THE UNDERGROUND WATER-SUPPLY
The Water-bearing Strata underlying the Libyan Desert —
Essential Conditions required to produce an Artesian Basin — The
Surface-Water Sandstone — Collecting Pits — Fissures — Yield
of Water — Flowing Wells from this Sandstone — Bores at El Dêr
el Ghennîma — Variable Quality of Water — Ancient Subterranean
Aqueducts — The Artesian-Water Sandstone — General Characters
— The Headquarters Area avoided by the Ancients — Drilling
Difficulties — Results of New Bores — Factors determining
Discharges of Wells — Temperature and Chemical Composition of
Artesian Waters.
When laboriously traversing the hot and arid plateaux of the Libyan
Desert, our thoughts divided between the fertile plains of the Nile
we have left behind and the still far distant oasis, it is difficult
to realize the presence, within a distance of a few hundred yards,
of an abundant supply of the purest water. Yet there is little doubt
that the water-bearing beds underlie practically the whole of the
Libyan Desert, though it is only on the floors of the depressions
that they lie within accessible distance of the surface. On the
high tablelands the cost of sinking bores to reach the sandstones
would be prohibitive, and without the aid of powerful and costly
pumps the water would not rise to the level of the ground. But
although the subterranean waters of the great desert plateaux
cannot economically be made available at the surface, it must not
be forgotten that the deeply buried sandstones of these regions must
act to a great extent as storage reservoirs capable of replenishing
the beds underlying the oases-depressions, from which large volumes
of water are continuously drawn by the numerous artesian wells.
The essential conditions required to produce an artesian basin,
which, when tapped by borings, will produce self-flowing wells,
are the presence of strata of sufficient porosity to carry water,
enclosed above and below by beds of sufficient impermeability to
prevent the escape of that water; the outcrop of the porous beds
in some higher and distant region, with an adequate exposure to
an abundant source of water, whether rain, river, or lake; and,
finally, the absence of an easy escape at a lower level, unless at
a considerable distance from the sites of the wells.
These requirements are well fulfilled in the Libyan Desert, where
the Nubian Series at the base of the Cretaceous consists of highly
porous sandstones, subdivided by impervious beds of shale, overlain
by a great thickness of absolutely water-tight beds, and underlain by
probably almost equally impermeable crystalline rocks. These porous
sandstones rise gradually to the south, and presumably outcrop in
Nubia and the Sudan, though whether, as we shall see later, they
derive their water-supplies from the Nubian reaches of the Nile,
from the great swamp regions of the Sudan, or from the rains of
Abyssinia or Darfur, is still, to some extent, an open question.
[Illustration: AN ARTESIAN WELL (BORE NO. 39).]
In Northern Kharga we find two similar but distinct sandstones,
separated by a 75-metre band of Impermeable Grey Shale. The upper
bed, which we have for convenience designated the ‘Surface-water
Sandstone,’ is exposed at the surface, and contains standing or
sub-artesian water; while the lower, from which the flowing wells
derive their supplies, never approaches within 80 metres of the
surface, and forms the true ‘Artesian-water Sandstone.’ It will
be convenient to give these sandstones separate consideration.
THE SURFACE-WATER SANDSTONE.
The stratigraphical position of the Surface-water Sandstone will be
seen by reference to the geological sequence given in a preceding
chapter, and to the section drawn across the oasis. The beds
forming this division have an average thickness of 45 metres, and
consist almost entirely of fine sandstones or coarser grits, often
containing an abundance of oxide of iron, and occasionally beds of
alum and Epsom salts, or, to be chemically exact, hydrous sulphates
of alumina and magnesia. Thin bands of shale are frequently met with,
intercalated in the sandstones, generally near the top or base of
the series. On the level or gently undulating floor of the oasis,
to the east of the line of disturbance passing through Jebel Têr,
the Surface-water Sandstone has a wide outcrop, while to the west,
where the general level of all the formations is higher, the same
stage forms the foot-hills of Jebel Tarif, of Jebel Têr, and of
the high cliffs which rise to the north of Um el Dabâdib and Ain
Lebekha. It is probable, moreover, that the sandstones which form
the surface of the desert between Kharga and Dakhla, and which cover
immense areas to the south of the oases, also belong to this group.
