Factors in a Ghyben-Herzberg System— Wentworth 
181 
would expect to find the middle of the zone 
of transition or diffusion at the same level it 
occupied initially. However, we are only 
theoretically interested in the middle of this 
zone; for practical purposes our interest in 
an operating Ghyben-Herzberg system is 
concentrated on that fringe nearest the fresh 
water, where the salinity is equivalent to the 
order of 1 per cent of sea water or less. It 
appears that any movement of the zone of 
transition causes it to widen. Thus the cen¬ 
ter of the zone may return to a former posi¬ 
tion after equal and alternate movements, 
but the near edge of the zone, judged by any 
defined standard, because of the previously 
stated principle of widening will not retract 
as readily as it advances. 
Thus it appears that in the practical sense, 
in relation to exploitation of potable or agri¬ 
culturally useful water, the encroachment of 
saline water, under the natural plus artificial 
and somewhat aggravated fluctuations, will 
take place more readily than the reverse 
process of elimination under a conservation 
program. Thus there is an element of irre¬ 
versibility, despite the difficulty which vari¬ 
ous workers, including the present writer, 
have had in seeing why the salt water could 
not be driven back by an equal period of 
reversed movement. Some have postulated 
trapping of salt water in pockets. It must, 
however, be pointed out that in a hydraulic 
system where water may move either way, 
trapping is not restricted to pocketing 
against the direction of gravity but could 
equally well work the other way with reverse 
direction of water movement. It is con¬ 
cluded here that no special theory of trap¬ 
ping, especially trapping in one direction, is 
needed, nor is any theory of directionality 
required. The condition seems fully ex¬ 
plained by the concept of symmetrical thick¬ 
ening, due to rinsing, plus the fixing of 
practical interest'in a position on the near, 
or upper, side of the zone of transition. 
DIFFERENCES IN SYSTEMS 
We need now to offer acceptable explana¬ 
tions of the differing qualities of water in 
various systems. It has been found in vari¬ 
ous places in Hawaii that the main part of 
some of the larger Ghyben-Herzberg sys¬ 
tems may for many years furnish water of 
surprising constancy of salinity. There are 
three most evident sources of sodium chlor¬ 
ide: (1) from normal rock weathering, (2) 
from salts left on the land from salt spray 
or from more saline irrigation waters, (3) 
from admixture in the aquifer with intrud¬ 
ing sea water. 
We are well acquainted with marked in¬ 
creases due to the third factor; there is an 
equal amount of evidence on aquifers which 
over a period of many years, and even with 
considerable reduction of head and increase 
of draft, continue not to be affected by (3) 
but represent a stable and not wholly defined 
combination of (1) and (2). This is true 
of some aquifers where the draft of water 
is from points several hundred feet below 
sea level. Obviously such points are in a part 
of the Ghyben-Herzberg lens that is not yet 
affected by saline encroachment (from 3) 
and cannot be a part of the transition zone. 
That in course of time, through thickening 
of the zone, they might become so is, unfor¬ 
tunately, one of the practical lessons we 
learn. 
On the other hand, we find aquifers in 
which the Ghyben-Herzberg lens at any 
level, from the top downward, yields water 
of high" salinity, often increasing markedly 
with draft, and appears to be deriving it 
from normal and induced admixture of sea 
water. In such places we can only conclude 
that the whole lens is a part of the zone of 
transition. It appears therefore that while 
some lenses have a considerable fraction of 
water not affected by the adjacent salt water, 
others do not. For convenience we may call 
the upper part the fresh-water core. 
