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 No. 1.Bore No. 2.Bore No. 5.Bore No. 6.
Total solids (grains per gallon)33303332
Composition of dissolved salts, per cent.
Silica4·24·54·63·4
Ferric oxide1·70·81·21·3
Lime6·57·05·45·6
Magnesia2·93·12·73·8
Sulphuric anhydride4·54·94·44·4

CHAPTER X
FLOWING WELLS: SOME EXPERIMENTS AND OBSERVATIONS

Total Water Discharge of Oasis — Water-Pressures — Static Head — Importance of Systematic Observations and Records of Bores — Sensitiveness of Wells — Experiments showing Mutual Interference of Wells — Bores Nos. 5 and 6 — Bores Nos. 4 and 42 — The Wells at El Dêr el Ghennîma — Decline in Discharge of New Wells — Effect of Closing Bores — Rate of Flow of Water in Sandstones — Danger of Over-Exploitation — Holding Capacity and Porosity — Experiments on Porosity of Nubian Sandstone — Large Quantities of Water held in Storage Beds — Economically only partially available at Surface.

At the present day there are about 230 native-owned wells in the oasis, yielding a total discharge of some 295 qirats. We have seen that the qirat has not a fixed value, but represents a discharge varying from 22 to 38 or more gallons a minute. Applying these values as far as possible to the old wells, and adding the known discharge of the new, we shall not be very far from the truth if we estimate the total discharge of the oasis wells at 8,000 gallons a minute, or 11,500,000 gallons (53,000 cubic metres) a day. The largest well in Kharga Oasis is Ain Estakherab at Gennâh, with a discharge of between 700 and 800 gallons per minute. The average yield of those which might be described as the best wells probably does not exceed 150 to 200 gallons, while there are a great many which only discharge 20 or 30 gallons per minute.

In some parts of the world the discharges of artesian wells are measured by thousands, not hundreds, of gallons. In Queensland and other parts of Australia, for instance, there are numerous bores throwing over 1,000 gallons a minute; in these regions the depth of individual bores in many cases amounts to thousands of feet, the pressure frequently rising as high as 100 pounds to the square inch. It must, moreover, be borne in mind that the artesian basins of Australia have only been exploited during a comparatively short period, and that from the nature of the country the wells are, as a rule, at considerable distances apart.

The exploitation of the artesian basin of the Libyan Desert has been in progress for hundreds and thousands of years, and it is probable, therefore, that in the oases-depressions the general average pressure has been very much reduced. At the present time the water-pressures seldom exceed a very few pounds per square inch, so that the static head, or absolute height to which the water will rise, is seldom more than a few metres above the actual surface of the ground. The outlets of the native wells are, unfortunately, of such a nature that it is not possible to make even approximate determinations of pressure; but after taking into account the influence of ground-level on the volume of flow, we still find differences which point to there being considerable variation in the absolute static head in different parts of the oasis—that is to say, the water will rise higher in some places than in others. This is probably partly to be accounted for by variations in the level of the underlying strata and by the presence of faults—for instance, the static head to the west of the central line of disturbance is certainly considerably higher than to the east.

In the absence of accurate data, the static head in a district can be approximately gauged from the ground-level and flow of any one of the higher and more isolated wells. The maximum static head judged in this way appears to be 88 metres above sea-level in the Kharga district on the west side of the fault; but in the Headquarters area, to the east of the fault, it can be definitely proved to be very much less.

In order to determine the extent of the local variations throughout the district where the recent boring operations have extended, I carried out a number of experiments. For this purpose four bores in different parts of the area were selected, of similar diameters, and of approximately equal depths into the water-sandstone. By means of a flange the casing of each bore was carried vertically upwards, the ordinary outlet being kept shut until the pressure ceased to rise, as indicated by a pressure-gauge. It was then found that in Bore No. 36 the water had risen to 61·6 metres, in Bore No. 38 to 59·55, in Bore No. 42 to 64·26, and in Bore No. 44 to 62·0 metres above sea-level. The average static head in this district may therefore be taken as 61·85 metres. The positions of the four bores in question, with regard to Headquarters, are as follows: No. 36, 7·7 kilometres N.; No. 38, 2·6 kilometres N.W.; No. 42, 1·6 kilometres S.E.; and No. 44, 3·9 kilometres W.N.W.

To the north of Meheriq the static head appears to rise, as flowing wells are found up to 76 metres above sea-level (Ain Mohammed Delaib). The water-level at Ain el Ghazâl is even higher (84·56), but this well does not actually run.

