All of the water supplied by BNL comes from beneath the ground and so is referred to as groundwater. Groundwater is stored in a sandy, geological formation known as an aquifer. Water in the aquifer originates as precipitation that percolated down through the soil.

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The Long Island aquifer system is made up of three primary formations lying one on top of the other (see diagram above): from the surface to about 150 feet down is the Upper Glacial, from 150 feet to 1,500 feet is the Magothy, and from 1,000 to 2,000 feet is the Lloyd. Drawing up to 1,000 gallons per minute, the Lab's six drinking-water wells tap into the upper Glacial aquifer.