Ground water is water underground in saturated zones beneath the land surface. Contrary to popular belief, ground water does not form underground "rivers." It fills the pores and fractures in underground materials such as sand, gravel, and other rock. If ground water flows from rock materials or can be removed by pumping from the saturated rock materials In useful amounts, the rock materials are called aquifers. Ground water moves slowly, typically at rates of 7 to 60 centimeters per day in an aquifer. As a result, water could remain in an aquifer for hundreds or thousands of years. Ground water is the source of about 40 percent of water used for public supplies and about 38 percent of water used for agriculture in the United States.