In the development of an artesian basin this question is of the utmost importance; only by a knowledge of the static head and the ground-level can we, with any likelihood of success, estimate beforehand the discharge to be looked for in any particular district. If, for instance, we sink a bore in a locality whose surface is above the static head, the result, so far as a flowing well is concerned, can only be failure, however great the amount of water existing in the underlying strata. If a sufficient number of observations are available, it is possible to construct charts showing the isopotential lines, or lines of equal pressure, and these may be of great value when boring is contemplated in intermediate districts.

It is seldom that accurate records of bores are preserved in the initial stages of the development of new artesian basins, and to this neglect is due much of the doubt which frequently arises at a later period as to the extent and permanence of the underground water-supplies.

BORE NO. 5.

BORE NO. 14.

In some countries, however, the water-resources have long been the subject of exhaustive examination, notably in the United States, where a most valuable and instructive series of water-supply reports has been issued by the Geological Survey. The chief difficulties arise from the fact that bores are generally the property of private individuals, who are seldom both able and willing to supply accurate information. Discharges, for instance, are usually given in the roundest of figures, and without regard to the conditions under which they were taken. In Australia more than one geologist has called attention to the matter, and quite recently Mr. G. H. Knibbs, F.R.A.S., of the University of Sydney, in a valuable and suggestive paper on the hydraulic aspect of the artesian problem, refers to the want of comprehensive and deliberate investigation in the past, and admits the inadequacy of the available data for the determination even of the one question only—i.e., the extent to which exploitation can be pushed without fear of exhausting the supply.

When drilling was first commenced in the Headquarters area, the bores were placed at an average distance apart of 500 metres; circumstances, however, led to there being a considerable variation in the depths of the wells, with the result that those of shallow depth and those situated on comparatively high ground were adversely affected by the deeper and more favourably placed bores. The sensitiveness of any one well to the influence of its neighbours is, I believe, far greater than is generally supposed, and appears to be especially dependent on the amount of difference between the depths, discharges, and surface-levels of the bores. In investigating this subject I made a number of experiments with the object of determining the mutual influence of wells, and perhaps some reference to these may not be without interest and value.

The first experiment to which I shall refer was made on wells situated comparatively close together. Bore No. 5 is 570 metres W.S.W. of Bore No. 6, the outlet of the former being at 57·38, that of the latter at 59·18, a difference of 1·8 metres. No. 5 has an internal diameter of 5⅝ inches, is 197 metres deep and 95 metres into the water-sandstone; No. 6 has a diameter of 8 inches, is 146 metres deep, and 61 metres into the sandstone. The two wells had been flowing continuously for a considerable period, and during the experiment neighbouring wells were kept shut down, so that there is no reason to suppose that the observations were affected by other bores.

Bore No. 5, discharging 114 gallons a minute, was shut down at 7 p.m. on June 12, 1907, and reopened at 7 a.m. on June 13. The hourly observations, as given in the following table, show the effects produced on Bore No. 6.

EXPERIMENT TO SHOW MUTUAL INTERFERENCE OF BORES.

Bore No. 5 closed at 7 p.m., June12, 1907.Bore No. 5 opened at 7 a.m., June 13, 1907.
Time.Discharge of Bore No. 6.Time.Discharge of Bore No. 6.Time.Discharge of Bore No. 6.Time.Discharge of Bore No. 6.
P.M.Gallons per Minute.A.M.Gallons per Minute.A.M.Gallons per Minute.P.M.Gallons per Minute.
7.061·21.076·67.083·71.069·0
8.1565·62.3077·48.078·42.067·7
9.068·43.079·29.075·03.066·8
10.069·64.079·710.073·04.066·2
11.073·25.082·111.070·85.066·6
12.074·76.083·112.069·66.065·3
7.064·0

From these figures it will be seen that the shutting down of a flowing or the opening of a closed well may produce a most marked effect on a neighbouring well within the short space of sixty minutes, even when the intervening distance is over 500 metres. In the above instance the rate of increase was most rapid at first, there being a gain of 7 gallons per minute, equivalent to about 12 per cent., in the first two hours. The total increase in the twelve hours amounted to 22½ gallons, or about 37 per cent. On reopening No. 5 it is seen that the discharge of No. 6 at once commenced to fall, the loss being nearly 9 gallons in the first two hours; afterwards the rate of decrease gradually diminished, until at 7 p.m., when the observations were discontinued, the flow had fallen to within 3 gallons of its normal.

