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Geological controls on water quality in arid Kuwait
Journal of Arid Environments (1998) 38: 187–204
Geological controls on water quality in arid Kuwait
F. Al Ruwaih*, S. Sayed† & M. Al-Rashed† *Kuwait University, Science College, Geology Department, P.O. Box 5969 13060, Safat, Kuwait †Water Resources Division, Kuwait Institute for Scientific Research, P.O. Box 24885 13109, Safat, Kuwait (Received 27 July 1997, accepted 3 November 1997) The objective of the study was to assess water quality of the Kuwait Group aquifer and outline the role of geological factors and processes and the subsurface structure on controlling the trend in water quality as discharge– recharge relations. Field pH, temperature and content of Ca2 + , Mg2 + , K + , – Na + , Cl–, SO2– 4 and HCO3 have been determined for 167 ground-water samples of the Kuwait Group aquifer. Hydrochemical characteristics of the study fields were investigated by means of extensive data processing in the form of equivalent per million percentage, equilibrium studies, as well as multivariate statistical analyses. It is found that the Kuwait Group aquifer is mainly occupied by brackish, NaCl and CaSO4 water types. Fresh water of Na2SO4, Ca(HCO3)2 and NaHCO3 water types occurred in the upper part of the Dibdibba Formation of the Kuwait Group. In addition, saline–brine ground-water is found in residential areas located east of Ahmadi anticline structure, where the total dissolved solids ranges from 10,000–115,000 mg l–1. A computer program, WATEQ, was utilized to compute the saturation indices of minerals with respect to a given water composition. It is found that the ground-water is supersaturated with respect to calcite, aragonite and dolomite, and undersaturated with respect to anhydrite, gypsum and halite. Calculated mean values of PCO2 for the fresh and brackish water fields are 4 3 10–3 and 5 3 10–3 (atm), respectively, indicating that the ground-water becomes charged with CO2 during infiltration processes. Generally, the salinity increases from 3000–100,000 mg l–1 in the direction of regional flow. ©1998 Academic Press Limited Keywords: multivariate statistical analyses; saline–brine ground-water; saturation indices
Introduction The state of Kuwait is located at the northern side of the Arabian Gulf and occupies an area of about 18,000 km2 of sandy and gravelly desert. The stratigraphic sequence of Kuwait (after Owen & Nasr, 1958) is listed in Table 1. Kuwait is a typical arid region, where rainfall is scarce and variable. Mean annual rainfall is 115 mm year–1, and potential evaporation is very high (2608 mm year–1). 0140–1963/98/020187 + 18 $25.00/0/ae970345
© 1998 Academic Press Limited
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Accordingly, Kuwait is short of natural water resources. Most water resources are brackish ground-water. The Eocene Dammam Formation and the Kuwait Group are the main sources of brackish water in Kuwait. Topography and geology Topography of Kuwait is generally flat, broken only by occasional low hills and shallow depressions. Elevations range from sea level in the east to nearly 300 m in the southwestern corner of the country. Generally, land surface of Kuwait slopes northeastward. The Jal Az-Zor escarpment forms one of the main topographic features in Kuwait (Fuchs, 1968). Another elevation is the Ahmadi Ridge of approximately 137 m separating the plain of Burgan from the Gulf coast, in addition to Wara hill. The major depression is Wadi Al-Batin, a broad shallow wadi making the western boundary of Kuwait. The drainage pattern becomes obscure in the south-western section of Kuwait, and the north-eastern drainage trend tends to be recognized in poorly developed wadis, as shown in Fig. 1. The surface of Kuwait is formed by sedimentary rocks and sediments ranging from Middle Eocene to Recent. The Dammam Formation represents the oldest exposed sedimentary rocks. The recent deposits of fine-grained beach sands covered the southern coast of Kuwait and the Neutral zone. General structure Kuwait is located on the Arabian Plate between the precambrian Arabian shield to the west and the Zagros fold belt to the east. According to Mitchell (1958), Kuwait and the Neutral zone are situated on the unstable shelf of the Arabian Shield. There was a gradual facies change from continental through mixed continental-marine to a marine continental lagoonal facies outward from the shield. Increasing subsidence in Table 1. Stratigraphic sequence of Kuwait (after Owen & Nasr, 1958)