Over a portion of the floor in the north of the depression these
sandstones have been entirely removed by denudation, so that in
this area the underlying grey shales form the desert surface. At
one time the sandstone was continuous across the depression, and
entirely covered by an impervious mantle consisting of the Purple
Shales. It was then, in all probability, fully charged with water
under pressure, as is the true Artesian-water Sandstone at the
present day; and it is possible, as we have already suggested,
that when the sandstones in question were first laid bare by the
active agents of erosion, innumerable springs broke forth, and
gave rise to a lake of considerable magnitude, which may, indeed,
have been that which persisted into the historic period.
In the district round Headquarters several pits have been sunk
in this sandstone, from which the water is lifted by power for
purposes of irrigation. In six pits the maximum level of the water
varies from 55·38 to 56·52, the average being 55·74 metres. From
Headquarters the general surface of the country falls to the north,
south, and east, and curiously the water-level in the sandstone
falls in the same directions. In a pit near Bore No. 43, about 3½
kilometres to the south-west, the water-level is 52·4 metres; at
points 640 metres and 2 kilometres to the west it was found to be
52·16 and 50·21 metres respectively; while near Bore No. 33, 4
kilometres to the north, the surface-water stands at 52·71 metres
above sea-level. The ground-levels at these points are 54·54,
57·90, 53·61, and 57·61 respectively, while that at Headquarters
averages 59·57 metres above sea-level.
For irrigation purposes an ordinary bore-hole or small pit is
quite useless, the inflow of water through the pores of this
sandstone being too slow to yield a pumping supply. When a large
pit is excavated, fissures, through which water is seen to be
freely circulating, are nearly always exposed. Experiments show
that—provided a sufficiently large collecting-tank is made,
measuring, for instance, 5 by 4 metres, and sunk from 1 to 2
metres below the standing water-level—a supply sufficient to
yield a continuous discharge of 8 gallons a minute (equivalent
to 11,500 gallons per day of twenty-four hours) can frequently be
obtained. Moreover, by sinking a bore to a depth of 7 or 8 metres
from the bottom of the pit, preferably on one of the fissures,
the supply can be increased threefold, the water emerging from the
bore in considerable volume, though with insufficient pressure
to carry it (in a length of well-casing fixed on the top of the
bore-hole) above the level of 55·74. The latter figure may, indeed,
be regarded as the static head in this neighbourhood of the water
in the Surface-water Sandstone. The fact that from this sandstone
we are unable to obtain an artesian well at the surface, but can
get flowing water at a depth of a few metres, is an important
illustration of the very slight difference in some cases between
flowing and non-flowing wells, and of their close connection with,
and dependence on, the absolute ground-level.
The extent to which this sub-surface source can be drawn on without
lowering its level has not yet been determined, though, in a pit
alongside Bore No. 2, a ‘saqia’ or Persian waterwheel (a native
contrivance adapted for lifting water, worked by oxen or camels),
has been working for about eighteen months without appreciably
affecting the supply.
We have seen that a flowing well can be obtained (in a pit) a few
metres below ground-level, and under certain conditions it may be
possible to obtain flowing wells at the surface from the sandstone
under consideration; the occurrence of artesian water at El Dêr
el Ghennîma, on the east side of the depression, may, indeed, be
a case in point. To the east of Headquarters, as may be gathered
from the section, the Surface-water Sandstone dips gently under
the Purple Shales, the thickness of the latter increasing as the
eastern escarpment is approached. Running north and south through
El Dêr is an anticlinal fold, and flowing water is obtainable
from wells sunk on its crest through the shales to the underlying
sandstone. The original depths of the ancient wells in this district
cannot be determined with certainty, but when one of those to the
north of the ruined fort was cleaned out and cased, flowing water
was obtained when the drill reached a depth of 41 metres. Below
this the ground appeared to be untouched, the bore being carried a
few metres deeper into fresh sandstone, which yielded an increased
supply. This well has given a steady discharge of about 40 gallons
a minute for over eighteen months. Quite recently the flow has been
augmented to just over 50 gallons per minute by its outlet being
lowered about 1½ metres, to 92½ metres above sea-level.
