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The problem of the palaeokarstic dammam limestone aquifer in Kuwait
Journal of Hydrology 6 (1968) 385-404; © North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
THE PROBLEM PALAEOKARSTIC
DAMMAM
OF T H E
LIMESTONE
AQUIFER
IN KUWAIT
D A V I D J. B U R D O N ,
Hydrogeologist, Food and Agriculture Organization of the United Nations, Rome and ABDULLAH AL-SHARHAN
Chief Engineer, Department of Water and Gas, Government of Kuwait 1. I N T R O D U C T I O N 2. T H E D A M M A M LIMESTONE A Q U I F E R , 2.1. The D a m m a m Limestone formation; 2.2. Origin and movement of groundwater; 2.3. Hydrochemistry of the D a m m a m Limestone waters; 2.4. Permeability of the D a m m a m Limestone Aquifer. 3. I N F L U E N C E OF THE PALAEOKARSTS ON G R O U N D W A T E R DEVELOPMENT, 3.1. Structure and erosion surfaces; 3.2. Karst Development in the EoceneOligocene; 3.3. The chert "cap-rock"; 3.4. Impermeable carapace to buried karst aquifers; 3.5. Collapses and associated structures; 3.6. Confined groundwater in a karst aquifer. 4. LINES O F INVESTIGATION, 4.1. Location of palaeokarstic zones; 4.2. Stratigraphy of the chert "cap-rock"; 4.3. Study of groundwater movement; 4.4. Hydrochemical investigations; 4.5. Possibilities of recharge and underground storage. 5. A C K N O W L E D G E M E N T S A N D REFERENCES, 5.1. Acknowledgements; 5.2. References. Abstract: The D a m m a m Limestone Aquifer has been developed in Kuwait to yield large supplies (27 million cubic metres in 1965) of poor-quality (4000 ppm of TDS) groundwater for use in Kuwait City and Ahmadi. It is a composite carbonate formation, subdivided into three units; there was an inter-formational emergence in the Middle Eocene and a prolonged post-formational emergence in the Upper Eocene and Oligocene. Karstification took place during both emergences, and the palaeokarsts then formed now appear to control and strongly influence present-day groundwater circulation, the yield to wells and even the chemical composition of the water in the D a m m a m Limestone Aquifer. Further groundwater development is planned from this anisotropic aquifer, and the problem is to locate the zones of palaeokarstification, so that they may be drilled and exploited to the full. Boreholes located in these karstic zones should have not only high yields but should produce better quality water due to more active flushing of the aquifer in the zones of higher permeability. In addition, it may be possible to store temporarily some of the water to be brought to Kuwait from Iraq in underground reservoirs in the D a m m a m Limestone, and so permit of more efficient use of this water. The paper sets out the lines along which the necessary investigations and experiments will be conducted. 385
386
DAVID J. BURDON AND A. A L - S H A R H A N
1. Introduction In studying the behaviour of groundwater in carbonate formations, the influence and importance o f karst networks formed in the past (fossil karsts or palaeokarsts) are often masked by the more recent karstic groundwater circulation networks formed under present-day or recently passed conditions. Opportunities to study the problems of groundwater circulating in pure palaeokarstified aquifers occur where the carbonate rock has been deeply buried and the groundwaters now reaching it are already in carbonate equilibrium with the enclosing media. Such a case occurs in the Dammam Limestone Aquifer of Kuwait, subjected to karstification during emergences in the Lower Eocene and again in the Oligocene, now buried under MioPliocene and Pleistocene formations, yet a major source of low-quality groundwater for the State of Kuwait. Over the sixteen years 1950-1965, some 160 million cubic metres of groundwater with total soluble salts in the 4000 ppm range have been pumped from the Dammam Limestone Aquifer in the Abduliya (for Ahmadi and the Kuwait Oil Company) and Sulaibiya (for Kuwait City and suburbs) water fields. Extraction rates are increasing rapidly to keep up with expanding demand; in 1965 extraction was 27 million cubic metres, and increasing at the rate of 15% per annum. These two water production fields are located in the south-eastern part of the State, close to the centres of consumption; but possibilities of extracting groundwater of better quality exist in the west and particularly in the south-west. So investigations have been initiated by Government to discover where high-yielding wells to tap the better-quality groundwater in the Dammam Limestone Aquifer should be sited. This paper sets out the problems which are being faced to achieve this objective as well as the lines of approach which are being followed and which are expected to lead to a satisfactory solution of the problem of optimum development of groundwater from the palaeokarstic Dammam Limestone Aquifer of Kuwait. 2. The Dammam Limestone Aquifer There is a considerable amount of data available on the geology, the hydrogeology, the hydrochemistry and the general behaviour of the Dammam Limestone Aquifer. However, the significant relationship of some of the phenomena observed to the development of the groundwater has not always been realized, while certain data has been misinterpreted in terms of groundwater investigation. The data available may be summarized under the following four headings.
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
2.1.
387
THE DAMMAM LIMESTONE FORMATION
The D a m m a m Limestone formation has been studied in outcrops in Kuwait (very limited area in the Ahmadi Quarries), in Saudi Arabia, in Bahrein and in Iraq; it has also been cut in numerous boreholes drilled mainly for oil. The type section is on the D a m m a m dome in Saudi Arabia, at N.26°17.3': E.50°07.7 '. The D a m m a m Limestone is of Middle Eocene age, and its position in the stratigraphical succession in Kuwait is shown in Table 1. In Kuwait the D a m m a m Limestone averages from 600 to 700 feet in thickness, and is essentially a limestone-dolomite formation showing some chert and silicification, and with some shales and sandy zones. In Kuwait, the formation is provisionally sub-divided into three units, which may be described as follows: U n i t 3 - Shelly, chalky to granular, porous limestone with hard siliceous limestone at top. Unit 2 Chalky, locally shelly limestone with thin cherty limestones at top and siliceous limestones with sandy beds at the base. U n i t 1 - Dense, dolomitic limestone, with lower fossiliferous (nummulitic) dolomitic limestone, thin anhydrite beds and green shale in the lower part. The D a m m a m Limestone formation shows a steady gentle regional dip to the north-east, averaging 0.3~o under the whole country. In south-west Kuwait, the top of the D a m m a n formation is at about 450 feet above sealevel and 250' below ground surface. It has fallen to 500' below sea-level under Kuwait Bay and is more than 1 000' below sea-level in north-eastern Kuwait. It is not clear whether the regional dip is to be attributed to tilting of the Arabian Shield, to geosynclinal subsidence in the Gulf, or to deposition on a sloping bottom, or to varying combinations of these factors. This general dip is interrupted by two types of folding. Deformation of the sea-bed commenced in the Middle Cretaceous, and there is a marked thinning in all subsequent formations over the crestal portion of areas of uplift, so that gentle folds and domes have been formed. During the Zargos orogeny in the Tertiary, there was a sharper type of folding, and one such fold has brought the top of the D a m m a m Limestone Formation to the surface of the ground at Ahmadi, where quarries expose its upper surface and its contact with the overlying Ghar Formation. Two surfaces of erosion have been carved into the D a m m a m Formation during emergences, one in the Middle Eocene between Units 1 and 2, and the second during the Upper Eocene and Oligocene when the top of the D a m m a m Limestone Formation was subjected to deep weathering and erosion. These erosion surfaces indicate emergence and uplift, and during
388
DAVID J. BURDON AND A. AL-SHARHAN TABLE 1
General stratigraphy of Kuwait (after Owen and Nasrl); Parsons 2), and Milton 3)), showing the position of the D a m m a m Limestone Formation, its sub-division into three units as well as the two unconformities which gave rise to karstification in the Middle Eocene and in Upper Eocene-Oligocene times Age
GrouPi
©
~=
Formation and thickness
Lit hology
Water
Dibdibba Formation Up to 350'
Coarse gravels of two provenances-volcanic and plutonic + m e t a morphic. Sands, gravels, minor clays and some gypsiferous sandy clays
Locally fresh water, due to infiltration of runoff; otherwise brackish to dry
Lower Fars Formation Up to 350' but unidentified in south
Cross-bedded alternating red and yellow sandstones; red and green clays ; fossiliferous
Generally dry or with brackish water
Gharformation Up to 900'
Current-bedded, coarse-grained to pebbley sandstone; a few green clay beds.
Brackish. Local upflow?
~ ~ .~ ~ ~d
6 "~
I
EROSIONAL U N C O N F O R M I T Y ] I
Q
,T~,,~
: Dammam Limestone Formation 600' to 700"
Unit 3. Shelly limestone; a "cap of chert is usually reported from boreholes Unit 2. Chalky Limestone Unconformity. Karstification
~ Q ~ ~
Karstification Major aquifer but TDS increase from 2500 ppm in southwest to + 100000 ppm in north
i
Unit 1. Dolomitic Limestone; anhydrite ! and green shale, i
¢L Rus Formation 250' to 400'
Anhydrite evaporites; limestone and some marl.
Brackish/Saline.
Radhuma Formation 600" to 1,400'
Marly limestone; dolomite; some anhydrite.
Brackish/Saline.
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
389
such times, the Dammam Limestone was not only eroded on surface but also karstified at depth, that is, its joints and openings were selectively enlarged and opened up by solution in actively circulating groundwater of meteoric origin. Such a zone of karstification would develop at some depth below the ground erosion surface leaving a zone of unkarstified and essentially and impermeable limestone, which has been called a "carapace'" (Burdon and Safadi 4) p. 339) between the top of the limestone and the actual water-bearing zone. Thus today there should exist two superimposed palaeokarstic networks which will continue to control the movement of groundwater in the Dammam Limestone Aquifer. If they can be located, they should prove to be the zone of most active circulation, of highest yield to boreholes and of greatest flushing so as to contain water of better chemical composition than other zones of the aquifer. 2.2.