Age
Group
Recent Pleistocene Pliocene Miocene Oligocene Eocene
Paleocene
K u w a i t Group
Formation Marine, aeolian and fluvial deposits Dibdibba Formation Lower Fars Formation Ghar Formation
H a s a
Dammam Formation
Group
Radhuma Formation
Rus Formation
Dominant lithology
Range of thickness
Sands, silts, clays and gravels Gravels and sands, mainly 120 m conglomerate, sandstone, siltstone, shale Calcareous sandstone, 100 m fossiliferous limestone, gypsiferous Quartzose sandstone, some 250 m shale in lower part Discontinuous chert cap, 185–217 m chalky and siliceous limestone, dolomite Anhydrite, limestone, 76–124 m marl Marly limestone, 185–434 m dolomite, anhydrite
WATER QUALITY IN KUWAIT
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the direction of geosyncline is indicated by the general thickening of the sedimentary beds in this direction. Within Kuwait, only very gentle structures have been found. According to Milton (1967), these structures (Manageesh, Rawdhatain, Umm Gudair, Ahmadi Ridge) have been growing almost steadily since at least Middle Cretaceous times and are possibly as old as upper Jurassic. An exception of this is the Ahmadi structure which appears to be the result of tangential movements in post Eocene times, probably related to the Zagros Orogeny. The conspicuous Jal-Az-Zor escarpment has been considered as an erosional surface because there is no indication of a tectonic origin. Ahmadi Ridge is considered as one of the five structural trends recognized in Kuwait. It is a rare north–north-west contraction trend probably related to the Zagros Orogeny mountain building, where it overprints the Kuwait Arch trend and is considered as the youngest fold structure in Kuwait, and has high structural amplitude at the Rus Formation. Figure 2 shows the structure contour on top of the Dammam Formation. Neogene–Quaternary system Kuwait Group (Neogene) The Kuwait Group consist of sand, gravels, sandstone, clay, silts and limestone or marls covering the entire surface of Kuwait and extending down to the top of the underlying Dammam limestone Formation. The thickness of the Kuwait Group
IRAQ Rawdatain
30 AZ
-Z
O
R
Bubiyan
JA L
18 0
WA D
IA
IR
AQ
l-B AT I
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Basra Road
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Umm Al-Aish
Failaka Kuwait Bay
18
N BIA
0
24
ARA
0
Kuwait City
LF GU
30
Shagaya
0
27
Sulibiya AHMADI RIDGE
Wara Hill
SAUDI ARAB IA
30 90
18 0
Drainage system: Contour interval: 30 m 0
25 km
Figure 1. The topographic and dominant north-east drainage patterns of Kuwait.
190
F. AL RUWAIH ET AL.
increases from about 150 m in the south-west to about 400 m in the north-east. The Kuwait Group is completely dry in the extreme south-west, and is almost saturated with water along the coast of the Arabian Gulf. In the north of Kuwait it is possible to divide Kuwait Group into three formations, based on the presence of an intermediate evaporite development. These divisions are the Dibdibba, Lower Fars, and Ghar Formations. The undivided Kuwait Group extends under all of Kuwait state with an extension eastwards beneath the Arabian Gulf where it may have an outcrop. A notable feature
0.5 0 –18165 –
50 –1 35 –1 0
–12 5 0 –10 –9
–7
32°50'
5 –60
0.6 0.4
–45 –3
–105 –90
0
5
–1
–75
32°30'
0.2
0
–6
2.4 5
–4
–30
–15 –45
32°10'
–30 –15
STRUCTURAL CONTOUR ON TOP OF DAMMAM FORMATION (DATUM M S.L) 0
Dip gradient in %
0
0
5
10 km 770
780
790
800
Figure 2. Structural contour map for the top of Dammam Formation.
WATER QUALITY IN KUWAIT
191
is the presence of a green, sandy-clay layer known as the Basal clay which rests unconformably on the eroded surface of the underlying Dammam Formation.
