First posted September 17, 2020 For additional information, contact: Director, New York Water Science CenterU.S. Geological Survey425 Jordan RoadTroy, NY 12180–8349 The U.S. Geological Survey, in cooperation with the New York State Department of Environmental Conservation, used surface and borehole geophysical methods to delineate the freshwater/saltwater interface in coastal plain aquifers along the southwestern part of Long Island, New York. Over pumping of groundwater in the early 20th century combined with freshwater/saltwater interfaces at the coastline created saltwater intrusion in the upper glacial, Jameco, Magothy, and Lloyd aquifers. This study documents, for the first time, extensive saltwater intrusion of the Lloyd aquifer along the southwestern coast of Long Island, N.Y. Several public-supply wells in the southern parts of Nassau, Queens, and Kings Counties have been adversely affected by saltwater intrusion causing supply wells to be shutdown and abandoned. Due to the ongoing groundwater pumping in southern Nassau County, the freshwater/saltwater interface requires delineation and monitoring for any inland movement.In 2015–17, the U.S. Geological Survey collected time domain electromagnetic soundings at 12 locations and borehole electromagnetic induction conductivity logs at 9 wells within the study area to delineate several saltwater intrusion wedges. The upper glacial, Jameco, and Magothy aquifers were grouped into one aquifer complex within the study area to simplify interpretations. The coastal plain sediments increase in thickness from west to east and north to south because of their regional dip toward the southeast. Three separate wedges, shallow, intermediate, and deep, of saltwater intrusion were delineated in the upper glacial, Jameco, and Magothy aquifer complex. In addition, analysis of geophysical logs collected in an open borehole of a test well in southern Queens County in 1989 revealed the Lloyd aquifer was nearly completely intruded by saltwater with an estimated chloride concentration of 15,000 milligrams per liter. The geophysical logs from this well provides, for the first time, definitive proof of saltwater intrusion of the Lloyd aquifer on Long Island’s south shore, suggesting the freshwater/saltwater interface was at the coastline and not miles offshore as theorized by previous studies.

First posted May 25, 2023 For additional information, contact: Director, New York Water Science CenterU.S. Geological Survey425 Jordan RoadTroy, NY 12180–8349 Digital maps of bedrock elevation and overburden thickness (depth to bedrock) were constructed for the five boroughs of New York City by the U.S. Geological Survey, in cooperation with the New York City Department of Design and Construction, from a compilation of historical and newly acquired data. Raster surfaces were interpolated from a point database containing data from more than 14,000 locations collected from a variety of sources. These data were collected between 1905 and 2021. These maps were constructed to supplement existing tools for the evaluation of potential construction of geothermal heat pump technology for buildings in New York City.The bedrock underlying the study area ranges from easily weathered to very resistant to weathering. This differential susceptibility to erosion, along with numerous north-northwest-trending faults, is believed to control the shape of the bedrock surface. Glacial scouring of the bedrock during the Pleistocene Epoch is the most recent control on the topography of bedrock surfaces. Overburden thickness is an important consideration for evaluation and construction of geothermal systems.Bedrock-surface elevation ranges from about 360 feet above sea level (in central Staten Island and northern Bronx) to 1,200 feet below sea level (in southern Queens) (North American Vertical Datum of 1988). The overburden thickness ranges from 0 foot thick at surface outcrops on Staten Island, Manhattan, and the Bronx, to 1,280 feet thick in southeastern Queens.

