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Figure ) : Coral atoll (hslf width). Original island almost cornpletely strbmerged. The
peripheral rim rises to a few rEtres above sea level and comprises cemented
beach rock, conal, Band, conglomerate, and storm debris. Thin soil and sparse
vegetation are usuallY Present.
Hydrology
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Most pacific island etolls are remote and spersely populated. Liquid requirements.of
the inhabitants have traditionally been furnished by coconuts or rein-water tanks. Brackish
water, mainly for washing and baUhing, is available in some instances from holes dug in beach
sand at the fresh-waterl-salr-water interface. While these sources have proven adequate' at
teasL to sustain life on the atolls, a limiled fresh waler resource might be exploited from the
water lens (Ghyben-Herzburg lens) underlying each atoll, the principle of which is discussed
under Raised Atolls (above).-Further investigations inLo developing this reso.urce is necessary
as tidal fluctuation, rainfall, and the latere-l extent, thickness, and hydraulic characteristics
of the lens itself will vary from aloll to atoll.
Mather Og7:' estabtished a model for the development of the ground-water resources
on atolls in which he assumed that "abstracting ground water from a fresh-water lens is
equivalent to reducing vertical recharge, and an Jstimate of the sr.rstainable yield of lhe bns
can be madet'.
Jacobson (1975)
Ocean, and calculated
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applied Mather's hypoLhesis to Cocos.-(Kee-ling) atoll in the Indian
that an effective rechange to the aquifer of 500 mm/y, and a water
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