ORIGIN AND MOVEMENT OF GROUNDWATER
Most, if not all, of the groundwater found in the Dammam Limestone Aquifer originates from infiltration into its outcrop zones. The most extensive of these occurs in the southern desert of Iraq (USGS Map 1-270 A), from 200 to 400 kilometres west of Kuwait, where the Dammam Formation covers some 30 000 square kilometres and comprises the Tuquaid, Radhuma, Chabd and Shawiya Limestones (Miletic 5) table 5). There is also a large outcrop in Saudi Arabia, covering some 1 200 square kilometres east of Khurays, about 400 kilometres south of Kuwait. Outcrops along the zone joining these two exposure areas are covered by Neogene beds of varying composition and origin; they are sufficiently permeable to permit infiltration to the Dammam Limestone they cover. This zone of covered Dammam Limestone "outcrop" might extend beneath the lower reaches of the Wadi Batin in Saudi Arabia; but it is not exposed in that Wadi. A study of the isopiezometric surface of the groundwater in the Dammam Limestone Aquifer (Fig. 1) indicates that it is a confined nappe and that the direction of pressure decline relative to the horizon is from south-west to north-east; this is interpreted as indicating movement from south-west (the zone of outcrop) to the north-east (possible discharge in the Gulf of Kuwait). There is some evidence that the pressure head of the water in the Dammam Limestone Aquifer is sufficient to cause upflow into the overlying Kuwait Group sandstone where the confining cap (?) of the Dammam Limestone is less impermeable or has been fractured. In the two water production fields, lowering of pressure heads by pumping may have induced some backflow downwards into the Dammam Limestone. It has been suggested (Aten and
390
DAVID J. BURDON AND A. AL-SHARHAN
Bergstrom 6) p. 30) that "the sediments and limestone act as an aquifer system and receive additional recharge in Kuwait" but this does not agree with the conception of the Dammam Limestone confining groundwater under pressure. 2.3. HYDROCHEMISTRY OF THE DAMMAM LIMESTONE WATERS Based on its total dissolved solids, the quality of the water in the Dammam Limestone Aquifer is seen to increase from less than 3 000 ppm in the southwestern part of Kuwait, through 40000 ppm (sea-water level) in the Bay of Kuwait zone to plus 100000 ppm (brine?) under north-eastern Kuwait,
Fig. 1. Piezometric surface contours and isosalinity (TDS in ppm) lines for the groundwater of the Dammam Limestone Aquifer in Kuwait; the zone where total dissolved solids (TDS) lie in the 10000-40000 ppm range has been hatchured.
Fig. 1. Groundwater averaging 4000 ppm is pumped from the Abduliya and Sulaibiya fields. Table 2 shows the anions and cations found in the Abduliya groundwater in 1961; water quality has shown but slight chemical variations over the years in this water field.
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
391
In general terms, the hydrochemical picture may be interpreted as a flushing of the aquifer by infiltrating meteoric waters down as far as present sea-level, the change diminishing down-flow. This is followed by a zone where pressure heads are close to that of present sea-level, where total soluble TABLE 2 Chemical composition of a representative water sample from the Abduliya Field ; sampled 4 January, 1961 ; Kuwait Oil Company laboratory number LW/1379.77 % of the salts may be equated to halite and anhydrite
Cation/Anion
Parts per Million
Milligram equiv./litre
% Reaction Value
Na Mg Ca CI SO4 HCO3
596 166 460 1 294 l 214 43
25.88 13.65 22.95 36.49 25.28 0.71
20.71% l 0.92 % 18.37 % 29.20 % 20.23 ~,,, 0.57 ~,,,
Totals
3 773
124.96
100.00%
salts are close to those of the Arabian Gulf sea-water so that sea-water may totally replace meteoric waters in the aquifer. The importance of this zone has been stressed by hatchuring on Fig. 1 the area between the 10000 and 40 000 ppm isosalinity lines. The zone to the north-east, where total soluble salts exceed those found in sea-water, would represent a non-flushed portion of the aquifer, filled with connate water or more probably with waters introduced during the post-Oligocene marine-lagoonal transgression. However, this general picture masks the fact that in certain boreholes the total soluble salts in the Dammam Limestone Aquifer can be greater or smaller than would be expected from the general picture. Such irregularities may be explained by differential flushing of the aquifer along zones of higher (karstified?) and lower permeability. Variations in permeability may also produce flow in directions not exactly perpendicular to the generalized isopiezometric contours. Variations in chemical composition in a vertical direction are also reported, the more mineralized water underlying the fresher water; there may be some variations of the Hertzberg phenomena in the aquifer. In Saudi Arabia, the Alat Aquifer (roughly corresponding to Unit 3) shows water with less than 3000 ppm of TDS as it approaches the Kuwait frontier, whereas the deeper Khobar Aquifer (roughly corresponding to Unit 2) contains water with + 6 000 ppm of TDS along the frontier, (Naimi 7) Figs. 7 and 8).
392
DAVID J. BURDON AND A. AL-SHARHAN
2.4.
PERMEABILITY OF THE DAMMAM LIMESTONE AQUIFER
Boreholes drilled into the Dammam Limestone Aquifer tend to have very variable yields; partial penetration of the aquifer is the rule, and little is known regarding different water production zones. Aten and Bergstrom~) (p. 29) report that Parsons made five complete test-pumpings, from which Transmissibility " T " was calculated as varying from 1 600 to l I 000 gallons per day per foot of aquifer-"coefficients of storage were consistent at about 0.0002, but some sites showed significant vertical permeability or leakage". In Saudi Arabia, aquifer coefficients have been determined for the Alat Aquifer and for the Khobar Aquifer. For the Alat Aquifer, Naimi 7) reports 30 test-pumpings, in the Ras Tanura area, some 300 kilometres south-east of Kuwait. There " T " ranged from 2 200 to 36 100 gallons per day per foot of aquifer, while the coefficient of storage ranged from 0.000132 to 0.000534. For the Khobar Aquifer, "a preliminary study in the Abqaiq area determined that the transmissibility and coefficient of storage were 49 118 gpd/ft and 1.3 x 10 -6 respectively". These figures come from too far away to have any real meaning in Kuwait beyond emphasising the extreme variability and irregularity of the permeability of the Dammam Limestone Aquifer.
3. Influence of the palaeokarsts on groundwater development All available data indicate that the karstic openings produced in the Dammam Aquifer during the Eocene and Oligocene emergences now control the movement of groundwater within the aquifer. Structure and lithology will have deeply influenced the evolution of topography during the emergences, and topography will have played its part in concentrating the precipitation and runoff, so as to have produced zones of karstification. Structure will also have controlled the movement of the groundwater after infiltration, and produced zones of karstification. Nothing which has subsequently occurred has seriously modified these palaeokarsts. There must have been some collapses, while in the subsequent transgression, infilling of opening and of collapses will have occurred, but with permeable sands. The relation of the chert "cap-rock" to the karstification process is not clear and indeed silicification of a karst erosion surface must be a very unusual if not unknown phenomenon. However, the fact that the karstified Dammam Limestone Aquifer is underlain by the impermeable Rus Formation and is capped by some form of impermeable or semi-impermeable cover means that the water is confined and this will facilitate development.
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
3.1.
393
STRUCTURE AND EROSION SURFACES
There has been a tendency to confuse structure and erosion as expressed by contours drawn on the upper surface of the D a m m a m Limestone Formation, that is on its contact surface with the overlying G h a r Formation. It has been usual to refer to such contours as structure contours, when they are really contours on a surface of erosion, presented clearly as "ElevationTop of D a m m a m Limestone" in Parsons 2) (Fig. 7-3, June, 1963) but liable to misinterpretation. In addition, the conception of a "chert cap" has led to the identification of the top of the D a m m a m as being the point where a borehole encounters chert or cherty limestone; as Fig. 2 shows, chert may BH.A.
BH,I
the
i
Chert
--
BH.C
BH.D
--
U ?
Fig. 2. Composite Hydrogeological Section, drawn to indicate the probable palaeokarstification and some groundwater development problems of the Dammam Limestone Aquifer in Kuwait. be encountered at more than one horizon and the boreholes may have been in the D a m m a m Formation for some depth before encountering chert. There is certainly a connection between structure and erosion. In an emergence, structural highs will be the first to be subjected to erosion, while the opening-up of joints and bedding along anticlinal structures will predispose the beds to quicker erosion. Thus, in the early stages of erosion, the process will tend to reduce structural highs to a more uniform topography; but later, there will tend to be valleys and depressions cut into the anticlines and domes, while highs may be composed of perched synclines. Fig. 2 has been so drawn as to emphasise these points. The magnitude of the relief produced by erosion on the surface of the
394
D A V I D J. B U R D O N A N D A. A L - S H A R H A N
D a m m a m Limestone is reported as " m o r e than 100 feet", and in places the whole of Unit 3 has been removed, so that the G h a r Formation may rest directly on Unit 2, as shown in Fig. 2. The macro-topography thus formed has dimensions which enable its surface to be indicated from spot levels in scattered boreholes; but clearly drilling alone cannot define this surface in detail and geophysical profiles should be drawn to fill in such details. Neither drilling or geophysics will reveal details of the karstified surface themselves (micro-topography and lapiez) for as seen in the Ahmadi exposures, the karst micro-relief is of the order of one ot two metres in depth, with ridges and depressions from two to four metres across. 3.2.
K A R S T D E V E L O P M E N T IN T H E E O C E N E - O L I G O C E N E
The first emergence took place after the deposition of the Middle Eocene nummulitic (N. gizehensis, N. discorbinus, Alveolina elliptica) limestone and the dolomitic limestone of Unit 1. It was a comparatively short emergence, and was followed by a transgression and a resumption of marine sedimentation similar to that which proceeded the emergence. But it was sufficient to expose the dolomite and limestones to erosion and some karstification. There have been no opportunities of studying this surface, for it is not exposed: but an idea of what it may have looked like, as well as its karstic groundwater network, is indicated on the theoretical section of Fig. 2. The second emergence took place after the deposition of the shelly, chalky and granular limestone of Unit 3, which are still of Middle Eocene age. It was a long emergence, and at a maximum estimate could have extended from the commencement of the Upper Eocene to the end of the Oligocene, equivalent to as much as fifteen million years. This was a period of marine regression and erosion thoughout most of the Middle East and has given rise to palaeokarsts (as in Syria, Burdon and Safadi 4) p. 332); in Kuwait it led to heavy erosion and karstification of the Middle Eocene limestones of Unit 3. Fig. 2 shows some of the ways in which karstification during this long emergence led to the development of a groundwater circulation network in the limestones. This karstification will have affected not only Unit 3 limestones, but by cutting down through Unit 2 will have re-activated the karstification of Unit 1 limestones and dolomites. The zones of karstification will have been controlled by the surface erosion features, such as the location of streams flowing over limestone outcrops and possible basins of closed drainage with swallow-holes and related karst phenomenon. In turn, the surface morphology will have been influenced by structure, so that in fact, these palaeokarstic zones should be related both to structures and to the Oligocene topography of what is now Kuwait.
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
3.3.
