Introduction | Location and Physical Setting | Freshwater | State of the Aquifer System | Interactive Content | References

One of the most important concepts to understand is that volumes of water pumped from a groundwater system must come from somewhere and must cause a change in the groundwater system. Another important concept is that water table aquifers are hydraulically connected to the streams that drain them. Therefore, pumping water from aquifers that are hydraulically connected with surface-water bodies can have a significant effect on those bodies by reducing groundwater discharges to surface water and possibly causing outflow from those bodies into the groundwater system. Thus, an evaluation of groundwater management strategies needs to involve consideration of surface-water resources, including closely related biological resources.

A key feature of Long Island’s groundwater system is the large volume of groundwater in storage, which allows the possibility of using aquifers for temporary storage, that is, managing inflow and outflow of groundwater in storage in a manner similar to surface-water reservoirs (Alley and others, 1999). The groundwater reservoir of Long Island is a wedged shaped mass of saturated unconsolidated deposits that overlie nearly impermeable consolidated bedrock and attain a maximum thickness of about 2,000 feet. The boundaries of the fresh groundwater reservoir are the water table, the fresh-salt water interfaces, the bedrock surface, and the streams. The estimated volume of material saturated with fresh groundwater is about 180 cubic miles, and an estimated 10-20 trillion gallons of freshwater would drain from these deposits if they could be completely dewatered (Franke and others, 1972).

From the standpoint of water use and water management, all groundwater is not equal--the suitability of water, as measured by its quality, is a key consideration in developing water-management strategies. Furthermore, determining water suitability (or unsuitability) requires detailed information on the three-dimensional distribution and concentrations of potential contaminants, both naturally occurring contaminants and those resulting from human activities (Alley and others, 1999).

Continued large withdrawals of water from an aquifer often result in undesirable consequences. From a management standpoint, water managers, stakeholders, and the public must decide the specific conditions under which the undesirable consequences can no longer be tolerated.

The effects of groundwater development may require many years to become evident. Thus, there is an unfortunate tendency to forego the data collection and analysis that is needed to support informed decision making until well after problems materialize (Alley and others, 1999).

  • Water Availability

      The foundation of any groundwater analysis, including those analyses whose objective is to propose and evaluate alternative management strategies, is the availability of high-quality data. Some, such as precipitation data, are generally available and relatively easy to obtain at the time of a hydrologic analysis. Other data and information, such as geologic and hydrogeologic maps, can require years to develop. Still other data, such as a history of water levels in different parts of groundwater systems, require foresight in order to obtain measurements over time, if they are to be available at all. Thus, a key starting point for assuring a sustainable future for any groundwater system is development of a comprehensive hydrogeologic database over time. These data ideally, should include depths and thicknesses of hydrogeologic units from lithologic and geophysical well logs, synoptic and historic water-level measurements to allow construction of pre-development water-level maps for major aquifers (as well as water-level maps at various times during development), groundwater quality analyses to document pre-development and post-development water quality, and simultaneous measurements of streamflow and stream quality during low flows to indicate possible impacts of discharging groundwater to surface-water quality (Alley and others, 1999).

    • Precipitation

        The National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center (NCDC) offers several types of climate information generated from examination of the data in the archives. These types of information include record temperatures, record precipitation and snowfall, climate extreme statistics, and other derived climate products. A collection of statistical weather and climate information tools developed and hosted by the NCDC are available. The Climate Data Online is a wonderful resource to retrieve climate products.

        The Global Historical Climatology Network (GHCN) is an integrated database of climate summaries from land surface stations across the globe. There are 344 climate divisions in the contiguous United States. A climate division area for Long Island NY was selected to conveniently represent annual time scale of the following data sets: Temperature, Precipitation and Palmer Hydrological Drought Index (PHDI) (figures 16A-C). This is an interactive tool providing historical information on precipitation and temperature for selected places, from cities to states to climate regions to the contiguous United States. A menu-driven system provides the history and trend for each place. Annual data can be further broken down by month and by season.

        The coastal region of Long Island from 1901-2000 had an estimated average precipitation of 44.35 inches, however during the 2005-2010 index period the estimated average precipitation was 48.38 inches.

        Long Island, New York Temperatures.
        Long Island, New York Precipitation.
        Long Island, New York Palmer Hydrological Drought Index (PHDI).

        Figure 16A-C: Graphs of temperature (A), Precipitation (B) and Palmer Hydrological Drought Index (C)
        for Long Island climate division (click to modify time period).

        Note: the Palmer Hydrological Drought Index (PHDI) is the monthly value (index) generated monthly that indicates the severity of a wet or dry spell. This index is based on the principles of a balance between moisture supply and demand. Man-made changes such as increased irrigation, new reservoirs, and added industrial water use were not included in the computation of this index. The index generally ranges from -6 to +6, with negative values denoting dry spells, and positive values indicating wet spells.

        Cooperative Observer Network (COOP) Climate stations have more than 10,000 volunteers take daily weather observations at National Parks, seashores, mountaintops, and farms as well as in urban and suburban areas. Weather records from climate stations were downloaded from http://www.ncdc.noaa.gov/cdo-web/datatools/findstation for Long Island. Monthly summaries, at selected climate stations, were selected to provide a sense of the variability of precipitation amounts, temporally and geographically.

        Long Island, New York Climate Stations.

        Figure 17: Locations of selected Cooperative Observer Network (COOP) on Long Island.

        The recorded monthly summaries from 1931 to 2010, at the selected climate stations, ranged between over 67 inches in 1996 at Bridgehampton to less than 23 inches in 1965 at both Mineola and New York's LaGuardia airport (figure 17). Monthly precipitations varied from over 20 inches in the October, 2005 at Riverhead farm to less than 0.5 inches at all of the selected station and occurred over 20 times, throughout the period.

        NOAA provides many forms of climatic data and Data tools to access the data collected. One example, the 1981-2010 Climate Normals are NCDC's latest three-decade averages of climatological variables, including temperature and precipitation. On Long Island, a selected Islip climate station at Islip, NY represents this 1981-2010 climate normals shown in figure 18. Monthly summaries from major airport weather stations that include a daily account of temperature extremes, degree days, precipitation and winds are also available through NOAA's Local Climatological Data Publication series at http://www.ncdc.noaa.gov/IPS/lcd/lcd.html.

        Islip, New York Climate Monthly Climate Normals (1981-2010).

        Figure 18: Monthly climate normals for precipitation and temperature (1981-2010) near Islip area, NY.
        Source: http://www.nws.noaa.gov/climate/xmacis.php?wfo=okx

        The 1981-2010 Climate Normals index period coincides with the gridded data product from the PRISM Climate Group which gathers climate observations from a wide range of monitoring networks, applies sophisticated quality control measures, and develops spatial climate datasets to reveal short and long-term climate patterns.

