Water Well Manual (USAID).pdf - The Water, Sanitation and Hygiene

Ground
surfuce7
Nonflowing
artesian
Woter-
Fig. 2.4 TYPES OF AQUIFERS.
Flowing
. .
artesian
Recharge area
at autc?opping
of formation
Water usually enters an artesian aquifer in an area where it rises to the
ground surface and is exposed (Fig. 2.4). Such an exposed area is called a
rechmge area and the aquifer in that area, being unconfined, would be of the
water-table type. Artesian aquifers may also receive water underground from
leakage through the confining layers and at intersections with other aquifers,
the recharge areas of which are at ground level.
Aquifer Functions
The openings and pores in a water-bearing formation may be considered as
a network of interconnected pipes through which water flows at very slow
rates, seldom more than a few feet per day, from areas of recharge to areas of
discharge. This network of pipes, therefore, serves to provide both storage
and flow or conduit functions in an aquifer.
Stooge fin&on: Related to the storage function of an aquifer are two
important properties known as porosity and specific yieZd.
The porosiZy of a water-bearing formation is that percentage of the total
volume of the formation which consists of openings or pores. For example,
the porosity of one cubic foot of sand which contains 0.25 cubic foot of
open spaces is 25 percent. It is therefore evident that porosity is an index of
the amount of ground water that can be stored in a saturated formation.
The amount of water yielded by, or that may be taken from, a saturated
formation is less than that which it holds and is, therefore, not represented by
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