Water in these areas, which may be comprised of rock types like sandstone, granite, limestone or shale, occurs in cracks, fractures and solution channels. The size of these openings and the degree to which they are interconnected can vary dramatically and will determine the extent to which the formation may serve as an aquifer. Two terms are important but often confused. Formations with very small and poorly connected open space yield little water and generally restrict the flow of water. As is the case with the terminology for groundwater and surface water, the language related to aquifers implies a straightforward and clear distinction between aquifers and aquitards and between confined and unconfined aquifers.
Reality offers more of a continuum than a simple, bright dividing line.
Geologic formations may be more or less permeable, more or less protected from pollution from adjoining or overlying layers. The average rate of groundwater movement through coarse sand, for example, may be about 360 feet per year. Through clay, groundwater may move less than one-half an inch per year. It can be difficult, if not impossible, to clean that sponge that you cannot see and you cannot squeeze. In this area, the water moves between soil particles and rocks that contain both air and water.