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Irrigation Association of New York

Long Island’s Groundwater – Understanding the Water Below

If you are in a water-related business on Long Island, there is a lot you should know about groundwater. Here is a thumbnail overview of key information about water use and the aquifer system that provides all Long Island’s water.

What is an aquifer? It is a geological formation (sand and gravel on Long Island) that stores, transmits and yields useable quantities of water, known as groundwater. Certain formations, such as clay, store large amounts of groundwater, but do not release it to pumping and thus, are not aquifers.

The aquifer system beneath Long Island – made up of three main aquifers: Upper Glacial, Magothy and Lloyd – is designated a sole-source aquifer, meaning it provides 50 percent or more of a community’s water. Long Island’s aquifers provide 100 percent of the region’s water. The designation is given by the U.S. Environmental Protection Agency.

All the freshwater in the aquifers comes from precipitation (rain, snow, etc.). Average annual rainfall on Long Island is about 44-46 inches. Half of this water (approx. 22-23 inches) will seep through the sandy soils and into the aquifers to become groundwater; a process is known as recharge. In 2015, rainfall was about nine inches below average, meaning that less water than normal replenished the aquifer system.

Per-person water use on Long Island is above the national average. Water use on Long Island swings between winter use, which is about 80-100 gallons per person per day, to summer use which increases by 200 to 400 percent, and is the highest in the Northeast. The majority of water use in the summer is outdoor use for irrigation, but also includes swimming pools and water features.

There are many misunderstandings about the consequences of taking too much water from the aquifers and how much water is available. Here are the basics.

The U.S. Geological Survey reports that the amount of water stored in the aquifers is around 90 trillion gallons. While it sounds like a tremendous amount, you should know that we can only withdraw between 5 to 10 percent of that water through pumping. That means the available water is about 4.5 to 9 trillion gallons. But, in order to protect the groundwater from saltwater intrusion and the loss of streams, wetlands, ponds and lakes, it is essential that the majority of the total groundwater should stay in the aquifers.

Thus, the total amount of water that the aquifers can safely produce is significantly less than the total amount of water in the system.

Another key point is that most groundwater recharge occurs during the months October through March. Once the weather warms and the growing season begins, very little recharge occurs. From April through September, when recharge is at its lowest, the water demand is at its highest. This puts great stress on the aquifers, lowering the water table, removing billions of gallons of water and reducing the pressure that groundwater exerts to keep the ocean from invading the coastal parts of the aquifers.

When it rains or when lawns and gardens are watered during the summer, virtually all the water that is not taken up by the plants evaporates or runs off and is not returned to the aquifers. This is contrary to the popular notion that excess lawn watering simply puts water back into the aquifers. So, water waste is a real downside to poor irrigation practices, because wasted water is lost from the aquifer system without any benefit. This is why it is so tragic to see lawn irrigation systems spraying full blast during a summer rain storm.

The challenge is to find better techniques and technologies that put the water in the soil where the plants need it and not in the air where it evaporates. The soil too warms and contributes to evaporation after plants have absorbed the water they need. This is why recharge is so low during the summer.

We do not have an unlimited amount of water in the aquifer system. Nassau County needs to reduce the overall amount of water taken from the aquifers. Suffolk County has more leeway regarding quantity, but if it wants to keep its rivers and streams full then there are limits to the amount of water that should be pumped from its portion of the aquifers too. Water conservation is an issue that will be receiving much more attention in the coming years. Summer water use will be the biggest target for limiting water waste and excessive use. We all need to work together to find the most effective and reasonable ways to preserve and manage this essential natural resource beneath Long Island.

Sarah Meyland is director of the Center for Water Resources Management at New York Institute of Technology in Old Westbury, N.Y