The water here seems to be derived from the Surface-water Sandstone,
though, considering that the well is situated on a line of folding,
it would not be advisable to entirely disregard the possibility
of the presence of fissures, through which the water might rise
directly from the Artesian-water Sandstone below. If, as appears to
be the case from the depth and position of the bore, the water does
have its origin in the Surface-water Sandstone, the explanation
may be that the necessary working head or pressure is furnished
by the difference of level of the sandstone here and in adjacent
districts. In the extreme north of the oasis the same sandstone lies
at a considerably higher level, and, at the same time, holds large
volumes of water, and it may be that the pressure of this water,
acting on that contained in the beds at lower levels, gives rise
to flowing wells when bores are sunk in favourable localities.
A very great drawback to the water of these upper sandstones is
its variable quality. In the Headquarters district it is usually
more or less ferruginous, and in some pits may contain as much as
63 grains of dissolved solids per gallon, the salts consisting of
iron, potash, and soda, with traces of lime and magnesia, mostly
in the form of sulphates and chlorides. Mr. William Garsed[9] has
calculated that this water would, if used for irrigation at the
rate of 3 gallons a minute per acre, effect an annual deposition
of over 3 tons of sulphate of potash and common salt on each acre
of land. Salts of this nature in such quantities would, of course,
have a very deleterious effect on the crops, not to mention the
mechanical binding action of the iron on the soil.
It appears that the want of uniformity in the quality of the water is
due to local causes, chief of which may be the relative abundance of
fissures, the purity of the sandstone, and the presence or absence
of mineralized shales. In those areas where the water is moving
only slowly—where, in fact, it tends towards stagnation in the
strata—it is probably liable to become more or less highly charged
with mineral salts; certainly the best qualities seem to coincide
with relative abundance, and the largest supplies are undoubtedly
found where the beds are traversed by fissures.
In many parts of the oasis perfectly sweet water is obtainable from
the sandstones of this series, and this source, as an auxiliary to
the artesian supplies obtained from deep borings, was taken full
advantage of in olden times. The ancients constructed the most
marvellous systems of subterranean aqueducts to tap these sources,
more especially in the neighbourhood of Um el Dabâdib, Qasr Lebekha,
and Qasr Gyb, where such works were especially applicable, owing to
the fact that the sandstones in those districts form extensive hills
above the general level of the surrounding cultivable ground. These
underground works are, in many respects, of far greater interest than
the ancient monuments, and they will therefore be fully described
in a later chapter.
THE ARTESIAN-WATER SANDSTONE.
The source of the great majority of the flowing wells of the
oasis is the group of sandstones underlying the Impermeable Grey
Shales. Although the beds of the series are nowhere visible to the
eye, their general characters can be judged by an examination of the
material brought to the surface during drilling operations. The
samples prove that, in general lithological characters, the
Artesian-water Sandstones do not essentially differ from those just
described. Throughout the area over which boring operations have
recently been carried out no well-defined, continuous, argillaceous
bands have been met with, though lenticular intercalations of
clayey strata are not uncommon. Up to the present time the base
of the Artesian-water Sandstone has not been reached, although
the deepest borings have been carried down to a depth of 122
metres below its junction with the confining shales above. The
different bands vary considerably in coarseness and porosity,
in hardness, and in the amount of cementing material between
the individual grains of the rock, all of which characters have
a marked influence on their capacity as water-carriers. Judging
from their unfossiliferous nature, from the presence of thin seams
of lignite associated with bands and nodules of iron pyrites, and
from certain other considerations, we are led to infer that these
sandstones were originally laid down on the bed of an immense inland
fresh-water lake.