A 1,200-foot deep well in southwestern Nassau County, Long Island, N.Y. was selected to evaluate the application of a nuclear magnetic resonance (NMR) logging tool. Technological advances in NMR borehole systems have allowed for reduced probe length and diameter, and focused measurement at specific diameters beyond the disturbed zone surrounding a well. This 3-inch-diameter NMR tool was specifically developed for use in deep 4-inch-diameter polyvinyl chloride cased wells common to Long Island. Selected intervals of the Magothy and Lloyd aquifers and the Raritan confining unit were logged for the evaluation. Unlike other petrophysical logs that respond to the rock matrix and fluid properties and are strongly dependent on mineralogy, NMR logs respond to the presence of hydrogen protons in the formation fluid to determine water fraction and pore-size distribution. NMR log analysis provided estimates of the clay-bound, capillary-bound, and mobile water fraction and hydraulic conductivity of aquifers and confining units penetrated by the well. NMR-estimated porosity and mobile water fraction for the Magothy aquifer (0.34 and 0.22 respectively), Magothy/Raritan(?) sand (0.35 and 0.30), Raritan confining unit (0.30 and 0.13), Raritan clay- and silt-rich lower part (0.23 and 0.01), and the Lloyd aquifer (0.27 and 0.19) was determined from the NMR log. conductivity was NMR-log the and sum of squared echoes equations with empirically derived constants for unconsolidated aquifers. Average hydraulic conductivity of the aquifer ft/d, the Raritan confining unit was 9.0 ft/d overall, the clay and silt-rich lower part was 0.24 ft/d, and the Lloyd aquifer was 56 ft/d. The sandy Magothy/Raritan(?) unit (not yet defined as a distinct unit) between the Magothy aquifer and the top of the Raritan confining unit had the highest hydraulic conductivity of 345 ft/d. The hydraulic-conductivity estimates from the NMR log analysis for the Magothy and Lloyd aquifers were consistent with published values and with the values estimated for the Lloyd aquifer from a specific-capacity test at the well site.

The U.S. Geological Survey used surface and borehole geophysical methods to delineate the freshwater-saltwater interface in coastal plain aquifers along the southwestern part of Long Island, New York. Overpumping of groundwater in the early 20th century combined with freshwater-saltwater interfaces at the coastline created saltwater intrusion in the upper glacial, Jameco, Magothy, and Lloyd aquifers. Our research indicates extensive saltwater intrusion of the Lloyd aquifer along the southwestern coast of Long Island, N.Y. Several public-supply wells in the southern parts of Nassau, Queens, and Kings Counties have been adversely affected by saltwater intrusion causing several supply wells to be shut down and abandoned. In 2015–17, the U.S. Geological Survey collected time domain electromagnetic soundings at 12 locations and borehole electromagnetic induction conductivity logs at 9 outpost wells within the study area to delineate several saltwater intrusion wedges. Three separate wedges (shallow, intermediate, and deep), of saltwater intrusion were delineated in the upper glacial, Jameco, and Magothy aquifer complex. In addition, analysis of geophysical logs collected in an open borehole of a test well in southern Queens County in 1989 revealed the Lloyd aquifer was nearly completely intruded by saltwater with an estimated chloride concentration of 15,000 milligrams per liter. This suggests the freshwater-saltwater interface was at the coastline and not miles offshore as theorized by previous studies.

Groundwater with chloride concentrations up to 15,000 mg/L has intruded the freshwater aquifer underlying southern Manhattan Island, New York. Historical (1940–1950) chloride concentration data of glacial aquifer wells in the study area indicate the presence of four wedges of saltwater intrusion that may have been caused by industrial pumpage. The limited recharge capability of the aquifer, due to impervious surfaces and the 22.7 million liters per day (mld) of reported industrial pumpage early in the 20th Century was probably the cause for the saltwater intrusion and the persistence of the historical saltwater intrusion wedges over time. Recent drilling of wells provided new information on the hydrogeology and extent of saltwater intrusion of the glacial aquifer overlying bedrock. The new observation wells provided ground-water level, chloride concentration, hydraulic conductivity, and borehole geophysical data of the glacial aquifer. The glacial sediments range in thickness from less than 0.3 m to more than 76.2 m within the study area. A linear relation between Electromagnetic-induction (EM) conductivity log response and measured chloride concentration was determined. Using this relation, chloride concentration was estimated in parts of the glacial aquifer where sampling was not possible. EM logging is an effective tool to monitor changes in saltwater intrusion wedges.