395
THE CHERT "CAP-ROCK"
An impression has arisen that the present surface of the D a m m a m Limestone is composed of a cap-rock of chert some 10 to 20 feet thick; it is also considered that this chert capping is the impermeable layer which confines the groundwater in the D a m m a m Aquifer. Neither hypothesis can be accepted. From exposures in the Ahmadi quarries, it can be seen that the upper surface of the D a m m a m Limestone is a karstic erosion surface developed in a chert-rich horizon. The chert is seen in irregular nodules up to 30 cm across, and infilling planar openings; there is also some silicification. Above the erosional surface, and so at the base of the overlying Ghar sandstones, nodules of brightly-coloured clay may be seen mainly in the hollows of the old surface; not infrequently these clay nodules enclose a chert nodule, showing some signs of weathering. It seems certain that these cherts were eroded from the underlying limestone matrix; the clays are residual from the chemical dissolution of the main body of the limestone and the cherts are residual from the chert-rich horizon. The evidence from these exposures at Ahmadi have been used as the basis for the D a m m a m - K u w a i t G r o u p contact features of Fig. 2; boreholes have been added to indicate the way in which evidence from drilling may be misinterpreted. Postulating three chert horizons within Units 2 and 3 of the D a m m a m Formation, boreholes A, B and C have each cut chert at a different stratigraphical horizon; yet they could be, and indeed appear to have been, equated with a continuous chert cap to the D a m m a m Limestone. Again, the first chert of boreholes A, B and D lies at the present surface of the D a m m a m Limestone; but borehole C will have gone some distance in the limestone before meeting chert. Here in C the mistake of considering the chert as the cap-rock of the limestone introduces still more errors. Yet care must be taken not to reject entirely the conception of chert at the surface of the D a m m a m Limestone, for it is clear that the chert horizons would have been erosion-resistant and so tended to form the land surface, while cherts derived from the limestones would also have rested on this land surface and been incorporated into the basal beds of the Kuwait Group. Beyond this point, there is but slight justification for the idea of chert forming after the erosional surface was carved into the D a m m a m Limestone but there is a little regional evidence which must not be entirely rejected. In general terms, silicification appears to have been widespread in the Near and Middle East in the Oligocene. In Bahrain, Fromant (1965) and Willis (1966) comment that "local silicification of the limestone (Zone A aquifer) at the surface has resulted in the formation of a prominent scarp in the Buri area
396
D A V I D J. B U R D O N A N D A. A L - S H A R H A N
of Bahrain Island". In Saudi Arabia, chert and geodal quartz are mentioned for the Dammam and Rus Formations in the legend of USGS Map 1-270-A. But under surface weathering, redeposition of chert as part of the karstification process is unknown to the authors. During the replacement of connate or reinfiltrated sea-water by fresh water in the aquifer, some chert might have formed when silica in colloidal solution came into contact with seawater; the amount would have been small and most unlikely to be concentrated at the top of the Dammam Limestone. Once the chert is accepted as being essentially older than the Oligocene erosion surface, then the hypothesis that it is an impermeable cap to the aquifer must be rejected. From the Ahmadi exposures, the limestone seems to be tectonized and joined after the deposition of the Kuwait Group sandstones and the chert zone is not impermeable. Elsewhere, however, the chert may occur in marly and chalky beds and these may be impermeable; drillers report water under confining pressure below the chert-rich zone. 3.4.
I M P E R M E A B L E C A R A P A C E TO B U R I E D K A R S T AQUIFERS
The phenomenon of confined groundwater in a deeply-buried karst aquifer is not restricted to the Dammam Limestone in Kuwait, but has also been reported from the Middle Cretaceous and other karst limestones in Syria (Burdon and Safadi 4)). There, careful records from 97 boreholes drilled into karst formations indicated that the water in the aquifer is usually under sufficient hydrostatic pressure to rise in the borehole, sometimes about the full thickness of the formation. Thus, in Syria, a karst formation may consist of a central part which is an open aquifer containing water under considerable pressure (even where general artesian-producing structures are absent), overlain by an upper portion (a carapace) in which the openings have never been enlarged or have been refilled so as to tender the carapace impermeable. The possibility of the carapace being produced by drilling mud sealing the openings around the borehole is rejected, and it is concluded that the probable explanation is that karstification has developed at some depth below the top of the limestone formation. Another less probable solution is that the upper portion of the limestone has been re-sealed by downward movement of terra rossa and other impermeable material. Taking such findings into consideration and applying them to the Dammam Limestone Aquifer in Kuwait suggests that the confining carapace in Kuwait may not be the chert "cap-rock", but may be due to the development of karst openings and the groundwater circulation network at some depth below the old surfaces of erosion. The possibility of the sealing of the openings in the upper portion of the limestone by downward movement of clay is less likely
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
397
to occur in Kuwait since the transgressive Miocene formation is composed of sands and not as in Syria of a marl. This conception of a carapace may be found to be a more correct explanation of the phenomenon of confined groundwater in the D a m m a m Limestone Aquifer than that offered by the conception of an impermeable chert-rich hirozon. 3.5.
COLLAPSES AND ASSOCIATED STRUCTURES
The D a m m a m Limestone is too deeply buried in Kuwait for any collapse structures to affect the present surface topography; in fact, it is considered that the main karstification took place in the Eocene-Oligocene and that dolines and other karst structures were thereafter infilled by the basal sandstones of the Kuwait Group. However, it is worth suggesting that the Bay of Kuwait, with its escarpment of the Jal Az-Zor on the north-west and no major fault dislocation, may be a major collapse structure in the zone where flushing-dissolution of the D a m m a m Limestone was a maximum in the area of groundwater discharge to the sea. However, it is more probable that the Bay of Kuwait was formed by selective marine erosion of some soft beds in the Kuwait G r o u p and undercutting of the harder beds which have collapsed. 3.6.
CONFINED GROUNDWATER IN A KARST AQUIFER
Development of groundwater from a confined karst aquifer is assisted by the fact that all the groundwater is in communication, and though only secondary fissures may be cut in drilling, still these smaller fissures are in hydraulic continuity with the major openings (Burdon and Papakis 1°) IV-2.2). While higher yields can be obtained and the number of boreholes reduced if the major karst channels can be cut, still missing these channels will not lead to completely unsuccessful boreholes as will occur in unconfined karst aquifers with no diffused circulation of the groundwater. The fact that the groundwater is confined in the D a m m a m Limestone will therefore be a favourable factor in drilling successful boreholes to develop its groundwater.
4. Lines of investigation In order to develop and use to the full the indicated special karstic properties of the D a m m a m Limestone Aquifer there are numerous problems to be solved. The palaeokarstic zones must be located, the mechanism causing the groundwaters to be confined below the top of the D a m m a m Formation (chert "cap-rock" or other aquiclude) must be elucidated, the movement of the groundwater must be studied including the possible use of tracers,
398
DAVID J. BURDON AND A. AL-SHARHAN
hydrochemical investigations including isotope determinations must be made, and the possibilities of underground storage of water by recharge to the Dammam Aquifer must be determined, and may involve experimental field work. Some of the required information will be obtained by direct methods; in other cases, data on related factors will be obtained and then used to solve the main problems. The proposed methods of investigation will include geophysical (seismic and electrical resistivity) surface surveys to obtain data on structural and erosional breaks as well as the presence or absence of fresh or brackish groundwater in the Dammam Limestone and in the sandstones of the overlying Kuwait Group. By drilling, data will be obtained on the lithology and stratigraphy of the formations; electrical logging will indicate water-bearing horizons and may locate marker horizons; test-pumping will determine aquifer constants, and if a sufficient number are made may indicate zones of varying permeability (and so karstification) in both the horizontal and vertical planes. Rock samples from the boreholes will be examined to determine and separate chert from silicification; micropalaeontological studies may enable the different chert horizons to be distinguished and identified; while heavy mineral residues may also locate marker horizons and fix exactly the passage from the basal Kuwait Group rocks to the underlying top of the Dammam Limestone Formation. Full mineral analysis of a sufficient number of water samples will yield data not only on the uses to which the water may be put, but also as to the genesis of the different waters; age determinations may be possible by studying the radioactive isotopes, including tritium, which must exist in these groundwaters. Field experiments can be made to determine the rate of recharge through existing boreholes, possibly in the Sulaibiya field, where heavy pumping has already lowered the water-table and by dewatering has created a potential underground reservoir. 4.1.
LOCATION OF PALAEOKARSTIC ZONES
On the hypothesis that karstification will have been selectively developed over anticlinal areas, at the bottom of synclines and beneath surface features such as valleys, the first task will be to determine the structure and buried topography of the two erosional surfaces carved into the Dammam Limestone in the Eocene and Oligocene. Some data exists already, especially as to the structure of the underlying Rus anhydrite formation. This will be supplemented by geophysical investigations, both seismic and electrical resistivity surveys. Boreholes, drilled right through the Dammam Limestone to the under-
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
399
lying Rus Formation, will test the karst zones indicated by the structuralerosional hypothesis; care will be taken to consider karstification in both its horizontal and vertical dimensions. The possibilities of karst horizons related to old water-tables and ancient seas rather than to stratigraphical horizons must be borne in mind, with an impermeable carapace of unkarstified primary limestone overlying the karst horizons. The aquifer coefficients from test-pumpings at different depths in each borehole should be sufficiently numerous to identify nappes with different transmissibility in the carbonate rocks forming the D a m m a m Limestone Aquifer. Thus in South Parnassos in Greece, a general diffused nappe (T equals 0.0184 lit/sec./metre width of aquifer), an intermediate nappe (T equals 0.1408) and true karstified horizons (T equals 1.7245) were identified by the analysis of data from 17 test-pumpings (Burdon11), Table 3). The recent paper by Hantush and Thomas 1',) indicates ways in which drawdown tests in an anisotropic nonleaky aquifer such as the D a m m a m Limestone can be analysed so as to yield figures for transmissivity and storage in different directions; it calls for a number of observation wells, and so such an investigation is costly, and may not be fully justified in Kuwait. The possibilities of infilling of karst openings by sands from the succeeding Kuwait G r o u p must not be overlooked in the interpretation of the aquifer coefficients in terms of karstic zones. Drilling will also produce rock samples which can be examined lithologically (including heavy mineral residue) and palaeontologically to fix their stratigraphic position and so assist in determining the structure of their formations. Geophysical logging of boreholes will also yield data which may be interpretable in stratigraphical and structural terms. The use of caliper logging should locate zones where the boreholes have intersected open permeable karstified fissures in the limestones, as in Lattman and Parizek 13), p. 85. Since karst zones should show low frictional resistance to the movement of groundwater, they may be reflected by lower gradients across the isopiezometric contours, and so the contours should lie further apart in karst zones. This will produce a ridge on the isopiezometric map, and may suggest flow away from the karst zone into the less karst areas, even though no such flow may occur. Since karst zones should be well-flushed, it is possible that groundwater from such zones will be less mineralized than from non-karstic portions of the aquifer. And if age determinations can be made, the younger water would be expected in the karstic zones. Thus, almost all the lines of investigation proposed here will be directed to one principle end - the location of palaeokarstic zones into which high-yielding development boreholes can be sunk. And if such karstic zones yield groundwater of better-than-average chemical composition, this is an added benefit.
