Science of Water : Concepts and Applications

All about Water 33
Ground
Confining layer
Clay
Flow
A confi ned aquifer is sandwiched between two impermeable layers that block the fl ow of water.
The water in a confi ned aquifer is under hydrostatic pressure. It does not have a free water table (see
Figure 2.5).
Confi ned aquifers are called artesian aquifers. Wells drilled into artesian aquifers are called
artesian wells and commonly yield large quantities of high quality water. An artesian well is any well
where the water in the well casing would rise above the saturated strata. Wells in confi ned aquifers
are normally referred to as deep wells and are not generally affected by local hydrological events.
A confi ned aquifer is recharged by rain or snow in the mountains where the aquifer lies close to
the surface of the Earth. Because the recharge area is some distance from areas of possible contamination,
the possibility of contamination is usually very low. However, once contaminated, confi ned
aquifers may take centuries to recover.
Groundwater naturally exits the Earth’s crust in areas called springs. The water in a spring can
originate from a water table aquifer or from a confi ned aquifer. Only water from a confi ned spring
is considered desirable for a public water system.
GROUNDWATER QUALITY
Artesian well
Bedrock
Clay
Recharge
area
Confined
aquifer
FIGURE 2.5 Confi ned aquifer. (From Spellman, F.R. 1996. Stream Ecology and Self-Purifi cation: An
Introduction for Wastewater and Water Specialists, Lancaster, PA, Technomic Publishing Company.)
Generally, groundwater possesses high chemical, bacteriological, and physical quality. Groundwater
pumped from an aquifer composed of a mixture of sand and gravel, if not directly infl uenced by
surface water, is often used without fi ltration. It can also be used without disinfection if it has a low
coliform count. However, as mentioned, groundwater can become contaminated. When septic systems
fail, saltwater intrudes, improper disposal of wastes occurs, improperly stockpiled chemicals
leach, underground storage tanks leak, and hazardous materials spill. Fertilizers and pesticides are
misplaced, and when mines are improperly abandoned, groundwater can become contaminated.
Rain