Flooding of streets and residential basements, and bacterial contamination of private-supply wells with Escherichia coli (E. coli) are recurring problems in the Rondout Valley near the Town of Wawarsing, Ulster County, New York. Leakage from the Rondout-West Branch (RWB) Water Tunnel and above-normal precipitation have been suspected of causing elevated groundwater levels and basement flooding. The hydrology of a 12-square-mile study area within the Town of Wawarsing was studied during 2008–10. A network of 41 wells (23 unconsolidatedaquifer and 18 bedrock wells) and 2 surface-water sites was used to monitor the hydrologic effects of four RWB Water Tunnel shutdowns. The study area is underlain by a sequence of northeast-trending sedimentary rocks that include limestone, shale, and sandstone. The bedrock contains dissolution features, fractures, and faults. Inflows that ranged from less than 1 to more than 9,000 gallons per minute from the fractured bedrock were documented during construction of the 13.5-foot-diameter RWB Water Tunnel through the sedimentary-rock sequence 710 feet (ft) beneath the studyarea valley. Glacial sediments infill the valley above the bedrock sequence and consist of clay, silt, sand, and gravel. The groundwater-flow system in the valley consists of both fractured-rock and unconsolidated aquifers. Water levels in both the bedrock and unconsolidated aquifers respond to variations in seasonal precipitation. During the past 9 years (2002–10), annual precipitation at Central Park, N.Y., has exceeded the 141-year mean. Potentiometric-surface maps indicate that groundwater in the bedrock flows from the surrounding hills on the east and west sides of the valley toward the center of the valley, and ultimately toward the northeast. On average, water levels in the bedrock aquifer had seasonal differences of 5.3 ft. Analysis of hydrographs of bedrock wells indicates that many of these wells are affected by the RWB Tunnel leakage. Tunnelleakage influences (water-level and temperature changes) in the bedrock aquifer were measured at distances up to 7,000 ft from the RWB Tunnel. Water levels in the bedrock changed as much as 12 ft within 0.5 hour during tunnel shutdowns. Nine of the 10 wells that responded to the shutdowns showed a water-level response of 5 ft or greater. Changes in water levels ranged from 1.5 to 12 ft, with tunnel-leakage influence delay times ranging from 0.5 to 60 hours. Many of the longest tunnel-influence delay times and smallest water-level changes were in wells located closest to the tunnel in shale. Tunnelinfluence response of the bedrock aquifer is consistent with its preliminary characterization as an anisotropic aquifer with greater transmissivity along bedding strike than across bedding strike. This tunnel-influence response is also consistent with the likely presence of discrete high-transmissivity networks along fractured limestone beds that have undergone dissolution. A lack of bedrock observation wells in half of the study area hampered a more thorough analysis of the extent of leakage from the RWB Tunnel in the study area. On average, water levels in the unconsolidated aquifer had a seasonal difference of 5 ft. Some unconsolidated-aquifer wells indicated water-level changes due to tunnel leakage. The locations of unconsolidated-aquifer wells with measurable water-level changes due to tunnel leakage correlated with those in the bedrock. Water levels in the unconsolidated aquifer changed as much as 2.5 ft within 18 hours of tunnel shutdowns, but water-level changes in some unconsolidatedaquifer wells were smaller or nonexistent. Introduction Seasonal flooding of streets and residential basements and bacterial contamination of private-supply wells with Escherichia coli (E. coli) are recurring problems in the Rondout Valley near the Town of Wawarsing, Ulster County, New York (fig. 1). Street flooding may be related to the low infiltration rates of local soils and ineffective stormwaterdrainage networks. The E. coli contamination of shallow 2 Effects of Shutdowns of the Rondout–West Branch Water Tunnel on the Groundwater-Flow System in Wawarsing, New York P O R T J E R V I S T R O U G H LON G IS LAN D SO UND Cannonsville Reservoir Pepacton Reservoir Neversink Reservoir Rondout Reservoir Wawarsing study area

