Ground water occurs in many different types of soil and rock strata. The amount of ground water that is stored and released from water-bearing strata depends on the porosity, size of pore spaces, and continuity of pores. Water-bearing porous soil or rock strata that yield significant amounts of water to wells are called aquifers. An aquifer can range from a few feet to hundreds of feet thick and can underlie a few acres or thousands of acres. For example, the Ogallala aquifer extends from Texas to South Dakota in the United States.
Water flowing through a soil or rock material follows the path of least resistance.
It will move through permeable materials and around impermeable ones. Ground water can flow slowly for hundreds of miles before emerging as a natural spring, seeping into a stream, being tapped by a well, or emerging into the ocean. Recharge areas may occur at large distances from the discharge areas of an aquifer. Because water moves through aquifers under the influence of gravity, recharge zones are always higher in elevation than areas of discharge. Recharge usually takes place in areas where permeable soil and rock materials are relatively close to the land surface and where there is an excess of water from precipitation.