        Source: http://www.prism.oregonstate.edu

        These gridded datasets are often used in a Geographic Information System (GIS) for analysis or as input needed in numerical models. The National Weather Service (NWS) National Digital Forecast Database (NDFD) consists of gridded forecasts of sensible weather elements (e.g., cloud cover, maximum temperature). These data are used in an interactive map which references the prism datasets and can view monthly precipitation amounts under observed, normal, departure from normal, and percent from normal conditions. The Northeast River Forecast Center also displays departures from normal and precipitation averages for each county.

        The USGS National Climate Change Viewer (NCCV), is a web application used for visualizing climate change for the Continental US at the state, county and watershed scale. Last year NASA released the NEX-DCP30 dataset, which statistically downscales air temperature and precipitation from CMIP5 global models to a high resolution 30-arcsecond grid. The USGS-NCCV creators have used the air temperature and precipitation data from the 30 CMIP5 models as input to a simple water-balance model to simulate changes in the surface water balance over the historical and future time periods. Combining the climate data with the water balance data in the NCCV provides further insights into the potential for climate-driven change in water resources.

        A USGS NCCV portal is available to assist the science community in conducting studies of climate change impacts at local to regional scales, and to enhance public understanding of possible future climate patterns and climate impacts at the scale of individual neighborhoods and communities.

    • NWIS the USGS Data Archive

        As part of the U.S. Geological Survey's (USGS) program for disseminating water data within USGS, to USGS cooperators, and to the general public, the USGS maintains a distributed network of computers and fileservers for the acquisition, processing, review, and long-term storage of water data. This water data is collected at over 1.5 million sites around the country and at some border and territorial sites. This distributed network of computers is called the National Water Information System (NWIS). Many types of data are stored in NWIS, including comprehensive information for site characteristics, well-construction details, time-series data for gage height, streamflow, groundwater level, precipitation, and physical and chemical properties of water. Additionally, peak flows, chemical analyses for discrete samples of water, sediment, and biological media are accessible within NWIS.

        The USGS shares these data within NWIS with the public through the USGS Water Data for the Nation Site. On this web site, data are displayed as graphs, in tables, or on maps. Data files can also be downloaded. Water Data for New York is a subset of Water Data for the Nation emphasizing New York’s water-resources data. One can search sites with data or, by category, to investigate current conditions, view site information, or use an interactive map which has symbolized site locations by station type, surface water, groundwater, springs, atmospheric or other site type.

        The most popular data queried on the Water Data for New York website include:

        Current streamflow conditions — Real-time data.
        Water levels in wells — Some water levels are available in real-time.
        Water quality of streams and groundwater — Some real-time data are available.

      • Surface Water - Streamflow

          Surface water current conditions are based on the most recent data from on-site automated recording equipment. Measurements are commonly recorded at a fixed interval of 15 to 60 minutes and transmitted by satallite uplink or telephone telemetry to the USGS every hour. Values may include "Approved" (quality-assured data that may be published) and/or more recent "Provisional" data (of unverified accuracy and subject to revision).

          Out of about 250 stream sites in New York, there are 9 stream sites on Long Island that provide current data collection, which is mostly provisional. This current data collection network of streams can be found by clicking on this link, streamflow current conditions or viewed in a map interface to download data. Another interactive map known as "Water Watch" uses the data collected at these stream sites to symbolize and rank the current flow conditions based on each stations historical data collection.

          Manual measurements of streamflow and gage height are used to supplement and (or) verify the accuracy of the automatically recorded observations, as well as to compute streamflow based on gage height. There are over 100 partial record stream sites with manual measurements, about 25 stream sites with annual maximum instantaneous peak streamflow and gage height data, and around 10 stream sites with historical instantaneous data on Long Island.

          Statistics are computed from approved daily mean data at each site. Statistical summaries of approved historical daily values for daily, monthly, and annual (water year or calendar year) time periods are also available. There are over 22 stream sites with daily, monthly, or annual streamflow statistics available throughout Long Island (figure 19).

          Long Island Streams and Average Flows for 1940-1965.

          Figure 19: Map of stream gaging locations with average annual streamflow for the entire period of record, on Long Island, New York.
          (click figure for interactive map and data)
      • Groundwater Levels

          Water-level measurements from observation wells are the principal source of information about the hydrologic stresses acting on aquifers and how these stresses affect groundwater recharge, storage, and discharge (Taylor and Alley, 2001). Water-level measurements are made by many Federal, State, and local agencies.

          The Active Groundwater Level Network contains water levels and well information from more than 20,000 wells that have been measured by the USGS or USGS cooperators at least once within the past 13 months. These wells are measured for a variety of disparate purposes, such as statewide monitoring programs, or more local effects like monitoring well drawdown, hydrologic research, aquifer tests, or even earthquake effects on water levels.

          Source: http://groundwaterwatch.usgs.gov/statemap.asp?sc=36&sa=NY

          On Long Island and in the five boroughs of New York City the USGS operates over 600 active groundwater data-collection stations that provide long-term, accurate, and unbiased information that meets the needs of many diverse users. Details of the current groundwater monitoring on Long Island and the five boroughs of New York City are available at http://ny.water.usgs.gov/projects/LI_PRJ/.

          Groundwater Watch.

          Figure 20: An example map of groundwater watch locations for Nassau County, New York.
          (click http://ny.water.usgs.gov/projects/LI_PRJ/ and choose “Project Data” for other Counties)

          These "groundwater watch" web pages group related wells and data from these active well networks, and provide basic statistics about the water-level data collected by USGS water science centers for Cooperative Programs, for Federal Programs, and from data supplied to us by our customers through cooperative agreements. USGS Groundwater Watch is maintained by the Office of Groundwater, and more groundwater watch web pages are available at http://groundwaterwatch.usgs.gov/ (figure 20). In addition to the active groundwater level network, all groundwater watch web pages for New York are available here: "Real-Time Groundwater Level Network" , "Below Normal Groundwater Levels" , "Long-Term Groundwater Data Network" or "Climate Response Network."

          The groundwater database within NWIS contains records from about 850,000 wells Nationwide that have been compiled during the course of groundwater hydrology studies over the past 100 years. Information from these wells is served via the Internet through the National Water Information System Web Interface, NWISWeb. NWISWeb provides all USGS groundwater data that are approved for public release. This large number of sites is excellent for some uses, but complicates retrievals when the user is interested in specific networks, or wells in an active water-level measurement program. The National Water Information System Web Interface, NWISWeb provides all USGS groundwater data that are approved for public release. Groundwater current conditions are based on the most recent data from on-site automated recording equipment. Measurements are commonly recorded at a fixed interval of 15 to 60 minutes and transmitted by satellite uplink or telephone to the USGS every hour. Values may include "Approved" (quality-assured data that may be published) and/or more recent "Provisional" data (of unverified accuracy and subject to revision).

