Chapter XI: The Origin of the Artesian Waters

Flow of Water through Porous Rocks — Importance of Pressure, Porosity, and Temperature — Intermittent Flows — Abundance of Extinct Wells — Former Prosperity — Possibility of increasing Present Total Discharge — Local Traditions regarding Origin of Artesian Waters — Possible Sources of Origin — Seepage from Nile into Nubian Sandstone — Sandstones as Storage Reservoirs — The Oasis Waters of Meteoric Origin — Fissures — Rate of Flow — Strongly-flowing Wells not necessarily dependent on Fissures — Local Pressure from Variation in Level of Water-Table — Rise of Water due to Hydraulic Pressure — Points requiring Investigation — High-Level Springs: Ain Amûr; on Escarpment near Beris; at Nakhail.

The rate of flow of water through porous rocks has been investigated by a number of engineers and geologists, among whom may be mentioned Darcy, Hazen, King, Slichter, Knibbs, and Baldwin-Wiseman. The subject is an extremely complicated one, and its study requires a combined knowledge of mathematics, physics, and geology. Various formulæ for determining the rate of flow under varying conditions have been devised, but it will be sufficient for our present purpose to remark that the average velocity of water in sands does not appear to be more than 3 or 4 kilometres a year. The importance of porosity, pressure, and temperature on the rate of flow can be illustrated by utilizing tubes containing equal columns of sand of different degrees of coarseness, and noting the volumes of water passed under different conditions of pressure and temperature. The coarse sands will be observed to permit the passage of water at a far greater rate than the finer varieties, while any sand can be made to markedly quicken its rate of discharge by increasing the head of water in the tube above. Moreover, the rate of flow will be found to increase with the temperature of the water. In the case of the ordinary water of the oasis, a difference of only a few degrees was found to cause a very great difference in the rate of flow; it seems, indeed, as if the water, when below a certain temperature, deposits its mineral contents in the pores of the sandstone, so as to block the passages to a large extent. I have not yet had an opportunity of making the necessary experiments to ascertain if this explanation is correct, but if it should prove to be so, the importance of temperature on the rate of underground flow can hardly be over-estimated.

We have already described the intermittent character of the flow in the case of some of the larger wells in the southern part of the oasis. The circumstance is apparently due to the temporary blocking of the bore-hole by sediment, and the consequent increase of water and gas pressure below, which at intervals forces the sediment from the channels and restores the normal flow. Something of the same nature was also noticed in Bore No. 42, in the course of experiments carried out to determine the height to which the water would rise in an open pipe fixed to the end of the casing. At intervals a distinct gurgling and bubbling took place in the pipe, the water at the same time rising to as much as 8 centimetres above its normal level, with distinct oscillations of a pressure-gauge attached to the well. The ebb and flow did not, however, take place at regular intervals, but at periods varying from four to nine minutes. I think a sufficient explanation of these phenomena is the probable variation in the amount of gas finding its way into the bore; and certainly, after several years’ observation of the flows of a large number of bores, I cannot admit that there is the slightest evidence in favour of the view that the flows of the wells have a periodicity dependent on the rise and fall of the Nile.

One frequently hears it stated that the oases were far more thickly populated and better watered in olden times than at the present day. This belief is based on the existence in many parts of the depression of extensive remains of temples, forts, and villages, on the widespread traces of formerly cultivated lands, and on the abundance of sanded-up wells. It must not, however, be forgotten that the remains in question belong to successive generations, and that there is as yet no evidence to enable us to determine how much of the land, or how many of the wells, were in use at one and the same time. The evidence is, however, sufficiently pronounced to justify the conclusion that under the Romans the oasis of Kharga was far more flourishing than in modern times, a large part of the population being engaged, not in agriculture, but in mining, boring, and in the excavation of subterranean aqueducts.

We have heard the most diverse and dogmatic opinions as to the feasibility of restoring the oases to their supposed former prosperity. For my part, I do not think that there is the slightest doubt that the total discharge of water could be very much increased, though to what extent it is impossible to say with the information at present available. One must consider the vast areas under which the water-bearing sandstones are known to extend, and the comparatively small extent of country over which the existing wells occur; that as yet the deepest bores have only penetrated the water-bearing beds to a depth of 122 metres; that the existing total discharge is mostly made up of insignificant flows from a great number of ancient and comparatively shallow wells, which for centuries have been subject to gradual decay; that so far as observed the flows increase in volume as deeper beds are struck; and that it might be possible to use artificial means of lifting the water to the surface, especially in districts lying above the level to which the artesian water will rise unaided. The extent to which the water-supply could be profitably augmented is, however, quite another question, and one depending on a great number of at present indeterminable factors.

According to local tradition, the waters of the Kharga wells come directly from the Nile through subterranean passages under the intervening plateaux, and the experience of an Arab trader is frequently related in support of this idea. The Bedawi in question, while engaged in filling his water-skins on the banks of the Nile, preparatory to setting out across the desert to the oasis, let fall his ‘tarbush,’ which was speedily engulfed in an eddy of the river. Although much annoyed at the time, our friend soon forgot the incident, until a few days later, when he was refreshing himself after his journey at a well in the neighbourhood of Beris, the identical piece of head-gear was borne up from its depths!