400
DAVID J. B U R D O N AND A. A L - S H A R H A N
4.2.
S T R A T I G R A P H Y OF THE CHERT " C A P - R O C K "
It is absolutely necessary that the nature of the reported impermeable "cap-rock" on the erosional surface of the D a m m a m Limestone be identified, and that it be distinguished from the chert-bearing horizons within the D a m m a m Limestone. It is also desirable that the different chert-bearing horizons be identified and distinguished one from another. The information desired will have to be obtained from rock samples obtained in drilling, though the Ahmadi outcrop should be studied and tested also. I f the geophysical work can identify positively the two erosional surfaces, then this will much facilitate the study of this impermeable cap-rock and also of the chert horizons. It is probable that the chert cap-rock will prove to be a valid conception where its erosion-resistant qualities have made the ancient land surface coincide with chert beds. Elsewhere solution-resistant chert boulders and cobbles may have been scattered over the erosion surface. But there should be many areas where the present top of the D a m m a m Formation is free of chert, as indicated in Fig. 2. A possibility of silicification of these surfaces cannot be entirely ruled out, but seems very improbable, despite that widespread silicification in the Oligocene reported in many places in the Near and Middle East. The need to distinguish chert from silicified limestone must be stressed. Micropalaeontological studies of the matrix surrounding the chert horizons may show that the different chert-bearing horizons can be distinguished one from the other by the presence of one or more diagnostic fossils. The heavy mineral residues, and possible sharks teeth, in the matrix may also be diagnostic for each individual chert horizon. However, there may be lithological changes across the area at the same horizon, and heavy mineral assemblages are not zone fossils. Likewise, it is possible that the character of the chert may differ from one chert-bearing horizon to another; but stratigraphical work based on such changes in chert appearance or even on specific trace elements in the chert nodules likewise does not rest on the sound basis of zone fossils. In carrying out these studies, the important factor from the water development viewpoint will be to determine the nature and extent of the "cap-rock" which is reported to confine the groundwater in the D a m m a m Limestone Aquifer. Study of the chert per se is of secondary importance. 4.3. STUDY OF GROUNDWATERMOVEMENT As shown on Fig. 1, the general direction of movement of groundwater in the D a m m a m Limestone Aquifer is known. However, this is a composite
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
401
picture, and more detailed work is required. Provided that there are still sufficient wells available for regular measurements, the isopiezometric surface should be plotted twice a year (possibly in February and August) for the whole State and more frequently for the development areas of Abduliya and Sulaibiya. Detailed plotting of this nature will indicate the zones of greatest permeability and horizontal movement. They may also prove that there is discharge from the Dammam Limestone in the zone to the south-east and under Kuwait Bay. Tracers might also be used, provided that there is a good possibility of positive recovery from boreholes, that they do not present health hazard and that their introduction will not affect future work on the hydrochemistry (including isotopes) of the waters. Radio-active isotopes have been used to determine direction of groundwater movement in a single borehole (as in Wurzel and Ward14)), and this technique may also have application to Kuwait. They have also been used to assist in measurements during test-pumpings, and the methods described by Hazza et aL ia) have already been applied with success in Kuwait. Groundwater movement in a vertical direction must also be studied, and such work is related to determining whether the top of the Dammam Aquifer is truly impermeable so as to produce fully confined and not leaky aquifer conditions. To determine this, the test-pumpings will have to be carried out with great care, and the results matched with the applicable formula. The basic principles are to be found in USGS Water-Supply Paper 1536-E (Ferris et al. 16) pp. 110-118), while the matter is treated at length in de Wiest's "Geohydrology ''17) pp. 271-282. 4.4.
HYDROCHEMICAL INVESTIGATIONS
The balanced full chemical analyses of a sufficient number of water samples will yield information essential to the investigation of the waters of the Dammam Limestone Aquifer. The evolution of the chemistry of the normal water should be determined, from its infiltration zones outside Kuwait to the zone where total soluble salts exceed 5 000 ppm and contamination by mixing becomes dominant over solution of minerals from the aquifer. This evolution of the chemistry of a groundwater has been termed "metasomatism", and has already been studied in adjoining countries, as in Syria (Burdon and Mazloum18)) and north-east Jordan (Lloyd19)). Particular attention should be paid to the bicarbonate and carbon dioxide ions, to see if the groundwater is saturated and no longer aggressive against calcite and dolomite when it reaches Kuwait; the method of determining saturation described by Back 2o) is very satisfactory and requires that pH and temperatures should also be measured. It is most probable that saturation has been reached, so
402
DAVID J. BURDON AND A. AL-SHARHAN
that the water can no longer impose a new karstification on those formed in the Eocene and Oligocene. Once the composition of the normal Dammam Limestone water has been established, abnormal changes due to mixing can be identified and localized. This work should yield data on downflow or upflow from the overlying or underlying beds, though high sulphate water at the base of the Dammam Aquifer may only mean that the water has come in contact with the gypsumanhydrite of the Rus Formation. In the badly flushed parts of the aquifer, there will be some mixing with older waters, whether connate or introduced in the post-Oligocene marine transgression. The brines in the Dammam Aquifer under northern Kuwait must represent such older waters, but the intermediate zone, from 10000 ppm to 40000 ppm total soluble salts, and hatchured on Fig. 1, may be due either to such mixing or to infiltration of sea-water. Clearly, such a study should identify zones of flushing and zones of stagnation, and it is most likely that the flushed zones will be the zones of palaeokarstification. The decay of radio-active isotopes occurring naturally in the groundwater may yield some information as to the age of the waters. Tritium is the best tool, but many of the waters must be more than 20 years old, while the increased tritium concentration in rainfall following the hydrogen bomb explosions in 1954 may not as yet have reached and altered the tritium content of the waters under examination. 4.5.
POSSIBILITIES OF RECHARGE AND UNDERGROUND STORAGE
Underground storage in Kuwait of the large amount of river water it is planned to bring from Iraq could influence greatly the use and value of the new water. Possibilities of recharge and underground storage in the NeogeneQuaternary of northern Kuwait are good (Bergstrom and Aten21)) but this does not preclude storage also in the old karstified openings of the Dammam Limestone. Recharge would have to be through boreholes located in or close to the existing water production feld of Sulaibiya; extraction would be from the existing groundwater pumping system. Experimental work is required to determine recharge rates under present conditions; some acidification might increase this rate and allow the new water to move rapidly from around the recharge bores to the large natural caverns and openings produced by the ancient karstification of the limestone.
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
403
5. Acknowledgements and references 5.1. ACKNOWLEDGEMENTS
The thanks of the authors are due to the Food and Agriculture Organization of the United Nations and to the Government of the State of Kuwait for permission to publish this paper. The work forms part of the technical assistance from the F.A.O. to the State of Kuwait. As can be seen from the following list of references, the authors have drawn heavily on previous work done in Kuwait as well as on relevant studies and work through the enlarging field of karst hydrogeology. 5.2. REFERENCES 1) R. M. S. Owen and Sami N. Nasr, Stratigraphy of the Kuwait-Basra Area in: Habitat of oil, Amer. Assoc. Pet. Geol. 0958) pp. 1252-1278 2) Parsons Corporation Ground-water resources of Kuwait, Vols. I and II (August, 1963) Report to Govt. of Kuwait 3) D. I. Milton, Geology of the Arabian Peninsula - Kuwait, describing part of the USGS Misc. Geol. Invest., (1956) Map 1-270-A 4) D.J. Burdon and S. Safadi, The karst groundwaters of Syria. J. Hydrol. 2 (1964) 324-347 5) P. Miletic, An outline of the geology and hydrogeology of the southern desert area of Iraq. Geoloski Vjesnik, Zagreb, 15 (1963) 369-390 6) R.E. Aten and R.E. Bergstrom, Groundwater hydrology of Kuwait, ASCE water Res. Eng. Con., Mobile (March, 1965) 7) All I. Naimi, The ground water of North-Eastern Saudi Arabia. Fifth Arab Pet. Congress, Cairo (March, 1965) 8) A.C. Fromant, The water supplies of Bahrain. J. Inst. Water Engin. 19 (1965) 579-585 9) R.P. Willis, Geology of the Arabian Peninsula-Bahrain, describing part of the USGS. Misc. Geol. Invest., (1966) Map 1-270-A 10) D. J. Burdon and N. Papakis, Handbook of karts hydrogeology. Inst. for Geol. and Subsurface Research, Athens (1963) 11) D.J. Burdon, Hydrogeology of some karstic areas of Greece. UNESCO Sym. Hydrol. Fractured Rocks, Dubrovnik Meeting 1965. (in press) 12) M. S. Hantush and R. G. Thomas, A Method of analyzing a Drawdown test in anisotropic aquifers. Water Resources Res. 2 (1965) 281-285 13) L.H. Lattman and R.P. Parizek, Relationship between fracture traces and the occurrence of ground water in carbonate rocks. J. Hydrol. 2 (1963) 73-91 14) P. Wurzel and P. R. B. Ward, A simplified method of groundwater direction measurement in a single borehole. J. Hydrol. 3 (1965) 97-105 15) I.B. Hazza, K.F. Saad, R.K. Girgis, A.A. Bakr and F.M. Swailem, Determination of the porosity of ground-water aquifers by the radioactive tracer technique. Int. J. appl. Radiation and Isotopes, 16 (1965) 487~,93 16) J.G. Ferris, D.B. Knowles, R H . Brown and R.W. Stallman, Theory of Aquifer Tests. USGS Water Supply Paper 1536-E (1962) 17) R . J . M . De Wiest, Geohydrology. John Wiley and Sons (1965) pp. 366
404
DAVID J. BURDON AND A. AL-SHARHAN