During the past half-century the natives have put down a considerable
number of deep bores in this and the neighbouring oasis. No written
records concerning these are, however, available, so that in seeking
information one has to rely on the memory of the men who sunk the
wells. By careful cross-questioning I have collected a large amount
of interesting and valuable information, but still, in describing the
artesian wells themselves, it will be more satisfactory to confine
our attention to those which have been drilled on the Headquarters
area during the last two or three years, and of which accurate and
reliable records have been preserved.
The Headquarters area, occupying the central part of the depression
between Kharga village and Jebel el Ghennîma, is one of the few
large districts entirely devoid of old wells and traces of former
cultivation. A combination of unfavourable circumstances appears to
have led the ancient well-borers to avoid this district. Firstly,
the general elevation is comparatively high, meaning small flows
from wells of ordinary depth; secondly, the superficial alluvial
deposit is clayey and heavy, necessitating a considerable expenditure
of time and labour to bring it into satisfactory condition for
cultivation; and, thirdly, and probably most important of all,
the presence of a copious supply of sub-surface water, which would
have greatly hampered, if not made impossible, the sinking of wells
by the ancient system.
It may therefore be assumed that, owing to the entire absence of
both ancient and modern wells, the sandstones of this district
were practically fully charged with water at the time the first
bore was sunk.
The junction of the Artesian-water Sandstone with the grey
shales above is usually fairly abrupt, the first flowing water
being obtained as soon as the drill strikes the top of the
sandstone. Where alternating sandstones and shales occur at or
near the junction, the former are generally charged with water
under feeble pressure, yielding flows at the surface of from 1
to 5 gallons a minute. Sometimes, indeed, the piercing of these
thin bands, prior to the main body of sandstone being entered,
merely results in a rise of water in the bore-hole, without actual
flow. On drilling into the sandstone proper, increments of the
flow are obtained at fairly frequent, though irregular, intervals
of depth. At times the discharge is seen to increase slowly but
steadily, while a particularly porous bed is being passed through;
at others the rate of increase is so rapid as to suggest that a
fissure charged with freely flowing water has been struck. As a rule,
hard beds of sandstone, (the ‘shells’ of American drillers)
overlie the best water-carrying layers, and though these act
locally as confining beds, there almost certainly is, nevertheless,
an intimate connection between different parts of the sandstone,
as no persistent argillaceous bands have been met with.
Loose uncemented sands may be encountered at any time, but
do not seem to coincide with marked increases of flow. These
‘quicksands’ form one of the greatest difficulties with
which drillers have to contend, and many bores have perforce to be
discontinued owing to the impossibility of drilling through them. The
loose sands ‘cave’—that is, run in from all sides—the whole
wall of the bore at times falling in, throughout a length of 5 or 10
metres. The only remedy against caving is the insertion of casing,
but this is generally undesirable, as it may effect the shutting off
of water already obtained from the upper bands of the sandstone;
in some cases, however, perforated lining may be satisfactorily
employed.
Of thirty-one bores finished in this district, none has failed to
strike water, though three have yielded such small flows that they
may be regarded as comparative failures. The average flow of the
thirty-one wells, the measurements being made in each case a week
or two after completion, was approximately 100 gallons a minute,
the maximum being 315 and the minimum 18 gallons per minute. All
bores have shown a marked decline in discharge for some time after
completion, when, if isolated, they have settled down to a fairly
steady flow, or at least to a flow which decreases at a constantly
diminishing rate.
Owing to the very adverse effect of some of the larger bores
(situated on the lower parts of the area) their flows have
been purposely reduced; and as a further precaution against
over-exploitation, and as a remedy to the waste consequent on
irrigation at night, all wells are, as far as possible, shut down
between sunset and sunrise. The average discharge, therefore, at the
present day is considerably less than the figure mentioned above,
amounting, in fact, to approximately 70 gallons a minute per bore.
