174 
PACIFIC SCIENCE, Vol. I, July, 1947 
other hand, if the rock at sea level is not 
permeable, there is no opportunity for a 
condition of hydraulic balance to become 
established. The fresh water cannot adjust 
itself to the salt water in relation to sea level 
and thus no Ghyben-Herzberg lens will be 
formed. 
The first requisite is then a suitable degree 
of permeability. This must be small enough 
to prevent the general mixing which would 
destroy such a system and large enough to 
permit fresh water under the existing head 
differences to move against sea water. Only 
then can fresh water progressively displace 
salt water in forming a Ghyben-Herzberg 
lens. It will be seen presently that the per¬ 
meability which proves effective is relative to 
other factors, and can be better discussed 
after these have been listed. 
Next to suitable permeability is an infil¬ 
tration of rainfall of sufficient amount and 
continuity to build and maintain a fresh 
basal ground-water body about a foot or 
more above sea level. According to the rain¬ 
fall and recharge, there is built a surcharge 
above sea level adequate to discharge the 
average daily or annual amount to the peri¬ 
phery of the island. The maintenance of 
this surcharge causes the downward accu¬ 
mulation of fresh water until an approxi¬ 
mate Ghyben-Herzberg lens is produced. 
The miscibility of the fresh and salt water 
tends to destroy the lens or the sharpness of 
its margin. The rate of addition of fresh 
water must be sufficient to overcome this 
tendency. 
The third requirement is a sufficiently 
small fluctuation in both ground-water heads 
and in sea-water levels to minimize the ef¬ 
fects of mixing and the spread of the zone 
of mixing through reversal of movement. 
Fluctuations in ground-water head are due 
to seasonal and other variations in rain¬ 
fall and recharge. Chief changes in sea level 
that we need to consider are those due to 
tides. It seems fairly certain that small 
islands, which with tidal range of 2 feet 
show moderate stability of the Ghyben- 
Herzberg lens, if subjected to a 15- or 20- 
Toot tidal range would show serious disturb¬ 
ances of the lens. 
It is difficult to give any categorical spe¬ 
cifications, but the best-known Ghyben- 
Herzberg water bodies in Hawaii do not have 
seasonal or annual fluctuations of water table 
exceeding perhaps one fifth of the total 
height of water table above sea level. It 
seems likely that any annual change such as 
half or two thirds of the mean value would 
tend to prevent growth of a lens from which 
any potable water could be taken. Both in 
their accomplishment and also in their effect 
on water quality, the amplitude of such fluc¬ 
tuations is obviously related not only to the 
infiltration changes but also to the per¬ 
meability. 
A fourth factor, and in some ways the 
most important of all, is that of comparative 
uniformity and regularity of permeability, 
and freedom from large and long openings 
crossing the boundary between fresh and 
salt water. Much of the effectiveness of the 
Ghyben-Herzberg mechanism depends on 
the maintaining of a fairly smooth and 
orderly boundary between the two liquids, as 
closely analogous as possible to the mathe¬ 
matically definable boundary between im¬ 
miscible liquids. It is fairly evident that the 
actual condition is somewhat remote from 
this and, with fluctuating directions of move¬ 
ment, that existing large fissures or tubes 
must carry long filaments of one sort of 
water into the realm of the other, and vice 
versa. Such conditions seem to explain the 
observed vagaries in the composition of 
water from different wells in the same dis¬ 
trict and even at the same depth. Despite 
such irregular and fluctuating interpenetra¬ 
tions, a broad regularity of boundary is 
shown in the more stable Ghyben-Herzberg 
systems such as those of Oahu. Certainly if 
changing heads tend to move the salt-fresh 