          Out of about 100 sites in New York, there are over 10 sites on Long Island that provide current data collection, which is mostly provisional. The data can be found by clicking on this link groundwater current conditions or viewed in a map interface to download data.

          Historical instantaneous groundwater data from these sites are also available. A summary of all data for each day (daily data) for the period of record and may represent the daily mean, median, maximum, minimum, and/or other derived value is available at about 50 groundwater sites on Long Island. Values may include "Approved" (quality-assured data that may be published) and/or more recent "Provisional" data (of unverified accuracy and subject to revision).

          Manual measurements of depth to water in wells are used to supplement and (or) verify the accuracy of the automatically recorded observations. There are over 4,300 groundwater sites on Long Island that can be selected which have at least one water level measurement (depth to water measurement).

          Statistics are computed from approved daily mean data at each site. Statistical summaries of approved historical daily values for daily, monthly, and annual (water year or calendar year) time periods are also available. There are around 50 groundwater sites on Long Island, with daily, monthly, or annual statistics of the measured groundwater elevation in feet above NGVD 1929.

        • Water Table and Surface Maps

    • Water Use

        The U.S. Geological Survey's National Water-Use Information Program (NWUIP) is responsible for compiling and disseminating the nation's water-use data. The USGS works in cooperation with local, State, and Federal environmental agencies to collect water-use information. USGS compiles these data to produce water-use information aggregated at the county, state, and national levels. Every five years, data at the county level are compiled into a national water-use data system and state-level data are published in a national circular. Over the history of these circulars, the water-use categories have had some changes.

        Data from these national circulars are represented as an interactive map interface from 1985-2005 at 5 year intervals. A table of water use data for each county on Long Island can be viewed.

        Source: TABLE 1985-2005 NWUIP DATA USED

        Interactive map interface of 5-yr water use data:

        Interactive Map - 1985
        Interactive Map - 1990
        Interactive Map - 1995
        Interactive Map - 2000
        Interactive Map - 2005
        Interactive Map - 2010

      • Public Water Supply Districts

          Public water supply refers to water withdrawn by public and private water suppliers that provide water to at least 25 people or have a minimum of 15 connections. Public-supply water is delivered to users for domestic, commercial, and industrial purposes, and also is used for public services and system losses. In New York State, the law requires any water withdrawal system with a capacity of 100,000 gallons per day or more to obtain a permit and send annual reports to the New York State Department of Environmental Conservation (NYSDEC).

          Between 70-80% of all the groundwater withdrawn beneath Long Island has been withdrawn for public supply usage for the 1985-2005 period. The responsibility of the water supply companies in Nassau and Suffolk Counties are shared between over 50 supply companies who are members of the Long Island Water Conference. These companies utilized over 1,100 large capacity wells to supply potable water to a population of over 2.6 million, and to light industries, such as office parks and other commercial business. The figure below represents a generalized distribution of supply wells that pumped water between 2007 and 2010, and are considered as active supply wells (figure 22).

          An interactive map of the water supply company service areas shows the generalized outline of the individual supply company service areas on Long Island. Withdrawal data reported to the NYSDEC, by law, was compiled to represent the most recent year’s average withdrawals over a calendar year. Over 415 Mgal/d total average annual withdrawal was reported for the 2010 calendar year. There were some supply areas where the 2010 withdrawal data was not available (at this time); however previous years withdrawals may give a rough estimate of the average withdrawals for that area. When 2010 withdrawal data were available an aquifer breakdown is shown as a pie chart. Island wide over 75% of the average 2010 groundwater withdrawals were from the Magothy aquifer.

          Long Island Pumping Wells.

          Figure 22: Generalized location of over 1,100 large capacity supply wells used since 2007.

          The trends in public supply withdrawals tend to follow population trends. The bar graph below represents the withdrawals reported to the NYSDEC from 1988-2010, in Nassau and Suffolk Counties (figure 23). There are instances when withdrawals are not reported, and are not represented in these annual totals; however these amounts are presumed to be a small percentage of the total public-supply withdrawals. The public supply annual withdrawal average from 2005 to 2010 was about 380 Mgal/d for Nassau and Suffolk Counties.

          Annual Pumpage for Nassau and Suffolk Counties.

          Figure 23: Graph of annual withdrawals for Nassau and Suffolk Counties, averaged daily from 1988-2010 (Source: New York State Department of Conservation).

          Seasonal reported withdrawals from 1988-2009 in Nassau County is represented in the bar graphs below (figure 24); the data used was provided by the Nassau County Department of Public Works. The monthly variability shows a cyclical pattern with the summer months average withdrawal rates ranging from 220 to over 340 Mgal/d; and the winter months ranging from 130 to 150 Mgal/d. The seasonal trend lines indicate a decreasing rate of withdrawal in the winters and an increasing rates of withdrawals in the summers, from 1988-2009.

          Monthly Withdrawal Rates for Nassau County (1988-2009).

          Figure 24: Graph of Nassau County total average monthly withdrawals, averaged daily from 1988-2009 (Source: Nassau County Department of Public Works).

          Seasonal reported withdrawals from 1988-2009 in Suffolk County represented in the bar graphs below (figure 25); the data used was provided by the Suffolk County Water Authority which serves groundwater to approximately 1.2 million people or 80 percent of the Suffolk County’s population. The monthly variability shows a cyclic pattern with the summer months average withdrawal rates ranging between 160 to over 360 Mgal/d; and the winter months ranging between 80 to 100 Mgal/d. The seasonal trend lines indicate a increasing rate of withdrawal in the winters and an increasing rates of withdrawals in the summers, from 1988-2009.

          Monthly Withdrawal Rates for Suffolk County (1988-2009).

          Figure 25: Graph of Suffolk County Water Authority monthly withdrawal, averaged daily from 1988-2009 (Source: Suffolk County Water Authority).
      • Domestic-Self Supply

          Self-supplied domestic water use is usually withdrawn from a private source, such as a well, or captured as rainwater in a cistern; in the United States 14 percent of the population, supplied their own water for domestic use in 2005. However on Long Island, about 200,000 people were estimated to have been using private domestic well water which totaled about 15 Mgal/d, in 2005 (Kenny and others, 2009).

