- Email: [email protected]
On the ancient water of the upper Nubian sandstone aquifer in Central Sinai and southern Israel
Journal of Hydrology 17 (1972) 353-374 © North-Holland Publishing Company Not to be reproduced by photoprint or microfilm without written permission f r o m the publisher
ON THE UPPER
ANCIENT
NUBIAN
IN CENTRAL
WATER
SANDSTONE
OF THE AQUIFER
SINAI AND SOUTHERN
ISRAEL
A. ISSAR, A. BEIN
Geological Survey of Israel, Ministry of Development and A. MICHAELI*
Hydrological Service, Ministry of Agriculture Abstract: The Nubian Sandstone, mainly that of Lower Cretaceous to Upper Jurassic age underlying the desert of Central Sinai and the Negev, was found to contain fresh water. The quantities stored in this aquifer are estimated at several hundred billion** cubic meters. Although only a very small fraction of this water has pumpage potential, this aquifer is of economic importance. The present arid climate of the area, the distances between the possible recharge areas and the areas where the water is found, as well as the large volume of water in storage, indicate that a large part of the water in this aquifer is fossil. The age of the water based on the 14C dating method, ranges between 13 000 years to more than 30 000 years. The 180/D ratios of this water are characteristic of precipitations in more temperate climates than the present one. Analysis of a calculated hypothetical decay curve indicates that although the water outflowing today has entered the aquifer more than 13 000 years ago, the rate of discharge is determined by the present rate of replenishment. Similar aquifers, containing fossil water in correlative stratigraphic sequences, are known in the Sahara and the Western Desert of Egypt.
Introduction The term Nubian
Sandstone
is a p p l i e d
in this p a p e r
to the " t h i c k
p r e d o m i n a n t l y s a n d y s e q u e n c e , m a i n l y o f terrestial origin, c h a r a c t e r i z e d by intensive and variable colours and composed
m a i n l y o f fine to c o a r s e
g r a i n e d , s o m e t i m e s gritty c o n g l o m e r a t i c , f r e q u e n t l y c r o s s - b e d d e d , m a i n l y soft b u t o c c a s i o n a l l y v e r y h a r d s a n d s t o n e s " (Lex. Strat. Israel, B e n t o r et al., 1960). T h e p r e s e n t s t u d y deals w i t h the u p p e r p a r t o f the s e q u e n c e , w h i c h is * Current address: T A H A L - Water Planning for Israel Ltd., 54, Ibn Gvirol Street, Tel Aviv. ** Billion = 109. 353
354
A. ISSAR, A. BEIN AND A. MICHAELI
of Upper Jurassic to Lower Cretaceous age. The Nubian Sandstone interfingers with marine sediments which become progressively more prominent with increasing distance from the Precambrian basement (Bentor et al., 1960). Although certain authors refer to the sequence as "Nubian Facies" (Knetsch et al., 1962) or even refrain altogether from using it (Pmeyrol, 1968; Weissbrod, 1970), it is considered the best short term, from the hydrogeological point of view, to be used for the above-mentioned sequence. The Nubian Sandstone is not restricted only to the northern margins of the Arabian massif (Picard, 1938) but extends all along the northern margin of the African shield from Egypt through Libya to the western Sahara (B.R.G.M., 1952; McKee, 1962). The Nubian Sandstone found throughout these areas is characterized by its predominance of clastics but has different proportions of sands, silts and shales. The occurrence of fresh water in the Cretaceous sandstone was discovered in the previous Century in Algiers by Ville and Rolland (1890, 1894). Ball (1927) described similar water occurrences from the Libyan Desert and explained it by a vast contemporary subterranean flow of water coming from Equatorial Africa. Heilstrom (1940) accepted this explanation but argued that a period of about 100000 years is needed for the water to flow from the assumed recharge region to the localities of the Western Desert. The hydrologists who studied these aquifers mainly in Algeria, Libya and Egypt, differ in their opinions as to whether the Nubian Sandstone aquifer is fossil or is still being recharged. In 1962 groundwater in the Nubian Sandstone aquifer of Eastern Sahara was reinvestigated using geological as well as geochemical methods, especially isotope analysis (Knetsch et al., 1962). The conclusions were that the rains which supplied the water probably fell during a Pluvial period about 25-35000 years ago, and that infiltration took place mainly in regions where the Nubian Sandstone is exposed and within the depressions of western Egypt. Himida (1970) concludes that the various water occurrences in Egypt, eastern Libya, northern Sudan and northeastern Chad are parts of a huge basin which is still being recharged in the highland of Chad, Sudan and probably the Tibesti plateau. Shiftan (1961), who studied the Nubian Sandstone aquifer in the Dead Sea region, suggested that the water there is of Pleistocene age.
Geography and climate The area studied has the form of a big triangle, its apex pointing southward and its base parallel to the northern coast line of Sinai and continuing eastward to the Negev, approximately along 31°05' latitude, and reaching the Dead Sea north of Mount Sedom. The western border of the triangle is
355
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
formed by the Canal and Gulf of Suez and its eastern border by the Gulf of Eilat and the Arava Valley. The central and northern areas form a high plateau, highest in its southern part where altitudes range from 1200 m to 1400 m, sloping gradually towards the north to altitudes of 300 to 400 m. North of this area a few anticlinal ridges rise to altitudes of 1000 m. The most northern part of the area slopes gradually towards the Mediterranean Sea, forming a wide coastal plain covered mostly by dunes. The mountainous region in the south is drained by many transverse ephemeral streams (Wadis) running in a general east-west direction to the gulfs of Suez and Eilat. Most of the central and northern areas are drained by Wadi el 'Arish which flows into the Mediterranean. The drainage of the eastern part of the Negev is towards the 'Arava and the Dead Sea. The area is arid. Precipitation is low, mostly below 100 mm/year (Table 1), and summers are hot and dry with temperatures reaching 40 °C. Rains fall in winter during very short periods, causing torrential floods in the wadis. On the mountains of Central Sinai snow falls only once in a few years, and it usually melts within a few days. TABLE 1
Mean monthly and annual totals of precipitation (mm) Station El Arish Kuntilla, El Nekhel Thamad, El
Tor, A. SantaKaterina
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual 19 3 7 3
20 1 7 8
12 3 4 6
2 2 2 1.5 1.4 10
7 2 4 2
2 0 1 1
0 0 0 0
0.3 0.1 0 7.9 6.0 drops
0 0 0 0
0 0 0 0
0 0 0 0
4 4 0 4
0 0
0 0
0 0
0.8 4.6
12 0 0 16
16 3 3 4
92 16 26 44
2 2 11 22.0 7.0 60.4
From "The Climate of Africa" Part 1, Air Temperature Precipitation - page 159, 1968. Main Administration of the Hydrometeorological Service of the U.S.S.R. Council of Ministers. A.I. Voeikov Main Geophysical Observatory. Israel Prog. Scientific Translation, Jerusalem 1970.
Geology The morphology, stratigraphy and structure of the area studied are closely related. The area may be divided into three different morphotectonic belts easily recognized on topographical and geological maps (Fig. 1). The belts are as follows: 1) The mountainous southern part of Sinai, which consists mainly of crystalline rocks, forms the northern extension of the A r a b o - N u b i a n massif. 2) The central high plateau, forming the "Stable Shelf" (Said, 1962) or
l'"
>
Z > Z
>.
>
t.m
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
357
"zone tabulaire" (Picard, 1938). This belt, which consists of sedimentary rocks with a gentle northward dip, is separated from the crystalline belt by the high E1-Tih Escarpment (Fig. 3). 3) The northern belt, forming the "Unstable Shelf" (Said, 1962). This belt consists of several parallel mountain chains trending east-north-east and belonging to an asymmetrical folding system. The sedimentary sequence may be divided into three divisions (Said, 1962) : 1) The Lower Clastic Division - a sequence of rocks, mainly clastics, overlying the igneous basement of pre-Cambrian age. The Nubian Sandstone discussed in this paper is found in the upper part of this division. 2) The Middle Calcareous Division - a sequence of rocks, mainly carbonates, of Cenomanian-Eocene age. 3) The Upper Clastic Division - a sequence of rocks, mainly clastics, of Neogene-Holocene age. The Nubian Sandstone sequence
The alternating beds of sandstones and shales forming the Nubian Sandstone sequence were deposited "largely in a marginal or continental environment, predominantly lagoonal, estuarine or lacustrine and fluviatile" (McKee, 19621). This depositional environment, which prevailed in the region during most of Palaeozoic and the beginning of Mesozoic times (Picard, 1943; McKee, 1962) and the facies change of the Mesozoic sequence from continental to marine, from south to north, make stratigraphical division and correlation quite difficult. The sandstone units in this study are equivalent mainly to the Kurnub group of Aharoni (1966), the upper variegated and white Nubian sandstone of Bentor et al. (1960), the upper Jurassic Nubian Sandstone and the Lower Cretaceous of Said (1962), and the Hatira Formation of Weissbrod (1970). The upper Nubian Sandstone is overlain by the Middle Calcareous Division (Said, 1962) which consists mainly of limestones, dolostones, chalks and marls. Its age ranges from Cenomanian to Upper Eocene. From a hydrogeological point of view this clastic sequence, called here the upper Nubian Sandstone, has aquifer properties. The lower boundary of this aquiferous sequence is not known, while its upper boundary is well marked by the calcareous sequence of marls and limestones, base of which has aquicludic properties. Since the lower boundary is not known, the exact thickness of the aquifer is unknown too, but it is at least 200 m. In most of the area this sequence forms a confined aquifer system.
358
A. ISSAR, A. BEIN AND A. MICHAELI
I~
I~
~
1~.
f l I I AriskArisk
lio0 mi/li~ Y
y Efe~ w,l'PIOm
D t CI L~O0 ml/I
o ! 1.-188l
~
211_
'
~
c,los,.dP r
eTim|r a ~.c-tO~ m
~e w.t ÷ ~ o i
~ I Mlkhte$t Q l t i a *.a.- s o m
A,d|t CI ~ • O0 ml/I ~
CO " t 2 0 - VBO m i A
i
wj.+aoom
.......
/,
,,,%
i !
Z
\
: I
t s A)in Muss l w.t,÷ 3 0 m
t
. ~ . Hlm|ik " ~ N lkhel I
mill
9
CI- $~11/I
. . t . e eO.Sm •
--i
.i t
r
I
fAl,,o,.o ]
¢-
4"'lined l i t e r f l o l dire¢llon in qh~ upper ~ . ~ .
A/~(ieoloiieal -
~
I~ Fig. 2.
/
~Nubiin
I ~.
Location map and hydrogeological data, Sinai and Southern Negev.
cro,s s¢cllon Sand~lone ~.quifer.
