Chloride.—Chloride concentrations in the 20 300 Magothy and upper glacial aquifers on the Great Neck Maximum contaminant level, 250 mg/L 200 peninsula generally range from 10 to 50 mg/L; those in 10 the Lloyd aquifer range from 5 to 10 mg/L. One 100 exception is public-supply well N5884, in the 0 0 southeastern part of the peninsula, which had a peak 19401950 1960 1970 YEAR chloride concentration of 163 mg/L in 1979 and a C. WELL N 1926 concentration of 82 mg/L in 1995 (fig. 16A). Seawater has an average chloride concentration of Figure 16. Chloride concentrations and pumpage at selected public-supply wells on the Great Neck, N.Y., about 19,000 mg/L (Drever, 1988). A sample from Long peninsula. A. Well N5884, 1957-95. B. Wells N30 and Island Sound had a chloride concentration of 13,995 mg/L, N31, 1952-86. C. Well N1926, 1948-67. (Data from indicating significant freshwater inflow into Long Island Nassau County Department of Health, Nassau County Sound. Water from three public-supply wells screened in Department of Public Works, and Great Neck Water the Lloyd aquifer (N30, N31, and N1926) had chloride Authority. Locations are shown in fig. 2.)

Hydrogeology 31 concentrations that exceeded the State MCL one observation well (N8052) screened in the North (maximum contaminant level, 250 mg/L for chloride) Shore aquifer exceeded the MCL’s for iron, lead, and and were shut down as a result (figs. 16B, 16C). Water manganese (fig. 2). No public-supply wells on the from two observation wells screened in the Lloyd Great Neck peninsula contained any inorganic aquifer (N12153, N12013) had chloride constituent in concentrations above the State MCL’s concentrations above the State MCL (4,998 and (Nassau County Department of Health, 1996). 737 mg/L, respectively); water from two observation wells screened in the North Shore aquifer (N12050, N12102) also had chloride concentrations above the EXTENT OF SALTWATER INTRUSION State MCL (13,748 and 12,250 mg/L, respectively). Volatile organic compounds.—Volatile organic Saltwater intrusion is the most common type of compounds (VOC’s) that have been detected in ground water-quality degradation in coastal-plain aquifers water in the Great Neck peninsula could be derived (Fetter, 1994). In coastal areas, the hydraulic head under from domestic cesspool and drain cleaners, and from predevelopment (nonpumping) conditions is higher on solvents and degreasers used by industries. The VOC’s land than in the surrounding saltwater embayments; most frequently detected in Nassau County public- thus, ground water flows from areas of high potential to supply wells (raw water) in 1995 were areas of lower potential, and fresh-water and saltwater tetrachloroethylene (PCE), trichloroethylene (TCE), meet at an equilibrium point that lies offshore. When the natural hydraulic gradient is reversed by pumping, and 1,1,1-trichlorethane (TCA) (Nassau County fresh ground water flows toward the pumping well Department of Health, 1996). NCDH data indicate that instead of flowing seaward, and the equilibrium point raw water from public-supply wells N700, N22, between freshwater and saltwater moves landward. If N4388, N5710, N4243, and N3905 (fig. 2), screened this point moves inland, saltwater intrusion results. in the Magothy and upper glacial aquifers, contained VOC’s in concentrations above the New York State The predevelopment chloride concentration of fresh ground water on Long Island was 10 mg/L or less MCL’s. Water from one public-supply well screened in (Lusczynski and Swarzenski, 1966). The ambient the Lloyd aquifer, N3443 (fig. 2), also had a VOC chloride concentration of shallow (upper-glacial) concentration above the State MCL’s. ground water in urbanized areas of Long Island is less In 1996, contamination by VOC’s was detected than 40 mg/L (Buxton and others, 1981; Heisig and in water from the upper glacial and Magothy aquifers. Prince, 1993); this increase is attributed to Water from one well screened in the Lloyd aquifer contamination from land-surface sources. In this (N3443) had possible VOC contamination that may report, “ambient” water is defined as ground water with have been caused by leakage of the well casing or a chloride concentration less than 40 mg/L; “brackish” hydraulic connection with the overlying upper glacial water has a chloride concentration of 40 to 250 mg/L, aquifer. In general, the Lloyd aquifer appears to be and “saltwater” has a chloride concentration greater protected from the downward movement of synthetic than 250 mg/L (Lusczynski and Swarzenski, 1966). An organic compounds from the overlying shallow aquifers increase above predevelopment concentrations of by the Raritan clay and North Shore confining unit. chloride in deep, confined aquifers on Great Neck can Water from three of the observation wells be indicative of saltwater intrusion from the screened in the upper glacial, Magothy, and Lloyd surrounding embayments in response to pumping. aquifers (N9098, N10390, and N10992) (fig. 2) had Historical chloride concentrations at public- VOC concentrations above the State MCL’s; water supply wells in Great Neck indicate that the from well N9098 had benzene and benzene background chloride concentration for the Lloyd derivatives, toluene, and xylene in concentrations of aquifer is 5 to 10 mg/L. Based on 50 years of chloride- hundreds to thousands of times above State MCL’s. concentration and pumpage data from public-supply Inorganic constituents and metals.—Water from wells, once a concentration of 50 mg/L is exceeded at four observation wells screened in the upper glacial a public-supply well in Great Neck, the concentrations aquifer (N9909, N9099, N9271, and N12464, fig. 2) will remain above 50 mg/L even if pumpage is exceeded the State MCL’s for iron, lead, and decreased. The technique of pumping only in alternate manganese; water from one Lloyd aquifer well years at wells with elevated chloride concentrations (N12082) exceeded the MCL for iron, and water from only slightly delays an inevitable rapid increase in