Mr. Patterson, whose knowledge of the folklore of the inhabitants is unique, recently related to me the following characteristic story: The natives of Beris, as the result of opening a long sanded-up well, obtained a very large flow of water. So terrified were they at the magnitude of the discharge—imagining, indeed, that they had tapped the Nile—that a deputation was hastily despatched to the Governor of Assiut, with profuse apologies for the damage done to the river. Needless to say, the Governor was somewhat taken aback, but realizing the solemnity of the mission, magnanimously informed its members that the waters of the Nile were so abundant that they might without fear take all they required.

At present any attempt to explain the origin of the artesian waters of the oases must be regarded as little better than speculation. More information is required concerning the geology of the country to the south of the oases, and as to the relative levels of the oases-depressions and the different parts of the Nile Valley and Libyan Desert, as far south as the more elevated regions of Kordofan, Darfur, and Tibesti. Little has been written on this subject, but the source generally assigned appears to have been Darfur. Possible sources of origin lie in the rainy districts of the Sudan, in the mountainous region of Abyssinia, in the great swamps of the Upper Nile, in the Nile River itself, and in past accumulations of water absorbed from the extensive lakes which covered parts of the country in the pluvial period which preceded the existing desert conditions.

In the present state of our knowledge I am personally inclined to agree with those who regard the Nile River as a present source of supply. It is known to flow for a considerable part of its course through a valley cut out in the Nubian Sandstone, and it is believed to lose an appreciable volume of water into that sandstone, though the exact amount has not been determined. Mr. J. I. Craig, of the Egyptian Survey, has estimated that at low Nile as much as 6,000 cubic metres of water (1,320,000 gallons) per minute drains back into the river from the sandstones on either side of the reach between Khartum and Wadi Halfa, and, as Captain Lyons remarks, this indicates that there is considerable percolation into the sandstones from the river when in flood.[10]

There is one point of the greatest importance to which we should like to draw attention, as it is generally entirely overlooked. The stores of water in the sandstones may represent the accumulations of hundreds and thousands of years, and the conditions to which the beds formerly owed their sources of supply may at the present time have become materially altered. It is quite conceivable that it may have required centuries or thousands of years to saturate the huge block of sandstone underlying the Libyan Desert, and even were the original sources of supply entirely cut off at any particular time, the effect on a few hundred bores, discharging only 50,000 cubic metres a day, would not necessarily be appreciable in one, or even five, centuries.

The total annual discharge of the whole of the wells of Kharga Oasis is barely equal in volume to the water which can be held by saturated beds underlying 1 square kilometre of surface, assuming the sandstone to be only 122 metres thick; that is to say, it would take between 3,000 and 4,000 years for the existing wells to discharge the water held by the beds underlying the depression alone, without considering the vast surrounding desert areas, where there is no reason to doubt that the water-tables are equally well developed. It must not, however, for a moment be supposed that bores would continue to discharge until the sandstones immediately surrounding them were completely depleted of water; they would, in all probability, pass into a sub-artesian condition in a very short time were the sandstones not replenished from more outlying districts.

In my opinion the subterranean water of the oasis is certainly of meteoric origin—that is to say, it is water which originally fell as rain, and has percolated underground from one of the possible sources above mentioned. It will, however, readily be admitted that the ordinary explanation of the flow and origin of artesian wells in regions of moderate or abundant rainfall, situated in well-defined basins, where the exact position, extent, and absorbing capacity of the water-table outcrop can be carefully determined, may be in some respects inadequate to account for the flowing wells of vast arid regions like the deserts of Africa and Australia; yet, after due consideration of the arguments for and against, I am unable to subscribe to Professor J. W. Gregory’s view[11] that a considerable portion of the waters in such regions is derived from magmatic or plutonic sources—that is to say, has its origin in the deep-seated crystalline rocks.

It may seem at first sight almost incredible that in those regions, where the outcrop of the water-bearing strata is so remote from the wells themselves and the dip over the intervening country so slight, the rise of the water in the wells could be due to direct pressure of water in the higher portions of the beds, unless on the supposition of large and continuous open fissures. That such fissures exist, and exist abundantly, is, I think, almost a matter of certainty; but it does not follow that their presence is essential to the production of flowing wells. Fissures are visible to the eye in the Surface-water Sandstone (which, as has been remarked, does not appear to differ in any important respect from the Artesian-water Sandstone), and it is through them that the bulk of the sub-surface water is obtained. The presence of fissures in the Artesian-water Sandstone is, moreover, in my opinion, almost demonstrated by the experiments on the mutual interference of wells. It seems hardly conceivable that the closing or opening of a bore could in the space of a minute or two affect the discharge of another well over ½ kilometre distant, if there did not exist a more or less open and direct connection between the two.

