Chapter IX: 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.

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 |
+-------------------------+------+------+------+------+