As distance from the stream increases, flow paths to the stream are longer and deeper. The horizontal scale of the figure could range from tens to hundreds of miles. In shallow, local-scale flow systems, ages of ground water at areas of discharge can vary from less than a day to a few hundred years. Surface and subsurface earth materials are highly variable in their degree of particle consolidation, the size of particles, the size and shape of pore or open spaces between particles and between cracks in consolidated rocks, and in the mineral and chemical composition of the particles. Ground water occurs both in loosely aggregated and unconsolidated materials, such as sand and gravel, and in consolidated rocks, such as sandstone, limestone, granite, and basalt.
Earth materials vary widely in their ability to transmit and store ground water.
For example, the interconnected pore spaces in sand and gravel are larger than those in finer grained sediments, and the hydraulic conductivity of sand and gravel is larger than the hydraulic conductivity of the finer grained materials. The ability of earth materials to store ground water also varies among different types of materials. For example, the volume of water stored in cracks and fractures per unit volume of granite is much smaller than the volume stored per unit volume in the intergranular spaces between particles of sand and gravel. Wells are the principal direct window to study the subsurface environment. Not only are wells used to pump ground water for many purposes, they also provide essential information about conditions in the subsurface.