Because of the significance of this groundwater for human use, we consider the behavior of water underground in some detail here. It might seem complex at first, but water flow follows very simple laws of physics.
Water Tables and Aquifers
Water at the surface of the Earth seeps slowly into the soil, a process known as percolation. Water will percolate through the uppermost layer of soil and loose material that contains air, the aerated zone, down to a level called the water table. The water table is at the top of the permanently waterlogged or saturated level.
The water table is a critical level because it defines the top of the aquifer, the body of water that can supply drinking water. The flow of water underground is controlled by a number of factors including the permeability of the aquifer and the hydraulic gradient. Explained simply, the hydraulic gradient between two wells is the difference in hydraulic pressure (known as hydraulic head) divided by the distance between them. If the difference in hydraulic head is high, water will flow readily; if the difference is nil, then water will only flow if pumped. The hydraulic gradient at points at the top of the water table is generally level.
The permeability of a rock is a function of a number of factors that include the amount of pore space, the arrangement of pores, and the amount of surface tension from grains, especially tiny (micron-sized) clay minerals that have very high surface area. The larger the pore space, the more connected the grains and the less clay, the higher the permeability, and the more easily water flows. Conversely, where pore space is tight and poorly connected, and there is a lot of clay, permeability is low and water cannot flow readily.
The best aquifers are often made of rocks with both high porosity and high permeability such as sandstone, but rocks with generally lower porosity can also be highly permeable. For example, limestone is often jointed and is readily dissolved by groundwater, leaving the rock highly permeable; rocks such as granite and basalt are often heavily fractured allowing water to flow readily. Some of the most productive aquifers are called “contained” or “confined,” and are sandwiched between low-permeability layers called “aquicludes.” Common aquicludes are shale and mudstone layers. Such contained aquifers can have a high hydraulic gradient because the aquicludes hold a significant hydraulic head; confined aquifers often produce wells called artesian wells that, owing to substantial confining pressure, produce water without pumping.
In other cases, the tops of aquifers are not confined by an impermeable layer. Such aquifers are called unconfined and will, all other things being equal, be characterized by less confining pressure. Groundwater is continuously exchanging with other reservoirs in the hydrological cycle. Aquifers are recharged with water from rain and snow percolating through the aerated zone. Conversely, groundwater flows back into rivers and lakes or into wells and springs in a process known as discharge. The time water spends underground is called the residence time, which varies from a few days to 10,000 years or more. As we will see later, the water table can move downward as a result of drought and this is happening in arid areas today.