Factors in a Ghyben-Herzberg System— Wentworth 
183 
taken therefrom, this becomes a new point 
of outflow, or sink. The quality of the 
water at any given point is determined by 
the rates and amounts of flow induced under 
hydraulic conditions from the several acces¬ 
sible sources. Availability of water from 
those sources depends on the sizes and num¬ 
ber of openings connecting them with the 
sampling point and on the proportionate 
hydraulic flow induced by draft at the 
sampling point. When the draft from a well 
is applied to the pre-existing flow pattern, 
that pattern is changed and the well com¬ 
petes for water against the other flow lines. 
The quality of water drawn depends on the 
flow pattern set up under the new conditions 
and on the compositions of the several 
waters available. 
The larger the intake surface (as in a long 
tunnel rather than a small well), the smaller 
the drawdown required and the smaller the 
disturbance of pre-existing flow lines re¬ 
quired to get the water. In such case the 
salinity of the water drawn will probably be 
less modified from that sampled from the 
aquifer under the original flow conditions. 
Particularly, the smaller the drawdown, the 
smaller the likelihood of inducing flow from 
lower parts of the lens, which may be more 
saline. 
In general, near the coast, the salinity of 
water taken from a well increases with in¬ 
crease of depth and with increase of draft. 
It also is greater at low regional heads than 
at high, and, as stated above, is greater at 
high drawdown than at low. Two wells of 
similar depth and location often show quite 
marked differences, owing to different, 
though often unknown, openings in the for¬ 
mations they penetrate. Occasionally wells or 
shafts are dug which encounter openings 
that are thought to run inland and in which 
the water becomes less saline as draft is in¬ 
creased; the reverse is far the more common 
situation. These exceptions do not invali¬ 
date any of the recognized principles; they 
merely emphasize the complexity of the 
hydrologic conditions involved, and our 
inability in most cases to make specific pre¬ 
dictions. 
A Ghyben-Herzberg lens is of great eco¬ 
nomic value when it has a fresh-water core 
of such size and stability as to permit the 
desired draft of fresh water and yet continue 
to maintain the freshness of the core. Where 
these conditions of stability exist and can be 
maintained, the amounts of water which can 
be taken out and the capacity of the system 
to sustain short period overdraft are truly 
astonishing. A great deal of exploration and 
usually much full-scale operation will be 
required in most such systems before the safe 
capacity or other conditions of operation can 
be determined. The chief requisite in any 
given case is a body of data that covers a 
sufficient range of facts and of time, to¬ 
gether with recognition of the principles in¬ 
volved. The present paper is offered as an 
elementary formulation of those principles 
as they appear at present. 
SUMMARY 
The Ghyben-Herzberg lens is essentially a 
gland, into which water moves from rainfall 
and out of which it moves through natural 
leaks and artificial discharge. Because of 
contact with salt water, with which it can 
mix, the lens of fresh water is in an inter : 
mediate condition of equilibrium. To sur¬ 
vive, it must not be stagnant; water must 
move through it. It can be destroyed by too 
little source water or by too rapid escape of 
water. In rocks that are too impermeable 
the dynamic equilibrium may not be set up. 
The formation and survival of such a lens in 
a given place depends on the values and 
mutual relations of the factors of permeabil¬ 
ity, rainfall, fluctuations in level, regularity 
of permeability, and presence or absence of 
a cap rock. Change of one of these condi¬ 
tions, such as fluctuation, may change the 
balance on which such a lens depends. 