      • Irrigation

          Irrigation water use includes water that is applied by an irrigation system to sustain plant growth in all agricultural and horticultural practices. Irrigation also includes water that is applied for pre-irrigation, frost protection, application of chemicals, weed control, field preparation, crop cooling, harvesting, dust suppression, leaching salts from the root zone, and water lost in conveyance. Irrigation of golf courses, parks, nurseries, turf farms, cemeteries, and other self-supplied landscape-watering uses also are included. Irrigation water use includes self-supplied withdrawals and deliveries from irrigation companies, irrigation districts, cooperatives, or governmental entities. In 2005, it was estimated that less than 10 Mgal/d was used for irrigation on Long Island (Kenny and others, 2009).

      • Commercial / Industrial-Self Supply

          Industrial water use includes water used for such purposes as fabricating, processing, washing, diluting, cooling, or transporting a product; incorporating water into a product; or for sanitation needs within the manufacturing facility. Some industries that use large amounts of water produce such commodities as food, paper, chemicals, refined petroleum, or primary metals. Water for industrial use may be delivered from a public supplier or be self-supplied. In this website, industrial use refers to self-supplied industrial withdrawals only. In 2005, about 67 Mgal/d was used for industrial use across Long Island (Kenny and others, 2009).

    • Groundwater Budget

        A groundwater system consists of a mass of water flowing through the pores or cracks below the Earth's surface. This mass of water is in constant motion. Water is constantly added to the system by recharge from precipitation, and water is constantly leaving the system as discharge to surface water and as evapotranspiration. Each groundwater system is unique in that the source and amount of water flowing through the system is dependent upon external factors such as rate of precipitation, location of streams and other surface-water bodies, and rate of evapotranspiration. The one common factor for all groundwater systems, however, is that the total amount of water entering, leaving, and being stored in the system must be conserved. An accounting of all the inflows, outflows, and changes in storage is called a water budget (Alley and others, 1999).

        Estimation of the amount of groundwater that is available for use requires consideration of two key elements. First, the use of groundwater and surface water must be evaluated together on a system wide basis. This evaluation includes the amount of water available from changes in groundwater recharge, from changes in groundwater discharge, and from changes in storage for different levels of water consumption. Second, because any use of ground water changes the subsurface and surface environment (that is, the water must come from somewhere), the public should determine the tradeoff between groundwater use and changes to the environment and set a threshold at which the level of change becomes undesirable. This threshold can then be used in conjunction with a system wide analysis of the groundwater and surface-water resources to determine appropriate limits for consumptive use.

        System wide hydrologic analyses typically use simulations (that is, computer models) to aid in estimating water availability and the effects of extracting water on the ground-water and surface-water system. Computer models attempt to reproduce the most important features of an actual system with a mathematical representation. If constructed correctly, the model represents the complex relations among the inflows, outflows, changes in storage, movement of water in the system, and possibly other important features. As a mathematical representation of the system, the model can be used to estimate the response of the system to various development options and provide insight into appropriate management strategies. However, a computer model is a simplified representation of the actual system, and the judgment of water-management professionals is required to evaluate model simulation results and plan appropriate actions (Alley and others, 1999).

        Estimates of the inflow and outflow into the Long Island groundwater system were evaluated using a compilation of data from 2005-2010 where available. Previous estimates from earlier studies and numerical models are also presented in this section as a comparison to a 2005-2010 index period.

      • Inflow to the Groundwater System

          Precipitation that infiltrates and percolates to the water table is Long Island's only natural source of freshwater because the groundwater system is bounded on the bottom by relatively impermeable bedrock and on the sides by saline ground water or saline bays and the ocean. About one-half the precipitation becomes recharge to the groundwater system; the rest flows as surface runoff to streams or is lost through evapotranspiration (Cohen and others, 1968).

          In determining the effects of pumping and the amount of water available for use, it is critical to recognize that not all the water pumped is necessarily consumed. For example, not all the water pumped for public supply is consumed. Some of the water returns to the groundwater system as infiltration through onsite-septic systems (recharge from wastewater return). Most other uses of groundwater are similar in that some of the water pumped is not consumed but is returned to the system. Thus, it is important to differentiate between the amount of water pumped and the amount of water consumed when estimating water availability (Alley and others, 1999).

          The estimated total inflow to the groundwater system in 2005-2010, is an accumulation of contributions of several sources. These sources include the natural groundwater recharge from precipitation, infiltration from onsite septic systems, infiltration from leaking water mains, and storm water runoff diverted to recharge basins. The estimated average daily inflow of fresh water into the groundwater system was about 1,500 Mgal/d for the 2005-2010 index period. This includes the natural groundwater recharge from precipitation of around 1,180 Mgal/d, and the remaining fresh water inflow of approximately 300 Mgal/d from other sources enumerated above.

        • Recharge from Precipitation

            Recharge from precipitation has been estimated to be about half of the total precipitation under natural pre-developed conditions. Impervious surfaces from urbanization across Long Island have altered the percentage of recharge from precipitation. However, new tools such as a Soil-Water-Balance (SWB) computer code have been used to calculate spatial and temporal variations in groundwater recharge. The SWB model calculates recharge by use of commonly available geographic information system (GIS) data layers in combination with tabular climatological data. The code is based on a modified Thornthwaite-Mather soil-water-balance approach, with components of the soilwater balance calculated at a daily time-step (Westonbroek and others, 2010). Recharge calculations are made on a rectangular grid of computational elements that may be easily imported into a regional groundwater-flow model. Recharge estimates calculated by the code may be output as daily, monthly, or annual values.

            A recent application of the SWB code was documented in the appendix section of Masterson and others, 2013; the modeled output included Long Island. The SWB calculated groundwater recharge used a 1-mile grid spacing across Long Island. The results of the SWB code show the spatial variability of the calculated average recharge across Long Island for the period 2005-2009 (figure 26). Cohen and others (1968) estimated that annual recharge ranged from about 10 to 35 inches of water, and is comparable to the output of the SWB program, as can be seen here.

            Long Island Recharge Map.

            Figure 26: Simulated output of Soil-Water-Balance Code (SWB) of recharge across Long Island, N.Y. from 2005-2009.
            Source: Masterson and others, 2013

            The SWB calculated average groundwater recharge across Long Island is represented as 1 square mile gridded output. Cumulatively summing each grid value provided the estimated daily average groundwater recharge for 2005-2009 totaled around 1,180 Mgal/d.

