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Hydrologic Data for Long Island
Freshwater is a vital natural resource. New York is a water-rich State; however, even
here, the economical use of water resources is needed to ensure there is enough water
of adequate quality for human and ecological needs—now and into the future. Nowhere
in New York is this more evident than on Long Island where public-water supply is
obtained from the sole-source aquifers directly beneath the 3 million people that
live there. Today [2023] in western Long Island’s Nassau County, groundwater is pumped
from these aquifers by about 46 water suppliers and hundreds of private wells to meet
the needs of about 1.4 million people (Masterson and Breault, 2019).
Groundwater on Long Island is the water that is present beneath land surface in pore
spaces between grains of clay, silt, sand, and gravel and supplies freshwater to streams
and rivers at points where the surface of the land intersects the water table. Areas
where groundwater can be extracted in sufficient quantities are referred to as aquifers.
On Long Island, groundwater is primarily stored in three such water-bearing units:
the upper glacial, Magothy, and Lloyd aquifers. Collectively, these three aquifers—and
other small, localized water-bearing units, along with intervening layers of poorly
permeable sediments (referred to as confining units)—are known as the Long Island
aquifer system (fig. 1).
Figure 1.Groundwater from Long Island’s aquifer system provides all the drinking water for
the nearly 1.4 million residents of Nassau County, New York, and supports the ecology
of groundwater-fed ponds, streams, wetlands, and coastal waters. All life on Long
Island is dependent on this single vital resource.
Long-term monitoring is needed to understand how to best manage Nassau County’s groundwater
resources. Fortunately, for more than 100 years, government entities, including the
Nassau County Department of Public Works (DPW), New York State, and several towns
and local water districts, have supported the collection of groundwater information.
It cannot be overstated how important these long-term data are to guiding the sustainable
management of the county’s water resources and to forecasting and preparing for the
potential effects of climate change.
The U.S. Geological Survey (USGS), the Nation’s premier Earth science organization,
works with partners to monitor, assess, research, and deliver information on a wide
range of water-resources issues and conditions. In Nassau County, the USGS partners
directly with the DPW and others to monitor streamflow, groundwater, water quality,
and water use. These partnerships currently [2023] support 6 streamgages, 1 lake-level
station, and 168 groundwater-monitoring wells that characterize conditions in the
upper glacial, Magothy, and Lloyd aquifers (fig. 2; U.S. Geological Survey, 2022). Frequent-interval (once every 15 minutes) measurements of streamflow (6 sites),
lake level (1 site), and groundwater elevation (18 sites) at 25 of the sites are electronically
transmitted directly to the USGS National Water Information System (NWIS) database
(U.S. Geological Survey, 2023) in near-real time. At the remaining groundwater sites, water levels are measured
by USGS hydrographers, either monthly or annually, and entered into NWIS manually.
Data in NWIS are stored and made available in perpetuity.
Figure 2.Map showing surface water and groundwater monitoring stations in the U.S. Geological
Survey hydrologic monitoring network in Nassau County, New York, in 2023; see also
U.S. Geological Survey (2022). DPW, Department of Public Works.
Groundwater level measurements go as far back as the early 1900s, becoming more frequent
starting in the 1930s (fig. 3). Although the number of monitoring stations and the frequency of data collection
has varied during the past 100 years, the continuous collection of water-resource
information using scientifically rigorous and nationally consistent methods by trained
USGS hydrographers has remained the same. Data collection, however, is only part of
the equation; the maintenance of long-term data collection networks requires tremendous
effort to ensure data are accurate, comparable, and representative.
Figure 3.Hydrographs from data for Nassau County, New York, from the U.S. Geological Survey
National Water Information System (U.S. Geological Survey, 2023). A, Stream discharge for the period of record (1936 to 2022) at Massapequa Creek at
Massapequa, N.Y. (station 01309500). B, Groundwater levels in the upper glacial aquifer for the period of record (1934 to
2022) at well N 53.1 in Rockville Centre, N.Y. (station 403929073382908). The drop
in water levels shown at well N 53.1 have been attributed to the severe drought in
the 1960s followed by the installation of large-scale sanitary sewer systems, which
lowered water levels throughout most of Nassau County.
The long-term maintenance of USGS data networks is required to ensure that data are
collected to the highest standards and involves multiple tasks that taken together
result in a high level of confidence in data collected by the USGS. These tasks involve
calibrating electronic and mechanical equipment by using manual measurements to ensure
collected data are accurate; surveying the elevation of reference marks to ensure
long-term data are comparable with data collected in the past; and reviewing and updating,
if necessary, long-term data records to account for any environmental changes that
may have affected data collection.
