A few comments are appropriate on the occurrence and movement of "groundwater", that is, water in pore spaces or open fractures in the subsurface, below the water table.
In most inhabited places on Earth, potable and safe groundwater is easily available in sufficient quantities for house hold use, for farms and for towns and small cities. About one-half of all water sources in the United States are water wells and about one-third of all Americans use groundwater. Over one-quarter of a million new successful wells are installed each year in the U.S.
Clearly, groundwater is a common, valuable and widely used resource. Groundwater availability depends on "aquifers", bodies or units in the subsurface that are saturated, that is, all openings such as pores and fractures are filled with water. The most common and widely exploited aquifers are porous sand and gravel deposits, perhaps old enough to have hardened into rock, and limestone, in which openings have been enlarged by solution. Limestone caverns are an extreme case of this type of aquifer. All of these types of aquifer usually are found as beds or strata that are continuous over a wide region. They are well known to local drillers and groundwater scientists and most of them have been described in the scientific literature and publications of state groundwater surveys intended specifically to inform the public.
In some regions, particularly those underlain by crystalline rocks such as granite and massive limestone, for example, groundwater occurrence is limited to deposits of sand and gravel in the lowlands or to open fractures in the rock. Fractures near the ground surface may be seen as "traces" on aerial photographs and specialists can sometimes identify intersections of traces, which are good targets for well drillers. Surface manifestations of fractures may be observed in the form of springs or seeps or changes in vegetation. In fact, all potable groundwater flows toward a discharge zone. All dry-weather flow of streams is ground water discharge (except of course for releases from reservoirs, and sewerage in some cases). Such streams and springs were the chosen sites for early settlements and still provide water for thousands of towns and cities.
In fact, in settled parts of the world it is not possible to dig a deep hole or drill a hole, without encountering groundwater. If you are in doubt, and have a sump pump in your basement, just turn it off and in due course you will be able to measure the level of the water table without leaving home.
Of course there are exceptions that prove the rule. The western great plains have extensive regions underlain by thick clay (or shale if it has hardbed to rock) which is not permeable enough to allow groundwater to flow to a well. In addition, the water in these regions may not be potable.
Robert N. Farvolden