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(a) Gravity springs
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Gravity springs occur in unconfined aquifers·. Gravity depression springs are found
where the ground surface dips below the water table. These springs have a limited
discharge and are likely to be ephemeral. Gravity overflow springs fed inter alia by
perched aquifers - a special type of unconfined aquifer - occur where the water is forced
to the ground surface by an underlying impervious layer of clay or solid bedrock, or a
change in bedrock type (geological contact). Perched aquifers are situated at some height
above the main groundwater body··. Gravity overflow springs have a higher and less
variable discharge than gravity depression springs, although the discharge may fluctuate
with low flows in the dry season (Hofkes, 1983).
(b) Artesian springs
Artesian springs occur in confined or partly confined aquifers where an overlying and
underlying impervious layer results in additional pressure, with the water unable to rise to
its free (piezometric) level. Artesian depression springs are similar to gravity depression
springs, although due to the pressure, the discharge is greater with less seasonal variation.
Artesian fissure springs are a sub-category of artesian depression springs, where water
issues from a fissure in the overlying impermeable layer. Artesian overflow springs, often
with a considerable recharge area at some distance from the eye, have a high discharge
(in view of the pressure) with little seasonal fluctuation. Artesian springs are less
susceptible to latrine effluent contamination because ofthe overlying impervious layer.
Secondly (as noted), the recharge area may be some distance away from the spring eye
(Hofkes, 1983).
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Readers are reminded that confined and unconfined aquifers are part of a single unified system, since
all confined aquifers have an unconfined area where most groundwater recharge takes place. The
water table in unconfined aquifers is at atmospheric pressure. The head at any place on the water
table is simply the elevation of the water table. The top of the groundwater therefore, is the water
table.
Perched water tables in level terrain indicate thatthe underlying material transmits water more slowly
than the rate at which the water enters the soil. Perched water tables in sloping land however,
indicate that both the underlying and downslope materials (either because they are less permeable
or because they are saturated), cannot transmit water at a rate which matches the rate of water
influx.