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shaped. [n the cenlre of the island (the old lagoon) lhe top surface of the aquifer is l.g] m
asl. The thickness of the fresh-waten lens, inferred from resistivity data, ranges from 40-80
m in the centre to 50-170 m beneath the former atoll rim, and sea level at thL coast. They
concluded that the inregular shape of the lens is probably due to variations in permeability
within the limestone.
. J;cobson and Hill (ibid) calculated that fresh water storage on Niue amounted to about
4-6 km-, and that punping and other Lests indicated a high permeability in the afluifer
(specific capacity of LZ.(A l/s/m at one drill site). They zuggehed a safe yietO ot ff mrTOlha
in their model.
On Tongatapu some 60 dug or drilled wells from a few rnelres to over 60 m deep
penetrated coral limestone. Pfeiffer (1971) eslimated that between 5% and I5% of the
averaqe annual rainfall of 1750 mm penetrated?to the aquifer, and by assuming a rechaqqerof
about l0% over ther,entire island (260 km') he estimated thal of the 45 x l0rm)/y
replacement, 25000 m'ld can be extracted from wells.
Waterhouse (1976) established that the lens zurface stood 0.5 m to 0.75 m asl in lhe
Tmgatapu Water Reserve, and frorn punp test data calculated a transmissivity at IZOO mtld.
The pump-test data, and rainfall figures were further analysed by Hunt (1978) who calculated
:_p^91""+ility of 1.5 cm/s, and estimated that 259t0 to ]0% of the average rainfall, or about
75000 m-, reached the aquifer.
From the above it appears that some 25000 ml to 75000 ,l/a rnigt,, be abstracted from
the aquifer underlying Tmgatapu.
Coral atolls
These mcur as a ring-shaped coral reef appearing as a low, roughly-circular, elliptical,
or horseshoe-shaped conal island, or a ring of closely-spaced coral isEts encircling or 'nearly
encireling a shallow lagoon. They may vary in diameter from I km to over 100 km and are
particularly common in the western and central Pacific. The zubnrerged, deeply-buried rock
on which the coral limestone originally grew is presunnd to be volcani- (Figure )).
Tfle Gilbett Islands (Kiribati), Tuvalu, Tokelau, Nonthern Cook Group, and French
Polynesia include good examples of true coral atolls.
Geology
Coral atolls are formed by contemporaneous upwards growth of reef coral during
gradual $bsidence of the volcanic s.rbstructure. Baillard (I9Sf) described the birth anO
gnowth of an atoll through its different stages of formation from the development of the
volcanic edifice m which the coral beceme established (probably in the mid Tertiary to the
present).
Under favourable conditions of climale, temperat.ure, and depth, coral larvae zuspended
in the sea water form scatLered colonies sr the volcanic flanks of the erbnnrged or emerging
island. They eventually form an encircling reef which itself is raised or lowered by the samJ
tectonie forceg controlling the elevation and srrbsidence of the volcano. The fins-l stages of
atoll formation are considered to be subsidence srd erosion, and es the volcano sinks to
greater depths the coral continues to grow upwards and ultimately cornpletely covers the
volcenie srbstructurc. Gradually the fringing barrier rcef restricts the circulation of water
between the open (rcean and the lagoon to the passages in the reef and lagoonal sediments
aceumub[e in the largely shallow, protected basin in which grow a few coral heads. The
outcr rim at this stage coffpriges cemented beach rock, coral, sand, conglomerate, storm
debris' and thin coil' end rarely etteins a height of npre than a few firetres *L
By virtue of thein sinking to great depths and the long time required to accornplish
thi!' etolls sre thought to be affrcrng the geologically oldeet of oceanie islandg. This
arsunptionr however, muet be treated wit,h caution beceuse eroeional criteria, me of the
tools ured in datingr (Kear 1957) are inpossible to apply when the volcano in queetion is
cotnpletely rubrrerged and capped by thick coral (1400 m m Eniwetok in the Marshall Islande)
(Menard f964). In addition, a KlAr date m a near etoll, Aitutaki, is younger than that for
Rrotonga (Dalrymple et el. 1975), a young volcano by any stenderd.