By far the most important factor determining the volume of flow is
the absolute ground-level at the mouth of the well. The floor of
the oasis in the district in question lies between 53 and 61 metres
above sea-level, the general slope being to the west, in the opposite
direction to the dip of the water-bearing sandstones. Although the
actual difference of level is so little, amounting only to 7 or 8
metres, the difference of flows from wells of equal depth on either
side of the area averages fully 100 per cent. This indicates that the
surface of this area is very near the static head or limit to which
water will rise from bores of this depth; in fact, if the ground were
raised by a very few metres, not one of the wells would discharge
at the surface. This I have proved by actual experiment, and it is,
moreover, borne out by the observed pressure, which even in the best
wells seldom amounts to more than 13 or 14 pounds to the square inch.
Before going further into this important matter of pressure, let
us briefly notice the temperature and chemical composition of the
artesian water. In Dakhla Oasis the temperature of the wells often
rises as high as 90° or 95° F., the highest recorded being 105°
F. in Bir el Dinaria, a bore sunk fifteen or sixteen years ago,
and the deepest and most northerly in that oasis. In Kharga it is
seldom that we meet with temperatures over 90° F., the well-waters
at Headquarters varying from 86° to 88° F. Identical figures were
obtained in the southern part of the oasis.
One of the most noticeable features of the artesian water is its
highly effervescent character when it reaches the surface. In most of
the newer bores the water is so strongly charged with minute bubbles
of gas that it closely resembles the contents of a newly-opened
bottle of highly aerated water, while in many of the older wells
the gas rises to the surface in a slow procession of large bubbles.
Analysis shows the gas to consist almost entirely of nitrogen, only
small quantities of oxygen and carbon dioxide being present; and it
has been estimated by rough experiment that the volume of gas issuing
from Bore No. 1 (internal diameter, 4¼ inches) amounts to half a
pint per minute. My own opinion is that this nitrogen represents
ordinary air deprived of its oxygen during the underground passage
of the water, and this explanation seems confirmed by Mr. Lucas,
F.C.S., Chemist to the Egyptian Survey Department, who refers me
to several cases in which air is believed to have been depleted of
its oxygen by pyrites, etc., during its passage underground.
The quality of the artesian water is in all respects excellent,
and when taken direct from a cased well forms, after cooling, a
palatable water free from all danger of contamination. Analyses by
Mr. Garsed of water samples from four bores show the total dissolved
solids to range from 43 to 47 parts per 100,000, equivalent to from
30 to 33 grains per gallon. In new bores the water is usually only
slightly ferruginous, though, as already mentioned, in some of the
ancient wells of certain districts it is so highly charged with
ferric oxide that thick deposits of ochre have been formed along
the irrigation channels.
The chemical composition of the dissolved salts, so far as
determined, is shown in the following table:
ANALYSES OF TYPICAL ARTESIAN WATER OF KHARGA OASIS (HEADQUARTERS
DISTRICT).
+-------------------------+------+------+------+------+
| | Bore | Bore | Bore | Bore |
| |No. 1.|No. 2.|No. 5.|No. 6.|
+-------------------------+------+------+------+------+
|Total solids (grains per | 33 | 30 | 33 | 32 |
| gallon) | | | | |
+-------------------------+------+------+------+------+
|Composition of dissolved | | | | |
|salts, per cent. | | | | |
| Silica | 4·2 | 4·5 | 4·6 | 3·4 |
| Ferric oxide | 1·7 | 0·8 | 1·2 | 1·3 |
| Lime | 6·5 | 7·0 | 5·4 | 5·6 |
| Magnesia | 2·9 | 3·1 | 2·7 | 3·8 |
| Sulphuric anhydride | 4·5 | 4·9 | 4·4 | 4·4 |
+-------------------------+------+------+------+------+