32 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York chloride concentrations. This results because once the log responses correlated with chloride data from filter- toe, or leading edge, of the saltwater wedge reaches press and well-screen samples. the public-supply well, it is upconed into the screen The toe of wedge A was penetrated at well zone of the well and responds to decreased pumping N12102, about 2,000 ft south of N12050 (fig. 2); here for only a very short time before stabilizing into a it is 50 ft thick (fig. 17). The maximum chloride rapid, continuous increase. concentration in filter-press samples was 13,200 mg/L, In Great Neck, overpumping of the Lloyd, North and that in a sample from an observation well screened Shore, and upper glacial aquifers has caused extensive at the base of the North Shore aquifer was 12,250 mg/L. saltwater intrusion (Stumm, 1993a, 1994; Stumm and The lack of a wide transition zone in wedge A within Lange, 1994). Filter-press samples obtained during this tidally affected aquifer indicates active saltwater drilling of 12 observation wells for this study were intrusion due to current overpumping of the Lloyd analyzed for chloride concentration and were aquifer in the central part of the peninsula. correlated with core samples from the screen zones, Saltwater wedge B is at the base of the Lloyd geologic logs, and geophysical (gamma, electric, and aquifer and was the cause of the shutdown and electromagnetic induction) logs to delineate the extent abandonment of public-supply wells N30 and N31 in of saltwater intrusion. Chloride concentrations of 1974 (figs. 2, 17). A graph of the chloride samples from public-supply wells also were analyzed. concentration and public-supply pumpage at this location from 1952 through 1986 (fig. 16B) indicates a typical pattern of saltwater intrusion—first a gradual Saltwater Wedges A, B, C, and D but steady increase in chloride concentration, and then rapid increases to concentrations above the State Four distinct wedge-shaped areas of saltwater MCL. Pumping of wells N30 and N31 began in the intrusion in Great Neck were identified; three extend early 1950’s and stopped during the late 1980’s. Well into the Lloyd and North Shore aquifers (fig. 17), and N30 and N31 had peak chloride concentrations of the fourth extends into the upper glacial aquifer. Three 1,600 mg/L in 1978 and 390 mg/L in 1983, additional and less distinct areas or wedges of respectively. Saltwater wedge B also was encountered saltwater in Great Neck have been detected through at the base of the Lloyd aquifer at wells N12079 and water-quality analyses and borehole-geophysical logs. N12013, which are 3,000 and 5,000 ft south of well The saltwater wedges typically form at the base of an N12102, respectively. Focused-induction logs indicate aquifer that overlies an impermeable layer (either wedge B to be about 20 ft thick at this location. The bedrock or a confining unit) because the density of sample from the screen zone at well N12013 had a saltwater is greater than that of freshwater. The chloride concentration of 737 mg/L, indicating wedges decrease in thickness landward and have saltwater intrusion. Sharp deflections in the induction relatively sharp freshwater-saltwater interfaces log at the bedrock surface supports the assumption that (transition zones) about 10 ft thick (Stumm, 1993a) the upper bedrock surface (saprolite zone) is a (fig. 18). The saltwater wedges were delineated on the relatively impermeable boundary because of the high basis of chloride concentrations in water samples from clay content. observation wells, hydrogeologic data, gamma and The western edge of saltwater wedge B is focused-electromagnetic induction logs, and interpreted to be near well N12079 (figs. 2, 17); potentiometric-surface data. offshore seismic-reflection, drill-core, and structure- Saltwater wedge A, in the northernmost part of contour-map data indicate a nearly 100-ft cuesta of Great Neck, was penetrated at the base of the North bedrock immediately south of N12102 that wedge A Shore aquifer at wells N12050 and N12102 (fig. 17). probably could not breach; thus, wedge B must Chloride concentrations from filter-press samples and originate just northeast of N12079. Whether saltwater estimated from induction-log response indicates that wedge B is directly associated with wedge A is wedge A at well N12050 is about 125 ft thick. The unknown. Filter-press samples and the induction log freshwater-saltwater interface here is sharp, and the of well N12079 indicate wedge B to be about 20 ft chloride concentration of a sample from the thick and to have a chloride concentration of 500 mg/L observation well screen was 13,750 mg/L (fig. 18), at the depth of the well screen; this corresponds to an which indicates severe saltwater intrusion. Induction- induction-log conductivity of about 50 mS/m. A third

