Long Island's Water: USGS Science for a Sole-Source Aquifer

Published by the U.S. Geological Survey in September 2021; the studies it describes were then under way.

The Long Island aquifer system is the only source of drinking water for nearly 3 million people. Its water, and the sensitive habitats that support the island's varied wildlife, are threatened by:

• nutrients;
• harmful algal blooms;
• synthetic chemicals such as per- and polyfluoroalkyl substances (PFAS) and 1,4-dioxane.

Protecting the water means understanding where these chemicals occur, what happens to them and how they move. The U.S. Geological Survey (USGS) collects and analyses surface-water and groundwater data and models groundwater flow, to help water professionals build a complete science strategy against contamination.

The data collection is funded by the USGS, the New York State Department of Environmental Conservation, the New York City Department of Environmental Protection, the Suffolk County Water Authority, the Nassau County Department of Public Works, other state and local agencies, and tribal and federal partners. Without their long-term commitment, judging sustainability and planning for future water needs would not be possible.

A sole-source aquifer

Long Islanders drink from three principal aquifers — from shallowest to deepest, the upper glacial, Magothy and Lloyd aquifers. Together they form a sole-source aquifer system: one that supplies more than 50 percent of the drinking water in its area and has no alternative if it becomes contaminated.

Figure 1. Long Island's major aquifers, the direction of groundwater flow, and the boundary between fresh and salt water; not to scale. Block diagram by Bethany Fuss, U.S. Geological Survey.

Can the aquifers sustain the demand?

About 50 water suppliers in Nassau and Suffolk Counties pump from the system, and as demand rises, more is pumped. There is concern that the water's quality and quantity may not be enough for both people and ecosystems — a concern made worse as climate change keeps altering the environment.

In 2016 the USGS, with the New York State Department of Environmental Conservation, began a study of whether the aquifer system can sustain current and projected withdrawals. Sustainable use means balancing human and environmental needs.

As part of it, the USGS is building a numerical groundwater-flow model of the whole island's aquifer system from the latest hydrologic and geologic information.

• New data. Deep monitoring wells drilled for the project yield water levels and the type, depth and thickness of sediments, updating what is known of the island's hydrology and geology.
• Uses. The model will test water-management strategies to help plan a sustainable future, and could also help estimate how much water is unusable because of poor quality.

A drill rig and geophysical logging at a groundwater-monitoring site. Photograph by the U.S. Geological Survey.

Long-term monitoring

The USGS has monitored water quality, groundwater levels, tide heights and streamflow on Long Island and in New York City for decades. The data guide water decisions across the region and are essential to building and maintaining the groundwater-flow model that managers can use to judge the aquifer system's sustainability.

• Near-real-time data. Upgrading the USGS data-collection platforms and groundwater-monitoring network to near real time would let managers make up-to-the-minute decisions.
• Long records. A commitment to the data collection needed to track long-term hydrologic change would improve understanding of the effects of climate change and how to manage them.

Measurements of (A) streamflow and (B) groundwater levels. Photographs by the U.S. Geological Survey.

Nutrients

Nitrogen and phosphorus contaminate the aquifer system. They come from fertilisers used on farms and suburban lawns, and from household septic systems. As groundwater flows through the aquifer and out into ponds, streams and coastal waters, the nutrients cause algal growth (eutrophication), and they are carried on into Long Island Sound, Peconic Bay and Great South Bay.

• Why the model matters. Restoring these waters depends on understanding how excess nutrients move and how far management can cut the loads carried by groundwater; the groundwater-flow model is a foundation for designing and testing nutrient-management strategies.
• Current work. Two USGS studies are using the island-wide model, still in development, to measure how nitrogen moves to Peconic Bay and Long Island Sound, to support a long-term strategy for legacy and future nutrient contamination.
• Coastal effects. Excess nutrients reaching coastal bays can cause harmful algal blooms and substantially change coastal ecology. Coupled with hydrodynamic and water-quality models, groundwater-flow models can help plan when and where to apply best management practices (BMPs) to limit the effect of nitrogen in groundwater discharge.

The USGS also studies harmful algal blooms in fresh and salt water.

Passive samplers used to detect toxins from harmful algal blooms. Photograph by the U.S. Geological Survey.

PFAS and 1,4-dioxane

Synthetic chemicals such as PFAS and 1,4-dioxane have been used by many industries on Long Island, and recent evidence shows that exposure can affect human and ecological health.

• What sampling found. Groundwater from a subset of wells across the island was analysed for PFAS and 1,4-dioxane using state-of-the-science field and laboratory methods. Every sampled well contained at least a few PFAS chemicals, 1,4-dioxane, or both — in some cases at levels unsafe to drink under New York State's regulatory limits.
• Filling the gaps with machine learning. Sampling the whole aquifer system is expensive and few wells are available, which makes the threat hard to gauge. A recently completed USGS modelling exercise showed that machine learning can help predict where 1,4-dioxane occurs in the aquifer system; sampling more areas could improve the model.

Collecting a water sample. Photograph by the U.S. Geological Survey.

Summary

Safeguarding the quantity and quality of water in the island's sole-source aquifer system takes a unified, multidisciplinary approach. High-quality, near-real-time data feed numerical and machine-learning models that increasingly inform water-management decisions. The models in turn show where more data are needed in the areas most vulnerable to problems of quantity or quality, and help managers meet human needs while limiting harm to the island's environment and coastal ecosystems.

Sources

• Breault, R.F., Masterson, J.P., Schubert, C.E., and Herdman, L.M., 2021, Managing water resources on Long Island, New York, with integrated, multidisciplinary science: U.S. Geological Survey Fact Sheet 2021–3044. https://doi.org/10.3133/fs20213044
• The definition of a sole-source aquifer is the U.S. Environmental Protection Agency's, as cited by the fact sheet.
• The diagram and photographs are taken from the fact sheet's PDF.
• Rewritten in hubnx's own words.