18) D.J. Burdon and S, Mazloum, Some Chemical Types of Groundwater from Syria. UNESCO Arid Zone Programme Salinity Problems in the Arid Zone, (1958) pp. 73-90 19) J.W. Lloyd, The hydrochemistry of the aquifers of North-East Jordan, J. Hydrol~ 3 (1965) 319-330 20) W. Back, Calcium carbonate saturation in ground water, from routine analyses. USGS Water-Supply Paper 1535-D (1961) 21) R.E. Bergstrom and R.E. Aten, Natural recharge and localization of fresh groundwater in Kuwait. J. Hydrol. 2 (1964) 213-231
THE PROBLEM PALAEOKARSTIC
DAMMAM
OF T H E
LIMESTONE
AQUIFER
IN KUWAIT
D A V I D J. B U R D O N ,
Hydrogeologist, Food and Agriculture Organization of the United Nations, Rome and ABDULLAH AL-SHARHAN
Chief Engineer, Department of Water and Gas, Government of Kuwait 1. I N T R O D U C T I O N 2. T H E D A M M A M LIMESTONE A Q U I F E R , 2.1. The D a m m a m Limestone formation; 2.2. Origin and movement of groundwater; 2.3. Hydrochemistry of the D a m m a m Limestone waters; 2.4. Permeability of the D a m m a m Limestone Aquifer. 3. I N F L U E N C E OF THE PALAEOKARSTS ON G R O U N D W A T E R DEVELOPMENT, 3.1. Structure and erosion surfaces; 3.2. Karst Development in the EoceneOligocene; 3.3. The chert "cap-rock"; 3.4. Impermeable carapace to buried karst aquifers; 3.5. Collapses and associated structures; 3.6. Confined groundwater in a karst aquifer. 4. LINES O F INVESTIGATION, 4.1. Location of palaeokarstic zones; 4.2. Stratigraphy of the chert "cap-rock"; 4.3. Study of groundwater movement; 4.4. Hydrochemical investigations; 4.5. Possibilities of recharge and underground storage. 5. A C K N O W L E D G E M E N T S A N D REFERENCES, 5.1. Acknowledgements; 5.2. References. Abstract: The D a m m a m Limestone Aquifer has been developed in Kuwait to yield large supplies (27 million cubic metres in 1965) of poor-quality (4000 ppm of TDS) groundwater for use in Kuwait City and Ahmadi. It is a composite carbonate formation, subdivided into three units; there was an inter-formational emergence in the Middle Eocene and a prolonged post-formational emergence in the Upper Eocene and Oligocene. Karstification took place during both emergences, and the palaeokarsts then formed now appear to control and strongly influence present-day groundwater circulation, the yield to wells and even the chemical composition of the water in the D a m m a m Limestone Aquifer. Further groundwater development is planned from this anisotropic aquifer, and the problem is to locate the zones of palaeokarstification, so that they may be drilled and exploited to the full. Boreholes located in these karstic zones should have not only high yields but should produce better quality water due to more active flushing of the aquifer in the zones of higher permeability. In addition, it may be possible to store temporarily some of the water to be brought to Kuwait from Iraq in underground reservoirs in the D a m m a m Limestone, and so permit of more efficient use of this water. The paper sets out the lines along which the necessary investigations and experiments will be conducted. 385
386
DAVID J. BURDON AND A. A L - S H A R H A N
1. Introduction In studying the behaviour of groundwater in carbonate formations, the influence and importance o f karst networks formed in the past (fossil karsts or palaeokarsts) are often masked by the more recent karstic groundwater circulation networks formed under present-day or recently passed conditions. Opportunities to study the problems of groundwater circulating in pure palaeokarstified aquifers occur where the carbonate rock has been deeply buried and the groundwaters now reaching it are already in carbonate equilibrium with the enclosing media. Such a case occurs in the Dammam Limestone Aquifer of Kuwait, subjected to karstification during emergences in the Lower Eocene and again in the Oligocene, now buried under MioPliocene and Pleistocene formations, yet a major source of low-quality groundwater for the State of Kuwait. Over the sixteen years 1950-1965, some 160 million cubic metres of groundwater with total soluble salts in the 4000 ppm range have been pumped from the Dammam Limestone Aquifer in the Abduliya (for Ahmadi and the Kuwait Oil Company) and Sulaibiya (for Kuwait City and suburbs) water fields. Extraction rates are increasing rapidly to keep up with expanding demand; in 1965 extraction was 27 million cubic metres, and increasing at the rate of 15% per annum. These two water production fields are located in the south-eastern part of the State, close to the centres of consumption; but possibilities of extracting groundwater of better quality exist in the west and particularly in the south-west. So investigations have been initiated by Government to discover where high-yielding wells to tap the better-quality groundwater in the Dammam Limestone Aquifer should be sited. This paper sets out the problems which are being faced to achieve this objective as well as the lines of approach which are being followed and which are expected to lead to a satisfactory solution of the problem of optimum development of groundwater from the palaeokarstic Dammam Limestone Aquifer of Kuwait. 2. The Dammam Limestone Aquifer There is a considerable amount of data available on the geology, the hydrogeology, the hydrochemistry and the general behaviour of the Dammam Limestone Aquifer. However, the significant relationship of some of the phenomena observed to the development of the groundwater has not always been realized, while certain data has been misinterpreted in terms of groundwater investigation. The data available may be summarized under the following four headings.
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
2.1.
387
THE DAMMAM LIMESTONE FORMATION
The D a m m a m Limestone formation has been studied in outcrops in Kuwait (very limited area in the Ahmadi Quarries), in Saudi Arabia, in Bahrein and in Iraq; it has also been cut in numerous boreholes drilled mainly for oil. The type section is on the D a m m a m dome in Saudi Arabia, at N.26°17.3': E.50°07.7 '. The D a m m a m Limestone is of Middle Eocene age, and its position in the stratigraphical succession in Kuwait is shown in Table 1. In Kuwait the D a m m a m Limestone averages from 600 to 700 feet in thickness, and is essentially a limestone-dolomite formation showing some chert and silicification, and with some shales and sandy zones. In Kuwait, the formation is provisionally sub-divided into three units, which may be described as follows: U n i t 3 - Shelly, chalky to granular, porous limestone with hard siliceous limestone at top. Unit 2 Chalky, locally shelly limestone with thin cherty limestones at top and siliceous limestones with sandy beds at the base. U n i t 1 - Dense, dolomitic limestone, with lower fossiliferous (nummulitic) dolomitic limestone, thin anhydrite beds and green shale in the lower part. The D a m m a m Limestone formation shows a steady gentle regional dip to the north-east, averaging 0.3~o under the whole country. In south-west Kuwait, the top of the D a m m a n formation is at about 450 feet above sealevel and 250' below ground surface. It has fallen to 500' below sea-level under Kuwait Bay and is more than 1 000' below sea-level in north-eastern Kuwait. It is not clear whether the regional dip is to be attributed to tilting of the Arabian Shield, to geosynclinal subsidence in the Gulf, or to deposition on a sloping bottom, or to varying combinations of these factors. This general dip is interrupted by two types of folding. Deformation of the sea-bed commenced in the Middle Cretaceous, and there is a marked thinning in all subsequent formations over the crestal portion of areas of uplift, so that gentle folds and domes have been formed. During the Zargos orogeny in the Tertiary, there was a sharper type of folding, and one such fold has brought the top of the D a m m a m Limestone Formation to the surface of the ground at Ahmadi, where quarries expose its upper surface and its contact with the overlying Ghar Formation. Two surfaces of erosion have been carved into the D a m m a m Formation during emergences, one in the Middle Eocene between Units 1 and 2, and the second during the Upper Eocene and Oligocene when the top of the D a m m a m Limestone Formation was subjected to deep weathering and erosion. These erosion surfaces indicate emergence and uplift, and during
388
DAVID J. BURDON AND A. AL-SHARHAN TABLE 1
General stratigraphy of Kuwait (after Owen and Nasrl); Parsons 2), and Milton 3)), showing the position of the D a m m a m Limestone Formation, its sub-division into three units as well as the two unconformities which gave rise to karstification in the Middle Eocene and in Upper Eocene-Oligocene times Age
GrouPi
©
~=
Formation and thickness
Lit hology
Water
Dibdibba Formation Up to 350'
Coarse gravels of two provenances-volcanic and plutonic + m e t a morphic. Sands, gravels, minor clays and some gypsiferous sandy clays
Locally fresh water, due to infiltration of runoff; otherwise brackish to dry
Lower Fars Formation Up to 350' but unidentified in south
Cross-bedded alternating red and yellow sandstones; red and green clays ; fossiliferous
Generally dry or with brackish water
Gharformation Up to 900'
Current-bedded, coarse-grained to pebbley sandstone; a few green clay beds.
Brackish. Local upflow?
~ ~ .~ ~ ~d
6 "~
I
EROSIONAL U N C O N F O R M I T Y ] I
Q
,T~,,~
: Dammam Limestone Formation 600' to 700"
Unit 3. Shelly limestone; a "cap of chert is usually reported from boreholes Unit 2. Chalky Limestone Unconformity. Karstification
~ Q ~ ~
Karstification Major aquifer but TDS increase from 2500 ppm in southwest to + 100000 ppm in north
i
Unit 1. Dolomitic Limestone; anhydrite ! and green shale, i
¢L Rus Formation 250' to 400'
Anhydrite evaporites; limestone and some marl.
Brackish/Saline.
Radhuma Formation 600" to 1,400'
Marly limestone; dolomite; some anhydrite.
Brackish/Saline.
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
389
such times, the Dammam Limestone was not only eroded on surface but also karstified at depth, that is, its joints and openings were selectively enlarged and opened up by solution in actively circulating groundwater of meteoric origin. Such a zone of karstification would develop at some depth below the ground erosion surface leaving a zone of unkarstified and essentially and impermeable limestone, which has been called a "carapace'" (Burdon and Safadi 4) p. 339) between the top of the limestone and the actual water-bearing zone. Thus today there should exist two superimposed palaeokarstic networks which will continue to control the movement of groundwater in the Dammam Limestone Aquifer. If they can be located, they should prove to be the zone of most active circulation, of highest yield to boreholes and of greatest flushing so as to contain water of better chemical composition than other zones of the aquifer. 2.2.
