Groundwater - Mathematics and Statistics for the Environment - page 580

Ground
Surface

Rain

Inltration

Percolation

Water Table

Water Table Well

Unconfined Aquifer

FIGURE 21.1 Unconfined aquifer. (From Spellman, F.R., Stream Ecology and Self-Purification, , Technomic,
Lancaster, PA, 1996.)

Recharge
Area

Rain

Ground

Artesian Well

Conning Layer

Clay

Clay

Conned
Aquifer

Flow

Bedrock

FIGURE 21.2 Confined aquifer. (From Spellman, F.R., Stream Ecology and Self-Purification, , Technomic,
Lancaster, PA, 1996.)

Confined aquifers are artesian aquifers. A well drilled in an artesian aquifer is called an arte-
sian well and commonly yields large quantities of high-quality water. A well in a confined aquifer
is normally referred to as a deep well and is not generally affected by local hydrological events. A
confined aquifer is recharged by rain or snow in the mountains where it is close to the surface of
the Earth. Because the recharge area is some distance from the area of possible contamination of
the confined aquifer, the possibility of contamination is usually very low. However, once contami-
nated, it may take centuries before it recovers. When groundwater exits the Earth's crust it is called
a spring . The water in a spring can originate from a water table aquifer or from a confined aquifer.
Only water from a confined aquifer spring is considered desirable for a public water system.