Kuwait Group aquifers Two aquifers, separated by an aquitard, have been identified in the Kuwait Group. An aquitard formation of clay and silty sand alternation separates the lower aquifer from the upper aquifer of the Kuwait Group, which is mostly made of gravelly sand. The sandy facies overlying the basal clayey zone forms the lower aquifer of the Kuwait Group. The initial piezometric heads of Kuwait Group aquifer indicates that water levels were about 120 m a.m.s.l. in the south-western corner of Kuwait. The piezometric heads slope downwards with a relatively uniform gradient in the southwest–north-east direction towards Kuwait Bay. Ground-water moves convergently towards Kuwait Bay to be discharged by evapo-transpiration at marsh lands along the shoreline and ultimately into the Bay and Gulf by seepage. The transmissivity of the Kuwait Group aquifer has been determined for the different water well fields utilizing the analytical pumping test methods (Kruseman & De Ridder, 1970). The resultant values and statistical summary of transmissivity are shown in Table 2. It is found that the transmissivity ranges from 295–3456 m2 day–1 and the transmissivity mostly decreases eastward.
Eocene system (Hasa Group) A marked erosional unconformity separates the Kuwait Group from the underlying Hasa Group which has a distinctive and widespread extent from Saudi Arabia via Kuwait and Iraq to Iran. A triple division of the Hasa Group has been made on the basis of lithology, from top to bottom to Dammam, Rus, and Radhouma Formations. The Dammam Formation underlies the whole of the state of Kuwait and forms the main aquifer in Kuwait. It consists of over 200 m of whitish grey, porous, dolomitized limestone, nummulitic limestone and soft chalky limestone. The top of the formation is generally marked by the presence of a hard, cherty layer with a persistent grey-green waxy shale horizon providing a distinctive base. The initial piezometric heads of the Dammam aquifer indicates that water levels were about 140 m a.m.s.l. in the south-western corner of Kuwait sloping towards the north-east. The head in the Dammam aquifer was 3 to 20 m higher than that in the aquifer of the Kuwait Group. Thus, vertical leakage upward between aquifers is expected.
Hydrochemical data One hundred and sixty-seven ground-water samples collected from different water well fields have been considered for chemical analyses (Fig. 3). In the selection of representative data, different lithologies, and uniform areal distribution of data locations are considered as a basic criteria. Water samples were analysed for major ions –1 such as Ca2 + , Mg2 + , Na + , K + , HCO–3, Cl– and SO2– 4 , expressed in mg 1 . Field measurements of electrical conductivity (EC), pH and laboratory determinations of total dissolved solids (TDS) are also included in the hydrochemical assessment of the
Shagaya A Shagaya B Shagaya E South-western wells Sulaibiya Umm-Gudair Wafra Residential areas Kabd Atraf Abdali All Kuwait Group wells
Well fields 17 2 18 6 4 151 4 6 6 13 3 230
Count 60·48 172·80 34·56 34·56 69·12 60·48 150·00 108·00 69·12 86·40 750·00 34·56
Minimum 345·60 777·60 777·60 864·00 259·20 3456·00 400·00 348·36 518·40 345·60 2400·00 3456·00
Maximum 285·12 604·80 743·04 829·44 190·08 3395·52 250·00 240·36 449·28 259·20 1650·00 3421·44
Range 139·26 475·20 304·32 299·52 211·68 618·36 275·00 200·51 285·12 225·97 1716·67 511·66
Mean
Table 2. Kuwait Group aquifer transmissivities (m2 day–1)
86·40 475·20 259·20 259·20 259·20 432·00 275·00 172·67 259·20 259·20 2000·00 345·00
Median
77·21 427·66 189·58 309·63 95·04 563·83 104·08 94·95 165·29 75·14 860·72 529·84
Standard deviation
192 F. AL RUWAIH ET AL.
WATER QUALITY IN KUWAIT
193
Kuwait Group aquifer. Results of some of these determination are presented in Table 3. All data were carefully checked for recording errors. The resulting ionic imbalances (cations–anions)/(cations + anions) generally did not exceed 5%. The areal distribution of the isosalinity of the aquifer shows an increase from 3000 mg l–1 in south-western fields to 10,000 mg l–1 eastward in Sulaibiya field. Total dissolved solids increased gradually to > 100,000 mg l–1 towards the east of the Kuwait Bay, north-east along the coastal line, and in the residential areas are located east of the Ahmadi anticline (Fig. 4). It is suggested that the presence of the structural element of Ahmadi anticline diverted the ground-water movement towards the north-east, making the ground-water in the residential areas stagnant and of low salinity. Therefore, the increase in TDS of the aquifer from south-west toward north and north-east may be attributed to the increased saturated thickness of the aquifer in this direction, to the presence of the Ahmadi structure, to the potentiometric level which reached near sea level, and to the nature of the marine environmental deposits of the aquifer sediments. In addition, and according to Chebotarev (1955), the salinity increases with time, and recharge–discharge area relations.