The data collected by highly trained USGS staff using proven methods provide essential
information needed to evaluate changes in the resource over time, to develop groundwater-flow
models and forecast trends, and to design, implement, and monitor the effectiveness
of groundwater management and protection programs (Taylor and Alley, 2001). An excellent contemporary [2023] example of the value of long-term groundwater-level
data—that could not have been anticipated decades ago when the hydrologic data collection
program started—is the use of the data in the development of a state-of-the-science
island-wide groundwater-flow model. This model can reproduce (or simulate) past and
current hydrologic conditions (groundwater levels and streamflows) and can be used
to predict the most likely outcomes from changes in pumping and sewering and other
potential changes in the environment, such as sea-level rise and prolonged droughts
(Masterson and Breault, 2019). Other possible uses of long-term surface-water and groundwater data, include:
• estimating long-term changes in groundwater recharge and storage;
• assessing the effects of climatic variability;
• estimating the regional effects of groundwater development;
• generating statistical analyses of water-level trends;
• measuring changes in groundwater-flow directions;
• characterizing groundwater and surface-water interactions; and
• mapping the depth to groundwater below land surface.
Mapping the depth to groundwater below land surface has been an important part of
the ongoing USGS–Nassau County partnership. Each spring, the USGS conducts a synoptic
round of water level measurements across Long Island to define the geometry of the
water table and potentiometric surfaces within the three main water-bearing units
in the Long Island aquifer system. These data are used to construct depth-to-water
maps (fig. 4; U.S. Geological Survey, 2019) that in turn are used in hydrogeologic investigations throughout the island and
are used by water-resources managers and public-water suppliers for aquifer management
and planning purposes (Como and others, 2018).
Figure 4.Map showing depth to the water on Long Island, New York, in April 2013; from U.S. Geological Survey (2019). <, less than; >, greater than.
In addition to streamflow and groundwater levels, the USGS, in partnership with Nassau
County, has been monitoring long-term trends in water-quality (fig. 5). This is particularly important in places like Nassau County with many legacy sources
of contamination and sandy soils that allow contaminants to move readily into the
aquifer system. Many new classes of contaminants, if present, are found at low levels
that require specialized labs and sampling methods to detect. The USGS has a long
history of monitoring and documenting the presence of contaminants in groundwater
on Long Island using cutting-edge analytical research methods. Contaminants introduced
at the land surface have to be documented in the groundwater system so that water
resource managers can plan for or respond to water-quality hazards to drinking water
supplies.
Figure 5.U.S. Geological Survey scientists collecting A, streamflow, B, groundwater-level, and C, water quality data.
Unbiased, high-quality hydrologic data collection is the trademark of the USGS. This
is of particular importance when there are competing needs for the same resource.
On Long Island, the quantity and quality of water is affected by human activities,
not only in Nassau County, but also in neighboring Suffolk County and New York City.
The USGS is a neutral, independent party that provides data that are robust and defensible.
References Cited
Como, M.D., Finkelstein, J.S., Rivera, S.L., Monti, J., Jr., and Busciolano, R., 2018,
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, 5 p., accessed May 3, 2023, at
https://doi.org/10.3133/sim3398.
Masterson, J.P., and Breault, R., 2019, Water for Long Island—Now and for the future:
U.S. Geological Survey Fact Sheet 2019–3052, 2 p., accessed May 3, 2023, at https://doi.org/10.3133/fs20193052.
Taylor, C.J., and Alley, W.M., 2001, Ground-water-level monitoring and the importance of long-term water-level data: U.S. Geological Survey Circular 1217, 68 p., accessed May 2, 2023, at https://doi.org/10.3133/cir1217.
U.S. Geological Survey, 2019, Long Island depth to water viewer: U.S. Geological Survey
web page, accessed May 16, 2023, at https://ny.water.usgs.gov/maps/li-dtw/.
U.S. Geological Survey, 2022, U.S. Geological Survey hydrologic monitoring on Long
Island, New York: U.S. Geological Survey web page, accessed May 16, 2023, at https://www.usgs.gov/centers/new-york-water-science-center/science/us-geological-survey-hydrologic-monitoring-long.
U.S. Geological Survey, 2023, USGS water data for the nation: U.S. Geological Survey database, https://doi.org/10.5066/F7P55KJN.
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