The island's sandy soils allow water to infiltrate rapidly to the water table and to store large amounts of ground water in the aquifer. Course-grained soils also allow rapid downward movement of nutrients and chemicals to the ground water and waterways. The flow rate through this permeable soil is a few feet per day. Clay and other fine-grained soils are less permeable. Water runs over the surface or infiltrates slowly at a rate of 1-3 feet per year.
Ground water is the island's only source of drinking water.
Nantucket taps into the ground water aquifer from public wellfields and from private on-site wells. A private well draws a small amount of water, sufficient to meet the needs of the individual property owner, and the drawdown area is localized. Residential areas and farms lying outside of town and Siasconset village are serviced by private wells. Wannacomet and Siasconset Water Companies draw large quantities of ground water from two public wellfields to supply residential and commercial drinkng water needs. The drawdown areas for each of these wellfields extends far beyond where the actual pumping occurs.