48°
47° Abdali
30° Rawdhatain
N
Bubiyan Island
IR
AQ
Umm Al-Aish
KUWAIT BAY
f tra
A
Residential Areas
Shagaya
A ab d
Fields
nm
Ur
SAUDI ARAB IA
K
B 29°
10 20 30 40 50 km
r
dai
Gu
0
Wafra
Legend Fresh ground-water well fields Brackish ground-water well fields South-western wells Northern wells Residential area of Kuwait
Figure 3. Location map of the study water fields.
ARABIAN GULF
Sulaibiya
E
Failaka Is.
W3-14 W5-4 W7-4994 WS11-46
AT-16 AT-19 AT-37 AT-40
Atraf
UG-1 UG-7 UG-15 UG-24
AB-9 AB-20 AB-35 AB-36
U-4 U-6 U-48 U-54
R-3 R-6 R-27 R-58
Well no.
Wafra
Umm-Gudair
Brackish water Abdali
Umm Al-Aish
Fresh water Rawdhatain
Well fields
7·50 7·35 7·45 7·50
7·20 7·20 7·70 7·60
7·70 7·60 7·50 7·50
7·00 7·40 7·20 7·40
7·90 8·00 8·00 7·80
7·50 7·60 7·70 7·60
pH
3753 4178 4442 3686
10,200 6600 4300 12,900
4790 4674 3637 3642
8320 4480 6464 5120
1086 403 1140 640
1737 773 1165 560
TDS
408·0 461·0 456·0 424·0
782·4 730·5 541·1 990·8
533·0 593·0 480·0 435·0
650·0 550·0 770·0 490·0
134·0 50·6 88·4 103·6
252·0 101·0 90·0 91·0
Ca2+
128·0 128·0 120·0 122·0
202·7 185·8 141·1 364·5
113·0 143·0 108·0 113·0
175·0 80·0 200·0 170·0
17·6 7·6 12·8 21·2
31·2 13·0 14·4 12·6
Mg2+
600·0 715·0 700·0 630·0
2163·2 1083·7 785·1 2402·8
710·0 640·0 625·0 510·0
2125·0 1145·0 1870·0 1460·0
140·0 65·0 230·0 65·0
190·0 120·0 255·0 30·0
Na+
15·2 15·0 14·3 14·5
96·3 63·5 44·9 162·4
16·0 16·5 15·0 16·0
40·0 25·0 21·0 20·0
4·0 3·0 6·5 4·0
6·5 5·0 5·0 4·0
K+
Table 3. Chemical analyses of the Kuwait Group aquifer (mg l –1)
144·0 107·0 103·0 132·0
143·0 144·0 140·0 250·0
61·0 70·0 70·0 78·0
339·0 165·0 122·0 122·0
131·3 157·0 133·2 203·0
129·8 191·7 165·4 204·9
HCO3–
1394 957 957 1218
2215 1682 1422 2706
1230 1065 1010 1020
2207 1544 3461 3461
380 86 375 108
713 260 375 108
SO2– 4
Water type
1026 1361 1361 1025
3120 2127 1538 4765
1617 1700 1238 1032
3578 1547 1994 1692
144 45 180 152
Na2SO4 NaCl NaCl NaCl
NaCl NaCl NaCl NaCl
NaCl CaCl2 NaCl Na2SO4
NaCl NaCl Na2SO4 Na2SO4
CaSO4 NaHCO3 Na2SO4 CaCL2
212 CaSO4 80 Na2SO4 152 Na2SO4 52 Ca(HCO3)2
Cl–
194 F. AL RUWAIH ET AL.
SM-10D-2 SM-09G-1 SM-11E-4 SM-03A
8·38 8·60 7·56 7·55
5·64 7·40 7·90 6·40
Residential areas Mishref MF-11D MF-16A MF-03A MF-05B
Shamiya
pH
Well no.