TABLE 2
27.12.67 23.2.70 3.4.68 9.11.70
÷ 4- 'Ayun Musa 1
+ q- Yotvata T/9
÷ Makhtesh Qatan 3
4- Makhtesh Qatan 4
÷ Analysis by " M e k o r o t h " . ÷ Jr- Analysis by Geological Survey of Israel.
27.12.67
9.1.63
q- Admon 2
÷ + ' A y u n Musa 2
8.11.67
÷ Tamar 5
27.12.67
13.1.70
q- q- Tamar 3
÷ Jr Nakhel 1
Date of analysis
Name of well
mg/l meq/l mg/l meq/l mg/l meq/l mg/l meq/l mg/l meq/l mg/1 meq/l mg/l meq/1 mg/l meq/l mg/l meq/l
196.0 9.8 153.1 7.6 233.0 11.6 116.0 5.8 386.0 19.3 278.0 13.9 118.0 5.9 154.9 7.7 215.2 10.74
Ca 30.4 2.5 60.7 5.0 103.0 8.5 92.3 7.6 86.0 7.07 37.7 3.10 85.0 7.0 70.0 5.7 79.0 6.5
Mg 401.08 17.4 386.5 16.8 1341.0 58.3 222.0 9.6 1210.0 52.6 520.0 22.62 370.0 16.1 460.0 20.0 492.0 21.4
Na 20.7 0.5 23.0 0.6 77.0 1.9 21.0 0.54 76.5 1.96 39.0 1.0 22.8 0.58 26.4 0.7 29.5 0.7
K 599.8 16.9 620.0 17.5 2069.0 58.3 355.0 10.0 2316.0 65.31 963.0 27.15 724.0 20.4 748.0 21.1 718.0 20.2
CI
Chemical composition
Chemical analysis of waters from various wells of the Nubian Sandstone aquifer
1.4 0.01 6.5 0.08 0.83 0.01 9.35 0.12 4.3 0.05 5.07 0.06 -
Br
413.9 8.6 391.7 8.1 895.0 18.6 485.0 10.0 760.0 15.82 530.0 11.03 83.5 1.74 374.9 7.8 673.2 14.0
SO4
305.0 5.0 274.3 4.5 182.0 3.0 220.0 3.6 110.0 1.8 146.0 2.39 79.0 1.3 317.0 5.2 284.7 4.6
HCOa
L/I
c
,-d © Z t"tl
Z
7r
G
7~
m
sr ,.d
rn
Z
-]
360
A. ISSAR, A. BEIN AND A. MICHAELI
Occurrences of groundwater in the Nubian Sandstone NORTHERN NEGEVAND DEAD SEA REGION A confined aquifer was first encountered in the Nubian Sandstone in southern Israel along the margins of the rift valley, west and southwest of the Dead Sea (Shiftan, 1961). This aquifer was also encountered in oil and gas exploration wells, situated north-west, west and south-west of the Dead Sea water wells, and in a few water wells located around the erosion cirques of Makhtesh Gadol and Makhtesh Qatan (Fig. 2). Isosalinity and potentiometric surface maps of this aquifer (Aharoni, 1966; Ettinger and Langozki, 1969) indicate a marked decrease in water head, accompanied by an increase in salinity, from the south to north-west, north and north-east. The only exception is the data from Avdat 1 (Fig. 2). According to Ettinger and Langozki (1969) the water head in this well, calculated from a D.S.T., is - 3 4 7 m and the chlorinity is about 500 mg/I. But the fact that this low head is the lowest known in the entire area (much lower even than those found in the Dead Sea region) and its combination with a relatively low salinity value (Table 2) makes these data unreliable. Ettinger and Langozki (1969) suggested a water-shed around the Makhtesh Gadol area. By omitting the data from Avdat 1 and taking into consideration the water head in Makhtesh Gadol water well (Fig. l), this watershed is not evident. Shiftan (1961) suggested that the relatively fresh water found in this aquifer dates from Pleistocene times. Furthermore, he maintained that the artesian water found in the limestone-dolomite aquifer overlying the Nubian Sandstone comes from the Nubian Sandstone aquifers. Arad (1966) suggests that "the sandstone aquifer is not a fossil body of water and that it has, in fact, an active and independent replenishment area". The replenishment areas are assumed to be the Judea Mountains and to a lesser degree the erosion cirques of the Makhteshim. He also suggests a system in which the present replenished waters mix with ancient ones and are responsible for possible groundwater movements (Arad, 1971, personal communication). YOTVATA~ SOUTHERN 'ARAVA VALLEY
Three water wells drilled in this region (Yotvata 1, 8 and 9) (Fig. 2) reached a confined Nubian Sandstone aquifer. The water table is about 90 m above M.S.L. and the water is of rather low salinity (700-900 mg/1 C1). The oldest rocks exposed in this region are limestones and dolostones of Cenomanian-Turonian age, but about 10 km to the south, in the Timna domal structure, small exposures of Nubian Sandstone are found. Larger Nubian Sandstone exposures are found about 30 km to the south.
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
361
CENTRAL SINAI
Three oil exploration wells were drilled in the vicinity of Nakhel (Fig. 1), in Central Sinai, by the Standard Oil Company of Egypt during the years 1944-1946 (Fig. 2). In these wells, which were spudded on shales of Paleocene age, the Nubian Sandstone sequence was encountered at a depth of 770-880 m. The water found in the Nubian Sandstone rose to about 230 m from the surface, between 190-200 m above M.S.L. The water chlorinity was about 350 mg/l. About 90 km north of the Nakhel area, Nubian Sandstones are exposed in the erosion cirque of the Gebel Helall anticline; here a shallow well encountered a confined water table in the upper Nubian Sandstone. The water rose to about 200 m above M.S.L. and it contained about 300 mg/l CI.
WESTERN SINAI
The spring and artesian wells of 'Ayun Musa (Fig. 2), about 15 km southeast of the town of Suez, are the most important occurrence of groundwater in northwestern Sinai. This region is covered by chalks and marls of Mio-Pliocene age, which are cut by wide joints from which the waters issue. The local inhabitants dig shallow holes at the peaks of the small hills, rising about 15 to 20 meters above the plain. These holes are filled by water seeping from the joints. The water is then transported by canals to the date-palm groves growing at the foot of these hills. Oil exploration wells have shown that the Nubian Sandstone is found at rather shallow depth (75-100 m) unconformably overlain by Miocene sediments. The Nubian Sandstone layers were found to contain water at artesian pressures with salinities ranging from 1 000 to 4000 mg/l CI. It is obvious that the water from the joints in the Mio-Pliocene rocks comes mostly from the Nubian Sandstone aquifer below.
Geochemistry of the Nubian Sandstone aquifer Table 2 and Fig. 4 present chemical analyses of water sampled from the Nubian Sandstone aquifer in the studied area. Except for the water from Nakhel 1, all these water analyses are comparable and may be grouped into one category, characterized by its ionic proportions: rCl>rSO4>rHCO 3 ;
rNa>rCa>rMg.
The water from the Nakhel 1 well is less saline than the others and also
362
A. 1SSAR, A. BEIN AND A. MICHAELI
differs in its ionic proportions:
rCl~-rSO4>rHC03;
rNa>rMg>rCa.
Measurements of the stable isotopes 180 and D (2H) in various aquifers along the rift valley have shown that the waters from the Nubian Sandstone aquifer in the Yotvata and T a m a r wells are significantly depleted in these isotopes as compared with groundwater from gravel aquifers in the 'Arava and those with recharge areas in the Judea Mountains (Gat and Dansgard, 1972) The depleted waters fall on a line on the 180 vs. D diagram (Fig. 5) which is assumed to represent meteoric water from a cooler period than the present one (Gat et al., 1969). This line is parallel with the non-depleted water line of the present-day meteoric water. Water taken from wells in which the Nubian Sandstone aquifer and the overlying limestone aquifer are pumped together (Admon 1, T a m a r 7, Tamar 3 before deepening) (Fig. 5) show an isotopic composition which falls between these two lines (Gat et al., 1969). The water from Nakhel 1, 'Ayun Musa and Makhtesh Qatan 3 wells are also depleted in their stable isotopic content and fall on the same line as the Nubian Sandstone aquifer in T a m a r and Yotvata wells (Fig. 5). It is interesting to note that the 180/D ratios of the water from the Nubian Sandstone aquifer of the western desert in Egypt also falls on the same line (Fig. 5).
14C Dating Dating of the waters has been carried out by measuring the 14C content in the bicarbonate dissolved in the water, and corrected according to the 13C content (Munnich and Vogel, 1962). These datings were made by Carmi TABLE 3 Water ages according to 14C content
Name
Depth
Age (B.P.)
Remarks
Tamar 3* Tamar 3* Yotvata 2* Makhtesh Qatan 3* Nakhel 3** 'Ayun Musa**
76m 400m 50m 763m 800m spring
14 900 :~ 600 28 500 ± 350 13 200 _~ 600 22 000 ± 1000 19 900 ± 3000 ~> 30 900
Waters of mixed origin Water of N.S. aquifer Water of mixed origin Water of N.S. aquifer Water of N.S. aquifer Water mainly of N.S.
* Carmi et al 1971. ** Kaufman, 1971 (personal communication). *** Ehhalt, 1963 and Munnich and Vogel, 1962.
-J200
-800
--400
o
400
0
8ooi
m 120o
-S-
ii0
xX X X
= 20
X
= 30
= 4o
carbonates
X XX
EI-Tih
5=o Km
escarpment
Cenomonion
Kc
t Middle
x x
j .L
Division
limestones
Calcareous
x
few
S a n d s t o n e s , shales
wells
•
Fig. 3.
rocks
Hydrogeological section A-A.
xXxx Precambrian , crystalline ___~Z Water table in N.S.
U,N.S. Lower C r e t a c e o u s - U p p e r Jurassic Nubian Sandstone I Lower P o - J Paleozoic - J u r a s s i c
Eocene Senonion Turonian
e $ Kt
~
Nokhel
sanastones Po-J
Clastic Division
--
t
Heloll
-..
