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Figure 2 t Raised coral atoll (half width). Subperged volcanic core capped by coral
limestone many hundreds of melres thick. No strearns are present and lain water
penetrates rapidly to the fresh-water lens. Overpumping, with corrsequent lowering
of the lens surface to on below sea level, will yield salt watec.
No sLreams are present on raised atolls and all water for local use is derived frorn roof
cat.chments, drillholes, duq wells, or brackish ponds and seeps. Of Lhese the rnost prornising
for future development are drilled wells for comrnunity/horticultunal purposes, with noof
catchment and shallow dug wells conLinuing Lo be used where appropriate.
Investigations of ground water resources by drilling on raised atolls over the past few
years has confirmed that a thin layer of fresh water is present 'floating'on sea watef. The
principle, expressed by [he Ghyben-Herzburg formulan is a ratio describing the static relation
of fresh ground water and sea watet and states that for each uniL rneasurernent of fnesh
water above sea level, the salt water surf ace will be displaced 40 tirnes that unit
measurement below sea level. Thus 0.5 m of fresh water asl rs theoretically balanced by 20 m
of fresh water bsl, but in praclice seldom, lf ever is lhis precise ratio ratio aLtained.
Never[heless, until funther studies in oceanic island grr:und waten dynarnics are undertakertt
this simple 4{J:I ratio is accepLable as an elemerltary yardstick.
Two islands on which solne ground-waten work has been done are Niue and Tongatapu .
On Niue, Jacobson and Hill (f980) showed the fresh-waler aquifer [o be doughnut
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