Cape Cod has what is called a sole-source aquifer. An aquifer is an underground rock or sand body that permits water to move through with ease. This ground water is the only source of drinking water for residents of western Cape Cod. Aquifers are classified as confined or unconfined. Confined aquifers are overlain by materials that have low permeability and receive little or no direct recharge from rainfall. The movement of water into and out of confined aquifers is slow. Unconfined aquifers are exposed to the atmosphere and are continually recharged by the percolation of rainfall, snowmelt, or water from streams or rivers. Recharge rates vary with the seasons and from year to year. Under natural conditions, discharge, or water flowing out of the aquifer, is balanced by recharge.
The unconfined Cape Cod aquifer is segmented into six sections called lenses. The upper limit of the lenses, or the water table, has about the same shape as the land above. The largest lens is on the Upper Cape. The highest point of this lens is right under the Massachusetts Military Reservation (MMR). Ground water flows perpendicular to the contours of the water table, much like rainwater would flow down hillsides above ground. It moves at the rate of about a foot a day until it reaches the sea.
Cape Codders get their drinking water from the aquifer. Public water supply systems bring drinking water to about 70 percent of the population of MMR and the towns of Bourne, Sandwich, Mashpee, and Falmouth. The other 30 percent of the population use domestic wells to supply their drinking water. The total average daily water demand on western Cape Cod is about 6.4 million gallons per day. This figure was calculated on an average of the amounts of water used during the vacation season and during the off season. During the summer season, water demand rises to 10.1 million gallons per day; during the off season, demand is at 5.2 million gallons per day.
Here's a paradox - ground water is the primary drinking supply for the Cape, but it is also the primary disposal area for wastewater generated by the population of the Cape.