zone is the water table. The depth to the water table is highly variable around the
planet, and humans have gravitated to the areas of large water supply close to the
surface. The depth to the water table varies seasonally and topographically.
Summer seasons usually result in the drop of the water table with winter recharge.
The depth of the water table generally follows the slope of the land above and is
closest to the surface in stream valleys. Humans have made impressive changes
in the water table. Around wells, there is usually a cone of depression but as many
wells tap the zone of saturation the water table is drawn inexorably lower. In Ft.
Worth, Texas, the water table has dropped 125 m in the last century. Such drops
require increasing energy use to draw the water to the surface and make use of
the water less economically efficient.
In some places, layers of impermeable sediments of rocks surround parts of the
saturated zone and so impede water from leaving that zone. If the amount of
groundwater so confined is large, this confined zone is an aquifer. Aquifers are
composed of materials like sandstone that are conducive to the movement of
water. Wells tapping an aquifer are kept supplied by groundwater moving toward
the wells. Major aquifers around the world include the Ogallala of the U.S. Great
Plains,theGreatArtesianBasinofAustralia, the Guarani aquifer of South
America, and the Nubian aquifer of Africa. In some cases, these aquifers do not
have substantial recharge, being the relicts of wetter times at the end of the Pleis-
tocene ice age thousands of years ago.
Beneath the saturated zone is the fourth zone, known as the waterless zone.
Usually, this zone begins a few kilometers under the surface and exists because
pressure from the overlying materials precludes the existence of pores in which
water can be stored.
The flow of groundwater is considerably slower than in surface water. Common
rates are between 15 and 125 m per day with some places having rates of centi-
metersper year. Rather than straight flow, groundwater flow is confined to path-
ways using the tiny openings existing in the surrounding materials. The flow is
energy by differential pressures. Unlike surface water, groundwater is able to
move up or down depending on the direction of the pressure gradient. Sometimes,
there is enough pressure involved that a break in a confining layer (by natural
circumstances or by a well) allows the groundwater to escape to the surface where
it is known as artesian water.
It has been estimated the world’s groundwater use is 600–700cubic kilometers
per year and represents the greatest tonnage of any extracted material. Use is quite
variable by circumstance. Just over a fifth of the United States water consumption
is supplied by groundwater withdrawal. In countries like Saudi Arabia, the ratio
exceeds four-fifths. In that many aquifers extend beneath national boundaries, the
scene is set for potential international tensions as groundwater becomes scarcer.
Groundwater 165