Extent of Saltwater Intrusion 33 73û46« 44« 42« H 12050(NS) D 13,800 N 40û50« U A O Manhasset Bay S 12102(NS) D N 12,300 H« ? LA ? IS G 12079 (L) N ? 500 LO ?

B 1000 12112(NS) 8

12013 (L) 100 737 10100(NSCU) 12076(L) <3 31(L) 8,000 ? 1,100 9776(L) ? GREAT8 NECK

C 2214(L) 10192(L) 58 12153(L) 3443(L) 8 5,000 40 12152(M) 100 140 48« 12735T D 12082(L) 687(L) <3 5884(UG) 12038(L) 86 82 23 1000 100 3 NASSAU CO. QUEENS CO.

Little Neck Bay 11730(L) 12154(L) 20 5 1298(L) 34 4388(M) <10

1618(L) EXPLANATION 13

LINE OF EQUAL CHLORIDE CONCENTRATION -- 100 Dashed where inferred. Concentration is in milligrams per liter. 12450(L) <3 H«HYDROGEOLOGIC CROSSÐSECTION 46« H PUBLIC-SUPPLY WELL 687(L) 0 2,000 4,000 FEET 86 0 500 1,000 METERS 11730 (L) OBSERVATION WELL -- Top number is well number; letter in 20 parentheses is aquifer designation. (L=Lloyd, M=Magothy, NSCU=North Shore confining unit, UG=upper glacial) Bottom number indicates chloride concentration in milligrams per liter. Base from New York State Department of Transportation, 1:24,000, 1981

Figure 17. Chloride concentrations in Lloyd, North Shore, and upper glacial aquifers, Great Neck, N.Y., 1996.

34 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York H H' FEET N12050 50 LONG

ISLAND UPPER GLACIAL AQUIFER

N8052 N12102 SOUND N8046 POND SEA SEA LEVEL LEVEL NORTH SHORE CONFINING UNIT

-50 -50

NORTH SHORE AQUIFER 18 mg/L -100 -100

FRESHWATER-SALTWATER INTERFACE

-150 -150

INDUCTION LOG 13 mg/L

-200 -200 INDUCTION LOG 13,800 mg/L

12,300 mg/L -250 -250

BEDROCK

Vertical Exaggeration X 10 0 500 FEET 0 50 100 METERS EXPLANATION SALTWATER WEDGE A

18 mg/L OBSERVATION WELL - Numbers indicate chloride concentration at screened interval (mg/L, milligrams per liter)