            Sutter (1937), one of the earliest regional water supply reports for Long Island, estimated 1,373 Mgal/d of precipitation recharged the groundwater system. Cohen and others (1968), evaluated a groundwater budget for the 1940-1965 period where they estimated about 820 Mgal/d entering as groundwater recharge; however the water budget area was smaller and excluded the areas of King and Queens Counties, and both the North and South Forks. Buxton and others (1999), used a numerical groundwater-flow model of Long Island’s groundwater system for the 1968-83 period where they estimated the average groundwater recharge was 1,126 Mgal/d. The area modeled included Kings and Queens Counties, however, excluded the north and south forks of Long Island. The SWB average daily groundwater recharge estimate therefore compares favorably with these previous published estimates of average daily recharge.

        • Recharge From Wastewater Return

            Nassau County’s Sewer and Storm Water Authority collects and treats most of the sewage generated in the County. Two of Nassau County’s largest treatment plants process 85% of the sewage collected within the County. These two plants each treat approximately 58 million gallons per day (Mgal/d), Ten other independent treatment facilities operate within the County, these ten facilities process 15% of the County’s effluent. The effluent treated is discharged into nearby estuaries, and therefore leaves the groundwater flow system.

            Source: Nassau Sewered Areas

            Approximately thirty percent of Suffolk County's residents have their sewage collected and treated by over 190 sanitary sewer systems, over 150 are private. The treatment amounts permitted are approximately 59 Mgal/d; 43 Mgal/d discharge to surface waters and about 16 Mgal/d discharge to the ground. The remaining seventy percent of Suffolk County's residents have on-site sanitary wastewater treatment and disposal system. These on-site systems include a septic tank or cesspool for solids settling, connected to leaching fields to allow clarified water to seep into the ground.

            Source: Suffolk Sewered Areas

            In unsewered areas of Nassau and Suffolk Counties, 85 percent of the water pumped for public supply is estimated to infiltrate back to the groundwater system, whereas in sewered areas, only about 20 percent returns. The amount of public-supply water that returns to the groundwater system in Nassau and Suffolk Counties varies spatially (Buxton and others, 1999). Approximately 1,100,000 people on Long Island are estimated to live in an unsewered area and are using onsite-septic systems during the period 2005-2010. These onsite-septic systems return about 74 Mgal/d of water from these systems back into the ground.

        • Recharge Basins

            Recharge basins are unlined excavations in the glacial deposits; they range from about 10 to 20 feet in depth and from less than 1 to about 30 acres in area. In Nassau and Suffolk Counties there are more than 2,000 recharge basins (figure 27).

            Recharge Basins in Nassau and Suffolk Counties.

            Figure 27: Generalized locations of 2,000 recharge basins across Long Island.

            Most of the runoff from highways in these counties is collected by storm sewers and routed to these recharge basins. A recharge basin is generally used only where the water table is sufficiently deep to remain below the floor of the basin at least most of the time. Therefore, only a few recharge basins are located in nearshore areas where the water table is within a few feet of the land surface. In addition, on Long Island many street storm water inlets are open bottomed (ie; not connected to a storm sewer) and therefore, function as small recharge basins (Franke and others, 1972).

            Seaburn (1970) studied the inflow of two recharge basins in residential developments in Nassau County. From the rainfall-inflow relation for one of these basins, he estimated that, on average about 15 percent of the total precipitation falling on the drainage area of the basin discharged into the basin.

            In 1968, Franke and McClymonds (1972) estimated that the total drainage area of all the recharge basins in Nassau and Suffolk Counties was probably on the order of 250 square miles. They assumed that 15 percent of total rainfall on this area enters recharge basins and that virtually all of this water recharges the groundwater reservoir, with the estimated daily average recharge to the groundwater system from these basins being on the order of 80 Mgal/d (Franke and others, 1972). The assumption that most of the water entering a recharge basin ultimately recharges the groundwater system is based on the observation that water entering most basins percolates into the ground fairly rapidly (commonly within a day or so).

            It is likely that during the 2005-2010 index period, the total drainage area of recharge basins on Long Island has increased since 1968 because of development, largely in Suffolk County. Population growth in Suffolk County increased 25% since 1970. If we simply increase the total recharge basin drainage area by 25%, then the total drainage area would equal 313 square miles in 2010 and, by using the same approach described above, the average estimated daily average recharge to the groundwater system from these basins is on the order of 100 Mgal/d.

        • Infrastructure Leaks

            Every day in America, we lose nearly six billion gallons of treated water due to crumbling infrastructure. Throughout the nation, leaky aging pipes, and outdated systems are wasting 2.1 trillion gallons annually. That's roughly 16% of our nation's daily water use (Center for Neighborhood Technology web link). Leakage is usually the largest component of distribution loss. Leakage in public-water-supply systems results in loss of purified drinking water but also means wasting the energy and material resources used in abstraction, transportation and treatment. Estimates of the loss of water for the period 2000-2010 through major city public-supply distributions systems average about 12.8 percent (http://growingblue.com/wp-content/uploads/2012/07/Leakage-Rate-US-Cities.png) nationally and 14.2% for New York City.

            Accounting for this loss of supply from infrastructure leaks is needed in evaluating a present day groundwater budget. Conservatively using an estimated leakage rate of around 11% for publically supplied water throughout Nassau and Suffolk Counties is equivalent of about 42 Mgal/d returned to the groundwater system on average during 2005-2010. Virtually all of Kings and Queens County has combined sewers, and the major source of returned water is leakage from water-supply and sewer lines, which carry 700 Mgal/d from a reservoir system in upstate New York. About 58 Mgal/d is estimated to enter the groundwater system throughout Kings and Queens Counties from this leaky infrastructure (Buxton and others, 1999).

      • Outflow from the Groundwater System

          The flow of water leaving, or discharging, the groundwater system of Long Island occurs naturally through streams, as base flow, at the coastline as shoreline discharge and sub-sea discharge, and through pumping wells as withdrawals. Estimates of each component of outflow from the groundwater system is presented and summarized in this section using streamflow measurements, and a compilation of reported or estimated withdrawals.

        • Streamflow

            There are over 100 stream channels on Long Island, typically less than 5 miles long, that flow to the tidewater that surrounds Long Island. The channels were formed by glacial melt water and therefore are more abundant along the southern shore than along the northern shore. Groundwater discharge to streams has a major effect on flow patterns within the groundwater system. Under pre-development conditions, about 21 percent of precipitation, equivalent to more than 40 percent of the groundwater leaving the system, was discharged to streams (Buxton and others, 1999).

