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Q) streems. Direct ebstrection via water intake in the stream bed
villages, or simply by collection in bucketg and tine.
(l) Gallerieo. Tren,cheg drg below the water tsble in streem valleys,
pipes and back filled. Ground weter seeps through the poroua pipei
via reaervoina or directly by pipeline to villages. -
End reticuleted to
lined with porous
and is reticulated
(4) Maui (okimming) Tmnels. Vertical or inclined shafts or adits dug to about the weter
tabler end me or more horizontal tunnels congtructed laterally iuat betow the water
level. Highly successful on Hawaii and also used 6t T$iti.
(5) Springs. Perched aquifere teaking laterally form ryrings at varioug levels around the
volcanic alopes. Ssne of these eleveted prings, well inland, end near villages, have
been tapped and the weter used for drinking, waahing, and bathing. At lower levele
where they emerge at impermeable dike, gill or flow contacts, aprini water is directed
into hand-dug rock-lined gtrallow 'troughsr and used for community purposes. Springe
also bubbb up through the eand around the foreshore, and holes 'scooped in -looie
material sre used for washing and bathing.
(6) Dug Wells. Hand-dug welle in weathered volcanic rocks, or in elluvial depoaits near the
coast provide small a.rpplies for individual familiee. Their yield is generally small
beceuse of the limite imposed by physically digging below the water tablJ.
(7) Driltholes. Generally eited on volcanics and drilled to intersect perched or neer
eea-level aquifers. Their yield is relatively small but following testing, it can be
regulated to euetain a constant flow for en indefinite period. The iepth of the hole end
pump inteke must be indicaled bef ore drilling commences, and the collar height
surveyed accurately relative to aea level.
In the Southern_ Cook Group, holes drilled in the volcanics from LJ m to 100 m depth
are being developed.for community supplies. Punp test analyses indicate that about f Vi is
available from the better holea, although in all csses tragsmissivity (gn indication of the
ability of the. aquifer to trensmit water) is low at b.Z m./d to ti mzla (Waternouse and
Petty, in prees).
Raiged coral atolls
Raised atolls comPrise a wbnrerged volcanic core slrrounded and capped by generally
thick coral limestone (Figure 2). The limestone, estimated to be J00 m'ffrict on-Niue, is
asgunEd to heve formed mainly in response to tectonic elevation and subsidence of the
volcanic edifice, and. to e lesser degree to eustatic fluctuations of sea level. Tmgatapu,
Niue, Nauru and Banaba are good examples of raised atolls.
Gcology
Sane raised atolls show the original atoll topography, as on Niue, with an old, elevated,
inland lagoon gJrrounded. by a peripheral reef ri'dg6. ' Odtt"rt are relatively flat, or gently
tloping, the maximum height generally being betwjen 60 m and B0 m asl., e.g. Tongitapr4
Banaba
Raised atolls are considered !o be intermediate in age between the young high islands
and old coral atolls because the volcanic base m which thl coral became established is now
nowhere in evidence. but it still represents a signif icant part of the island's mass. progressive
overell eubsidence, during which time the coraf continued to grow upwards, may have been in
exce8s of I kilometre over the last l0 million years, as thL oldest corels recognized are
Oligocene to Miocene in age.
Hydrology
The highly-permeable limestone or corel-sand crrface m raised atolls allowe rainfall to
drein rapidly downwards to the fresh-water lena near sea level. In a f ew instancee
impermeable clays and small, impernreable depressions pond the rain water locally, but laterel
runoff into nnre porous beds and high-evapoiation rates contribute t" ."Jii-**"ce drying.