Water-quality samples collected in an area prone to groundwater flooding in Wawarsing, New York, were analyzed and assessed to better understand the hydrologic system and to aid in the assessment of contributing water sources. Above average rainfall over the past decade, and the presence of a pressurized water tunnel that passes about 700 feet beneath Wawarsing, could both contribute to groundwater flooding. Water samples were collected from surface-water bodies, springs, and wells and analyzed for major and trace inorganic constituents, dissolved gases, age tracers, and stable isotopes. Distinct differences in chemistry exist between tunnel water and groundwater in unconsolidated deposits and in bedrock, and among groundwater samples collected from some bedrock wells during high head pressure and low head pressure of the Rondout–West Branch Tunnel. Samples from bedrock wells generally had relatively higher concentrations of sulfate (SO4 2-), strontium (Sr), barium (Ba), and lower concentrations of calcium (Ca) and bicarbonate (HCO3 -), as compared to unconsolidated wells. Differences in stable-isotope ratios among oxygen-18 to oxygen-16 (δ18O), hydrogen-2 to hydrogen-1 (δ2H), sulfur-34 to sulfur-32 (δ34S) of SO4 2-, Sr-87 to Sr-86 (87Sr/86Sr), and C-13 to C-12 (δ13C) of dissolved inorganic carbon (DIC) indicate a potential for distinguishing water in the Delaware–West Branch Tunnel from native groundwater. For example, 87Sr/86Sr ratios were more depleted in groundwater samples from most bedrock wells, as compared to samples from surface-water sources, springs, and wells screened in unconsolidated deposits in the study area. Age-tracer data provided useful information on pathways of the groundwater-flow system, but were limited by inherent problems with dissolved gases in bedrock wells. The sulfur hexafluoride (SF6) and (or) chlorofluorocarbons (CFCs) apparent recharge years of most water samples from wells screened in unconsolidated deposits and springs ranged from 2003 to 2010 (current) and indicate short flow paths from the point of groundwater recharge. All but three of the samples from bedrock wells had interference problems with dissolved gases, mainly caused by excess air from degassing of hydrogen sulfide and methane. The SF6 and (or) CFC apparent recharge years of samples from three of the bedrock wells ranged from the 1940s to the early 2000s; the sample with the early 2000s recharge year was from a flowing artesian well that was chemically similar to water samples collected at the influent to the tunnel at Rondout Reservoir and the most hydraulically responsive to water tunnel pressure compared to other bedrock wells. Data described in this report can be used, together with hydrogeologic data, to improve the understanding of source waters and groundwater-flow patterns and pathways, and to help assess the mixing of different source waters in water samples. Differences in stable isotope ratios, major and trace constituent concentrations, saturation indexes, tritium concentrations, and apparent groundwater ages will be used to estimate the proportion of water that originates from Rondout– West Branch Tunnel leakage.

Advanced borehole-geophysical methods were used to assess the geohydrology of fractured crystalline bedrock in 31 of 64 boreholes on the southern part of Manhattan Island, NY in preparation of the construction of a new water tunnel. The study area is located in a highly urbanized part of New York City. The boreholes penetrated gneiss, schist, and other crystalline bedrock that has an overall southwest-to northwest-dipping foliation. Most of the fractures intersected are nearly horizontal or have moderate- to high-angle northwest or eastward dip azimuths. Heat-pulse flowmeter logs obtained under nonpumping ( ambient) and pumping conditions, together with other geophysical logs, delineated transmissive fracture zones in each borehole. Water-level and flowmeter data suggest the fractured-rock ground-water-flow system is interconnected. The 60 MHz directional borehole-radar logs delineated the location and orientation of several radar reflectors that did not intersect the projection of the borehole. A total of 53 faults intersected by the boreholes have mean orientation populations of N12 degrees W, 66 degrees W and N11 degrees W, 70 degrees E. A total of 77 transmissive fractures delineated using the heat-pulse flowmeter have mean orientations of N11 degrees E, 14 degrees SE ( majority) and N23 degrees E, 57 degrees NW ( minority). The transmissivity of the bedrock boreholes ranged from 0.7 to 870 feet squared (ft(2)) per day (0.07 to 81 metres squared (m(2)) per day).

Advanced borehole-geophysical techniques were used to assess the geohydrology of crystalline bedrock in 26 of 29 boreholes on the southern part of Manhattan Island, N.Y., in preparation for construction of a third water tunnel for New York City. The borehole-logging techniques included natural gamma, single-point resistance, short-normal resistivity, mechanical and acoustic caliper, magnetic susceptibility, borehole-fluid temperature and resistivity, borehole-fluid specific conductance, dissolved oxygen, pH, redox, heat-pulse flowmeter (at selected boreholes), borehole deviation, acoustic and optical televiewer, and borehole radar (at selected boreholes). Hydraulic head and specific-capacity test data were collected from 29 boreholes. The boreholes penetrated gneiss, schist, and other crystalline bedrock that has an overall southwest to northwest-dipping foliation. Most of the fractures penetrated are nearly horizontal or have moderate- to high-angle northwest or eastward dip azimuths. Fracture population dip azimuths are variable. Heat-pulse flowmeter logs obtained under pumping and nonpumping (ambient) conditions, together with other geophysical logs, indicate transmissive fracture zones in each borehole. The 60-megahertz directional borehole-radar logs delineated the location and orientation of several radar reflectors that did not intersect the projection at nine selected boreholes. Fracture indexes range from 0.12 to 0.93 fractures per foot of borehole. Analysis of specificcapacity tests from each borehole indicated that transmissivity ranges from 1 to 459 feet squared per day..

Introductory paragraph for this paper is available only in the PDF and GZipped PS files

Introductory paragraph for this paper is available only in the PDF and GZipped PS files