            Continuous streamflow records, ranging in length from about 7 to 73 years, are available for the 22 streams shown below (Table 1). Unless otherwise stated, all values of streamflow are for total streamflow and therefore, include direct runoff. The average base flow of the streams (that is, seepage from the groundwater reservoir) is about 90-95 percent of total average streamflow (Spinello and Simmons, 1992; Reynolds, 1982). The average measured streamflow for the entire period of record at each stream gage station is presented below. On average about 188 Mgal/d of cumulative measured discharge occurred at these stations. Assuming that 90-95 percent of this streamflow is base flow, approximately 170 to 179 Mgal/d of groundwater leaves the Long Island aquifer system as stream base flow.

            A large amount of additional water undoubtedly seeps from the groundwater system into the lower tidal reaches of the streams in the nearshore areas (estuarine seepage) particularly the southern nearshore area. Most of this water is derived from precipitation that recharges the groundwater system in the nearshore areas, that is, it is not part of the deep circulating groundwater system. The estimated average amount of this additional unmeasured streamflow is on the order of 40-80 Mgal/d for the southern nearshore area and 10-15 Mgal/d for the northern nearshore area (Franke and other,1968).

            Therefore, the combined estimated groundwater discharge to streams and as nearshore estuarine seepage is on average around 220 to 280 Mgal/d for the period 2005-2010. This amount is about 60 Mgal/d less than the 26 year period (1940-65) described in Franke and others (1972). Buxton and others (1999), estimated the average groundwater discharge into the streams was 460 Mgal/d in a predevelopment period, and 325 Mgal/d from 1968-1983; in other words, a 135 Mgal/d reduction in base flow from the predevelopment period.

            Streamflow stations.

            Table 1: USGS stream gage stations which had continuous record for a period of record, the average daily discharge was calculated
            using the entire period of record. (Gage locations are shown in figure 10 and 18)
        • Groundwater Withdrawals

            Groundwater pumpage on Long Island and water usage presented under the “Water Availability-Water Use” section is summarized here. The public water supply withdrawal data reported to the New York State Department of Environmental Conservation (NYSDEC) in 2010 and the USGS National Water Use Information Program (NWUIP) estimates of 2005 (Kenny and others, 2009) groundwater withdrawals for domestic-self supply, industrial and irrigation were used to calculate the average amount of water pumped from the groundwater reservoir. Even though these data have different time periods, it is an assumption that they are approximate amounts. The estimated average groundwater withdrawals throughout Long Island is over 500 Mgal/d, for 2005-2010 index period.

            The following is a compilation of groundwater withdrawal data from several reports represented in an animation. A recharge amount of 22 inches per year (estimate of long term natural recharge) was used throughout the animation, as the natural source of fresh water entering the system. The animation shows the average annual withdrawals for each county from 1900-2007, and the percentage of the natural recharge going towards withdrawals, for each county. This approach provides a simple comparison of the natural groundwater recharge with the withdrawals for each county; and is not representing returned water through leaks, recharge basins or on-site septic systems.

            Content on this page requires a newer version of Adobe Flash Player.

            Get Adobe Flash player

        • Coastline and Sub-Sea Discharge

            Subsurface outflow (sub-sea) under natural (pre-development) conditions mainly included the subsurface movement of groundwater northward to Long Island Sound, and southward (a) to the swampy lowlands bordering the south-shore bays, (b) directly into the bays, and (c) directly into the Atlantic Ocean.

            Sufficient data are not available to directly estimate subsurface outflow, which may be the largest element of natural groundwater discharge from the water-budget area. However, an indirect method based on a water-budget concept suggests that total subsurface outflow from the water-budget area under natural conditions was on the order of 470 Mgal/d (Cohen and others, 1968). Buxton and others in 1999 estimated 503 Mgal/d discharged to the shore and 58 Mgal/d as sub-sea discharge for the 1968-83 period.

            Assuming storage removal is negligible, we estimate that 690 Mgal/d is discharged to the shoreline and as subsea discharge for the 2005-2010 index period. This was calculated using the indirect method based on the water-budget concept, as in previous investigations, to estimate coastline and sub-sea discharge.

  • Water Suitability

      Groundwater quality may be affected by natural and human factors (Johnston, 1988). Although the vulnerability of groundwater to contamination from the land surface is influenced by many factors, the degree of aquifer confinement, the depth of the well, and the surrounding land use are primary key factors that influence shallow groundwater quality. Unconfined aquifers generally are much more vulnerable to contamination than confined aquifers. For a well in a confined aquifer, the farther the well is from the unconfined area, the less vulnerable it is to contamination. Generally, the deeper the well, the less vulnerable it is to contamination. Finally, because human activities greatly affect the quality of water that recharges an aquifer, the amount and type of land use in the area that contributes water to the well is a key factor in determining aquifer vulnerability to contamination (Clawges and others, 1999).

      Two of the factors that have the greatest effect on groundwater quality are the land-use practices in the recharge area above the aquifer(s) and the groundwater-flow patterns within the aquifer(s) (Haefner, 1992).

      Pollution Potentially Impacting Groundwater Sites on Long Island.

      Figure 28: Generalized locations of contamination sites that have the potential to impact groundwater quality, in Nassau and Suffolk Counties, N.Y.
      (Source: U.S. Environmental Protection Agency).

      Groundwater quality data on Long Island has been collected for many years by the USGS and other Federal, State and local agencies. A recent map of contamination sites that have the potential to impact groundwater across Long Island is shown in Figure 28. This map was created by the U.S. Environmental Protection Agency, using information maintained by the New York State Department of Environmental Conservation.

    • Water Quality Data

        The USGS collects and analyzes chemical, physical, and biological properties of water, sediment and tissue samples from across the Nation. The Water Data for the Nation discrete sample database is a compilation of over 4.4 million historical water quality analyses in the USGS district databases through September 2005. The discrete sample data is a large and complex set of data that has been collected by a variety of projects ranging from national programs to studies in small watersheds.

        At selected surface-water and groundwater sites, the USGS maintains instruments that continuously record physical and chemical characteristics of the water including pH, specific conductance, temperature, dissolved oxygen, and percent dissolved-oxygen saturation. Supporting data such as air temperature and barometric pressure are also available at some sites. At sites where this information is transmitted automatically, data are available from the current data system.

        There are over 4,200 sites on Long Island that had a water quality sample measurement taken since the early 1900's. The largest sample collections occurred between 1970-1990, where over 2,100 sites were sampled. Currently, for the period 2010-2014, only 117 sites have a water quality sample. Real time water quality data map service is available for several field parameters like water temperature.

      • Groundwater

          Groundwater will normally look clear because the ground naturally filters out particulate matter. However, both natural and anthropogenic compounds can be found in groundwater. As groundwater flows through the ground, metals such as iron and manganese are dissolved and may later be found in high concentrations in the water. Industrial discharges, urban activities, agriculture, groundwater pumpage, and disposal of waste all can affect groundwater quality.