ORIGIN AND MOVEMENT OF GROUNDWATER
Most, if not all, of the groundwater found in the Dammam Limestone Aquifer originates from infiltration into its outcrop zones. The most extensive of these occurs in the southern desert of Iraq (USGS Map 1-270 A), from 200 to 400 kilometres west of Kuwait, where the Dammam Formation covers some 30 000 square kilometres and comprises the Tuquaid, Radhuma, Chabd and Shawiya Limestones (Miletic 5) table 5). There is also a large outcrop in Saudi Arabia, covering some 1 200 square kilometres east of Khurays, about 400 kilometres south of Kuwait. Outcrops along the zone joining these two exposure areas are covered by Neogene beds of varying composition and origin; they are sufficiently permeable to permit infiltration to the Dammam Limestone they cover. This zone of covered Dammam Limestone "outcrop" might extend beneath the lower reaches of the Wadi Batin in Saudi Arabia; but it is not exposed in that Wadi. A study of the isopiezometric surface of the groundwater in the Dammam Limestone Aquifer (Fig. 1) indicates that it is a confined nappe and that the direction of pressure decline relative to the horizon is from south-west to north-east; this is interpreted as indicating movement from south-west (the zone of outcrop) to the north-east (possible discharge in the Gulf of Kuwait). There is some evidence that the pressure head of the water in the Dammam Limestone Aquifer is sufficient to cause upflow into the overlying Kuwait Group sandstone where the confining cap (?) of the Dammam Limestone is less impermeable or has been fractured. In the two water production fields, lowering of pressure heads by pumping may have induced some backflow downwards into the Dammam Limestone. It has been suggested (Aten and
390
DAVID J. BURDON AND A. AL-SHARHAN
Bergstrom 6) p. 30) that "the sediments and limestone act as an aquifer system and receive additional recharge in Kuwait" but this does not agree with the conception of the Dammam Limestone confining groundwater under pressure. 2.3. HYDROCHEMISTRY OF THE DAMMAM LIMESTONE WATERS Based on its total dissolved solids, the quality of the water in the Dammam Limestone Aquifer is seen to increase from less than 3 000 ppm in the southwestern part of Kuwait, through 40000 ppm (sea-water level) in the Bay of Kuwait zone to plus 100000 ppm (brine?) under north-eastern Kuwait,
Fig. 1. Piezometric surface contours and isosalinity (TDS in ppm) lines for the groundwater of the Dammam Limestone Aquifer in Kuwait; the zone where total dissolved solids (TDS) lie in the 10000-40000 ppm range has been hatchured.
Fig. 1. Groundwater averaging 4000 ppm is pumped from the Abduliya and Sulaibiya fields. Table 2 shows the anions and cations found in the Abduliya groundwater in 1961; water quality has shown but slight chemical variations over the years in this water field.
PALAEOKARSTIC DAMMAM LIMESTONE AQUIFER IN KUWAIT
391
In general terms, the hydrochemical picture may be interpreted as a flushing of the aquifer by infiltrating meteoric waters down as far as present sea-level, the change diminishing down-flow. This is followed by a zone where pressure heads are close to that of present sea-level, where total soluble TABLE 2 Chemical composition of a representative water sample from the Abduliya Field ; sampled 4 January, 1961 ; Kuwait Oil Company laboratory number LW/1379.77 % of the salts may be equated to halite and anhydrite
Cation/Anion
Parts per Million
Milligram equiv./litre
% Reaction Value
Na Mg Ca CI SO4 HCO3
596 166 460 1 294 l 214 43
25.88 13.65 22.95 36.49 25.28 0.71
20.71% l 0.92 % 18.37 % 29.20 % 20.23 ~,,, 0.57 ~,,,
Totals
3 773
124.96
100.00%
salts are close to those of the Arabian Gulf sea-water so that sea-water may totally replace meteoric waters in the aquifer. The importance of this zone has been stressed by hatchuring on Fig. 1 the area between the 10000 and 40 000 ppm isosalinity lines. The zone to the north-east, where total soluble salts exceed those found in sea-water, would represent a non-flushed portion of the aquifer, filled with connate water or more probably with waters introduced during the post-Oligocene marine-lagoonal transgression. However, this general picture masks the fact that in certain boreholes the total soluble salts in the Dammam Limestone Aquifer can be greater or smaller than would be expected from the general picture. Such irregularities may be explained by differential flushing of the aquifer along zones of higher (karstified?) and lower permeability. Variations in permeability may also produce flow in directions not exactly perpendicular to the generalized isopiezometric contours. Variations in chemical composition in a vertical direction are also reported, the more mineralized water underlying the fresher water; there may be some variations of the Hertzberg phenomena in the aquifer. In Saudi Arabia, the Alat Aquifer (roughly corresponding to Unit 3) shows water with less than 3000 ppm of TDS as it approaches the Kuwait frontier, whereas the deeper Khobar Aquifer (roughly corresponding to Unit 2) contains water with + 6 000 ppm of TDS along the frontier, (Naimi 7) Figs. 7 and 8).
392
DAVID J. BURDON AND A. AL-SHARHAN
2.4.
PERMEABILITY OF THE DAMMAM LIMESTONE AQUIFER
Boreholes drilled into the Dammam Limestone Aquifer tend to have very variable yields; partial penetration of the aquifer is the rule, and little is known regarding different water production zones. Aten and Bergstrom~) (p. 29) report that Parsons made five complete test-pumpings, from which Transmissibility " T " was calculated as varying from 1 600 to l I 000 gallons per day per foot of aquifer-"coefficients of storage were consistent at about 0.0002, but some sites showed significant vertical permeability or leakage". In Saudi Arabia, aquifer coefficients have been determined for the Alat Aquifer and for the Khobar Aquifer. For the Alat Aquifer, Naimi 7) reports 30 test-pumpings, in the Ras Tanura area, some 300 kilometres south-east of Kuwait. There " T " ranged from 2 200 to 36 100 gallons per day per foot of aquifer, while the coefficient of storage ranged from 0.000132 to 0.000534. For the Khobar Aquifer, "a preliminary study in the Abqaiq area determined that the transmissibility and coefficient of storage were 49 118 gpd/ft and 1.3 x 10 -6 respectively". These figures come from too far away to have any real meaning in Kuwait beyond emphasising the extreme variability and irregularity of the permeability of the Dammam Limestone Aquifer.
3. Influence of the palaeokarsts on groundwater development All available data indicate that the karstic openings produced in the Dammam Aquifer during the Eocene and Oligocene emergences now control the movement of groundwater within the aquifer. Structure and lithology will have deeply influenced the evolution of topography during the emergences, and topography will have played its part in concentrating the precipitation and runoff, so as to have produced zones of karstification. Structure will also have controlled the movement of the groundwater after infiltration, and produced zones of karstification. Nothing which has subsequently occurred has seriously modified these palaeokarsts. There must have been some collapses, while in the subsequent transgression, infilling of opening and of collapses will have occurred, but with permeable sands. The relation of the chert "cap-rock" to the karstification process is not clear and indeed silicification of a karst erosion surface must be a very unusual if not unknown phenomenon. However, the fact that the karstified Dammam Limestone Aquifer is underlain by the impermeable Rus Formation and is capped by some form of impermeable or semi-impermeable cover means that the water is confined and this will facilitate development.
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393
STRUCTURE AND EROSION SURFACES
There has been a tendency to confuse structure and erosion as expressed by contours drawn on the upper surface of the D a m m a m Limestone Formation, that is on its contact surface with the overlying G h a r Formation. It has been usual to refer to such contours as structure contours, when they are really contours on a surface of erosion, presented clearly as "ElevationTop of D a m m a m Limestone" in Parsons 2) (Fig. 7-3, June, 1963) but liable to misinterpretation. In addition, the conception of a "chert cap" has led to the identification of the top of the D a m m a m as being the point where a borehole encounters chert or cherty limestone; as Fig. 2 shows, chert may BH.A.
BH,I
the
i
Chert
--
BH.C
BH.D
--
U ?
Fig. 2. Composite Hydrogeological Section, drawn to indicate the probable palaeokarstification and some groundwater development problems of the Dammam Limestone Aquifer in Kuwait. be encountered at more than one horizon and the boreholes may have been in the D a m m a m Formation for some depth before encountering chert. There is certainly a connection between structure and erosion. In an emergence, structural highs will be the first to be subjected to erosion, while the opening-up of joints and bedding along anticlinal structures will predispose the beds to quicker erosion. Thus, in the early stages of erosion, the process will tend to reduce structural highs to a more uniform topography; but later, there will tend to be valleys and depressions cut into the anticlines and domes, while highs may be composed of perched synclines. Fig. 2 has been so drawn as to emphasise these points. The magnitude of the relief produced by erosion on the surface of the
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D a m m a m Limestone is reported as " m o r e than 100 feet", and in places the whole of Unit 3 has been removed, so that the G h a r Formation may rest directly on Unit 2, as shown in Fig. 2. The macro-topography thus formed has dimensions which enable its surface to be indicated from spot levels in scattered boreholes; but clearly drilling alone cannot define this surface in detail and geophysical profiles should be drawn to fill in such details. Neither drilling or geophysics will reveal details of the karstified surface themselves (micro-topography and lapiez) for as seen in the Ahmadi exposures, the karst micro-relief is of the order of one ot two metres in depth, with ridges and depressions from two to four metres across. 3.2.
K A R S T D E V E L O P M E N T IN T H E E O C E N E - O L I G O C E N E
The first emergence took place after the deposition of the Middle Eocene nummulitic (N. gizehensis, N. discorbinus, Alveolina elliptica) limestone and the dolomitic limestone of Unit 1. It was a comparatively short emergence, and was followed by a transgression and a resumption of marine sedimentation similar to that which proceeded the emergence. But it was sufficient to expose the dolomite and limestones to erosion and some karstification. There have been no opportunities of studying this surface, for it is not exposed: but an idea of what it may have looked like, as well as its karstic groundwater network, is indicated on the theoretical section of Fig. 2. The second emergence took place after the deposition of the shelly, chalky and granular limestone of Unit 3, which are still of Middle Eocene age. It was a long emergence, and at a maximum estimate could have extended from the commencement of the Upper Eocene to the end of the Oligocene, equivalent to as much as fifteen million years. This was a period of marine regression and erosion thoughout most of the Middle East and has given rise to palaeokarsts (as in Syria, Burdon and Safadi 4) p. 332); in Kuwait it led to heavy erosion and karstification of the Middle Eocene limestones of Unit 3. Fig. 2 shows some of the ways in which karstification during this long emergence led to the development of a groundwater circulation network in the limestones. This karstification will have affected not only Unit 3 limestones, but by cutting down through Unit 2 will have re-activated the karstification of Unit 1 limestones and dolomites. The zones of karstification will have been controlled by the surface erosion features, such as the location of streams flowing over limestone outcrops and possible basins of closed drainage with swallow-holes and related karst phenomenon. In turn, the surface morphology will have been influenced by structure, so that in fact, these palaeokarstic zones should be related both to structures and to the Oligocene topography of what is now Kuwait.