Figure 18. Hydrogeologic section showing extent of saltwater wedge A, Great Neck, N.Y. (Location is shown in fig. 19.) well (N12112) was drilled about 3,000 ft east of public-supply well N1926 (fig. 16C) indicates rapid N12013 and screened at the base of the North Shore saltwater intrusion in 1965 with little warning; this aquifer (figs. 2, 17), but geophysical logs and water intrusion may have been because the well was samples from this well did not indicate saltwater screened fairly close to the base of the aquifer in an intrusion; thus, the eastern boundary of saltwater area where the freshwater-saltwater interface was just wedge B probably does not extend this far east. offshore. The pumpage and chloride graph for public- Saltwater wedge C is at the base of the Lloyd supply well N2214 (fig. 19B), about 3,000 ft southeast aquifer along the western shore of Great Neck of N1926 (fig. 17), also indicates saltwater intrusion a (fig. 17). The wedge has affected two public-supply few years later, in 1968. Samples from this well had a wells (N1926 and N2214) and a private well (N7613). peak chloride concentration of 180 mg/L in 1975. The A plot of pumpage and chloride concentrations at reduced-pumpage technique has been applied at

Extent of Saltwater Intrusion 35 300 90 induction-log response and correlation with PUMPAGE stratigraphy at well N1926 indicate the saltwater CHLORIDE 80 wedge to have a thickness of about 30 ft at this 250 70 location and an estimated chloride concentration of about 8,000 mg/L. The induction log and the water- 200 60 sample data from the screen zone at well N12153, 50 which is 4,000 ft southeast of N12076, indicate a 150 wedge thickness of about 20 ft and a chloride 40 concentration of 5,000 mg/L. 100 30 Saltwater wedge D is on the eastern shore of the peninsula within the upper glacial aquifer (fig. 17). 20 The graph of pumpage and chloride concentrations at 50 10 public-supply well N5884 (fig. 16A) indicates an increase in pumpage in the late 1960’s, followed by a 0 0 gradual, then rapid, increase in chloride concentration. 19501960 1970 1980 1990 2000 A . WELL N 687 Well N5884 had a peak chloride concentration of 300 200 160 mg/L in 1979. A reduction in pumpage since 1979 PUMPAGE has lowered the concentration, but not to pre-intrusion CHLORIDE 180 levels. Observation well N12152 (next to N5884) was 250 160 drilled and screened at the base of the Magothy aquifer; data from drill-core and filter-press samples 140 200 and geophysical logs indicate a 30-ft-thick wedge of 120 brackish water resting on a thin clay layer within the

PUMPAGE, IN MILLIONS OF GALLONS PER YEAR 150 100 upper glacial aquifer. The maximum filter-press chloride concentration was 141 mg/L. Gamma logs 80 CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER from well N5884 and an adjacent, recently installed 100 60 test well N12735T (figs. 2, 17) indicate that the thin

40 clay layer is continuous and extends some distance 50 inland. Well-screen-construction records indicate that 20 well N5884 has two screen zones—one in the upper 0 0 glacial aquifer and the other in the Magothy aquifer 19501960 1970 1980 1990 2000 below. The two screen zones are separated by the thin YEAR B. WELL N 2214 clay layer. Although well N5884 is officially listed as screened in the Magothy aquifer, it pumps water from Figure 19. Chloride concentrations and pumpage at two the upper glacial and the Magothy aquifers. The public-supply wells on Great Neck, N.Y., 1950-96: source of saltwater at this well is the well screen in the A. Well N687. B. Well N2214. (Data from Nassau upper glacial aquifer. County Department of Health, Nassau County Depart- ment of Public Works, and Great Neck Water Authority.) Other Saltwater Wedges

Two potential public-supply test wells, N2214 since 1972 (fig. 19B) but has not resulted in a N12580T and N12581T, were drilled at the Fresh decrease in chloride concentrations to below about Meadow Country Club, just south of the east end of 50 mg/L. Two observation wells on the western shore the peninsula. The borehole-geophysical logs of of the peninsula (N12076 and N12153, fig. 2) were these wells suggest several wedges of dense, drilled within wedge C. Well N12076 was screened in conductive ground water at the base of the upper the Lloyd aquifer above wedge C, but a sump was glacial aquifer resting upon a clay layer (fig. 12). The installed to bedrock to obtain an induction log through possible saltwater wedges increase in conductivity the entire thickness of the Lloyd aquifer. The with depth, rest upon confining units, and are within aquifers that are locally capped by clay layers. In the

