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

IC FQRXA~NMIIS AS AQUIFERS
For convenience, . :)iogists describe all earth materials as roclis. Rocks
may be of the ~~onti.J ‘,-refl type (held firntiy together by compaction,
cementation and otLe:, K A~.,P sut!~ :ts granite. sandr’,>r-* e;\d limestone or
rtrncomoiidated type (IL, .-:aterials) such as clay, sand Jnd gravel. The terms
hml and soft are also usr,~: r.o describe consolidated and unconsolidated rocky
respectively.
Aquifers may be composed of consolidated or unconsolidated rocks. The
rock materiais must be sufficiently porous (contain a reasonably high
proportion of pores or other openings to solid material) and be sufficiently
permeable (the openings must be interconnected to permit the travei of water
through them).
Rock Classification
Rocks may be classified with respect to their or+;? into the three main
categories cf sedimentary rocks, igneous rocks, and lxidmorphic rocks.
Sedimentary rocks are the deposits of material derived from the
weathering and erosion of other rocks. Though constituti;.; only about 5
percent of the earth’s crust they contain an estimated 95 percent of the
available ground water.
Sedimentary rocks may be consolidated or unconsolidated depending
upon a number of ?‘zctors such as the type of parent rock, mode of
weathering, means of transport, mode of deposition, and the extent to which
packing, compactiorl, and cementation have taken place. Harder rocks
generally produce sediments of coarser texture than softer ones. Web;>erin;
by mechanical disintegration (e.g. rock fracture due to temperature varlu
tions) produces coarser sediments than those produced by chemical decom-
position. Deposition in water provides more sorting and better packing of
materials than does deposition directly onto land. Chemical constituents in
the parent rocks and the environment are responsible for the cemerltatioll of
unconsolidated rocks into hard, consolidated ones. These factors aiso
influence the water-bearing capacity of sedimentary rocks. Disintegrated shale
sediments are usually fine-grained and make poor aquifers while sediments
derived from granite or other crystalline rocks usually form good sand and
gravel aquifers, particularly when considerable water transportation has
resulted in well-rounded and sorted particles.
Sand, gravel, and mixtures of sand and gravel are among the unconsoli-
dated sedimentary rocks that form aquifers. Granular and unconsolidated,
they va.ry in particle size and in the degree of sorting and rounding of the
particies. Consequently, their water-yielding capabilities vary considerably.
However, they consitute the best water-bearing formations. They are widely
distributed throughout the world and produce very significant proportions of
the water used in many countries.
Other unconsolidated sedimentary aquifers include marine deposits,
alluvial or stream deposits (including deltaic deposits and alluviai fans), glacial
drifts and wind-blown deposits such as dune sand and loess [very fine silty
deposits). Great variations in the water-yielding capabilities of these forma-
tions can also be expected. For example, the yield from wells in sand dunes
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