-N-
364
A. ISSAR, A. BEIN AND A. MICHAELI
et al., (1971), Ehhalt (1963) and Kaufman (1971, personal communication), (Table 3). It can be seen that the water found in Cenomanian aquifers but replenished from the Nubian Sandstone aquifer (Tamar 3 at 76 m, Yotvata 2) show a younger date than that of the water coming directly from the Nubian Sandstone (Tamar 3 at 400 m, Makhtesh Qatan, Nakhel). This is due to the mixture with younger waters of the Cenomanian aquifer. Discussion The above data are from Nubian Sandstone aquifers in four areas: Central Sinai, the ~uez Graben, southern Arava Valley (Yotvata wells) and Central Negev - Dead Sea. The question is whether these waters are connected within one aquiferial system or whether they represent local unconnected aquifers. The following points may indicate a connection between the Nubian Sandstone aquifers found in the different areas: 1) Age: The water found in all these localities is fossil, ranging in age from about 13000 years to 30000 years. 2) Chemistry of the water: All the water analyses except that of Nakhel 1 are similar and may be grouped into one category. The different and sweeter water found in Nakhel 1 may represent a water development from a recharge area in the south through Central Sinai (Nakhel 1) towards the Gulf of Suez and the Arava Valley. 3) Stable isotope content: The water in the four regions are depleted in ~80 and D content as compared with present-day recharged aquifers. This depletion indicates that the waters found in these aquifers accumulated in the past in a cooler period. This cooler period was characterized also by higher rainfall as compared with present times. This is evident from archaeological data from Kharga Oasis, Egypt, which indicate an abrupt decline of the water heads in the Nubian Sandstone aquifer approximately 10000 years ago (Caton-Thompson and Gardner, 1932). Additional evidence for a more humid climate prevailing in the past is the presence of lacustrine flesh water carbonates in the highland of southern Sinai doted as 20000 years (Kaufman, 1971, personal communication). 4) Water heads: The water heads are highest in Central Sinai while those found in the Gulf of Suez and the 'Arava Valley are the lowest. This may indicate water flow from the highland of Central Sinai towards the Grabens (see also point 6). 5) Recharge areas: Large Nubian Sandstone exposures are found only in southern Sinai and Israel. These exposures are the only possible inlet for the water found in the storage of these aquifers. This is at least true for Central
THE ANCIENT WATER OF THE NUB1AN SANDSTONE AQUIFER
365
Sinai, Ayun Musa and southern 'Arava Valley. In the Central Negev-Dead Sea region, possible inlets are also the erosion cirques and perhaps the Judea Mountains. 6) Continuity of the Nubian Sandstones aquifer: The geological and topographical maps of the area indicate a continuity of the Nubian Sandstone sequence from Central Sinai towards Ayun Musa in the west and towards the southern 'Arava Valley in the East. The sequence is mostly below the potentiometric surface suggested by the water head data given above. In the central Negev the three anticlines of Hatzera, Hatira and Ramon (Fig. 1), raise the upper Nubian Sandstone above this assumed regional water table. The water heads in the Hatira and Hatzera anticlines (Makhtesh Qatan and Makhtesh Gadol wells) may represent a conning of the water table due to local replenishment. Continuity of the aquifer is effected apparently along the synclines. The continuity of the Nubian Sandstone aquifer beyond northern Sinai seems unlikely due to the presence of large anticlines trending east-north-east in this area. The Nubian Sandstone sequence is raised along these anticlines to levels sufficiently high to form a hydrological barrier (Fig. 3). Further to the north the Nubian Sandstone sequence plunges to great depth gradually losing its aquifer properties due to facies change into marine shales (the Gevar'Am Formation). Considering all the points presented above it seems reasonable to conclude that the various Nubian Sandstone aquifers found in the investigated area are parts of one aquifer system. This huge aquifer underlying the desert of Sinai and southern Israel was replenished mainly in the past, more than 13 000 years ago, in a colder climate. The hydrogeology of this aquifer, i.e. the present recharge and the factors determining the outflow from this aquifer, are discussed below.
Hydrogeology of the Nubian Sandstone aquifer RECENT REPLENISHMENT
The main replenishment areas for this aquifer are the Nubian Sandstone exposures found between the igneous basement of southern Sinai and the E1-Tih escarpment. Smaller replenishment areas exist in the erosion cirques of central Sinai and the Negev where the Nubian Sandstones are exposed. This replenishment causes bodies of fresher water and conning of the water table in the vicinity of these areas, as was observed in Makhtesh Qatan and Helall. Although the climate of the region is arid and average annual precipitation on the Nubian Sandstone outcrops is estimated to be between 50 and 100 mm (Table 1), recharge takes place even under the present climatic conditions.
366
A. ISSAR, A. BEIN A N D A. M I C H A E L I
meq~ i 0.0
Tamar.
3 ---4 ----I --'4
Tamar.
5
Ac[mon.
2 ................
Nakh¢l
I t....._
.................
9 8
7 6
/,
\
]
)
Ayun
Musa
2 ,
Ayun
Musa
I
t
Makhtesh Qatan
3 . . . . . . .
Makhtesh Qalan
4 ............
2
.:/ .~,.ty'~..~.,~.:;.~,,'..~. \\ #
'\, 10, 9
ii.4"
~,'~
."/ ,.'I~Y
',\
"~. ","
~,'\ \""\~
ot,ata
"r/9
..........
',,.,.~. \,.
...,//Z.",,~x"
....
8
7 6 5
"% :~,, ~,,'.. v .,.....~
-.~~ ~:,,,.....,.I ; -"~,'V'-::/: \ \"..../ / "
",\ /: ,,\/: ,v.: ;'
4
3
\5" ',\\
\\
\
2
i
!
Ca
F i g . 4.
Mg
)
i
i
Na÷K
CI
So,
i HCO.z
Chemical composition of waters from the Nubian Sandstone aquifer.
This is proven by the many shallow wells and springs found in the highland of Sinai. The most spectacular is the spring of Ein Furtaga, issuing from jointed granite and alluvium in eastern Sinai and discharging 70 to 120 m 3 per hour. In the vicinity of Abu Rudeis (Fig. 2) near the western shore of Sinai, a well field is utilizing the underflow of the Wadi Sidri river bed. This underflow was found to reach about 600000 m 3 per year (Kafri, 1968). The
-g
"6
-10
c-.O
o -4
-20
-21
--
0 --
I
J
I
I
(1963)
Department
-14
I_
of
-12
Fig. 5. zsO-Deuterium
Y. Gat
D. Ehhalt
Io
I
to
-16
According
-18
X According
•
I
"" '%" \
aquifer and drainage from Negev M . . . . . ins
I
I
$'~o
-IO
o
relationships
I -g
I
-6
I
.....
n
¢.
%/
~ ~'/
Y S
~
-4
)nstilute
I
of
-Z
I
Science
I
I
Rehovot
I
aqu ~rer I G. . . . .
in Sinai and Southern
t
Acava
/
deepening
.. %/
2 (N.~. "aqff,fer) Musa l(N,S aqu,rer~
groundwaters
Weizmman
of various
Research
% o
1
¢e-Wes l e r n D e s e r t
ALnias'2 ~ m /~__~Ayun T ara~,,t-~,,,"~S
Isotopic
(N S. aqu=fer)
Sheva
rocks cenlr~l Sinai f~j.~ ~" amar 7 IG . . . . d is sarl .~ ~ l l ~ j ~ / l a m a r Dead S e a w e l l f i e l d . h , ~ . J ~ . . ~ " -/ ( G a l el al 1969) , / ~ O g Qm~L'Tamar 3 I B e f o r e
Beer
I 0
Israel.
Israel
I
e2
I
aquifer)
k~ "
N.S.
~o"
19691
to
¢.,r.
~.¢.¢
.o ~
C~
Frl
>
z
©
z
> Z
_=
7
.-r
©
> -]
z
> 7
"4
368
a. ISSAR, A. BEIN AND A. MICHAELI
drainage area of Wadi Sidri is about 600 km 2. The rate of replenishment is thus 1000 m 3 per 1 km 2 per year. The total area of Nubian Sandstone outcrops and replenishable subcrops is about 3000 km 2. By using the above rate of recharge the replenishment of the Nubian Sandstone aquifers is in an order of magnitude of about 3.106 m 3 per year, but this estimate is by no means an accurate one. The real replenishment of this aquifer may in fact reach even two or more times this value. FACTORS DETERMINING THE OUTFLOW FROM THE AQUIFER
The rate of outflow from the Nubian Sandstone aquifer is controlled mainly by the hydraulic characteristics of the aquifer, and the rate of its replenishment. There is no reason to assume that the hydraulic properties of the aquifer changed significantly since the time when the humid climatic conditions - which presumably prevailed in the past (see page p. 364) - changed into arid conditions. On the other hand, this climatic change, must have caused a sharp decrease of the aquifer recharge. This significant change in the rate of recharge took place approximately 10000 years ago (page 364). In this aquifer the distance from the recharge area to the outlet is very great and the quantities of water stored in the aquiferous media are huge (Bein and Issar, 1968). This situation leads to a long gentle depletion of the outflow discharge. The following analysis is made in order to check if even under these conditions there is any chance that the existing outflow is still significantly affected by the high recharges rates which prevailed in the past. For the purpose of the analysis, estimates of hydraulic characteristics of the aquifer and several assumptions are used. Estimates and assumptions are made so that the calculated depletion pattern is not going to be more rapid than the real one. As a first step, it will be assumed - only for the sake of the analysis - that the high rates of recharge which prevailed in the past were suddenly completely stopped so that all the present-day outflow is a depletion of the volumes stored in the aquifer during the humid period. The flow in the Nubian Sandstone aquifer from its phreatic area through
°5',
=b
'~---~
~
c
~
-.
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
369
the confined part to the outlets can be schematically represented as follows: The discharge, Q, of the aquifer is: Q-
a+b+f
×K×D×B,
h
B= K = D = h =
Width of the aquifer, Darcy coefficient, Thickness of the aquifer, at the confined zone, The driving head of the aquifer from the phreatic zone to the outlets. Since the length of the aquifer, L = b + c is a few orders of magnitude longer than the maximum value of h, the above equation can be written as follows : h
Q=
L
K x D x B.
(1)
During the depletion period the outflow of the aquifer is equal to the change in the volume of water stored both in the phreatic and the confined parts of the aquifer caused by the depletion of h; therefore, the flow equation, neglecting the second order of magnitude components, can be written as follows : O xdt=(-dh)
x D'x Bx n +
BxD
x L x n x (-dp) kw
+
B x D x Lx
(-dp)
k,
(2)
where:
n = effective porosity of the aquifer, kw -- bulk modules of elasticity of water, k, = bulk modules of elasticity of sandstone, p =pressure caused by the driving head in the confined area, D' = thickness of the aquifer in the phreatic zone. F r o m Eqs. (1) and (2) the following can be derived:
h
K x D
x Bxdt=i--dh)
x m x D x Bx
n
L
+
e×D×L×n×7×(-dh)
+
B×B×D×L×n×7×(-dh)
kw
kw
where: 7= dh
k
-
K
dp dh
fl--
kw n x k~
kw
L mxnxkw+VxLxn(l+fl)
m-
D' D " dt =
l to
x dt
(3)
370
A. ISSAR, A. BEIN AND A. MICHAELI
where: L x m x n t o --
K
? +
X L 2 x n (1 + fl)
Kxk
w
to has time dimensions, In ( h o / h ) = t/to is the solution of Eq. (3), where: In = natural logarithm ho = driving head at the beginning of the depletion period, in the past, t = time elapsed from the above date, h = existing head and as Q o / Q = h o / h , it follows that O = Qo x e -'/'°
(4)
where: Q = outflow during the depletion period e = base of natural logarithm. The formula obtained is similar to the well-known formula for the springs fed by phreatic aquifers, but to is determined by different properties of the aquiferous complex. In order to calculate to, the relevant properties of the aquifer have been determined, as follows: L = the distance from the replenishment area to the outlet line does not exceed 200000 m at any point of the line; K = values of K for similar sandstone layers obtained in pumping tests in the southern part of Israel, are about 1 m/day or 365 m/year: n = values of n for the above layers are about 10 -1 (Issar, Bein and Michaeli, 1969). kw = the modulus is about 20000 kg/cm 2 7 = when p units are kg/cm 2 and h units are metres, y = 10-1; fl = kwl(,, × k 3 . k s values for sandstone may vary between 1 0 5 - 5 x 105 kg/cm 2. There is no specific known value for the discussed aquifer. The lowest value for ks with the assumed value for n yields fl=2, and smaller values of fl are obtained for higher values of k s (Ettingen, 1951). m = the thickness D of the aquifer which is about 200 m does not change significantly throughout the area, the value of m is determined therefore by the slope of the aquifer, rising towards the phreati¢ zone. The slope is about 5% and the resulting value of m is about 20. The resulting t o is 1 1 0 0 + 1 6 5 ~ 1 3 0 0 years of which the major part (1 100 years) is due to volume changes in the phreati¢ zone.