36 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York absence of nearby landfills or heavy industry, 35 saltwater intrusion is a likely explanation, particularly because several golf-course wells 30 screened in the same zone were abandoned a few years earlier. Neither test well was completed as a 25 public-supply well because analysis of borehole- geophysical logs indicated saltwater intrusion. The 20 source of the conductive ground water is unknown because three shallow public-supply wells 15 (N7747-58, N7961-71, and N12555) at the south end of Manhasset Bay (fig. 2) have not indicated elevated 10 chloride concentrations. A sixth possible wedge of saltwater appears to 5 have begun to affect public-supply well N687, screened in the Lloyd aquifer 2,000 ft from the 0 19501960 1970 1980 1990 2000 southwestern coast (fig. 2). Pumpage and chloride A. WELL N 1298 graphs indicate that increased pumping of this well 50 during the 1980’s was followed by a rapid increase in chloride concentrations (fig. 19A). Similar trends in 45 pumpage, and chloride plots of six other public-supply 40 wells on the peninsula, indicate that a wedge of saltwater has begun to affect public-supply well N687. 35 No outpost wells have been installed in that area to 30 delineate this wedge, however. Public-supply well N1298 is the southernmost 25 well on Little Neck Bay (fig. 2). The chloride- CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER 20 breakthrough curve for this well indicates an increasing trend in chloride concentrations since the 15 1950’s (fig. 20A). This increase reflects the landward movement of the freshwater-saltwater interface (a 10 possible seventh wedge) toward the well. This well 5 1950 1960 1970 1980 1990 2000 typically is pumped during the summer, when water YEAR levels in the Lloyd aquifer are at their lowest. The B. WELL N 3443 Lloyd aquifer in this area may be in jeopardy of saltwater intrusion in the near future. Public-supply well N3443 is near the center of Figure 20. Chloride-breakthrough curves for two the peninsula and screened in the Lloyd aquifer. The public-supply wells screened in the Lloyd aquifer, chloride-breakthrough curve for this well also Great Neck, N.Y.: A. Well N1298, 1952-96. B. Well N3443,1951-96. (Locations are shown in fig. 2.) indicates an increase in chloride concentrations—from 5 to 10 mg/L in the 1950’s (fig. 20B) to the 30- to 40 mg/L range by 1995. This increase could have three explanations: (1) the toe of saltwater wedge B has begun to reach N3443; (2) the confining units above the Lloyd aquifer are more permeable in this area than on the rest of the affected the Lloyd aquifer there. The second peninsula; and (3) the annular seal surrounding the explanation is not supported by the drill-core data, casing may have failed. Although the first explanation which indicate the Raritan clay to be a substantial is possible, chloride concentrations at well N9776, confining unit. The third explanation seems the most 3,000 ft upgradient of N3443, are still between 5 to plausible. Drawdowns at N3443 during pumping 10 mg/L and, thus, indicate that wedge B has not reach 30 to 40 ft below sea level; thus, failure of the

