The U.S. Geological Survey (USGS) is providing online maps of water-table and potentiometric-surface altitude in the upper glacial, Magothy, Jameco, Lloyd, and North Shore aquifers on Long Island, New York, April-May 2016. Also provided is a depth-to-water map for Long Island, New York, April-May 2016. The USGS makes these maps and geospatial data available as REST Open Map Services (as well as HTTP, JSON, KML, and shapefile), so end-users can consume them on mobile and web clients. A companion report, U.S. Geological Survey Scientific Investigations Map 3398 (Como and others, 2018; https://doi.org/10.3133/sim3398) further describes data collection and map preparation and presents 68x22 in. Portable Document Form (PDF) versions, 4 sheets, scale 1:125,000. The USGS, in cooperation with State and local agencies, systematically collects groundwater data at varying measurement frequencies to monitor the hydrologic conditions on Long Island, New York. Each year during April and May, the USGS completes a synoptic survey of water levels to define the spatial distribution of the water table and potentiometric surfaces within the three main water-bearing units underlying Long Island -- the upper glacial, Magothy, and Lloyd aquifers (Smolensky and others, 1989) -- and the hydraulically connected Jameco (Soren, 1971) and North Shore aquifers (Stumm, 2001). These data and the maps constructed from them are commonly used in studies of the hydrology of Long Island and are used by water managers and suppliers for aquifer management and planning purposes. Sheets 1-4 in U.S. Geological Survey Scientific Investigations Map 3398 (Como and others, 2018; https://doi.org/10.3133/sim3398) were prepared using water-level data measured at 424 groundwater monitoring wells (observation and supply) and 15 streamgages across Long Island during April and May of 2016. Additionally, digital datasets were derived from the water-level observations that include (1) contour lines and a continuous raster of the depth to water table in the upper glacial and Magothy aquifers, (2) contour lines of the potentiometric surface in the middle to deep Magothy aquifer and the hydraulically connected Jameco aquifer, (3) contour lines of the potentiometric surface in the Lloyd aquifer and hydraulically connected North Shore aquifer, and (4) point feature classes for the 424 groundwater-monitoring wells and 15 streamgages where water levels were collected. Data Sources Water-level measurements made in 424 monitoring wells (observation and supply wells), 13 streamgages, and 2 lake gages across Long Island during April-May 2016 were used to prepare the maps in this report. Groundwater measurements were made by the wetted-tape or electric-tape method to the nearest hundredth of a foot. Many of the supply wells are in continuous operation and, therefore, were turned off for a minimum of 24 hours before measurements were made to allow the water levels in the wells to recover to ambient (nonpumping) conditions. Full recovery time at some of these supply wells can exceed 24 hours; therefore, water levels measured at these wells are assumed to be less accurate than those measured at observation wells, which are not pumped (Busciolano, 2002). In addition to pumping stresses, density differences (saline water) also lower the water levels measured in certain wells (Lusczynski, 1961). In this report, all water-level altitudes are referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29). Analysis Contours of water-table and potentiometric-surface altitudes were created using the groundwater measurements. The water-table contours were interpreted using water-level data collected from 13 streamgages, 2 lake gages, 275 observation wells, and 1 supply well screened in the upper glacial aquifer or the shallow Magothy aquifer. The potentiometric-surface contours of the Magothy aquifer were interpreted from measurements at 88 wells (61 observation wells and 27 supply wells) screened in the middle to deep Magothy aquifer and the contiguous and hydraulically connected Jameco aquifer. The potentiometric-surface contours of the Lloyd aquifer were interpreted from measurements at 60 wells (55 observation wells and 5 supply wells) screened in the Lloyd aquifer and the contiguous and hydraulically connected North Shore aquifer. Editing of the geospatial data was done in ArcGIS Desktop (Esri, 2015a) using geographic information system (GIS) tools. Depth to water map A GIS was used to create a continuous surface of the water table using an iterative finite-difference interpolation technique (Esri, 2015b) with measurements from 13 streamgages, 2 lake gages, 275 observation wells, 1 supply well, interpreted 10-foot (ft) contour intervals, and the coastline. The land surface altitude, or topography, was obtained from National Oceanic and Atmospheric Administration (2016). The data were collected using light detection and ranging (lidar) and were used to produce a three-dimensional digital elevation model (DEM). The lidar data have a horizontal accuracy of 1.38 ft and a vertical accuracy of 0.40 ft at a 95-percent confidence level for the "open terrain" land-cover category. The DEM was developed jointly by the National Oceanic and Atmospheric Administration and the U.S. Geological Survey as part of the Disaster Relief Appropriations Act of 2013 (Pub.L. 113-2, H.R. 152, 127 Stat. 4). Land surface altitude is referenced to the North American Vertical Datum of 1988 (NAVD 88). On Long Island, NAVD 88 is approximately 1 foot higher than NGVD 29. The continuous surface of the water table was adjusted for the vertical datum differences across Long Island. This surface was then subtracted from the land surface elevation (from the DEM) at the same location. The results are shown as a continuous depth to water-table map. References Cited Busciolano, Ronald, 2002, Water-table and potentiometric-surface altitudes of the upper glacial, Magothy, and Lloyd aquifers on Long Island, New York, in March-April 2000, with a summary of hydrogeologic conditions: U.S. Geological Survey Water-Resources Investigations Report 01-4165, 17 p., 6 pl. Como, M.D., Finkelstein, J.S., Simonette L. Rivera, Monti, Jack, Jr., and Busciolano, Ronald, 2017, Water-table and potentiometric-surface altitudes in the upper glacial, Magothy, and Lloyd aquifers of Long Island, New York, April-May 2016: U.S. Geological Survey Scientific Investigations Map 3398, 4 sheets, scale 1:125,000, 6-p. pamphlet, https://doi.org/10.3133/sim3398. Esri, 2015a, ArcMap 10.3.1: Redlands, Calif., Esri, software. Esri, 2015b, Topo to Raster Tool: Redlands, Calif., Esri, software. Lusczynski, N.J., 1961, Head and flow of ground water of variable density: Journal of Geophysical Research, v. 66, no. 12, p. 4247-4256. National Oceanic and Atmospheric Administration, 2016, NCEI Hurricane Sandy digital elevation models: National Centers for Environmental Information, digital data, accessed January 8, 2016, at https://www.ngdc.noaa.gov/mgg/inundation/sandy/sandy_geoc.html Smolensky, D.A., Buxton, H.T., and Shernoff, P.K., 1989, Hydrologic framework of Long Island, New York: U.S. Geological Survey Hydrologic Investigations Atlas HA-709, 3 sheets, scale 1:250,000. Soren, Julian, 1971, Results of subsurface exploration in the mid-island area of western Suffolk County, Long Island, New York: Oakdale, N.Y., Suffolk County Water Authority, Long Island Resources Bulletin 1, 60 p. Stumm, Frederick, 2001, Hydrogeology and extent of saltwater intrusion of the Great Neck peninsula, Great Neck, Long Island, New York: U.S. Geological Survey Water-Resources Investigations Report 99-4280, 41 p.