Ronald J. Busciolano | USGS Science

USGS2009

Water-Table and Potentiometric-Surface Altitudes in the Upper Glacial, Magothy, and Lloyd Aquifers beneath Long Island, New York, March-April 2006

The U.S. Geological Survey (USGS), in cooperation with State and local agencies, systematically collects ground-water data at varying measurement frequencies to monitor the hydrologic situation on Long Island, New York. Each year during March and April, the USGS conducts a synoptic survey of hydrologic conditions 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. These data and the maps constructed from them are commonly used in studies of Long Island's hydrology, and by water managers and suppliers for aquifer management and planning purposes. Water-level measurements made in 502 wells across Long Island during March-April 2006, were used to prepare the maps in this report. Measurements were made by the wetted-tape method to the nearest hundredth of a foot. Water-table and potentiometric-surface altitudes in these aquifers were contoured using these measurements. The water-table contours were interpreted using water-level data collected from 341 wells screened in the upper glacial aquifer and (or) shallow Magothy aquifer; the Magothy aquifer's potentiometric-surface contours were interpreted from measurements at 102 wells screened in the middle to deep Magothy aquifer and (or) contiguous and hydraulically connected Jameco aquifer; and the Lloyd aquifer's potentiometric-surface contours were interpreted from measurements at 59 wells screened in the Lloyd aquifer or contiguous and hydraulically connected North Shore aquifer. Many of the supply wells are in continuous operation and, therefore, were turned off for a minimum of 24 hours before measurements were made so that the water levels in the wells could recover to the level of the potentiometric head in the surrounding aquifer. 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. In this report, all water-level altitudes are referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29).

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USGS2005

Statistical analysis of long-term hydrologic records for selection of drought-monitoring sites on Long Island, New York

Ground water is the sole source of water supply for more than 3 million people on Long Island, New York. Large-scale ground-water pumpage, sewering systems, and prolonged periods of below-normal precipitation have lowered ground-water levels and decreased stream-discharge in western and central Long Island. No method is currently (2004) available on Long Island that can assess data from the ground-water-monitoring network to enable water managers and suppliers with the ability to give timely warning of severe water-level declines. This report (1) quantifies past drought- and human-induced changes in the ground-water system underlying Long Island by applying statistical and graphical methods to precipitation, stream-discharge, and ground-water-level data from selected monitoring sites; (2) evaluates the relation between water levels in the upper glacial aquifer and those in the underlying Magothy aquifer; (3) defines trends in stream discharge and ground-water levels that might indicate the onset of drought conditions or the effects of excessive pumping; and (4) discusses the long-term records that were used to select sites for a Long Island drought-monitoring network. Long Island’s long-term hydrologic records indicated that the available data provide a basis for development of a drought-monitoring network. The data from 36 stations that were selected as possible drought-monitoring sites—8 precipitation-monitoring stations, 8 streamflow-gaging (discharge) stations, 15 monitoring wells screened in the upper glacial aquifer under water-table (unconfined) conditions, and 5 monitoring wells screened in the underlying Magothy aquifer under semi-confined conditions—indicate that water levels in western parts of Long Island have fallen and risen markedly (more than 15 ft) in response to fluctuations in pumpage, and have declined from the increased use of sanitary- and storm-sewer systems. Water levels in the central and eastern parts, in contrast, remain relatively unaffected compared to the western parts, although the effects of human activity are discernible in the records. The value of each site as a drought-monitoring indicator was assessed through an analysis of trends in the records. Fifty-year annual and monthly data sets were created and combined into three composite-average hydrographs—precipitation, stream discharge, and ground-water levels. Three zones representing the range of human effect on ground-water levels were delineated to help evaluate islandwide hydrologic conditions and to quantify the indices. Data from the three indices can be used to assess current conditions in the ground-water system underlying Long Island and evaluate water-level declines during periods of drought.

Scientific Investigations Report↗