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395
THE CHERT "CAP-ROCK"
An impression has arisen that the present surface of the D a m m a m Limestone is composed of a cap-rock of chert some 10 to 20 feet thick; it is also considered that this chert capping is the impermeable layer which confines the groundwater in the D a m m a m Aquifer. Neither hypothesis can be accepted. From exposures in the Ahmadi quarries, it can be seen that the upper surface of the D a m m a m Limestone is a karstic erosion surface developed in a chert-rich horizon. The chert is seen in irregular nodules up to 30 cm across, and infilling planar openings; there is also some silicification. Above the erosional surface, and so at the base of the overlying Ghar sandstones, nodules of brightly-coloured clay may be seen mainly in the hollows of the old surface; not infrequently these clay nodules enclose a chert nodule, showing some signs of weathering. It seems certain that these cherts were eroded from the underlying limestone matrix; the clays are residual from the chemical dissolution of the main body of the limestone and the cherts are residual from the chert-rich horizon. The evidence from these exposures at Ahmadi have been used as the basis for the D a m m a m - K u w a i t G r o u p contact features of Fig. 2; boreholes have been added to indicate the way in which evidence from drilling may be misinterpreted. Postulating three chert horizons within Units 2 and 3 of the D a m m a m Formation, boreholes A, B and C have each cut chert at a different stratigraphical horizon; yet they could be, and indeed appear to have been, equated with a continuous chert cap to the D a m m a m Limestone. Again, the first chert of boreholes A, B and D lies at the present surface of the D a m m a m Limestone; but borehole C will have gone some distance in the limestone before meeting chert. Here in C the mistake of considering the chert as the cap-rock of the limestone introduces still more errors. Yet care must be taken not to reject entirely the conception of chert at the surface of the D a m m a m Limestone, for it is clear that the chert horizons would have been erosion-resistant and so tended to form the land surface, while cherts derived from the limestones would also have rested on this land surface and been incorporated into the basal beds of the Kuwait Group. Beyond this point, there is but slight justification for the idea of chert forming after the erosional surface was carved into the D a m m a m Limestone but there is a little regional evidence which must not be entirely rejected. In general terms, silicification appears to have been widespread in the Near and Middle East in the Oligocene. In Bahrain, Fromant (1965) and Willis (1966) comment that "local silicification of the limestone (Zone A aquifer) at the surface has resulted in the formation of a prominent scarp in the Buri area
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of Bahrain Island". In Saudi Arabia, chert and geodal quartz are mentioned for the Dammam and Rus Formations in the legend of USGS Map 1-270-A. But under surface weathering, redeposition of chert as part of the karstification process is unknown to the authors. During the replacement of connate or reinfiltrated sea-water by fresh water in the aquifer, some chert might have formed when silica in colloidal solution came into contact with seawater; the amount would have been small and most unlikely to be concentrated at the top of the Dammam Limestone. Once the chert is accepted as being essentially older than the Oligocene erosion surface, then the hypothesis that it is an impermeable cap to the aquifer must be rejected. From the Ahmadi exposures, the limestone seems to be tectonized and joined after the deposition of the Kuwait Group sandstones and the chert zone is not impermeable. Elsewhere, however, the chert may occur in marly and chalky beds and these may be impermeable; drillers report water under confining pressure below the chert-rich zone. 3.4.
I M P E R M E A B L E C A R A P A C E TO B U R I E D K A R S T AQUIFERS
The phenomenon of confined groundwater in a deeply-buried karst aquifer is not restricted to the Dammam Limestone in Kuwait, but has also been reported from the Middle Cretaceous and other karst limestones in Syria (Burdon and Safadi 4)). There, careful records from 97 boreholes drilled into karst formations indicated that the water in the aquifer is usually under sufficient hydrostatic pressure to rise in the borehole, sometimes about the full thickness of the formation. Thus, in Syria, a karst formation may consist of a central part which is an open aquifer containing water under considerable pressure (even where general artesian-producing structures are absent), overlain by an upper portion (a carapace) in which the openings have never been enlarged or have been refilled so as to tender the carapace impermeable. The possibility of the carapace being produced by drilling mud sealing the openings around the borehole is rejected, and it is concluded that the probable explanation is that karstification has developed at some depth below the top of the limestone formation. Another less probable solution is that the upper portion of the limestone has been re-sealed by downward movement of terra rossa and other impermeable material. Taking such findings into consideration and applying them to the Dammam Limestone Aquifer in Kuwait suggests that the confining carapace in Kuwait may not be the chert "cap-rock", but may be due to the development of karst openings and the groundwater circulation network at some depth below the old surfaces of erosion. The possibility of the sealing of the openings in the upper portion of the limestone by downward movement of clay is less likely
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to occur in Kuwait since the transgressive Miocene formation is composed of sands and not as in Syria of a marl. This conception of a carapace may be found to be a more correct explanation of the phenomenon of confined groundwater in the D a m m a m Limestone Aquifer than that offered by the conception of an impermeable chert-rich hirozon. 3.5.
COLLAPSES AND ASSOCIATED STRUCTURES
The D a m m a m Limestone is too deeply buried in Kuwait for any collapse structures to affect the present surface topography; in fact, it is considered that the main karstification took place in the Eocene-Oligocene and that dolines and other karst structures were thereafter infilled by the basal sandstones of the Kuwait Group. However, it is worth suggesting that the Bay of Kuwait, with its escarpment of the Jal Az-Zor on the north-west and no major fault dislocation, may be a major collapse structure in the zone where flushing-dissolution of the D a m m a m Limestone was a maximum in the area of groundwater discharge to the sea. However, it is more probable that the Bay of Kuwait was formed by selective marine erosion of some soft beds in the Kuwait G r o u p and undercutting of the harder beds which have collapsed. 3.6.
CONFINED GROUNDWATER IN A KARST AQUIFER
Development of groundwater from a confined karst aquifer is assisted by the fact that all the groundwater is in communication, and though only secondary fissures may be cut in drilling, still these smaller fissures are in hydraulic continuity with the major openings (Burdon and Papakis 1°) IV-2.2). While higher yields can be obtained and the number of boreholes reduced if the major karst channels can be cut, still missing these channels will not lead to completely unsuccessful boreholes as will occur in unconfined karst aquifers with no diffused circulation of the groundwater. The fact that the groundwater is confined in the D a m m a m Limestone will therefore be a favourable factor in drilling successful boreholes to develop its groundwater.
4. Lines of investigation In order to develop and use to the full the indicated special karstic properties of the D a m m a m Limestone Aquifer there are numerous problems to be solved. The palaeokarstic zones must be located, the mechanism causing the groundwaters to be confined below the top of the D a m m a m Formation (chert "cap-rock" or other aquiclude) must be elucidated, the movement of the groundwater must be studied including the possible use of tracers,
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hydrochemical investigations including isotope determinations must be made, and the possibilities of underground storage of water by recharge to the Dammam Aquifer must be determined, and may involve experimental field work. Some of the required information will be obtained by direct methods; in other cases, data on related factors will be obtained and then used to solve the main problems. The proposed methods of investigation will include geophysical (seismic and electrical resistivity) surface surveys to obtain data on structural and erosional breaks as well as the presence or absence of fresh or brackish groundwater in the Dammam Limestone and in the sandstones of the overlying Kuwait Group. By drilling, data will be obtained on the lithology and stratigraphy of the formations; electrical logging will indicate water-bearing horizons and may locate marker horizons; test-pumping will determine aquifer constants, and if a sufficient number are made may indicate zones of varying permeability (and so karstification) in both the horizontal and vertical planes. Rock samples from the boreholes will be examined to determine and separate chert from silicification; micropalaeontological studies may enable the different chert horizons to be distinguished and identified; while heavy mineral residues may also locate marker horizons and fix exactly the passage from the basal Kuwait Group rocks to the underlying top of the Dammam Limestone Formation. Full mineral analysis of a sufficient number of water samples will yield data not only on the uses to which the water may be put, but also as to the genesis of the different waters; age determinations may be possible by studying the radioactive isotopes, including tritium, which must exist in these groundwaters. Field experiments can be made to determine the rate of recharge through existing boreholes, possibly in the Sulaibiya field, where heavy pumping has already lowered the water-table and by dewatering has created a potential underground reservoir. 4.1.
LOCATION OF PALAEOKARSTIC ZONES
On the hypothesis that karstification will have been selectively developed over anticlinal areas, at the bottom of synclines and beneath surface features such as valleys, the first task will be to determine the structure and buried topography of the two erosional surfaces carved into the Dammam Limestone in the Eocene and Oligocene. Some data exists already, especially as to the structure of the underlying Rus anhydrite formation. This will be supplemented by geophysical investigations, both seismic and electrical resistivity surveys. Boreholes, drilled right through the Dammam Limestone to the under-
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lying Rus Formation, will test the karst zones indicated by the structuralerosional hypothesis; care will be taken to consider karstification in both its horizontal and vertical dimensions. The possibilities of karst horizons related to old water-tables and ancient seas rather than to stratigraphical horizons must be borne in mind, with an impermeable carapace of unkarstified primary limestone overlying the karst horizons. The aquifer coefficients from test-pumpings at different depths in each borehole should be sufficiently numerous to identify nappes with different transmissibility in the carbonate rocks forming the D a m m a m Limestone Aquifer. Thus in South Parnassos in Greece, a general diffused nappe (T equals 0.0184 lit/sec./metre width of aquifer), an intermediate nappe (T equals 0.1408) and true karstified horizons (T equals 1.7245) were identified by the analysis of data from 17 test-pumpings (Burdon11), Table 3). The recent paper by Hantush and Thomas 1',) indicates ways in which drawdown tests in an anisotropic nonleaky aquifer such as the D a m m a m Limestone can be analysed so as to yield figures for transmissivity and storage in different directions; it calls for a number of observation wells, and so such an investigation is costly, and may not be fully justified in Kuwait. The possibilities of infilling of karst openings by sands from the succeeding Kuwait G r o u p must not be overlooked in the interpretation of the aquifer coefficients in terms of karstic zones. Drilling will also produce rock samples which can be examined lithologically (including heavy mineral residue) and palaeontologically to fix their stratigraphic position and so assist in determining the structure of their formations. Geophysical logging of boreholes will also yield data which may be interpretable in stratigraphical and structural terms. The use of caliper logging should locate zones where the boreholes have intersected open permeable karstified fissures in the limestones, as in Lattman and Parizek 13), p. 85. Since karst zones should show low frictional resistance to the movement of groundwater, they may be reflected by lower gradients across the isopiezometric contours, and so the contours should lie further apart in karst zones. This will produce a ridge on the isopiezometric map, and may suggest flow away from the karst zone into the less karst areas, even though no such flow may occur. Since karst zones should be well-flushed, it is possible that groundwater from such zones will be less mineralized than from non-karstic portions of the aquifer. And if age determinations can be made, the younger water would be expected in the karstic zones. Thus, almost all the lines of investigation proposed here will be directed to one principle end - the location of palaeokarstic zones into which high-yielding development boreholes can be sunk. And if such karstic zones yield groundwater of better-than-average chemical composition, this is an added benefit.
