60
Confined aquifers, also known as artesian or pressure aquifers, occur where
groundwater is confined under pressure greater than atmospheric pressure by
overlying relatively impermeable strata. In such cases when a well penetrates
upto the confining bed, water level will rise above the bottom of the
impermeable bed. Fig. 6.2 depicts the actual disposition of confined and
unconfined aquifers. It will be seen therefrom that the pressure exerted on the
confining bed is equivalent to the water table of the recharge zone of the
confining aquifer and may be far away from the area where the well is dug. The
pressure level or pizeometric surface as it is called for confined aquifers may
be higher than the water table of the area. When the pizeometric surface goes
above the given level, 'autoflow' or 'artesian' conditions occur.
Perched Water Table
In a thick saturated zone, there may be localized zone where the water table
may be different than the regional water table because of presence of a local
impermeable bed hindering vertical flow of groundwater and thus creating an
independent saturated zone. Such localized water table is known as perched
water table (Fig. 7.2)
Fig. 6.2. Confined and unconfined aquifers
(After Karanth, 2004)
Recharge
area
water
table
Piezometic surface
Ground
surface
Flowing
well
Water
table
well
Artesian
well
Water table
Impermeable
strata
Ground surface
Perched water tables
Impermeable strata
Unconfined
aquifer
Confining stratum
Confined aquifer
Water table