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
371
The maximum possible value of h could theoretically be achieved in the past when and if the water table in the phreatic zone reached the level of the exposed sandstone, where the replenishment of the aquifer takes place, i.e. about 750 m above the estimated existing water table. The value of B is about 100 kin. The volume that can be gained by the 750 m depletion is about 30000 × 106 m 3 in the phreatic zone and about 1 500 × 106 m 3 in the confined zone, a total of about 31 500 10 6 m 3. Integration of formula (4), by time, produces Vo = to ((20 - Qe)
where: Qo = outflow at the beginning of the depletion period Qe = the present outflow
Oo- Qe=
Vo to
after a depletion period longer than five times t o Qe
Qo
< 0.0068
and
Qo ~
Vo
to
•
The resulting Qo is about 25 x 106 m3/year; and Qe< 170000 m 3. It can be seen that Qe is an insignificant portion of the replenishment of the aquifer which is estimated at about 3 × 106 m3/year. The main conclusion drawn from the analysis, is that although the water outflowing today has been recharged to the aquifer more than 13000 years ago, the present rate of outflow is determined almost entirely by the present rate of replenishment. HYDROLOGICAL ANALYSIS OF PRODUCTION EFFECTS
General situation
The phreatic surface of the Nubian Sandstone aquifer in Southern-Central Sinai is mostly found about 600-700 m below the ground level. In most other areas it is confined and the depth of the strata, which is about 600-800 m, does not enable to create, with ordinary vertical pumps, phreatic conditions over most of the pumping area. In such a situation the huge quantities of former storage confined in the Nubian Sandstone aquifer are almost valueless as far as productivity is concerned. A hydrological analysis of the production patterns is attempted in order to follow changes in aquifer behaviour and thus supply the basic data for calculations of the formerly available reserves. The analysis does not deal with hydrological aspects
372
a. ISSAR, A. BEIN AND A. MICHAELI
relating to the feasibility of pumpage, as this problem lies beyond the scope of this paper.
Pattern of production from the confined area Production from the confined area may have an influence on the water head over very large areas. When continuous pumping takes place the gradients in the aquifer will reach a new equilibrium after a relatively short period. The changes in head over the aquifer due to pumpage at one spot, can be obtained by the following equation: S = - -Q In R 27rT R~
(1)
where: Q = discharge at the spot, T = transmissibility of the aquifer, Re = effective radius of well, R = distance from well, S = drawdown of water level or head at distance R. The gradients in the aquifer will be as follows: dS Q 1 dR - 2~T K"
(2)
When reasonable values or a reasonable range of values, for Q and T are applied in Eq. (2), it can be seen that at a short distance from the pumpage point the gradients decline to less than one per mill. This leads to the conclusion that when a stable discharge is applied over a set of pumping wells spread over the area, the water head after a while will be depleted practically at the same rate over the whole area. This phenomenon will occur almost regardless of the distance between wells, and intensity of pumpage. The difference in water head between the lower spots where the wells are located and the highest spots of head, i.e. the m a x i m u m drawdown, depends mainly on the discharge, and only to a small extent on the distance between wells (Eq. (1)). The confined storativity of the aquifer, which determines the volumes gained by the change of compression of the water and the aquifer's media, which follows the decrease in the head, is smaller by two or three orders of magnitude than the effective porosity of the aquifer; therefore, in spite of the fact that a significant head depletion may take place over wide areas, the water pumped will come from the vicinity of the pumping wells, and the chemical properties of water during long periods would not be affected by possible changes in the chemical properties of water over a wide area.
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
373
Interference on phreatic zones o f the aquifer
Direct production of water from the phreatic zone (south of the confined area) is not possible because of the high ground level in this area (Fig. 3). But depletion of head in the confined aquifer, results in depletion of head in the phreatic zone in the south. The high effective porosity of the phreatic zone may yield a relatively large volume of one time reserves for low depletions of the water head. In this case the volume of the effective former reserves depends on the rate of pumping. If the pumpage from the aquifer is small, even gentle gradients from the phreatic zone to the new depleted level of the production zone, will ensure a sufficient flow; thus, a big decline of water level in the phreatic zone will not reduce the rate of flow below the required minimum. It follows that large volumes of one time reserves, are available when low pumping rates are applied, and, vice versa, small one-time reserves will be available when high pumping rates are applied. Therefore, the optimal exploitation and the resulting available one time reserves, depend on the length of the period for which exploitation from this source is planned. F r o m a rough calculation (Issar, Bein and Michaeli, 1969) based on the above patterns, it was found that although the aquifer is more or less fossil and deeply confined, a quantity of an order of magnitude of about 20 x 1 0 6 m 3 per year can be utilized for a period of about 30 years.
Acknowledgements We acknowledge with thanks the information and advice given by Prof. Y. Gat, Head of the Isotope Department, Weizman Institute of Science. We are also grateful to Mr. M. Goldschmidt, former Head of the Hydrological Service, to Mr. I. Perath, Dr. A. Arad and Mr. T. Weissbrod from the Geological Survey of Israel, for constructive criticism of the manuscript.
Note After completion of the present paper, a water well north of Nakhel, sited according to the conclusions presented in this paper, reached the Nubian Sandstone aquifer at a depth of about 800 m. The confined water table is at a depth of about 230 m from the surface and the chlorinity is about 800 mg/1.
References Arad, A., 1966. Hydrogeochemistry of groundwater in central Israel. Bull. LA.S.H. XI: 122-146. Aharoni, E., 1966. Oil and gas prospects of the Kurnub Group (Lower Cretaceous) in southern Israel. Bull. Amer. Assoc. Petrol. Geol. 50: 2388-2403.
374
A. ISSAR, A. BEIN AND A. MICHAELI
Ball, J., 1927. Problems of the Libyan Desert. Geog. J. 70 (1,2, 3), London. Bein, A. and lssar, A., 1968. Development possibilities of the Nubian Sandstone aquifer in Sinai and the Negev. Geol. Survey of Israel Report. Bentor, Y. K., et al., 1960. Lexique stratigraphic international, V. III, Asie, Fasc. 10C2, Israel : Centre National de la Recherche Scientifique, Paris. B. R. G. M., 1952. Carte Gdologique lnternationalde l'Afrique, 1:5 000 000 (Paris). Carrel, I., Noter, Y. and Schlesinger, R., 1971. Rehovoth radiocarbon measurements 1. Dep. Radiocarbon, 13 (2). Caton-Thompson, G. and Gardner, E. W., 1932. The prehistoric geography of Kharga oasis. Geogr. J., London. Ehhalt, D., 1963. Deuteriumgehalt in Hydrosph~ire und Atmosph~ire. Inaugural Dissertation, Heidelberg. Ettingen, S. B., 1954. Engineering Handbook. V. 1. Massadah, Tel-Aviv, (in Hebrew). Ettinger, M. and Langozki, Y., 1969. Hydrodynamics of the Mesozoic formations in the Northern Negev, Israel. Bull. Geol. Surv. Israel, 46. Gat, J. R. and Dansgaard, W., 1972, Stable isotope variations in water sources of Israel. J. Hydrol. 16: 177-212. Gat, J. R., Mazor, E. and Tzur, Y., 1969. The stable isotope composition of mineral waters in the Jordan Rift Valley, Israel. J. Hydrol. 76: 334-352. Hellstrom, B., 1940, The subterranean water in the Libyan Desert. Geog. Annaler Stockholm 3-4: 206-239. Himida, I. H., 1970, The Nubian Artesian Basin, its regional hydrogeological aspects and palaeohydrogeological reconstruction. J. Hydrol. 9:89-116. Issar, A., Bein, A. and Michaeli, A., 1969. The Nubian Sandstone aquifer in Sinai and southern Negev. Geol. Survey Israel Report. Kafri, U., 1968. Report on pumping tests in the Wadi Sidri field. Geol. Survey Israel Report. Knetsch, G., 1962, Geologische Uberlegungen zu der Frage des Artesischen Wassers in der Westlichen Agyptischen Wfiste. Geol. Rund. 52: 640-650. Knetsch, G., Shata, A., Munnich, K. O., Vogel, J. C. and Shazly, M. M., 1962. Untersuchungen an Grundwasser der Ost-Sahara. Geol. Rund. 52: 587-610. McKee, E. C., 1962, Origin of the Nubian and similar sandstones. Geol. Rund. 52:551-586. Munnich, K. O. and Vogel, J. C,, 1962. Untersuchungen an Pluvialen Wassern der OstSahara. Geol. Rund. 52:611-624. Picard, L., 1938. Synopsis of stratigraphic terms in Palestinian geology. Bull. Hebrew Univ. Geol. Dept. 2. Pomeyrol, R., 1968. Nubian Sandstone. Bull. Amer. Assoe. Petrol. Geol. 52: 589-600. Said, R., 1962. The Geology o f Egypt. Elsevier Publ. Co. Amsterdam. Shiftan, Z., 1961, New data on the artesian aquifers of the southern Dead Sea Basin. Bull. Res. Counc. o f lsrae110G : 267-291. Weissbrod, T., 1970. Nubian Sandstone, Discussion. Bull. Amer. Assoc. Petrol. Geol. 55 : 890-891. Weissbrod, T., 1970. The stratigraphy of the Nubian Sandstone in southern Israel (Timena, Eilat area). Geol. Survey Israel Report OD/2/70 and Inst. for Petroleum Research and Geophysics Report No. 1047. Ville, M. and Rolland, G., 1890, 1894. Gdologie et Hydrologie du Sahara Algdrien, Paris.