A second series of observations was made between two bores considerably farther apart, No. 4 being 835 metres N.N.W. of No. 42. The difference of level in this case was found to be 1·18 metres, the outlet of No. 4 being 60·74, and that of No. 42, 59·56 metres. Bore No. 4 has an internal diameter of 4¼ inches, is 141 metres deep, and draws from 19 metres of sandstone; Bore No. 42 is 6 inches in diameter, 218 metres deep, and 69 metres into the water-sandstone. Previous to the experiment, No. 4 was flowing 36·75, and No. 42 about 68·5 gallons per minute. Precautions were taken against other wells influencing the results, the nearest bores having been opened twenty-four hours previously and being kept in the same condition throughout the experiment. Bore No. 42 was closed down at 9 a.m. on March 4, 1908, periodical observations being then made of the discharge of No. 4 during the next thirty-six hours.

Briefly stated, the result of this experiment was as follows: The discharge of No. 4 had not perceptibly increased at the end of the first half-hour, but had done so after one hour. It continued to increase at a very slow rate, the net gain after thirty-six hours being only 3 gallons, or between 8 and 9 per cent. In this case the mutual interference is very much less than that between Nos. 5 and 6, doubtless largely owing to the greater distance apart, and to the lesser difference between the outlet-levels of the wells. In all probability there are many other conditions which combine with the above in determining the amount of interference, such as the positions of the wells with regard to the main lines of underground flow, the relative depths of the bores, and the thicknesses of sandstone from which they draw their supplies.

The most marked example of interference with which I have met was in the case of two ancient wells at El Dêr el Ghennîma, situated only 88 metres apart, on the crest of an anticlinal fold running north and south. These wells had been sanded-up for centuries, but were recently taken in hand and cleaned out. The difference of level in the outlets is 2·07 metres, the higher well being 34½ metres in depth, the lower 41 metres. The opening or closing of the lower well produces an almost instantaneous effect on the higher, the difference in flow of the latter within thirty seconds amounting to as much as 11 per cent.

A great many observations were made, but the following are sufficient to show the rates of decrease and increase:

Gallons per Minute.
Upper well flowed13·2when lower wellwas open (flow 50·3 g.p.m.).
„ „20·5„ „had been closed10minutes.
„ „23·7„ „„ „20„
„ „26·3„ „„ „30„
„ „39·4„ „„ „24hours.
„ „35·6„ „had been open30seconds.
„ „32·5„ „„ „2minutes.
„ „26·7„ „„ „7„
„ „24·9„ „„ „9„
„ „23·4„ „„ „16½„
„ „19·7„ „„ „45„

The closing down of the lower well is thus seen to have influenced the discharge of the upper to the extent of 100 per cent, in the short space of thirty minutes, while the flow was trebled in twenty-four hours. On opening the lower well the discharge of the upper fell to within 50 per cent. of its normal within forty-five minutes.

As already mentioned, most bores show a marked decline in discharge for some time after completion, and except in special cases it seems doubtful if large bores can be expected to maintain their original flows for long periods of years. During the early part of its existence a well draws its supplies from fully saturated beds, the water being forced into it from every side, not only through the pores of the sandstone, but through any fissures the bore may have struck. The flow of water through a compact sandstone is, however, extremely slow, and it is probable that as time goes on every bore becomes more and more dependent on fissures for the maintenance of its supply. This supersaturation of the water-bearing beds, if we may be permitted to use the term, is well illustrated by the closing of a bore for a few days. The water at once commences to accumulate around it, and when the bore is reopened the discharge will generally be found to have increased to a very great extent. As an example of this I may mention Bore No. 14, which, on April 19, 1907, was flowing at the rate of 225 gallons per minute. The well was then closed down for five days; on reopening the discharge was found to be 370 gallons per minute, an increase of 145 gallons, or about 65 per cent., the pressure during the same time having risen from below 9 to nearly 16 pounds per square inch. The discharge took about twelve hours, or one-tenth of the time, to fall to its normal. On another occasion the same well had its output increased from 217 to 339 gallons by being closed for twenty-four hours, a gain amounting to 55 per cent.

The rate of flow of water through an underground sandstone depends upon a number of conditions, the most important being the size of the pores or spaces between the component grains, the porosity or water-holding capacity of the sandstone, the temperature of the water, and the pressure acting on it. The yield of a well will depend, of course, not only on all these factors, but also on the diameter of the bore, its depth into the water-stratum, the size and number of fissures passed through, and, last and most important of all, on the absolute height of its outlet. Large pores, high average porosity, and high temperatures make for strong flows, though in the absence of pressure greater than that due to a column of water equal in height to the distance between the water-stratum and the outlet of the well, they are in themselves of no avail in the production of an artesian flow. Moreover, although some of the above conditions may be known beforehand, the resistance to flow of the strata immediately surrounding a bore can never be more than approximately conjectured, as the size and mode of arrangement of the individual grains of any sedimentary rock must always vary, both horizontally and vertically, to a very great extent, and on these factors depends in very large measure the capacity of the strata to transmit water.