Extent of Saltwater Intrusion 37 annular seal could allow water from the upper glacial Most of the Magothy aquifer has been aquifer to reach the well screen and could explain the completely removed in Great Neck by glacial erosion gradual increase in chloride concentrations and the except in the southernmost part and along the anomalous VOC contamination at the well. In southeastern coast of the peninsula. comparison, the maximum chloride concentration at The North Shore confining unit is a sequence of the other public-supply wells screened in the Lloyd Pleistocene-age clay and silt deposits that occur aquifer (N8342, N1618, and N9308, fig. 2) in 1994 locally along the northern parts of Queens, Nassau, was 13 mg/L, which is indicative of ambient and Suffolk Counties. Offshore glacial-lake deposits conditions in the aquifer. were correlated, through offshore seismic-reflection surveys, borehole-geophysical logs, and drill-core samples, with buried valleys that extend across the SUMMARY AND CONCLUSIONS Great Neck peninsula. These deposits infill two sharply defined buried valleys that truncate the The intrusion of saltwater toward several public- surrounding coarse-grained deposits. The valleys are supply wells within the Great Neck peninsula from the asymmetrical, steep sided, northwest-southeast surrounding embayments (Little Neck Bay, Long trending, and are 2-4 mi long and about 1 mi wide; Island Sound, and Manhasset Bay, N.Y.) prompted a they extend down to bedrock and are more than 200 ft cooperative study by the U.S. Geological Survey in below sea level. 1991-96 with the Nassau County Department of The upper glacial aquifer consists of various till, Public Works. The cooperative study collected hydrogeologic, geophysical and water-quality data, sand, gravel, silt and clay deposits underlain in some and delineated the subsurface geology and extent of areas by the North Shore confining unit. Offshore saltwater intrusion in Great Neck. Fifteen observation seismic-reflection surveys and drill-core data indicate wells were drilled during 1991-96, and continuous that the upper glacial aquifer does not extend offshore high-resolution seismic-reflection surveys were and does not infill the valleys beneath Manhasset Bay completed offshore in the embayments surrounding and Little Neck Bay, as proposed in previous studies. the Great Neck and Manhasset Neck peninsulas in The water table is in the upper glacial aquifer in all but 1993 and 1994. The seismic-reflection surveys were the southernmost part of Great Neck, where it is in the used to delineate the hydrogeologic framework Magothy aquifer. The water table is not tidally underlying Little Neck Bay, Long Island Sound, and influenced throughout Great Neck. Manhasset Bay. Borehole-geophysical logs (gamma, Ground water in the Lloyd and North Shore electric, and EM induction) were used to delineate aquifers is confined by the Raritan clay and North hydrogeologic units and the extent of saltwater Shore confining unit, respectively. Water-level data intrusion at selected observation wells on the Great from the tidal-effect studies indicate that both aquifers Neck peninsula. are hydraulically connected and are greatly affected by The new drill-core data led to the proposal of tides and public-supply pumping. two hydrogeologic units—the North Shore aquifer and The potentiometric surface of the Lloyd and the North Shore confining unit; these names are North Shore aquifers in August 1995 contained four defined for the first time in this report. The North large cones of depression throughout the peninsula Shore aquifer is a sequence of Pleistocene-age that reflect public-supply pumping. Potentiometric sediments that were penetrated during drilling in the levels measured in most wells were below sea level northernmost part of Great Neck. The Lloyd aquifer and extended over large parts of the peninsula and was not found in the northernmost part of the into the surrounding saltwater embayments. Water peninsula or offshore. levels ranged from 4 ft above to almost 40 ft below The Raritan clay overlies and confines ground sea level. Increased pumping in summer causes water in the Lloyd aquifer. The Raritan clay has been ground-water flow in the Lloyd and North Shore severely eroded or completely removed in many parts aquifers to reverse direction; that is, to flow inland of the peninsula by glacial scouring. A glacially from the surrounding saltwater embayments toward eroded buried valley appears to have incised the the pumping wells. This reversal of the normal Raritan clay from the northwestern corner to the seaward flow is the primary cause of past and present southeastern corner of Great Neck. saltwater intrusion in the peninsula.