ON THE UPPER
ANCIENT
NUBIAN
IN CENTRAL
WATER
SANDSTONE
OF THE AQUIFER
SINAI AND SOUTHERN
ISRAEL
A. ISSAR, A. BEIN
Geological Survey of Israel, Ministry of Development and A. MICHAELI*
Hydrological Service, Ministry of Agriculture Abstract: The Nubian Sandstone, mainly that of Lower Cretaceous to Upper Jurassic age underlying the desert of Central Sinai and the Negev, was found to contain fresh water. The quantities stored in this aquifer are estimated at several hundred billion** cubic meters. Although only a very small fraction of this water has pumpage potential, this aquifer is of economic importance. The present arid climate of the area, the distances between the possible recharge areas and the areas where the water is found, as well as the large volume of water in storage, indicate that a large part of the water in this aquifer is fossil. The age of the water based on the 14C dating method, ranges between 13 000 years to more than 30 000 years. The 180/D ratios of this water are characteristic of precipitations in more temperate climates than the present one. Analysis of a calculated hypothetical decay curve indicates that although the water outflowing today has entered the aquifer more than 13 000 years ago, the rate of discharge is determined by the present rate of replenishment. Similar aquifers, containing fossil water in correlative stratigraphic sequences, are known in the Sahara and the Western Desert of Egypt.
Introduction The term Nubian
Sandstone
is a p p l i e d
in this p a p e r
to the " t h i c k
p r e d o m i n a n t l y s a n d y s e q u e n c e , m a i n l y o f terrestial origin, c h a r a c t e r i z e d by intensive and variable colours and composed
m a i n l y o f fine to c o a r s e
g r a i n e d , s o m e t i m e s gritty c o n g l o m e r a t i c , f r e q u e n t l y c r o s s - b e d d e d , m a i n l y soft b u t o c c a s i o n a l l y v e r y h a r d s a n d s t o n e s " (Lex. Strat. Israel, B e n t o r et al., 1960). T h e p r e s e n t s t u d y deals w i t h the u p p e r p a r t o f the s e q u e n c e , w h i c h is * Current address: T A H A L - Water Planning for Israel Ltd., 54, Ibn Gvirol Street, Tel Aviv. ** Billion = 109. 353
354
A. ISSAR, A. BEIN AND A. MICHAELI
of Upper Jurassic to Lower Cretaceous age. The Nubian Sandstone interfingers with marine sediments which become progressively more prominent with increasing distance from the Precambrian basement (Bentor et al., 1960). Although certain authors refer to the sequence as "Nubian Facies" (Knetsch et al., 1962) or even refrain altogether from using it (Pmeyrol, 1968; Weissbrod, 1970), it is considered the best short term, from the hydrogeological point of view, to be used for the above-mentioned sequence. The Nubian Sandstone is not restricted only to the northern margins of the Arabian massif (Picard, 1938) but extends all along the northern margin of the African shield from Egypt through Libya to the western Sahara (B.R.G.M., 1952; McKee, 1962). The Nubian Sandstone found throughout these areas is characterized by its predominance of clastics but has different proportions of sands, silts and shales. The occurrence of fresh water in the Cretaceous sandstone was discovered in the previous Century in Algiers by Ville and Rolland (1890, 1894). Ball (1927) described similar water occurrences from the Libyan Desert and explained it by a vast contemporary subterranean flow of water coming from Equatorial Africa. Heilstrom (1940) accepted this explanation but argued that a period of about 100000 years is needed for the water to flow from the assumed recharge region to the localities of the Western Desert. The hydrologists who studied these aquifers mainly in Algeria, Libya and Egypt, differ in their opinions as to whether the Nubian Sandstone aquifer is fossil or is still being recharged. In 1962 groundwater in the Nubian Sandstone aquifer of Eastern Sahara was reinvestigated using geological as well as geochemical methods, especially isotope analysis (Knetsch et al., 1962). The conclusions were that the rains which supplied the water probably fell during a Pluvial period about 25-35000 years ago, and that infiltration took place mainly in regions where the Nubian Sandstone is exposed and within the depressions of western Egypt. Himida (1970) concludes that the various water occurrences in Egypt, eastern Libya, northern Sudan and northeastern Chad are parts of a huge basin which is still being recharged in the highland of Chad, Sudan and probably the Tibesti plateau. Shiftan (1961), who studied the Nubian Sandstone aquifer in the Dead Sea region, suggested that the water there is of Pleistocene age.
Geography and climate The area studied has the form of a big triangle, its apex pointing southward and its base parallel to the northern coast line of Sinai and continuing eastward to the Negev, approximately along 31°05' latitude, and reaching the Dead Sea north of Mount Sedom. The western border of the triangle is
355
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
formed by the Canal and Gulf of Suez and its eastern border by the Gulf of Eilat and the Arava Valley. The central and northern areas form a high plateau, highest in its southern part where altitudes range from 1200 m to 1400 m, sloping gradually towards the north to altitudes of 300 to 400 m. North of this area a few anticlinal ridges rise to altitudes of 1000 m. The most northern part of the area slopes gradually towards the Mediterranean Sea, forming a wide coastal plain covered mostly by dunes. The mountainous region in the south is drained by many transverse ephemeral streams (Wadis) running in a general east-west direction to the gulfs of Suez and Eilat. Most of the central and northern areas are drained by Wadi el 'Arish which flows into the Mediterranean. The drainage of the eastern part of the Negev is towards the 'Arava and the Dead Sea. The area is arid. Precipitation is low, mostly below 100 mm/year (Table 1), and summers are hot and dry with temperatures reaching 40 °C. Rains fall in winter during very short periods, causing torrential floods in the wadis. On the mountains of Central Sinai snow falls only once in a few years, and it usually melts within a few days. TABLE 1
Mean monthly and annual totals of precipitation (mm) Station El Arish Kuntilla, El Nekhel Thamad, El
Tor, A. SantaKaterina
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Annual 19 3 7 3
20 1 7 8
12 3 4 6
2 2 2 1.5 1.4 10
7 2 4 2
2 0 1 1
0 0 0 0
0.3 0.1 0 7.9 6.0 drops
0 0 0 0
0 0 0 0
0 0 0 0
4 4 0 4
0 0
0 0
0 0
0.8 4.6
12 0 0 16
16 3 3 4
92 16 26 44
2 2 11 22.0 7.0 60.4
From "The Climate of Africa" Part 1, Air Temperature Precipitation - page 159, 1968. Main Administration of the Hydrometeorological Service of the U.S.S.R. Council of Ministers. A.I. Voeikov Main Geophysical Observatory. Israel Prog. Scientific Translation, Jerusalem 1970.
Geology The morphology, stratigraphy and structure of the area studied are closely related. The area may be divided into three different morphotectonic belts easily recognized on topographical and geological maps (Fig. 1). The belts are as follows: 1) The mountainous southern part of Sinai, which consists mainly of crystalline rocks, forms the northern extension of the A r a b o - N u b i a n massif. 2) The central high plateau, forming the "Stable Shelf" (Said, 1962) or
l'"
>
Z > Z
>.
>
t.m
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
357
"zone tabulaire" (Picard, 1938). This belt, which consists of sedimentary rocks with a gentle northward dip, is separated from the crystalline belt by the high E1-Tih Escarpment (Fig. 3). 3) The northern belt, forming the "Unstable Shelf" (Said, 1962). This belt consists of several parallel mountain chains trending east-north-east and belonging to an asymmetrical folding system. The sedimentary sequence may be divided into three divisions (Said, 1962) : 1) The Lower Clastic Division - a sequence of rocks, mainly clastics, overlying the igneous basement of pre-Cambrian age. The Nubian Sandstone discussed in this paper is found in the upper part of this division. 2) The Middle Calcareous Division - a sequence of rocks, mainly carbonates, of Cenomanian-Eocene age. 3) The Upper Clastic Division - a sequence of rocks, mainly clastics, of Neogene-Holocene age. The Nubian Sandstone sequence
The alternating beds of sandstones and shales forming the Nubian Sandstone sequence were deposited "largely in a marginal or continental environment, predominantly lagoonal, estuarine or lacustrine and fluviatile" (McKee, 19621). This depositional environment, which prevailed in the region during most of Palaeozoic and the beginning of Mesozoic times (Picard, 1943; McKee, 1962) and the facies change of the Mesozoic sequence from continental to marine, from south to north, make stratigraphical division and correlation quite difficult. The sandstone units in this study are equivalent mainly to the Kurnub group of Aharoni (1966), the upper variegated and white Nubian sandstone of Bentor et al. (1960), the upper Jurassic Nubian Sandstone and the Lower Cretaceous of Said (1962), and the Hatira Formation of Weissbrod (1970). The upper Nubian Sandstone is overlain by the Middle Calcareous Division (Said, 1962) which consists mainly of limestones, dolostones, chalks and marls. Its age ranges from Cenomanian to Upper Eocene. From a hydrogeological point of view this clastic sequence, called here the upper Nubian Sandstone, has aquifer properties. The lower boundary of this aquiferous sequence is not known, while its upper boundary is well marked by the calcareous sequence of marls and limestones, base of which has aquicludic properties. Since the lower boundary is not known, the exact thickness of the aquifer is unknown too, but it is at least 200 m. In most of the area this sequence forms a confined aquifer system.
358
A. ISSAR, A. BEIN AND A. MICHAELI
I~
I~
~
1~.
f l I I AriskArisk
lio0 mi/li~ Y
y Efe~ w,l'PIOm
D t CI L~O0 ml/I
o ! 1.-188l
~
211_
'
~
c,los,.dP r
eTim|r a ~.c-tO~ m
~e w.t ÷ ~ o i
~ I Mlkhte$t Q l t i a *.a.- s o m
A,d|t CI ~ • O0 ml/I ~
CO " t 2 0 - VBO m i A
i
wj.+aoom
.......
/,
,,,%
i !
Z
\
: I
t s A)in Muss l w.t,÷ 3 0 m
t
. ~ . Hlm|ik " ~ N lkhel I
mill
9
CI- $~11/I
. . t . e eO.Sm •
--i
.i t
r
I
fAl,,o,.o ]
¢-
4"'lined l i t e r f l o l dire¢llon in qh~ upper ~ . ~ .
A/~(ieoloiieal -
~
I~ Fig. 2.
/
~Nubiin
I ~.
Location map and hydrogeological data, Sinai and Southern Negev.
cro,s s¢cllon Sand~lone ~.quifer.
TABLE 2
27.12.67 23.2.70 3.4.68 9.11.70
÷ 4- 'Ayun Musa 1
+ q- Yotvata T/9
÷ Makhtesh Qatan 3
4- Makhtesh Qatan 4
÷ Analysis by " M e k o r o t h " . ÷ Jr- Analysis by Geological Survey of Israel.