Data are as yet far too insufficient to warrant an attempt to calculate the supply which can safely be drawn from a given area without unduly reducing the pressure, lowering the average static head, and endangering the continuance of the artesian supply. In some parts of the oasis there are bores many hundreds of years old still pouring forth their hundreds of gallons a minute; such wells are probably situated in exceptionally favourable localities, and are very possibly fed to a great extent by fissures. At the same time it must not be forgotten that there are throughout the oasis scores of wells which have ceased to run, either through local exhaustion of the water-bearing strata, or through failure to keep the bore-channels open; possibly through a combination of both circumstances. In some cases time seems to have remedied matters, as it is not uncommon to meet with instances where new bores, sunk in the immediate neighbourhood of long extinct wells, have produced strong discharges of considerable volume.

As I have already stated, the rate of flow of water is largely influenced by both the size of the pores and the porosity of the rock, the capacity to transmit water being very much greater for large than for small pores, and for high than for low porosity. It must, however, be pointed out that large pores and high porosity do not necessarily go together, and that small pores in a rock frequently accompany high holding capacity. For instance, a fine-grained sample of Nubian Sandstone will absorb from 25 to 28 per cent. of water, a medium-grained sample 20 per cent., while a very coarse sample may take up as little as 15 per cent. The pores and transmitting capacity of the coarse-grained variety will, however, be very much greater than in the case of either of the others.

In order to arrive at some sort of idea as to the holding capacity of the artesian-water strata of the oasis I made an examination of between sixty and seventy samples of sand brought up from varying depths from a few selected bores in the oasis. As, however, owing to the methods of drilling employed, only powdered samples were available, it was necessary in the first instance to ascertain the relative porosity of sandstone in its ordinary state and broken up into the form of sand. For this purpose I collected eight specimens of the Surface-water Sandstone from various points in Northern Kharga, and subjected them to absorption tests, both in the whole and in the powdered states. In six out of the eight examples the absorption was more when powdered than when whole, the average for the eight rock samples being 22·44 per cent.; for the same when powdered, 23·55 per cent. There is, moreover, no reason to suspect that in ordinary lithological characters the Surface-water Sandstone differs in any important respect from the Artesian-water Sandstone, so that if we estimate the porosity of the latter from powdered samples, we shall obtain a figure only about 5 per cent. too high.

The average porosity of sixty-four samples, collected from Bores 14, 16, 18, 31, 39, and 44, at various depths in the water-bearing strata, was found to be 19·45 per cent. If we confine our attention to a single bore, and examine a representative sample from every stratum of the water-bearing sandstone, and, in deducing the porosity, take into account the thickness of each individual bed, we shall obtain a still more reliable figure. This was done in the case of Bore No. 18, which passed through 122 metres of water-bearing strata, thirty-one samples being collected and subjected to examination. The absorptions obtained varied from 15·3 to 25·5 per cent., the average porosity of the whole column being calculated as 19·6 per cent. I believe this figure may be accepted without misgiving as a satisfactory working value for the porosity of powdered Nubian Sandstone, that of the solid rock being taken as 5 per cent. lower, or, say, 18·5 per cent.

The Artesian-water Sandstone has been proved to reach a thickness of 122 metres, and probably its total thickness is considerably more. Assuming, however, a vertical extent of only 122 metres, the water-bearing beds under 1 square kilometre would, if fully saturated, hold 4,965,000,000 gallons, which is the equivalent of the water which would be discharged in ninety-four years by a well flowing at the rate of 100 gallons a minute. When we consider that the area of the floor of the depression is some thousands of square kilometres, and that the water-sandstones, except in the immediate neighbourhood of the existing wells, are probably fully saturated, we realize the vast amount of water which is stored under the depression alone, irrespective of the still greater quantities which underlie the surrounding plateaux. The movement of the water through the sandstones, except along fissures and through particularly porous beds, is, however, very slow, so that the amount of water which can economically be made available at the surface is more or less limited, self-flowing wells being only obtainable over that portion of the area lying below the general static head.

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.

LANDS UNDER RECLAMATION AT BORE NO. 39.

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.

AIN AMUR, ON THE UPPER DAKHLA ROAD.