          Contaminants can be human-induced, as from leaking fuel tanks or toxic chemical spills. Pesticides and fertilizers applied to lawns and crops can accumulate and migrate to the water table. Leakage from septic tanks and/or waste-disposal sites also can introduce bacteria to the water, and pesticides and fertilizers that seep into farmland can eventually end up in water drawn from a well. In some instances, a well might have been drilled in proximity to land that was once used for a landfill or chemical dump site. In any case, if you use your own well to supply drinking water to your home, it is wise to have your well water tested for chemicals and contaminates.

      • Surface Water

          As watersheds are urbanized, much of the vegetation is replaced by impervious surfaces, thus reducing the area where infiltration to groundwater can occur. Thus, more stormwater runoff occurs — runoff that must be collected by extensive drainage systems that combine curbs, storm sewers, and ditches to carry stormwater runoff directly to streams. More simply, in a developed watershed, much more water arrives into a stream much more quickly, resulting in an increased likelihood of more frequent and more severe flooding.

          As it flows over the land surface, stormwater picks up potential pollutants that may include sediment, nutrients (from lawn fertilizers), bacteria (from animal and human waste), pesticides (from lawn and garden chemicals), metals (from rooftops and roadways), and petroleum by-products (from leaking vehicles). Pollution originating over a large land area without a single point of origin and generally carried by stormwater is considered non-point source pollution.

    • Case Studies

        A collection of studies that focused on the quality of groundwater and surface water, are presented in this section. The reports associated with these areas of water quality concerns are linked as an online source for further reading.

      • Saltwater Occurrence

        • Saltwater Occurrence and Intrusion in the Aquifers of Long Island, New York

            Freshwater on the island discharges along most of the periphery of the island, which prevents saltwater from entering the aquifers. In western Long Island, however, saltwater wedges that are hydraulically connected to the sea are found in aquifers on the Atlantic Ocean side of the island. The saltwater wedge in the Lloyd aquifer extends seaward of the wedges in the overlying aquifers because of the relatively impermeable clays of the Raritan confining unit, and the high potentiometric head in the Lloyd Aquifer, which force freshwater in the Lloyd aquifer to extend seaward of the island (figure 29). The positions of the saltwater wedges have been attributed mainly to natural conditions that prevailed long before the start of groundwater development in western Long Island. Groundwater pumping, however, has caused a landward migration of the freshwater-saltwater interface in aquifers in western Long Island since the late 1890s (Lusczynski and Swarzenski, 1966; Buxton and Shernoff, 1999). Saline groundwater is also probably migrating downward into the Lloyd aquifer from the overlying Jameco and Magothy aquifers in areas of heavy pumping. In the Forks areas of eastern Long Island, saltwater underlies freshwater in lens-shaped reservoirs that resemble those that underlie outer areas of Cape Cod, Massachusetts (Nemickas and Koszalka, 1982).

            Cross Section of Queens-Nassau Border, Long Island, N.Y.

            Figure 29: Generalized cross section on Long Island with estimated salty groundwater areas (Barlow, 2003).

            Recent investigations which focused on saltwater concerns within public supply system are presented in Stumm (2001) and Stumm & others (2002).

      • Pesticide Occurrence

          Pesticide-Related Chemicals Detected in Long Island Groundwater 1996-2010
          by USGS, SCDHS, and SCWA:

          Detailed water quality monitoring data, from monitoring conducted by Suffolk County and the U.S. Geological Survey, are available in a document titled “Water Quality Monitoring for Pesticides, Historical Monitoring Data for the Long Island Pesticide Pollution Prevention Strategy”. Datasets in that document indicate minimum, maximum and median concentration levels of pesticides and degradates detected from about 1997 to 2011.

          Source: Link to Online Report

          The U.S. Geological Survey (USGS) National Water Quality Assessment (NAWQA) program is designed to assess the status of the Nation's water quality, describe trends in water quality, and provide a sound scientific understanding of the primary natural and human factors that affect the quality of the Nation's water resources. One component of the NAWQA program is the study of pesticides to determine their occurrence, concentrations, and seasonal variability in surface and groundwater throughout the country. The study was conducted as part of the Long Island-New Jersey (LINJ) coastal drainages NAWQA project. The LINJ study area is one of 59 areas studied nationwide.

          Source: Link to Online Report

          The NAWQA Pesticide National Synthesis Project, which began in 1992, is a national-scale assessment of the occurrence and behavior of pesticides in streams and groundwater of the United States and the potential for pesticides to adversely affect drinking-water supplies or aquatic ecosystems.

          Source: Nawqa report
          Source: Nawqa fact sheet
      • Nitrogen Loading

          Although nitrogen is abundant naturally in the environment, it is also introduced into aquifers through sewage and fertilizers. Chemical fertilizers or animal manure is commonly applied to crops to add nutrients. It may be difficult or expensive to retain on site all nitrogen brought on to farms for feed or fertilizer and generated by animal manure. Unless specialized structures have been built on the farms, heavy rains can generate runoff containing these materials into nearby streams and lakes. Wastewater-treatment facilities that do not specifically remove nitrogen can also lead to excess levels of nitrogen in surface or groundwater.

          Several USGS investigations have evaluated the nitrogen loads and trends entering Long Island's surrounding estuaries from groundwater and surface water. Trends in nitrogen concentration and nitrogen loads entering the South Shore Estuary Reserve (Monti and Scorca, 2003) were evaluated on 13 major south-shore streams in Nassau and Suffolk Counties, Long Island, New York with adequate long-term (1971-1997) water-quality records. Furthermore, 192 south-shore wells with sufficient water-quality data, were selected for analysis of geographic, seasonal, and long-term trends in nitrogen concentration. An example of the long-term trend in one of the 13 streams is shown in figure 30.

          Nitrogen Levels at Bellmore Creek.

          Figure 30: Annual mean nitrogen load calculated for Bellmore Creek, Nassau County, N.Y. (Monti and Scorca, 2003)

          Seasonal and long-term trends of nitrogen loads entering Long Island Sound from groundwater and streams on Long Island, New York were analyzed for four major streams on the north shore of Long Island that have long-term discharge and water-quality records for the period 1985-1996 (Scorca and Monti, 2001).

      • Volatile Organic Compounds

          Volatile Organic Compounds (VOCs) "are organic compounds that can be isolated from the water phase of a sample by purging the water sample with inert gas, such as helium, and, subsequently, analyzed by gas chromatography. Many VOCs are human-made chemicals that are used and produced in the manufacture of paints, adhesives, petroleum products, pharmaceuticals, and refrigerants. They often are compounds of fuels, solvents, hydraulic fluids, paint thinners, and dry-cleaning agents commonly used in urban settings. VOC contamination of drinking water supplies is a human-health concern because many are toxic and are known or suspected human carcinogens — (U.S. Geological Survey, 2005).

