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. In addition, earth materials can be sampled directly at any depth during the drilling of the well. Consider the elevations above sea level at points A and B in an unconfined aquifer and C in a confined aquifer. Now consider the addition of wells with short screened intervals at these three points. The vertical distance from the water level in each well to sea level is a measure of hydraulic head or head, referenced to a common datum at each point A, B, and C, respectively.