182 
PACIFIC SCIENCE, Vol. I, July, 1947 
MAINTAINING THE CORE 
The fresh-water core is a unit through 
which passes annually the amount of water 
added to the water table. Some of this water 
passes down the slope of the water table to 
escape near the coast, but there is no ques¬ 
tion that there is considerable deeper circu¬ 
lation. However, except for the migration 
of the zone of transition with fluctuations of 
rainfall and draft, and the effect of a few 
large openings with unbalanced pressures, 
the water does not move through the transi¬ 
tion zone. More properly we can assume 
that the water in the fresh core circulates 
past the upper fringe of the transition zone. 
Presumably this circulation in some lenses 
is sufficiently active to offer considerable 
resistance to the thickening of the transition 
zone. This would operate by rinsing away 
any slight increases in salinity that might 
persist if the invading fringe of saline com¬ 
position were penetrating a truly static body. 
It appears that in some of the thicker and 
more functional Ghyben-Herzberg lenses, 
the transition zone has not become thick 
enough to reach the top of the lens. The cir¬ 
culation in this upper fresh-water core is 
active enough to provide an adequate rins¬ 
ing action against the upper fringe of the 
transition zone. Hence the integrity and 
water quality of the fresh-water core are well 
maintained. In other lenses, usually much 
thinner and perhaps developed under less 
favorable conditions, the transition zone has 
either thickened to encompass the whole 
thickness of the lens or perhaps has always 
had some such thickness. Here there is no 
fresh-water core, at least not at the coast or 
where exploration has penetrated. 
Experience in finding small amounts of 
water of low salinity at the very top of such 
water bodies by no means invalidates the 
distinctions set forth above. Undoubtedly 
in wet weather a certain amount of rain 
water would move down the slope of the 
water table with only slight mixing with the 
prevailing ground water; such would be the 
source of a fresh-water layer, but the layer 
would be insufficient in amount. Not un¬ 
commonly water bodies which will yield 
fresh water in small samples are found on 
drilling and pumping to yield only water of 
considerably higher salinity. For this reason 
preliminary or bailed samples sometimes 
lead to hopes that are later not realized. 
It is not intended here to treat the various 
complexities of water development from the 
Ghyben-Herzberg lens, but the few ele¬ 
ments which enter into the problem may be 
mentioned. We start by emphasizing that 
the lens is a storage body or gland through 
which water is moving, and that under bal¬ 
anced conditions the inflow and outflow 
are equal in amount. 
Because the upper surface slopes toward 
the ocean and the rocks are permeable, there 
is a steady loss proportional to the head 
differences through various openings, whose 
locations are usually not known in detail. So 
long as this head is maintained these open¬ 
ings, if not plugged, will discharge the same 
amount of water. If water is to be taken by 
man, the head must be lowered until the loss 
from various openings has been reduced by 
the amount taken artificially. The lowering 
of head makes water temporarily available 
from storage at the upper surface; if we fol¬ 
low the Ghyben-Herzberg principle we can¬ 
not doubt that in due course the bottom of 
the lens must shrink to reach a new equi¬ 
librium. This will yield, over a long period, 
very large amounts of water. This condition 
appears to explain the remarkable stability 
against draft which is shown by some large 
systems (Wentworth: 1942). 
EFFECT OF DRAFT ON QUALITY 
We have seen that the water of the lens 
is in motion, toward points of outflow, 
which in hydraulic terms are called sinks. 
When a well or shaft is dug and water is \ 
