Geoscience Reference
In-Depth Information
Upland recharge area
Water infiltrates underground
Perched water table on clay lens
Springs
Unsaturated zone
River or lake
Saturated zone
Aquiclude
Bedrock
Aquiclude
Sandstone aquifer
Figure 15.3 Water tables and aquifers.
A water table is the top of the saturated zone of rock and soil.
Depending on the kind of rock deeper in the ground, water may collect in saturated zones called aqui-
fers, enclosed by layers of impermeable rock called aquicludes. An aquifer is a saturated zone that holds
enough water to serve as a source for irrigation or drinking water.
Human Interactions
with Groundwater
In some places, the groundwater system is simple and con-
sists of a single saturated zone of broad regional extent that
overlies an aquiclude. At other places, however, the situation
may be more complex with a layered assemblage of saturated
zones, aquicludes, and unsaturated zones (Figure 15.3). Where
such complexity occurs, saturated zones deep within the system
may be slowly
recharged
by water gradually moving from the
saturated zones higher in the column or from sources of water
far away. In still other settings, a saturated zone may be highly
localized because it occurs in association with an isolated
aquiclude such as a body of clay-rich sediment deposited by
an ancient glacier or stream. Such a localized saturated zone
is called a
perched water table
because it is located above
the primary groundwater body. Regardless of the setting of a
specific saturated zone, or whether it is regionally extensive or
more isolated, if it is sufficiently large to be a significant source
of water for people, it is called an
aquifer
.
The High Plains Aquifer
The most extensive aquifers occur in regions where abundant
sandy sediments or sandstone rocks are found across a broad
geographical area. Aquifers are one of the most important natu-
ral resources on Earth. Perhaps the best-known and most heav-
ily utilized aquifer in North America is the
High Plains Aquifer
.
Also known as the
Ogallala Aquifer
, this groundwater reservoir
is found in the Great Plains region of the United States and
underlies portions of eight states, ranging from South Dakota to
Texas (Figure 15.4). The High Plains Aquifer is largely formed
in the thick, porous deposits of unlithified sand and gravel of the
Ogallala Formation, which consists of sediments that weathered
and washed out from the Rocky Mountains between 19 and 5
million years ago. These sediments covered the western part of
the Great Plains and slowly filled with water from rainfall and
snowmelt during the ensuing millennia. Much of this groundwa-
ter accumulated during the ice ages, between about 1.6 million
and 10,000 years ago, when the regional climate was much wet-
ter than it is today.
Perched water table
A localized area of saturated sediment
that is elevated above the regional water table by a zone of
impermeable rock or sediment.
Aquifer
A geological formation that contains a suitable
amount of water to be accessed for human use.