38 Hydrogeology and Extent of Saltwater Intrusion of the Great Neck Peninsula, Great Neck, Long Island, New York Water from six public-supply wells and three aquifer. EM logs indicate that the wedge is about 20 ft observation wells screened in the upper glacial and thick at the base of the Lloyd. Magothy aquifers contained several VOC’s in Saltwater wedge C is in the Lloyd aquifer along concentrations greater than the New York State the western shore of Great Neck. It has affected Department of Health maximum contaminant levels public-supply wells N1926 and N2214 and private (MCL’s), and water from one public-supply well well N7613. Two observation wells were drilled (N3443) screened in the Lloyd aquifer also contained within wedge C—N12076 and N12153. The saltwater a VOC in concentrations above the State MCL’s. Water wedge is about 20 to 30 ft thick at the base of the from observation well N9098 contained benzene and Lloyd aquifer with an estimated chloride benzene derivatives, toluene, and xylene in concentration of about 8,000 mg/L at N12076 and concentrations of hundreds to thousands of times in 4,998 mg/L at N12153. excess of the State MCL’s. Saltwater wedge D is on the eastern shore of the Chloride concentrations in the Magothy and peninsula within the upper glacial aquifer; it is 30 ft upper glacial aquifers in the Great Neck peninsula thick and had a maximum chloride concentration of typically ranged from 10 to 50 mg/L. One exception is 141 mg/L. It has affected public-supply well N5884, at public-supply well N5884, in the southeastern part which has two screen zones—one in the upper glacial of the peninsula, where the peak chloride concentration aquifer and one in the underlying Magothy aquifer. was 163 mg/L in 1979 and 82 mg/L in 1995. Three The source of saltwater at this well is the well screen in the upper glacial aquifer. public-supply wells (N1926, N30, and N31) screened in the Lloyd aquifer have been shut down because the Two potential public-supply wells—N12580T chloride concentrations exceeded the State MCL of and N12581T—were drilled in the southeastern part of Great Neck. Interpretation of the geophysical 250 mg/L. Chloride concentrations at four observation logs suggests that this area may contain several wells screened in the Lloyd and North Shore aquifer saltwater wedges. (N12050, N12102, N12153, and N12013) exceeded the State MCL; concentrations were 13,748, 12,250, Other possible wedges of saltwater in the Lloyd aquifer appear to have begun to affect public-supply 4,998, and 737 mg/L, respectively. wells N687 and N1298, screened in the aquifer. Overpumping of the Lloyd, North Shore, and Pumpage and chloride-concentration graphs indicate upper glacial aquifers on Great Neck has caused that increased pumpage during the 1980’s was extensive saltwater intrusion. Four distinct wedge- followed by a rapid increase in chloride shaped areas of saltwater intrusion have been concentrations; this increase indicates that the identified in Great Neck; three have extended into the freshwater-saltwater interface is moving landward Lloyd and North Shore aquifers, and the fourth has toward the wells. extended into the upper glacial aquifer. Three other The chloride-breakthrough curve for public- areas, or wedges, of saltwater have been detected supply well N3443 indicates a gradual increase in through water-quality and geophysical-data analyses. chloride concentrations since the 1950’s; this increase The wedges decrease in thickness landward, and the may be due to a failure of the annular seal surrounding transition zones are about 10 ft thick. the casing. Saltwater wedge A, in the northernmost part of Great Neck, was encountered at the base of the North Shore aquifer. The wedge is about 125 ft thick at well REFERENCES CITED N12050 with a chloride concentration at the well screen of 13,750 mg/L. The chloride concentration of Baskerville, C.A., 1992, Bedrock and engineering geologic the Long Island Sound sample was 13,995 mg/L. maps of Bronx County and parts of New York and Queens Counties, New York: U.S. Geological Survey Saltwater wedge B was the cause of the Miscellaneous Map I-2003, 2 sheets, scale 1:24,000. shutdown and abandonment of public-supply wells Buxton, H.T., Soren, Julian, Posner, Alex, and Shernoff, N30 and N31, adjacent to observation well N12013. P.K., 1981, Reconnaissance of the ground-water The chloride concentration at well N30 was resources of Kings and Queens Counties, New York: 1,600 mg/L in 1978. Wedge B was penetrated at wells U.S. Geological Survey Open-File Report 81-1186, N12079 and N12013, screened at the base of the Lloyd 64 p.

References Cited 39 Chu, Anthony, and Stumm, Frederick, 1995, Delineation of Kilburn, Chabot, 1979, Hydrogeology of the Town of North the saltwater-freshwater interface at selected locations Hempstead, Nassau County, Long Island, New York: in Kings and Queens Counties, Long Island, New York, Mineola, N.Y. Nassau County Department of Public through use of borehole geophysical techniques, in Works, Long Island Water Resources Bulletin 12, 87 p. Geology of Long Island and Metropolitan New York, Kimmel, G.E., 1973, Change in potentiometric head in the April 22, 1995, Program with Abstracts: Stony Brook, Lloyd aquifer, Long Island, New York: U.S. Geological N.Y., Long Island Geologists, p. 21-30. 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