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4.2.
S T R A T I G R A P H Y OF THE CHERT " C A P - R O C K "
It is absolutely necessary that the nature of the reported impermeable "cap-rock" on the erosional surface of the D a m m a m Limestone be identified, and that it be distinguished from the chert-bearing horizons within the D a m m a m Limestone. It is also desirable that the different chert-bearing horizons be identified and distinguished one from another. The information desired will have to be obtained from rock samples obtained in drilling, though the Ahmadi outcrop should be studied and tested also. I f the geophysical work can identify positively the two erosional surfaces, then this will much facilitate the study of this impermeable cap-rock and also of the chert horizons. It is probable that the chert cap-rock will prove to be a valid conception where its erosion-resistant qualities have made the ancient land surface coincide with chert beds. Elsewhere solution-resistant chert boulders and cobbles may have been scattered over the erosion surface. But there should be many areas where the present top of the D a m m a m Formation is free of chert, as indicated in Fig. 2. A possibility of silicification of these surfaces cannot be entirely ruled out, but seems very improbable, despite that widespread silicification in the Oligocene reported in many places in the Near and Middle East. The need to distinguish chert from silicified limestone must be stressed. Micropalaeontological studies of the matrix surrounding the chert horizons may show that the different chert-bearing horizons can be distinguished one from the other by the presence of one or more diagnostic fossils. The heavy mineral residues, and possible sharks teeth, in the matrix may also be diagnostic for each individual chert horizon. However, there may be lithological changes across the area at the same horizon, and heavy mineral assemblages are not zone fossils. Likewise, it is possible that the character of the chert may differ from one chert-bearing horizon to another; but stratigraphical work based on such changes in chert appearance or even on specific trace elements in the chert nodules likewise does not rest on the sound basis of zone fossils. In carrying out these studies, the important factor from the water development viewpoint will be to determine the nature and extent of the "cap-rock" which is reported to confine the groundwater in the D a m m a m Limestone Aquifer. Study of the chert per se is of secondary importance. 4.3. STUDY OF GROUNDWATERMOVEMENT As shown on Fig. 1, the general direction of movement of groundwater in the D a m m a m Limestone Aquifer is known. However, this is a composite
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picture, and more detailed work is required. Provided that there are still sufficient wells available for regular measurements, the isopiezometric surface should be plotted twice a year (possibly in February and August) for the whole State and more frequently for the development areas of Abduliya and Sulaibiya. Detailed plotting of this nature will indicate the zones of greatest permeability and horizontal movement. They may also prove that there is discharge from the Dammam Limestone in the zone to the south-east and under Kuwait Bay. Tracers might also be used, provided that there is a good possibility of positive recovery from boreholes, that they do not present health hazard and that their introduction will not affect future work on the hydrochemistry (including isotopes) of the waters. Radio-active isotopes have been used to determine direction of groundwater movement in a single borehole (as in Wurzel and Ward14)), and this technique may also have application to Kuwait. They have also been used to assist in measurements during test-pumpings, and the methods described by Hazza et aL ia) have already been applied with success in Kuwait. Groundwater movement in a vertical direction must also be studied, and such work is related to determining whether the top of the Dammam Aquifer is truly impermeable so as to produce fully confined and not leaky aquifer conditions. To determine this, the test-pumpings will have to be carried out with great care, and the results matched with the applicable formula. The basic principles are to be found in USGS Water-Supply Paper 1536-E (Ferris et al. 16) pp. 110-118), while the matter is treated at length in de Wiest's "Geohydrology ''17) pp. 271-282. 4.4.
HYDROCHEMICAL INVESTIGATIONS
The balanced full chemical analyses of a sufficient number of water samples will yield information essential to the investigation of the waters of the Dammam Limestone Aquifer. The evolution of the chemistry of the normal water should be determined, from its infiltration zones outside Kuwait to the zone where total soluble salts exceed 5 000 ppm and contamination by mixing becomes dominant over solution of minerals from the aquifer. This evolution of the chemistry of a groundwater has been termed "metasomatism", and has already been studied in adjoining countries, as in Syria (Burdon and Mazloum18)) and north-east Jordan (Lloyd19)). Particular attention should be paid to the bicarbonate and carbon dioxide ions, to see if the groundwater is saturated and no longer aggressive against calcite and dolomite when it reaches Kuwait; the method of determining saturation described by Back 2o) is very satisfactory and requires that pH and temperatures should also be measured. It is most probable that saturation has been reached, so
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that the water can no longer impose a new karstification on those formed in the Eocene and Oligocene. Once the composition of the normal Dammam Limestone water has been established, abnormal changes due to mixing can be identified and localized. This work should yield data on downflow or upflow from the overlying or underlying beds, though high sulphate water at the base of the Dammam Aquifer may only mean that the water has come in contact with the gypsumanhydrite of the Rus Formation. In the badly flushed parts of the aquifer, there will be some mixing with older waters, whether connate or introduced in the post-Oligocene marine transgression. The brines in the Dammam Aquifer under northern Kuwait must represent such older waters, but the intermediate zone, from 10000 ppm to 40000 ppm total soluble salts, and hatchured on Fig. 1, may be due either to such mixing or to infiltration of sea-water. Clearly, such a study should identify zones of flushing and zones of stagnation, and it is most likely that the flushed zones will be the zones of palaeokarstification. The decay of radio-active isotopes occurring naturally in the groundwater may yield some information as to the age of the waters. Tritium is the best tool, but many of the waters must be more than 20 years old, while the increased tritium concentration in rainfall following the hydrogen bomb explosions in 1954 may not as yet have reached and altered the tritium content of the waters under examination. 4.5.
POSSIBILITIES OF RECHARGE AND UNDERGROUND STORAGE
Underground storage in Kuwait of the large amount of river water it is planned to bring from Iraq could influence greatly the use and value of the new water. Possibilities of recharge and underground storage in the NeogeneQuaternary of northern Kuwait are good (Bergstrom and Aten21)) but this does not preclude storage also in the old karstified openings of the Dammam Limestone. Recharge would have to be through boreholes located in or close to the existing water production feld of Sulaibiya; extraction would be from the existing groundwater pumping system. Experimental work is required to determine recharge rates under present conditions; some acidification might increase this rate and allow the new water to move rapidly from around the recharge bores to the large natural caverns and openings produced by the ancient karstification of the limestone.
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5. Acknowledgements and references 5.1. ACKNOWLEDGEMENTS
The thanks of the authors are due to the Food and Agriculture Organization of the United Nations and to the Government of the State of Kuwait for permission to publish this paper. The work forms part of the technical assistance from the F.A.O. to the State of Kuwait. As can be seen from the following list of references, the authors have drawn heavily on previous work done in Kuwait as well as on relevant studies and work through the enlarging field of karst hydrogeology. 5.2. REFERENCES 1) R. M. S. Owen and Sami N. Nasr, Stratigraphy of the Kuwait-Basra Area in: Habitat of oil, Amer. Assoc. Pet. Geol. 0958) pp. 1252-1278 2) Parsons Corporation Ground-water resources of Kuwait, Vols. I and II (August, 1963) Report to Govt. of Kuwait 3) D. I. Milton, Geology of the Arabian Peninsula - Kuwait, describing part of the USGS Misc. Geol. Invest., (1956) Map 1-270-A 4) D.J. Burdon and S. Safadi, The karst groundwaters of Syria. J. Hydrol. 2 (1964) 324-347 5) P. Miletic, An outline of the geology and hydrogeology of the southern desert area of Iraq. Geoloski Vjesnik, Zagreb, 15 (1963) 369-390 6) R.E. Aten and R.E. Bergstrom, Groundwater hydrology of Kuwait, ASCE water Res. Eng. Con., Mobile (March, 1965) 7) All I. Naimi, The ground water of North-Eastern Saudi Arabia. Fifth Arab Pet. Congress, Cairo (March, 1965) 8) A.C. Fromant, The water supplies of Bahrain. J. Inst. Water Engin. 19 (1965) 579-585 9) R.P. Willis, Geology of the Arabian Peninsula-Bahrain, describing part of the USGS. Misc. Geol. Invest., (1966) Map 1-270-A 10) D. J. Burdon and N. Papakis, Handbook of karts hydrogeology. Inst. for Geol. and Subsurface Research, Athens (1963) 11) D.J. Burdon, Hydrogeology of some karstic areas of Greece. UNESCO Sym. Hydrol. Fractured Rocks, Dubrovnik Meeting 1965. (in press) 12) M. S. Hantush and R. G. Thomas, A Method of analyzing a Drawdown test in anisotropic aquifers. Water Resources Res. 2 (1965) 281-285 13) L.H. Lattman and R.P. Parizek, Relationship between fracture traces and the occurrence of ground water in carbonate rocks. J. Hydrol. 2 (1963) 73-91 14) P. Wurzel and P. R. B. Ward, A simplified method of groundwater direction measurement in a single borehole. J. Hydrol. 3 (1965) 97-105 15) I.B. Hazza, K.F. Saad, R.K. Girgis, A.A. Bakr and F.M. Swailem, Determination of the porosity of ground-water aquifers by the radioactive tracer technique. Int. J. appl. Radiation and Isotopes, 16 (1965) 487~,93 16) J.G. Ferris, D.B. Knowles, R H . Brown and R.W. Stallman, Theory of Aquifer Tests. USGS Water Supply Paper 1536-E (1962) 17) R . J . M . De Wiest, Geohydrology. John Wiley and Sons (1965) pp. 366
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18) D.J. Burdon and S, Mazloum, Some Chemical Types of Groundwater from Syria. UNESCO Arid Zone Programme Salinity Problems in the Arid Zone, (1958) pp. 73-90 19) J.W. Lloyd, The hydrochemistry of the aquifers of North-East Jordan, J. Hydrol~ 3 (1965) 319-330 20) W. Back, Calcium carbonate saturation in ground water, from routine analyses. USGS Water-Supply Paper 1535-D (1961) 21) R.E. Bergstrom and R.E. Aten, Natural recharge and localization of fresh groundwater in Kuwait. J. Hydrol. 2 (1964) 213-231