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

Fragmental materials discharged by volcanos. such as ash and cinders, have
been known to form excellent aquifers where particle sizes are sufficiently
large. Their water-yielding capabilities vary considerably, depending on the
complexity of stratification, the range of particle sizes, and shape of the
particles. Examples of excellent aquifers of this type are to be found in
Central America.
Metanwrphic rock is the name given to rocks of all types, igneous or
sedimentary, which have been altered by beat and pressure. Examples of
these are quart&e or metamorphosed sandstone, slate and mica schist from
shale, and gneiss from granite. Generally, these form poor aquifers with water
obtained only from cracks and fractures. Marble, a metamorphosed lime-
stone, can be a good aquifer when fractured and containing solution channels.
With the above description of the three main rock types, it should now be
easier to understand why an estimated 95 percent of the available ground
water is to be found in sedimentary rocks which constitute only about 5
percent of the earth’s crust. The wells described in this manual will be those
constructed in unconsolidated sedimentary rocks which are undoubtedly the
most important sources of water for small community water supply systems.
Role of Geologic Processes in Aquifer Formation
Geologic processes are continually, though slowly, altering rocks and rock
formations. So slowly are these changes taking place that they are hardly
perceptible to the human eye and only barely measurable by the most
sensitive instruments now available. Undoubtedly, however, mountains are
being up-lifted and lowered, valleys filled or deepened and new ones created,
sea shores advancing and retreating, and aquifers created and destroyed.
These changes are more obvious when referred to a geologic timetable with
units measured in thousands and ,millions of years and to which reference can
be made .in almost any book on geology.
Geologically old as well as young rocks may form aquifers but generally
the younger ones which have been subjected to less compression and
cementation are the better producers. Geologic processes determine the
shape, extent, and hydraulic or flow characteristics of aquifers. Aquifers in
sedimentary rock formations for example vary considerably depending upon
whether the sediments are terrestrial or marine in nature.
Terrestrial sediments, or materials deposited on land, include stream, lake,
glacial, and wind-blown deposits. With but few exceptions they are usually of
limited extent and discontinuous, much more so than are marine deposits.
Texture variations both laterally and vertically are characteristic of these
formations.
AZZMaI or stream deposits are generally long and narrow. Usually
SUbSUrface, or below the valley floor, they may also be in the form of terraces
indicating the existence of higher stream beds in the geologic past. The
material in such aquifers may range in size from fine sand to gravel and
boulders. Abandoned stream courses and their deposits are sometimes buried
under wind-home or glacial depos,ts with no visible evidence of their
existence. Where a rapidly flowing stream such as a mountain stream
encounters a rapid reduction of slope, the decrease in velocity causes a
9