Rapid flow through a compact sandstone is impossible owing to friction, which increases as the size of the channels decreases; but, as pointed out by Knibbs and others, the hydrostatic pressure can never entirely disappear through friction, the rate of loss of head being dependent on the rate of flow. It therefore by no means follows that a strongly-flowing well cannot be obtained from an unfissured sandstone, for a rapid flow from the bore itself does not in any way depend on an equally rapid flow of the water through the sandstone surrounding the bore. For instance, Mr. Knibbs has calculated[12] that although in a 10-inch bore, discharging 700 gallons a minute from a 10-foot stratum, the water would have a velocity of 5½ feet a second at the bore itself, at the distance of one mile it would only be moving through the stratum at the rate of about ¹⁄₂₀₀ inch per second, or 18 inches an hour. In other words, water flowing through a 10-foot bed of sandstone from all sides towards a 10-inch bore, need, at the distance of one mile, only have a velocity of 18 inches an hour to produce a discharge from the well of 700 gallons a minute.

The importance of local pressure arising from variations in level of a fully saturated water-table in adjacent areas may be of itself quite adequate to cause flowing wells, especially if assisted by the presence of large volumes of gas under compression, such as occur in the Kharga waters. Another theory which has at times been brought forward as an adequate cause of flowing wells is rock-pressure— _i.e._, pressure due to the weight of the overlying strata. The objections to this theory seem to me, however, so cogent that we may at once dismiss it from our minds. There is indeed little doubt that the waters of the oasis are of meteoric origin, have travelled immense distances underground, and rise through bores placed in favourable localities by means of hydrostatic or hydraulic pressure, acting both through the pores of the rock and through open fissures. Although the rate and direction of flow through the sandstones as a whole may remain more or less matters of conjecture, it seems probable that the water of any one bore is derived from all sides, rising as the result of the pressure exerted by the water held in the same bed situated at higher levels, whether in the immediate neighbourhood or at a considerable distance.

I am conscious of having done little more than indicate the possible origin of the oasis waters and suggest the causes to which the flowing wells are due; more than this it is at present impossible to state with any confidence. The points to which attention should be directed as likely to throw further light on the subject are as follows: The area and position of the outcrops of the Impermeable Grey Shales and the underlying sandstone, and their relations to possible sources of water, whether rain, river, or lake; the nature of the bed of the swamp region of the Upper Nile; the amount and distribution of the rainfall of all surrounding regions; the volume of water lost in the different reaches of the Nile over and above that which can be directly accounted for by evaporation and by water abstracted for purposes of irrigation; and the total thickness of the water-bearing beds, the presence within them of impervious strata, and their relation to the underlying crystalline rocks.

HIGH-LEVEL SPRINGS.

Before leaving the subject of water-supply we must briefly refer to two or three remarkable instances where water is found at very high elevations. The most important of these is on the upper road between Kharga and Dakhla, where a spring, known as Ain Amûr, occurs near the summit of the plateau at about 460 metres above sea-level. The water, which is quite potable, occupies the bottom of a hole about 3 or 4 metres in depth, at the base of a clump of palm-stubs, a few paces to the south of a solitary tree; it does not run, and the water-level is said to fall considerably in the summer. A small patch of green rushes lies a few paces to the west, and there is a good deal of scrub in the neighbourhood, from the position of which one is led to infer that there are several small springs thrown out along one of the bedding-planes, at the summit of the marly and clayey beds which underlie the limestones forming the uppermost portion of the cliff.

The high-level spring on the eastern wall of the oasis, in the neighbourhood of Beris, consists of a pool of clear sweet water at the base of a large fallen block of limestone, in a desolate rocky dingle. Ball determined its height as being 180 metres above Beris, or 260 metres above sea-level. The pool itself is overgrown with weeds, and, scattered about on the sides and bottom of the valley, there is a good deal of vegetation, mostly in the form of coarse grasses, prickly scrub, and tamarisk bushes. Geologically, the spring appears to be similarly situated to Ain Amûr—that is to say, it emerges near, or at the summit of, the argillaceous Exogyra Series, and below the overlying Danian limestones.

The water obtainable at Nakhail, 60 kilometres S.S.E. of the oasis, also appears near the summit of the Exogyra Series, and may therefore be considered to have a similar origin to the springs just described.

These elevated occurrences, thrown out along more or less definite geological horizons, must be regarded as natural springs, quite distinct from the artificially-made wells of the oasis-floor. As the springs lie several hundred feet above the static head of the artesian water, and are separated from the water-bearing sandstones by a great thickness of argillaceous impermeable strata, it is perhaps permissible to assume that their waters are derived from an entirely different source. The occurrences known are of very limited number, and the water only appears in small quantities, so that it is not unlikely that it is derived from the very occasional rains which fall on the plateaux. A portion of these rains would doubtless find its way downwards through fissures in the limestone, and, in areas where the dip of the beds was towards the oasis, might travel underground and occasionally be thrown out as springs on the walls of the depression, at the junction of the limestones and underlying clays.