Factors in a Ghyben-Herzberg System— Wentworth 
173 
portance of the lens is great because in it a 
much greater thickness of fresh or near¬ 
fresh water is accumulated at sea level than 
would otherwise be the case. In fact the 
fresh water is perched on salt water. Be¬ 
cause of the wide extent of such possible 
perching, these basal water bodies lying in 
Ghyben-Herzberg balance are the largest 
ground water resources of the region. 
In rocks not favorable for developing 
such a lens, there may be springs of fresh 
water at sea level. These springs are mostly 
small and unimportant and are occasioned 
by the cutting of the sea against the land, by 
an action such as that which produces valley- 
side springs. In Hawaii the basal water 
which is in Ghyben-Herzberg balance may, 
or may not, be in part artesian. Where a 
cap rock exists, the thickness of the lens is 
increased by its retardation; but the cap rock 
is not essential. The development of hydro¬ 
static balance occurs even where the lighter 
water of the lens is somewhat brackish. Such 
a system may be of true Ghyben-Herzberg 
form even where the land water carries a 
considerable fraction of sea water. But for 
practical use only the more perfect systems 
where the upper water has less salt than cor¬ 
responds to about 1 to 2 per cent of sea water 
are of interest. Salinity of the Honolulu 
city supply corresponds to approximately 
1/400 sea water. 
Often water supply systems are planned 
where there is some indication of fresh land 
water at the coast. Here it is supposed that 
only drilling or tunneling may be needed to 
secure a potable supply. Often such excava¬ 
tion after prolonged test may show that 
despite some degree of Ghyben-Herzberg 
functioning the resulting developed water is 
too saline for the proposed use. Or, the 
water may grow progressively more saline 
and show that the hydrostatic system is not 
sufficiently stable to stand the disturbance of 
even a moderate draft of water. Engineers 
and others who have accepted the principle 
are naturally disappointed and in turn ques¬ 
tion it after such adverse tests. But in the 
writer’s view it is not surprising that the 
fresh-water lens in certain areas is either 
lacking or fails to meet the extremely severe 
test of yielding potable water continuously. 
Rather, it continues as one of the natural 
wonders that such systems as that at Hono¬ 
lulu and a few other places have been de¬ 
veloped in course of geological time, in such 
perfection and with such ability to withstand 
modification through artificial development 
of the water. These systems are the excep¬ 
tions rather than the rule. 
Observations in various parts of Hawaii 
and a growing knowledge of basal water 
conditions on other Pacific islands permit 
some broad generalizations concerning the 
conditions essential to an effective Ghyben- 
Herzberg system. An attempt is made in this 
paper to outline the requisite conditions. 
In the course of compiling this discussion, 
the manuscript has been read and criticized 
by Charles V. Theis, Arthur M. Piper, L. H. 
Herschler, and Gordon A. Macdonald, each 
of whom has made valuable suggestions. 
The writer is indebted particularly to the lat¬ 
ter, with whom he has had many most profit¬ 
able discussions on this and related problems 
over a period of years. 
OUTLINE OF FACTORS 
In outlining the factors affecting salinity, 
attention is first drawn to the fact that the 
Ghyben-Herzberg lens consists of a lighter 
liquid, floating on a heavier liquid and 
miscible with it. If the two liquids were not 
miscible, they could maintain their separate 
character and their common boundary indef¬ 
initely even without being restrained in a 
permeable aquifer. However, as they are 
miscible, if the permeable aquifer were not 
present the two liquids would become mixed 
and diffused in a short time so that the 
lighter lens would be destroyed. On the 