Well fields
12,200 8400 13,000 10,500
18,000 5800 3600 76,000
TDS
778·0 579·0 834·0 707·0
1057·0 627·0 614·0 3862·0
Ca2+
369·0 212·0 393·0 345·0
231·0 126·0 50·0 1340·0
Mg2+
Na+
2581·0 1458·0 2862·0 2174·0
3401·0 815·0 315·0 18660·0
Table 3. Continued
64·0 46·0 71·0 46·0
188·0 38·0 15·0 635·0
K+
148·0 135·0 153·0 149·0
104·0 499·0 59·0 414·0
HCO3–
2559 2290 3240 3200
1540 2559 1750 3178
SO2– 4
4291 2948 6183 3098
8665 700 418 40,416
Cl–
NaCl NaCl NaCl NaCl
NaCl Na2SO4 CaSO4 NnCl
Water type
WATER QUALITY IN KUWAIT 195
196
F. AL RUWAIH ET AL.
Water types relationships For each ground-water sample the dominant cations and anions expressed in meq 1–1 are determined for classification purposes. The water composition of the Kuwait Group aquifer can be categorized into four main groups namely: NaCl, CaSO4, Na2SO4, and CaCl2. CaSO4 and NaCl are the most abundant water types in the brackish ground-water fields, while Ca(HCO3)2 and NaHCO3 are the water types representing fresh water fields. The presence of these different water types are mainly due to the cation exchange process, where water quality changes from NaCl to CaCl2 water type. The reverse process takes place when fresh water flushes marine aquifer water, the Ca2 + is taken up from the water and Na + is retained forming NaHCO3 and Na2SO4 water types. The extension of chloride from NNE to the SW of the study area is due to the farthest distance from the naturally recharged area, and to the increased saturated thickness of aquifer along this direction. Trilinear diagram The Piper trilinear diagram (1944) constitutes a useful tool in water analysis interpretation. The diagram is used as a means of generally indicating similarities and
47°
48°
400
,0
0 ,00 0 0
50
00
40
6000
00
2000
2 30 000 00 0
N
3000
30°
4000 500 60 0 0 70 0 00
IR
AQ
20
10
10,000
6000
30
00
4000
500
0
ARABIAN GULF
00
10,0
KINGDOM O
29°
F SAUDI AR
ABIA 400
0
00
Contour interval (mg l )
30
1000
–1
Well location 0
10
20
30
40 km 5000
Figure 4. Areal distribution map of isosalinity of the Kuwait Group aquifer.
WATER QUALITY IN KUWAIT
197
differences in the composition of water from certain geologic and hydrologic units. The trilinear diagram of the water chemistry of the study fields is shown in Fig. 5. It is clear from this diagram that calcium sulphate and sodium chloride are the most dominant water types. All the brackish ground-water fields show a concentration of strong acid exceeding weak acid, and alkalies exceed alkaline earth’s. While the fresh water samples exhibited a concentration of strong acids exceeding weak acids, but alkaline earth’s exceed alkalies. While the residential areas showed that the alkalis (Na + + K + ) exceed the alkaline earth’s and the non-carbonate alkali exceed 50%, that is, the chemical properties are dominated by alkalis and strong acids, usually ocean water and brines are related to this area of plot. The Durov diagram has been constructed in Fig. 6 to show the chemical water composition and prevailing chemical processess of the Kuwait Group aquifer. Most of the brackish water fields exhibited a dissolution or mixing process and water is related to the reverse ion exchange of Na + –Cl– water. The dominant of Cl– and Na + indicated an end-point water (Lloyd & Heathcote, 1985). In the residential areas Na + and Cl– are the dominant ions. However, the fresh water fields show a high concentration of SO2– 4 indicating a probable mixing influence of the aquifer material with water of meteoric origin, and some samples show a dominance of 2+ SO2– which may be due to simple mixing or dissolution process. 4 and Ca
50
Mixing
2+
Fresh
g
50
–M
SO 2– 4 – C
2+
Ca
l ––
3
NO –
Fresh water fields Brackish water fields Residential areas
Sea
2+
O–
3
HC
50
Ca2+
Figure 5. Ground-water composition using Piper diagram.