27.12.67
9.1.63
q- Admon 2
÷ + ' A y u n Musa 2
8.11.67
÷ Tamar 5
27.12.67
13.1.70
q- q- Tamar 3
÷ Jr Nakhel 1
Date of analysis
Name of well
mg/l meq/l mg/l meq/l mg/l meq/l mg/l meq/l mg/l meq/l mg/1 meq/l mg/l meq/1 mg/l meq/l mg/l meq/l
196.0 9.8 153.1 7.6 233.0 11.6 116.0 5.8 386.0 19.3 278.0 13.9 118.0 5.9 154.9 7.7 215.2 10.74
Ca 30.4 2.5 60.7 5.0 103.0 8.5 92.3 7.6 86.0 7.07 37.7 3.10 85.0 7.0 70.0 5.7 79.0 6.5
Mg 401.08 17.4 386.5 16.8 1341.0 58.3 222.0 9.6 1210.0 52.6 520.0 22.62 370.0 16.1 460.0 20.0 492.0 21.4
Na 20.7 0.5 23.0 0.6 77.0 1.9 21.0 0.54 76.5 1.96 39.0 1.0 22.8 0.58 26.4 0.7 29.5 0.7
K 599.8 16.9 620.0 17.5 2069.0 58.3 355.0 10.0 2316.0 65.31 963.0 27.15 724.0 20.4 748.0 21.1 718.0 20.2
CI
Chemical composition
Chemical analysis of waters from various wells of the Nubian Sandstone aquifer
1.4 0.01 6.5 0.08 0.83 0.01 9.35 0.12 4.3 0.05 5.07 0.06 -
Br
413.9 8.6 391.7 8.1 895.0 18.6 485.0 10.0 760.0 15.82 530.0 11.03 83.5 1.74 374.9 7.8 673.2 14.0
SO4
305.0 5.0 274.3 4.5 182.0 3.0 220.0 3.6 110.0 1.8 146.0 2.39 79.0 1.3 317.0 5.2 284.7 4.6
HCOa
L/I
c
,-d © Z t"tl
Z
7r
G
7~
m
sr ,.d
rn
Z
-]
360
A. ISSAR, A. BEIN AND A. MICHAELI
Occurrences of groundwater in the Nubian Sandstone NORTHERN NEGEVAND DEAD SEA REGION A confined aquifer was first encountered in the Nubian Sandstone in southern Israel along the margins of the rift valley, west and southwest of the Dead Sea (Shiftan, 1961). This aquifer was also encountered in oil and gas exploration wells, situated north-west, west and south-west of the Dead Sea water wells, and in a few water wells located around the erosion cirques of Makhtesh Gadol and Makhtesh Qatan (Fig. 2). Isosalinity and potentiometric surface maps of this aquifer (Aharoni, 1966; Ettinger and Langozki, 1969) indicate a marked decrease in water head, accompanied by an increase in salinity, from the south to north-west, north and north-east. The only exception is the data from Avdat 1 (Fig. 2). According to Ettinger and Langozki (1969) the water head in this well, calculated from a D.S.T., is - 3 4 7 m and the chlorinity is about 500 mg/I. But the fact that this low head is the lowest known in the entire area (much lower even than those found in the Dead Sea region) and its combination with a relatively low salinity value (Table 2) makes these data unreliable. Ettinger and Langozki (1969) suggested a water-shed around the Makhtesh Gadol area. By omitting the data from Avdat 1 and taking into consideration the water head in Makhtesh Gadol water well (Fig. l), this watershed is not evident. Shiftan (1961) suggested that the relatively fresh water found in this aquifer dates from Pleistocene times. Furthermore, he maintained that the artesian water found in the limestone-dolomite aquifer overlying the Nubian Sandstone comes from the Nubian Sandstone aquifers. Arad (1966) suggests that "the sandstone aquifer is not a fossil body of water and that it has, in fact, an active and independent replenishment area". The replenishment areas are assumed to be the Judea Mountains and to a lesser degree the erosion cirques of the Makhteshim. He also suggests a system in which the present replenished waters mix with ancient ones and are responsible for possible groundwater movements (Arad, 1971, personal communication). YOTVATA~ SOUTHERN 'ARAVA VALLEY
Three water wells drilled in this region (Yotvata 1, 8 and 9) (Fig. 2) reached a confined Nubian Sandstone aquifer. The water table is about 90 m above M.S.L. and the water is of rather low salinity (700-900 mg/1 C1). The oldest rocks exposed in this region are limestones and dolostones of Cenomanian-Turonian age, but about 10 km to the south, in the Timna domal structure, small exposures of Nubian Sandstone are found. Larger Nubian Sandstone exposures are found about 30 km to the south.
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
361
CENTRAL SINAI
Three oil exploration wells were drilled in the vicinity of Nakhel (Fig. 1), in Central Sinai, by the Standard Oil Company of Egypt during the years 1944-1946 (Fig. 2). In these wells, which were spudded on shales of Paleocene age, the Nubian Sandstone sequence was encountered at a depth of 770-880 m. The water found in the Nubian Sandstone rose to about 230 m from the surface, between 190-200 m above M.S.L. The water chlorinity was about 350 mg/l. About 90 km north of the Nakhel area, Nubian Sandstones are exposed in the erosion cirque of the Gebel Helall anticline; here a shallow well encountered a confined water table in the upper Nubian Sandstone. The water rose to about 200 m above M.S.L. and it contained about 300 mg/l CI.
WESTERN SINAI
The spring and artesian wells of 'Ayun Musa (Fig. 2), about 15 km southeast of the town of Suez, are the most important occurrence of groundwater in northwestern Sinai. This region is covered by chalks and marls of Mio-Pliocene age, which are cut by wide joints from which the waters issue. The local inhabitants dig shallow holes at the peaks of the small hills, rising about 15 to 20 meters above the plain. These holes are filled by water seeping from the joints. The water is then transported by canals to the date-palm groves growing at the foot of these hills. Oil exploration wells have shown that the Nubian Sandstone is found at rather shallow depth (75-100 m) unconformably overlain by Miocene sediments. The Nubian Sandstone layers were found to contain water at artesian pressures with salinities ranging from 1 000 to 4000 mg/l CI. It is obvious that the water from the joints in the Mio-Pliocene rocks comes mostly from the Nubian Sandstone aquifer below.
Geochemistry of the Nubian Sandstone aquifer Table 2 and Fig. 4 present chemical analyses of water sampled from the Nubian Sandstone aquifer in the studied area. Except for the water from Nakhel 1, all these water analyses are comparable and may be grouped into one category, characterized by its ionic proportions: rCl>rSO4>rHCO 3 ;
rNa>rCa>rMg.
The water from the Nakhel 1 well is less saline than the others and also
362
A. 1SSAR, A. BEIN AND A. MICHAELI
differs in its ionic proportions:
rCl~-rSO4>rHC03;
rNa>rMg>rCa.
Measurements of the stable isotopes 180 and D (2H) in various aquifers along the rift valley have shown that the waters from the Nubian Sandstone aquifer in the Yotvata and T a m a r wells are significantly depleted in these isotopes as compared with groundwater from gravel aquifers in the 'Arava and those with recharge areas in the Judea Mountains (Gat and Dansgard, 1972) The depleted waters fall on a line on the 180 vs. D diagram (Fig. 5) which is assumed to represent meteoric water from a cooler period than the present one (Gat et al., 1969). This line is parallel with the non-depleted water line of the present-day meteoric water. Water taken from wells in which the Nubian Sandstone aquifer and the overlying limestone aquifer are pumped together (Admon 1, T a m a r 7, Tamar 3 before deepening) (Fig. 5) show an isotopic composition which falls between these two lines (Gat et al., 1969). The water from Nakhel 1, 'Ayun Musa and Makhtesh Qatan 3 wells are also depleted in their stable isotopic content and fall on the same line as the Nubian Sandstone aquifer in T a m a r and Yotvata wells (Fig. 5). It is interesting to note that the 180/D ratios of the water from the Nubian Sandstone aquifer of the western desert in Egypt also falls on the same line (Fig. 5).
14C Dating Dating of the waters has been carried out by measuring the 14C content in the bicarbonate dissolved in the water, and corrected according to the 13C content (Munnich and Vogel, 1962). These datings were made by Carmi TABLE 3 Water ages according to 14C content
Name
Depth
Age (B.P.)
Remarks
Tamar 3* Tamar 3* Yotvata 2* Makhtesh Qatan 3* Nakhel 3** 'Ayun Musa**
76m 400m 50m 763m 800m spring
14 900 :~ 600 28 500 ± 350 13 200 _~ 600 22 000 ± 1000 19 900 ± 3000 ~> 30 900
Waters of mixed origin Water of N.S. aquifer Water of mixed origin Water of N.S. aquifer Water of N.S. aquifer Water mainly of N.S.
* Carmi et al 1971. ** Kaufman, 1971 (personal communication). *** Ehhalt, 1963 and Munnich and Vogel, 1962.
-J200
-800
--400
o
400
0
8ooi
m 120o
-S-
ii0
xX X X
= 20
X
= 30
= 4o
carbonates
X XX
EI-Tih
5=o Km
escarpment
Cenomonion
Kc
t Middle
x x
j .L
Division
limestones
Calcareous
x
few
S a n d s t o n e s , shales
wells
•
Fig. 3.
rocks
Hydrogeological section A-A.
xXxx Precambrian , crystalline ___~Z Water table in N.S.
U,N.S. Lower C r e t a c e o u s - U p p e r Jurassic Nubian Sandstone I Lower P o - J Paleozoic - J u r a s s i c
Eocene Senonion Turonian
e $ Kt
~
Nokhel
sanastones Po-J
Clastic Division
--
t
Heloll
-..
