The aquifer may be a layer of gravel or sand, a layer of sandstone or cavernous limestone, a rubbly zone between lava flow, or even a large body of massive rock, such as fractured granite. The quantity of water that a given rock can contain depends on the rock's porosity - the total measure of the spaces among the grains or in cracks that can fill with water. If water is to move through rock, the pores must be connected to one another. If the rock has many connected pore spaces big enough that water can move freely through them, the rock is said to be permeable. Although it moves slowly, it may travel for miles before it emerges as a spring, seeps unseen into a stream, or is tapped by a well.
Ground water is simply water that fills pores or cracks in subsurface rocks.
It is ultimately replenished by precipitation, according to the local climate and geology, and is unevenly distributed in both quantity and quality. When rain falls or snow melts, plants and soils take up water. Some water is evaporated to the atmosphere from plant leaves, most runs off to streams, and some percolates down into the pores or cracks in rocks. When rain falls, the first water that enters the soil replaces water that has been evaporated or used by plants during a preceding dry spell. After the thirsty plants and soil have had enough water and if rain continues to fall, the excess water will drain to the water table, the top of the zone in which openings in rocks are saturated.