groundwater is out of sight and moves much slower than streams, its nature is not
appreciated by most people. Yet, its importance looms large in that there is two-
and-a-half times more groundwater than in all the streams and lakes on the planet.
Not all groundwater is potable. The contact of underground water with rock
leads to the solution of salts into the water. In some cases, the saltiness is much
higher than that found in the ocean. In other places with moderately “hard” water
containing positively charged ions (cations) of calcium and magnesium, the salts
can be dropped out of solution by using water softener, thus making it usable for
human purposes.
Groundwater can be found underneath most of Earth’s surfaces but not in consis-
tently developable amounts by either depth or geographical region. Most ground-
water is within a kilometer of the surface, but some water has been found as deep
as 10 km. With depth, however, the pore spaces in rocks and sediments become con-
siderably smaller because of the pressure of overlying materials, and groundwater is
essentially trapped in place. The deeper groundwater is connate or “fossil water”
that was trapped as the rock layer was laid down. This deep groundwater is usually
brought to the surface as an unavoidable consequence of drilling for oil and natural
gas. It is almost invariably salty because of the immense amounts of time the water
has had to dissolve surrounding materials. Sometimes these brines can be economi-
cally tapped and important materials such as iodine extracted.
Groundwater is sometimes conceived as a gigantic underground lake or stream,
but this is not close to the truth. Near Earth’s surface, there are four zones differ-
entiated with respect to groundwater: the zone of aeration, the zone of saturation,
the zone of confined water, and the waterless zone. The depths, amount of water,
and flow characteristics have immense variations according to the type and struc-
ture of the underground materials.
The zone of aeration is the topmost zone and abuts the surface. It is composed
of solid materials with pore spaces occupied by air and water. As the sky precipi-
tates, water infiltrates into the zone of aeration, filling the pore spaces for a while.
The pore spaces drain of water via gravity and via evaporation directly to the
atmosphere and via transpiration from plants whose roots absorbed water from
the zone of aeration. The nature of the zone of aeration changes dramatically after
each precipitation event. It has vertical depths that can extend to hundreds of
meters or much less than a meter. This is not a zone in which one would normally
situate the bottom of a groundwater withdrawal well in that the amount of water
present is so highly variable.
The zone of saturation is beneath the zone of aeration, and it is this layer into
which wells are drilled to tap the groundwater supply. The zone of saturation has
gained its water gravitationally from above and all of its spaces are completely
filled by water, which is properly known as groundwater. The top of the saturated164 Groundwater