-N-
364
A. ISSAR, A. BEIN AND A. MICHAELI
et al., (1971), Ehhalt (1963) and Kaufman (1971, personal communication), (Table 3). It can be seen that the water found in Cenomanian aquifers but replenished from the Nubian Sandstone aquifer (Tamar 3 at 76 m, Yotvata 2) show a younger date than that of the water coming directly from the Nubian Sandstone (Tamar 3 at 400 m, Makhtesh Qatan, Nakhel). This is due to the mixture with younger waters of the Cenomanian aquifer. Discussion The above data are from Nubian Sandstone aquifers in four areas: Central Sinai, the ~uez Graben, southern Arava Valley (Yotvata wells) and Central Negev - Dead Sea. The question is whether these waters are connected within one aquiferial system or whether they represent local unconnected aquifers. The following points may indicate a connection between the Nubian Sandstone aquifers found in the different areas: 1) Age: The water found in all these localities is fossil, ranging in age from about 13000 years to 30000 years. 2) Chemistry of the water: All the water analyses except that of Nakhel 1 are similar and may be grouped into one category. The different and sweeter water found in Nakhel 1 may represent a water development from a recharge area in the south through Central Sinai (Nakhel 1) towards the Gulf of Suez and the Arava Valley. 3) Stable isotope content: The water in the four regions are depleted in ~80 and D content as compared with present-day recharged aquifers. This depletion indicates that the waters found in these aquifers accumulated in the past in a cooler period. This cooler period was characterized also by higher rainfall as compared with present times. This is evident from archaeological data from Kharga Oasis, Egypt, which indicate an abrupt decline of the water heads in the Nubian Sandstone aquifer approximately 10000 years ago (Caton-Thompson and Gardner, 1932). Additional evidence for a more humid climate prevailing in the past is the presence of lacustrine flesh water carbonates in the highland of southern Sinai doted as 20000 years (Kaufman, 1971, personal communication). 4) Water heads: The water heads are highest in Central Sinai while those found in the Gulf of Suez and the 'Arava Valley are the lowest. This may indicate water flow from the highland of Central Sinai towards the Grabens (see also point 6). 5) Recharge areas: Large Nubian Sandstone exposures are found only in southern Sinai and Israel. These exposures are the only possible inlet for the water found in the storage of these aquifers. This is at least true for Central
THE ANCIENT WATER OF THE NUB1AN SANDSTONE AQUIFER
365
Sinai, Ayun Musa and southern 'Arava Valley. In the Central Negev-Dead Sea region, possible inlets are also the erosion cirques and perhaps the Judea Mountains. 6) Continuity of the Nubian Sandstones aquifer: The geological and topographical maps of the area indicate a continuity of the Nubian Sandstone sequence from Central Sinai towards Ayun Musa in the west and towards the southern 'Arava Valley in the East. The sequence is mostly below the potentiometric surface suggested by the water head data given above. In the central Negev the three anticlines of Hatzera, Hatira and Ramon (Fig. 1), raise the upper Nubian Sandstone above this assumed regional water table. The water heads in the Hatira and Hatzera anticlines (Makhtesh Qatan and Makhtesh Gadol wells) may represent a conning of the water table due to local replenishment. Continuity of the aquifer is effected apparently along the synclines. The continuity of the Nubian Sandstone aquifer beyond northern Sinai seems unlikely due to the presence of large anticlines trending east-north-east in this area. The Nubian Sandstone sequence is raised along these anticlines to levels sufficiently high to form a hydrological barrier (Fig. 3). Further to the north the Nubian Sandstone sequence plunges to great depth gradually losing its aquifer properties due to facies change into marine shales (the Gevar'Am Formation). Considering all the points presented above it seems reasonable to conclude that the various Nubian Sandstone aquifers found in the investigated area are parts of one aquifer system. This huge aquifer underlying the desert of Sinai and southern Israel was replenished mainly in the past, more than 13 000 years ago, in a colder climate. The hydrogeology of this aquifer, i.e. the present recharge and the factors determining the outflow from this aquifer, are discussed below.
Hydrogeology of the Nubian Sandstone aquifer RECENT REPLENISHMENT
The main replenishment areas for this aquifer are the Nubian Sandstone exposures found between the igneous basement of southern Sinai and the E1-Tih escarpment. Smaller replenishment areas exist in the erosion cirques of central Sinai and the Negev where the Nubian Sandstones are exposed. This replenishment causes bodies of fresher water and conning of the water table in the vicinity of these areas, as was observed in Makhtesh Qatan and Helall. Although the climate of the region is arid and average annual precipitation on the Nubian Sandstone outcrops is estimated to be between 50 and 100 mm (Table 1), recharge takes place even under the present climatic conditions.
366
A. ISSAR, A. BEIN A N D A. M I C H A E L I
meq~ i 0.0
Tamar.
3 ---4 ----I --'4
Tamar.
5
Ac[mon.
2 ................
Nakh¢l
I t....._
.................
9 8
7 6
/,
\
]
)
Ayun
Musa
2 ,
Ayun
Musa
I
t
Makhtesh Qatan
3 . . . . . . .
Makhtesh Qalan
4 ............
2
.:/ .~,.ty'~..~.,~.:;.~,,'..~. \\ #
'\, 10, 9
ii.4"
~,'~
."/ ,.'I~Y
',\
"~. ","
~,'\ \""\~
ot,ata
"r/9
..........
',,.,.~. \,.
...,//Z.",,~x"
....
8
7 6 5
"% :~,, ~,,'.. v .,.....~
-.~~ ~:,,,.....,.I ; -"~,'V'-::/: \ \"..../ / "
",\ /: ,,\/: ,v.: ;'
4
3
\5" ',\\
\\
\
2
i
!
Ca
F i g . 4.
Mg
)
i
i
Na÷K
CI
So,
i HCO.z
Chemical composition of waters from the Nubian Sandstone aquifer.
This is proven by the many shallow wells and springs found in the highland of Sinai. The most spectacular is the spring of Ein Furtaga, issuing from jointed granite and alluvium in eastern Sinai and discharging 70 to 120 m 3 per hour. In the vicinity of Abu Rudeis (Fig. 2) near the western shore of Sinai, a well field is utilizing the underflow of the Wadi Sidri river bed. This underflow was found to reach about 600000 m 3 per year (Kafri, 1968). The
-g
"6
-10
c-.O
o -4
-20
-21
--
0 --
I
J
I
I
(1963)
Department
-14
I_
of
-12
Fig. 5. zsO-Deuterium
Y. Gat
D. Ehhalt
Io
I
to
-16
According
-18
X According
•
I
"" '%" \
aquifer and drainage from Negev M . . . . . ins
I
I
$'~o
-IO
o
relationships
I -g
I
-6
I
.....
n
¢.
%/
~ ~'/
Y S
~
-4
)nstilute
I
of
-Z
I
Science
I
I
Rehovot
I
aqu ~rer I G. . . . .
in Sinai and Southern
t
Acava
/
deepening
.. %/
2 (N.~. "aqff,fer) Musa l(N,S aqu,rer~
groundwaters
Weizmman
of various
Research
% o
1
¢e-Wes l e r n D e s e r t
ALnias'2 ~ m /~__~Ayun T ara~,,t-~,,,"~S
Isotopic
(N S. aqu=fer)
Sheva
rocks cenlr~l Sinai f~j.~ ~" amar 7 IG . . . . d is sarl .~ ~ l l ~ j ~ / l a m a r Dead S e a w e l l f i e l d . h , ~ . J ~ . . ~ " -/ ( G a l el al 1969) , / ~ O g Qm~L'Tamar 3 I B e f o r e
Beer
I 0
Israel.
Israel
I
e2
I
aquifer)
k~ "
N.S.
~o"
19691
to
¢.,r.
~.¢.¢
.o ~
C~
Frl
>
z
©
z
> Z
_=
7
.-r
©
> -]
z
> 7
"4
368
a. ISSAR, A. BEIN AND A. MICHAELI
drainage area of Wadi Sidri is about 600 km 2. The rate of replenishment is thus 1000 m 3 per 1 km 2 per year. The total area of Nubian Sandstone outcrops and replenishable subcrops is about 3000 km 2. By using the above rate of recharge the replenishment of the Nubian Sandstone aquifers is in an order of magnitude of about 3.106 m 3 per year, but this estimate is by no means an accurate one. The real replenishment of this aquifer may in fact reach even two or more times this value. FACTORS DETERMINING THE OUTFLOW FROM THE AQUIFER
The rate of outflow from the Nubian Sandstone aquifer is controlled mainly by the hydraulic characteristics of the aquifer, and the rate of its replenishment. There is no reason to assume that the hydraulic properties of the aquifer changed significantly since the time when the humid climatic conditions - which presumably prevailed in the past (see page p. 364) - changed into arid conditions. On the other hand, this climatic change, must have caused a sharp decrease of the aquifer recharge. This significant change in the rate of recharge took place approximately 10000 years ago (page 364). In this aquifer the distance from the recharge area to the outlet is very great and the quantities of water stored in the aquiferous media are huge (Bein and Issar, 1968). This situation leads to a long gentle depletion of the outflow discharge. The following analysis is made in order to check if even under these conditions there is any chance that the existing outflow is still significantly affected by the high recharges rates which prevailed in the past. For the purpose of the analysis, estimates of hydraulic characteristics of the aquifer and several assumptions are used. Estimates and assumptions are made so that the calculated depletion pattern is not going to be more rapid than the real one. As a first step, it will be assumed - only for the sake of the analysis - that the high rates of recharge which prevailed in the past were suddenly completely stopped so that all the present-day outflow is a depletion of the volumes stored in the aquifer during the humid period. The flow in the Nubian Sandstone aquifer from its phreatic area through
°5',
=b
'~---~
~
c
~
-.
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
369
the confined part to the outlets can be schematically represented as follows: The discharge, Q, of the aquifer is: Q-
a+b+f
×K×D×B,
h
B= K = D = h =
Width of the aquifer, Darcy coefficient, Thickness of the aquifer, at the confined zone, The driving head of the aquifer from the phreatic zone to the outlets. Since the length of the aquifer, L = b + c is a few orders of magnitude longer than the maximum value of h, the above equation can be written as follows : h
Q=
L
K x D x B.
(1)
During the depletion period the outflow of the aquifer is equal to the change in the volume of water stored both in the phreatic and the confined parts of the aquifer caused by the depletion of h; therefore, the flow equation, neglecting the second order of magnitude components, can be written as follows : O xdt=(-dh)
x D'x Bx n +
BxD
x L x n x (-dp) kw
+
B x D x Lx
(-dp)
k,
(2)
where:
n = effective porosity of the aquifer, kw -- bulk modules of elasticity of water, k, = bulk modules of elasticity of sandstone, p =pressure caused by the driving head in the confined area, D' = thickness of the aquifer in the phreatic zone. F r o m Eqs. (1) and (2) the following can be derived:
h
K x D
x Bxdt=i--dh)
x m x D x Bx
n
L
+
e×D×L×n×7×(-dh)
+
B×B×D×L×n×7×(-dh)
kw
kw
where: 7= dh
k
-
K
dp dh
fl--
kw n x k~
kw
L mxnxkw+VxLxn(l+fl)
m-
D' D " dt =
l to
x dt
(3)
370
A. ISSAR, A. BEIN AND A. MICHAELI
where: L x m x n t o --
K
? +
X L 2 x n (1 + fl)
Kxk
w
to has time dimensions, In ( h o / h ) = t/to is the solution of Eq. (3), where: In = natural logarithm ho = driving head at the beginning of the depletion period, in the past, t = time elapsed from the above date, h = existing head and as Q o / Q = h o / h , it follows that O = Qo x e -'/'°
(4)
where: Q = outflow during the depletion period e = base of natural logarithm. The formula obtained is similar to the well-known formula for the springs fed by phreatic aquifers, but to is determined by different properties of the aquiferous complex. In order to calculate to, the relevant properties of the aquifer have been determined, as follows: L = the distance from the replenishment area to the outlet line does not exceed 200000 m at any point of the line; K = values of K for similar sandstone layers obtained in pumping tests in the southern part of Israel, are about 1 m/day or 365 m/year: n = values of n for the above layers are about 10 -1 (Issar, Bein and Michaeli, 1969). kw = the modulus is about 20000 kg/cm 2 7 = when p units are kg/cm 2 and h units are metres, y = 10-1; fl = kwl(,, × k 3 . k s values for sandstone may vary between 1 0 5 - 5 x 105 kg/cm 2. There is no specific known value for the discussed aquifer. The lowest value for ks with the assumed value for n yields fl=2, and smaller values of fl are obtained for higher values of k s (Ettingen, 1951). m = the thickness D of the aquifer which is about 200 m does not change significantly throughout the area, the value of m is determined therefore by the slope of the aquifer, rising towards the phreati¢ zone. The slope is about 5% and the resulting value of m is about 20. The resulting t o is 1 1 0 0 + 1 6 5 ~ 1 3 0 0 years of which the major part (1 100 years) is due to volume changes in the phreati¢ zone.