          Source: Nawqa Reports on VOCs analysis.
      • Pharmaceuticals Occurrence

          In 2002, the U.S. Geological Survey (USGS), in cooperation with the Suffolk County Water Authority (SCWA), began a 4-year study to document the occurrence of pharmaceutically active compounds in groundwater wells throughout Suffolk County. Benotti and others (2006) collected seventy (70) water samples from 61 wells in the upper glacial and Magothy aquifers (9 wells were sampled twice) during 2002–2005 and analyzed for 24 pharmaceuticals. Wells were selected for their proximity to known wastewater-treatment facilities that discharge to the shallow upper glacial aquifer. Of the 70 samples taken, pharmaceuticals were detected in 28, of which 19 contained one compound, and 9 contained two or more compounds. The report summarizes the results from the study and relates the concentrations and frequencies of detection to those reported from a 1998–2000 nationwide study of streams that receive wastewater (Kolpin and others, 2002).

      • Perchlorate Occurrence

          Perchlorate (ClO4-) is a common groundwater constituent with both synthetic and natural sources. A potentially important source of ClO4- is past agricultural application of ClO4-3- fertilizer imported from the Atacama Desert, Chile, but evidence for this has been largely circumstantial. Bohlke and others (2009), reported perchlorate was present in all samples collected in the study, and some concentrations exceeded the New York drinking-water guidance level of 50 nmol/L (5 μg/L). The highest concentrations were from the areas where fireworks disposal and military activities were potential sources.

  • Potential Hazards

      Hazards which may impact the ground water system adversely are presented in this web page. The impacts of these hazards are only shown here as a topic for further discussion and may need to be investigated with further details.

    • Sea Level Rise

        The Center for Operational Oceanographic Products and Services has been measuring sea level for over 150 years, with tide stations of the National Water Level Observation Network operating on all U.S. coasts. Changes in Mean Sea Level (MSL), either a sea level rise or sea level fall, have been computed at 128 long-term water level stations using a minimum span of 30 years of observations at each location. These measurements have been averaged by month to remove the effect of higher frequency phenomena (e.g. storm surge) in order to compute an accurate linear sea level trend. The trend analysis has also been extended to 240 global tide stations using data from the Permanent Service for Mean Sea Level (PSMSL). In the vicinity of Long Island, the rate of sea-level rise ranges from about 2.35 to 3.9 mm/year based on long-term trend data.

        Changes in climate and sea level will drive changes to the coastal groundwater system that will impact both human populations and coastal ecosystems. Increases in sea-level will raise the fresh water table in many coastal regions (figure 31). Impacts to humans may include an increase in the potential for basement or septic system failure. Sea-level rise can also contaminate groundwater supplies due to landward and upward movement of sea-water in coastal aquifers. The intrusion of saltwater into groundwater systems will also impact coastal ecosystems such as marshes by changing the elevation of the freshwater-saltwater interface.

        A major concern for water managers on Long Island is the potential adverse effect of sea-level rise on the depth to the freshwater-saltwater interface near public groundwater supply wells. Pumping from public-supply wells in coastal aquifers underlain by saltwater can lower the water table with respect to sea level, decreasing the depth to the freshwater-saltwater interface beneath the pumping well. This increases the potential for saltwater intrusion, and potentially limits the amount of potable water available from the well.

        Sea Level Rise.

        Figure 31: A rise in sea-level will affect groundwater flow in coastal aquifers (1). An increase in the elevation of the water table (dashed blue line) may result in basement flooding and compromise septic systems (2). A rise in sea level may also result in an upward and landward shift in the position of the freshwater-saltwater interface (3). Where streams are present, an increase in the water-table elevation also may increase groundwater discharge to streams and result in local changes in the underlying freshwater-saltwater interface (4). (USGS)

        The USGS Center of Excellence for Geospatial Information Science (CEGIS) provides a viewer animating a sea-level rise with hypothetical depths of rise from 0-30 meters. An example for New York can be found at this link. The animation demonstrates the population amounts that might be impacted by sea-levels rising to a certain height.

    • Groundwater Flooding

        The groundwater table rises and falls because of increases and decreases in recharge or discharges (pumping wells). Average total precipitation (and temperature) has been above normal on Long Island from 2004-2010 based on data provided by National Oceanic and atmospheric administration (NOAA) National Climatic Data Center (NCDC). The above normal precipitation has brought the groundwater levels to near record highs in some parts of Long Island. The impacts of a rising water table may include an increase in the potential for subsurface structure flooding (subway tunnels, basements) or on-site septic system failure. In 2010, the depth to the water table was estimated to show areas that have the potential for groundwater flooding (figure 32). An interactive mapping tool allows the user to select a location and retrieve the estimated depth to water at that location.

        Long Island 2010 Water Table.

        Figure 32: Map of the estimated depth to water in 2010, Long Island, NY.
    • Droughts

        A drought is a period of drier-than-normal conditions that results in water-related problems. When rainfall is less than normal for several weeks, months, or years, the flow of streams and rivers declines, water levels in lakes and reservoirs fall, and the depth to water in wells increases. If dry weather persists and water-supply problems develop, the dry period can become a drought.

        The term "drought" can have different meanings to different people, depending on how a water deficiency affects them. Droughts have been classified into different types such as:

        Meteorological Drought - Lack of precipitation
        Agricultural Drought - Lack of soil moisture
        Hydrologic Drought - Reduced streamflow or groundwater levels

        It is not unusual for a given period of water deficiency to represent a more severe drought of one type than another type. For example, a prolonged dry period during the summer may substantially lower the yield of crops due to a shortage of soil moisture in the plant root zone but have little effect on groundwater storage replenished the previous spring.

        A groundwater drought typically refers to a period of decreased groundwater levels that results in water-related problems. The amount of groundwater decline that would be considered a drought varies regionally and locally due to differences in groundwater conditions and groundwater needs for humans and the environment.

        For additional USGS background on drought, see:

        Drought, USGS Open-File Report 93-642
        The Hydrology of Drought – Frequently Asked Questions
        Sustainability of ground-water resources, Box B - Droughts, Climate Change, and Ground-Water Sustainability
        Ground-Water Depletion Across the Nation, USGS Fact Sheet 103-03
        Statement of Robert Hirsch, USGS Associate Director for Water, at 2002 Drought Summit

        The National Drought Mitigation Center (NDMC), provides several drought monitoring tools and information to help people assess drought severity and impacts.