Rt€@lll

-6t
table glevation ranging from 0.I7 to 0.16 m asl, would produce a sustaineble yield of about
2oo m'ld.
The factors governing the shape of fresh-water lens are rainfall periodieity of
droughts, tides, seepage, and abstraction nates. To minimize drawdown, Jecobson (ibid)
recommends infiltretion galleries rather than dug wells. Punping should be cemied out
continuously and at a rate such that. the thickness of the fresh-water lens is maintained at
more than half the original thickness. It follows that abstraction points should be located in
places where the lens is thickest.
Because of the delicate hydraulic balance at the fresh-water/salt-water interface, the
pumping-water level should be carefully monitored. The punp intake should never be set
below mean sea level because when the cone of pumping depression intersects sea-level
datumr all the available fresh waier is exhausted and upwelling sea water will enter the hole.
Once contaminatedr the hole may take years before a balance is re-established, although in a
few instances it may be possible to accelerate the process by artificial recharge.
Clearly' the ground-water lens configuration will vary from island to islend and much
investigational work remains to be done. However, it is proven that the fresh-water principle
is valid in coral island situations and its abstraction is possible under given conditione.
Weter qualitv
It ig clear thet we cen classify the geological struct,ure of Pacific islands into three
broad groups. Both the type of waier resource and broad chemical characteristics of water
quality also conform to this grouping.
There is little information on the bacteriological quality of water on different islands
although unconfirrned accounts by technicians working in hospital laboratories in the region
indicate Lhe presence of coliform bacteria in many samples and a general lack of treatment
of water supplies.
Our interest has been in providing drinking waler of an acceptable standard and the
World Health Organizationrs tecommendations have been used to judge water quality from a
chemical point of view. Ssne of these crileria are given in Teble I (World Health
Organization l97I).
B::?.T" z.o-8.5
I"1ffilii1" 6.i-s.2
.Naturally
Table I : WHO quality for drinkipg water
(except for pH all as g/m-)
pH Ca Mg Cl SOr,
Total
Hardness
as Ca CO, Mn Fe Cu Zn
75
200
50 200
I50 600
200
400
r00 0.05 0.1
500 0.5
occuming waten erpplies vary somewhat from these figures. Turtidity (not
listed) is a useful measure of alteration to the aquifer, so is the -presence of niirogen
compounds. Heevy metal contamination mey indicate other variation from good quality.
Generally there ere aesthetic contaminants which make water less attractive to drink or
toxic naterial which renders it dangerous to drink. Normal rain water has a pH of 5.6.
Overall we have few chemicel analyses and are unable to pnesent more than examples
of tests from specific sites. There are no deta from sarnples of the same source taken at
different times. We can, however, trace the changes which rain water undergoes as it pasees
into different aquifers.
1.0
0.05
t.5
5.0
r5.0