A Century of Water Data on Western Long Island: How Nassau County's Groundwater Is Watched

Freshwater is a vital resource, and even in water-rich New York it has to be used carefully so there is enough, of good enough quality, for people and ecosystems now and later. Nowhere in the state is that clearer than on Long Island, where public water supply comes from the sole-source aquifers directly beneath the 3 million people who live there.

In western Long Island's Nassau County (as of 2023), about 46 water suppliers and hundreds of private wells pump groundwater from these aquifers for about 1.4 million people.

The Long Island aquifer system

On Long Island, groundwater fills the pore spaces between grains of clay, silt, sand and gravel, and feeds streams and rivers where the land surface meets the water table. Layers that yield water in useful amounts are aquifers, and three hold most of it: the upper glacial, Magothy and Lloyd aquifers. With smaller local water-bearing units and the poorly permeable layers between them (confining units), they make up the Long Island aquifer system.

Figure 1. The aquifer system supplies all the drinking water for Nassau County's nearly 1.4 million residents and sustains its groundwater-fed ponds, streams, wetlands and coastal waters. USGS.

Who measures what

Managing the county's groundwater well depends on long-term monitoring. For more than 100 years, government bodies — the Nassau County Department of Public Works (DPW), New York State, and several towns and local water districts — have paid for groundwater data to be collected. Those long records are central to sustainable management and to preparing for the possible effects of climate change.

The U.S. Geological Survey (USGS) works with the DPW and others in Nassau County to monitor streamflow, groundwater, water quality and water use. As of 2023 the partnership supports:

• At 25 of these sites — all 6 streamgages, the lake and 18 wells — readings are taken every 15 minutes and sent in near-real time to the USGS National Water Information System (NWIS).
• At the other wells, USGS hydrographers measure water levels monthly or yearly and enter them into NWIS by hand.
• Data in NWIS are kept and made available permanently.

Figure 2. The USGS surface-water and groundwater monitoring network in Nassau County, 2023. USGS.

A record back to the early 1900s

Groundwater levels were measured as early as the early 1900s, and more often from the 1930s. The number of stations and how often they are read have changed over the century; what has not is careful, nationally consistent measurement by trained USGS hydrographers.

Figure 3. A, stream discharge at Massapequa Creek at Massapequa, 1936–2022 (station 01309500). B, groundwater levels in the upper glacial aquifer at well N 53.1, Rockville Centre, 1934–2022 (station 403929073382908). The drop at N 53.1 is attributed to the severe drought of the 1960s followed by the building of large-scale sanitary sewers, which lowered water levels across most of the county. USGS.

Collecting the data is only part of the work. Keeping a long network accurate, comparable and representative takes steady effort:

• calibrating electronic and mechanical instruments against manual measurements;
• surveying the elevation of reference marks, so new data match old;
• reviewing long records and revising them where environmental changes may have affected the measurements.

What long records make possible

These data show how the resource changes over time, feed groundwater-flow models and forecasts, and let managers design, run and test groundwater management and protection programs.

A recent example no one could have foreseen when collection began: the records went into a state-of-the-science groundwater-flow model of the whole island. It can reproduce past and present groundwater levels and streamflows, and predict the likeliest results of changes in pumping and sewering, and of sea-level rise or long droughts.

Long surface-water and groundwater records can also be used for:

• estimating long-term changes in recharge and storage;
• assessing the effects of climate variability;
• estimating the regional effects of groundwater development;
• statistical analysis of water-level trends;
• measuring changes in flow direction;
• describing how groundwater and surface water interact;
• mapping the depth to groundwater.

Depth-to-water maps. Every spring, the USGS measures water levels across Long Island in a single round, to map the shape of the water table and the potentiometric surfaces in the three main aquifers. The resulting maps are used in hydrogeologic studies across the island and by water managers and public suppliers to plan and manage the aquifers.

Figure 4. Depth to water on Long Island, April 2013. USGS.

Water quality

With Nassau County, the USGS also tracks long-term water-quality trends. That matters in a county with many legacy sources of contamination and sandy soils that let contaminants reach the aquifers easily. Many newer classes of contaminants, where present, occur at low levels that only specialized labs and sampling methods can detect. The USGS has long monitored contaminants in Long Island's groundwater with advanced analytical methods, so that water managers can plan for, or respond to, threats to drinking-water supplies.

Figure 5. USGS scientists collecting streamflow, groundwater-level and water-quality data.

Why a neutral source matters

On Long Island, the quantity and quality of water are shaped by human activity not only in Nassau County but in neighbouring Suffolk County and New York City. Where needs compete for the same water, the USGS serves as a neutral, independent source of reliable, defensible data.

Sources

Based on the U.S. Geological Survey fact sheet "A century of hydrologic data collection prepares western Long Island for current and future water-resources challenges," U.S. Geological Survey; a work of the United States government in the public domain. It cites Masterson and Breault (2019), Water for Long Island — Now and for the Future (Fact Sheet 2019–3052); Taylor and Alley (2001), Ground-Water-Level Monitoring and the Importance of Long-Term Water-Level Data (Circular 1217); Como and others (2018), Scientific Investigations Map 3398; the USGS Long Island depth-to-water viewer; and USGS water data for the nation.