An aquifer is a body of sand or porous rock capable of storing and producing significant quantities of water. An aquifer may be a layer of loose gravel or sand, a layer of porous sandstone, a limestone layer, or even an igneous or metamorphic body of rock. An aquifer may be only a few feet to hundreds of feet thick. Aquifers occur near the surface or buried thousands of feet below the surface. It may have an aerial extent of thousands of square miles or a few acres. The key requirements are that the layer or body has sufficient porosity to store the water, sufficient permeability to transmit the water, and be at least partly below the water table. The water table is the elevation of the top of the completely saturated (phreatic) zone. Above the water table is the vadose or unsaturated zone where the pore spaces...