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
371
The maximum possible value of h could theoretically be achieved in the past when and if the water table in the phreatic zone reached the level of the exposed sandstone, where the replenishment of the aquifer takes place, i.e. about 750 m above the estimated existing water table. The value of B is about 100 kin. The volume that can be gained by the 750 m depletion is about 30000 × 106 m 3 in the phreatic zone and about 1 500 × 106 m 3 in the confined zone, a total of about 31 500 10 6 m 3. Integration of formula (4), by time, produces Vo = to ((20 - Qe)
where: Qo = outflow at the beginning of the depletion period Qe = the present outflow
Oo- Qe=
Vo to
after a depletion period longer than five times t o Qe
Qo
< 0.0068
and
Qo ~
Vo
to
•
The resulting Qo is about 25 x 106 m3/year; and Qe< 170000 m 3. It can be seen that Qe is an insignificant portion of the replenishment of the aquifer which is estimated at about 3 × 106 m3/year. The main conclusion drawn from the analysis, is that although the water outflowing today has been recharged to the aquifer more than 13000 years ago, the present rate of outflow is determined almost entirely by the present rate of replenishment. HYDROLOGICAL ANALYSIS OF PRODUCTION EFFECTS
General situation
The phreatic surface of the Nubian Sandstone aquifer in Southern-Central Sinai is mostly found about 600-700 m below the ground level. In most other areas it is confined and the depth of the strata, which is about 600-800 m, does not enable to create, with ordinary vertical pumps, phreatic conditions over most of the pumping area. In such a situation the huge quantities of former storage confined in the Nubian Sandstone aquifer are almost valueless as far as productivity is concerned. A hydrological analysis of the production patterns is attempted in order to follow changes in aquifer behaviour and thus supply the basic data for calculations of the formerly available reserves. The analysis does not deal with hydrological aspects
372
a. ISSAR, A. BEIN AND A. MICHAELI
relating to the feasibility of pumpage, as this problem lies beyond the scope of this paper.
Pattern of production from the confined area Production from the confined area may have an influence on the water head over very large areas. When continuous pumping takes place the gradients in the aquifer will reach a new equilibrium after a relatively short period. The changes in head over the aquifer due to pumpage at one spot, can be obtained by the following equation: S = - -Q In R 27rT R~
(1)
where: Q = discharge at the spot, T = transmissibility of the aquifer, Re = effective radius of well, R = distance from well, S = drawdown of water level or head at distance R. The gradients in the aquifer will be as follows: dS Q 1 dR - 2~T K"
(2)
When reasonable values or a reasonable range of values, for Q and T are applied in Eq. (2), it can be seen that at a short distance from the pumpage point the gradients decline to less than one per mill. This leads to the conclusion that when a stable discharge is applied over a set of pumping wells spread over the area, the water head after a while will be depleted practically at the same rate over the whole area. This phenomenon will occur almost regardless of the distance between wells, and intensity of pumpage. The difference in water head between the lower spots where the wells are located and the highest spots of head, i.e. the m a x i m u m drawdown, depends mainly on the discharge, and only to a small extent on the distance between wells (Eq. (1)). The confined storativity of the aquifer, which determines the volumes gained by the change of compression of the water and the aquifer's media, which follows the decrease in the head, is smaller by two or three orders of magnitude than the effective porosity of the aquifer; therefore, in spite of the fact that a significant head depletion may take place over wide areas, the water pumped will come from the vicinity of the pumping wells, and the chemical properties of water during long periods would not be affected by possible changes in the chemical properties of water over a wide area.
THE ANCIENT WATER OF THE NUBIAN SANDSTONE AQUIFER
373
Interference on phreatic zones o f the aquifer
Direct production of water from the phreatic zone (south of the confined area) is not possible because of the high ground level in this area (Fig. 3). But depletion of head in the confined aquifer, results in depletion of head in the phreatic zone in the south. The high effective porosity of the phreatic zone may yield a relatively large volume of one time reserves for low depletions of the water head. In this case the volume of the effective former reserves depends on the rate of pumping. If the pumpage from the aquifer is small, even gentle gradients from the phreatic zone to the new depleted level of the production zone, will ensure a sufficient flow; thus, a big decline of water level in the phreatic zone will not reduce the rate of flow below the required minimum. It follows that large volumes of one time reserves, are available when low pumping rates are applied, and, vice versa, small one-time reserves will be available when high pumping rates are applied. Therefore, the optimal exploitation and the resulting available one time reserves, depend on the length of the period for which exploitation from this source is planned. F r o m a rough calculation (Issar, Bein and Michaeli, 1969) based on the above patterns, it was found that although the aquifer is more or less fossil and deeply confined, a quantity of an order of magnitude of about 20 x 1 0 6 m 3 per year can be utilized for a period of about 30 years.
Acknowledgements We acknowledge with thanks the information and advice given by Prof. Y. Gat, Head of the Isotope Department, Weizman Institute of Science. We are also grateful to Mr. M. Goldschmidt, former Head of the Hydrological Service, to Mr. I. Perath, Dr. A. Arad and Mr. T. Weissbrod from the Geological Survey of Israel, for constructive criticism of the manuscript.
Note After completion of the present paper, a water well north of Nakhel, sited according to the conclusions presented in this paper, reached the Nubian Sandstone aquifer at a depth of about 800 m. The confined water table is at a depth of about 230 m from the surface and the chlorinity is about 800 mg/1.
References Arad, A., 1966. Hydrogeochemistry of groundwater in central Israel. Bull. LA.S.H. XI: 122-146. Aharoni, E., 1966. Oil and gas prospects of the Kurnub Group (Lower Cretaceous) in southern Israel. Bull. Amer. Assoc. Petrol. Geol. 50: 2388-2403.
374
A. ISSAR, A. BEIN AND A. MICHAELI
Ball, J., 1927. Problems of the Libyan Desert. Geog. J. 70 (1,2, 3), London. Bein, A. and lssar, A., 1968. Development possibilities of the Nubian Sandstone aquifer in Sinai and the Negev. Geol. Survey of Israel Report. Bentor, Y. K., et al., 1960. Lexique stratigraphic international, V. III, Asie, Fasc. 10C2, Israel : Centre National de la Recherche Scientifique, Paris. B. R. G. M., 1952. Carte Gdologique lnternationalde l'Afrique, 1:5 000 000 (Paris). Carrel, I., Noter, Y. and Schlesinger, R., 1971. Rehovoth radiocarbon measurements 1. Dep. Radiocarbon, 13 (2). Caton-Thompson, G. and Gardner, E. W., 1932. The prehistoric geography of Kharga oasis. Geogr. J., London. Ehhalt, D., 1963. Deuteriumgehalt in Hydrosph~ire und Atmosph~ire. Inaugural Dissertation, Heidelberg. Ettingen, S. B., 1954. Engineering Handbook. V. 1. Massadah, Tel-Aviv, (in Hebrew). Ettinger, M. and Langozki, Y., 1969. Hydrodynamics of the Mesozoic formations in the Northern Negev, Israel. Bull. Geol. Surv. Israel, 46. Gat, J. R. and Dansgaard, W., 1972, Stable isotope variations in water sources of Israel. J. Hydrol. 16: 177-212. Gat, J. R., Mazor, E. and Tzur, Y., 1969. The stable isotope composition of mineral waters in the Jordan Rift Valley, Israel. J. Hydrol. 76: 334-352. Hellstrom, B., 1940, The subterranean water in the Libyan Desert. Geog. Annaler Stockholm 3-4: 206-239. Himida, I. H., 1970, The Nubian Artesian Basin, its regional hydrogeological aspects and palaeohydrogeological reconstruction. J. Hydrol. 9:89-116. Issar, A., Bein, A. and Michaeli, A., 1969. The Nubian Sandstone aquifer in Sinai and southern Negev. Geol. Survey Israel Report. Kafri, U., 1968. Report on pumping tests in the Wadi Sidri field. Geol. Survey Israel Report. Knetsch, G., 1962, Geologische Uberlegungen zu der Frage des Artesischen Wassers in der Westlichen Agyptischen Wfiste. Geol. Rund. 52: 640-650. Knetsch, G., Shata, A., Munnich, K. O., Vogel, J. C. and Shazly, M. M., 1962. Untersuchungen an Grundwasser der Ost-Sahara. Geol. Rund. 52: 587-610. McKee, E. C., 1962, Origin of the Nubian and similar sandstones. Geol. Rund. 52:551-586. Munnich, K. O. and Vogel, J. C,, 1962. Untersuchungen an Pluvialen Wassern der OstSahara. Geol. Rund. 52:611-624. Picard, L., 1938. Synopsis of stratigraphic terms in Palestinian geology. Bull. Hebrew Univ. Geol. Dept. 2. Pomeyrol, R., 1968. Nubian Sandstone. Bull. Amer. Assoe. Petrol. Geol. 52: 589-600. Said, R., 1962. The Geology o f Egypt. Elsevier Publ. Co. Amsterdam. Shiftan, Z., 1961, New data on the artesian aquifers of the southern Dead Sea Basin. Bull. Res. Counc. o f lsrae110G : 267-291. Weissbrod, T., 1970. Nubian Sandstone, Discussion. Bull. Amer. Assoc. Petrol. Geol. 55 : 890-891. Weissbrod, T., 1970. The stratigraphy of the Nubian Sandstone in southern Israel (Timena, Eilat area). Geol. Survey Israel Report OD/2/70 and Inst. for Petroleum Research and Geophysics Report No. 1047. Ville, M. and Rolland, G., 1890, 1894. Gdologie et Hydrologie du Sahara Algdrien, Paris.