50
(Na+ + K+)
50
Cl– – NO–3
2–
+
50
SO 4
+K
M
+
50
50
Ca2+
Na
g 2+
Mg
198
F. AL RUWAIH ET AL.
Chemical activities The chemical activity of an ion is equal to the molal concentrations times the activity coefficient, α = γM, where α is the chemical activity, M is the molal concentration and γ is the activity coefficient. Once the ionic strength of a solution of electrolysis is known, the activity coefficient of the individual ion can be computed from the Debye-Huckel equation (Fetter, 1994) provided that some parameters in this equation are taken from tables for any specific ions. The ion activity product (Kiap), which is the product of the measured activities, is calculated for the water samples to test for saturation. The value of Kiap for a mineral may be compared with the value of the solubility product of that mineral (Ksp). In order to avoid this long procedure, a computer program WATEQ4F (Ball & Nordstorm, 1992) was utilized in this study to calculate the ionic strength, chemical activity, activity coefficients Ksp and Kiap and saturation index (SI) for about 167 samples of different locations of the Kuwait Group aquifer. Table 4 indicates the logarithms of ion activity product for calcite, anhydrite, gypsum, halite, aragonite and dolomite. Comparison of the activity products with respective logarithmic solubility products (Ksp) of the ions provides an indication of equilibrium or non-equilibrium as undersaturation or oversaturation. If Kiap/Ksp is greater than 1, the solution is supersaturated, if less than 1, it is undersaturated and if equal to 1, the solution is in equilibrium. Furthermore, the ionic activity products have been computed for aragonite, dolomite and halite. Accordingly, it is found that the Kuwait Group aquifer Mg2+ 50%
Ca2+ 25% Mg2+ 25%
Ca2+ 50%
1
Ca2+ 25% + Na 25%
Na+ 25%
2
3
M
HCO–3 25% – Cl 25%
Ca2+ 25% + Na 25%
6
E
PL LU SO
IS D
HCO–3 25% Cl– 25%
N O
5
G
IN
IX
4
R
Cl– 25% SO42– 25%
O
TI
SO42– 50%
Na+ 25% Mg2+ 25%
M SI
HCO–3 25% SO42– 25%
– HCO3 50%
Fresh water fields Brackish water fields Residential areas
REVERSE ION EXCHANGE
8
9
Cl– 50%
7
Figure 6. Durov diagram showing the results of chemical analyses of ground-water of the Kuwait Group aquifer.
Atraf
Wafra
Umm-Gudair
Brackish water Abdali
Umm Al-Aish
Fresh water Rawdhatain
Well fields
AB-9 AB-20 AB-35 AB-36 UG-1 UG-7 UG-15 UG-24 W3-14 W5-4 W7-4994 WS11-46 AT-16 AT-19
U-4 U-6 U-48 U-54
R-3 R-6 R-27 R-58
Well no.
–0·345 –0·414 –0·114 –0·35 –0·483 –0·512 –0·568 –0·584 –0·275 –0·331 –0·445 –0·156 –0·516 –0·62
–1·165 –2·004 –1·529 –1·717
–0·771 –1·382 –1·334 –1·719
Anhydrite CaSO4
0·207 0·299 –0·116 0·007 0·194 0·211 0·049 0·063 0·114 0·149 0·544 0·809 0·264 0·066
0·364 0·166 –0·608 0·433
0·148 0·156 0·091 0·211
Aragonite CaCO3
0·351 0·443 0·027 0·154 0·338 0·354 0·192 0·207 0·258 0·293 0·683 0·953 0·404 0·209
0·508 0·314 –0·465 0·577
0·292 0·344 0·235 0·355
0·484 0·391 –0·193 0·179 0·346 0·438 0·081 0·171 0·279 0·337 1·126 1·822 0·643 0·208
0·473 0·142 –6·534 0·813
0·013 0·051 0·014 0·198
–0·129 –0·196 0·103 –0·132 –0·265 –0·293 –0·349 –0·365 –0·058 –0·113 –0·226 0·064 –0·297 –0·402
–0·945 –1·784 –1·309 –1·497
–0·551 –1·162 –1·115 –1·499
Phase / Chemical formula Calcite Dolomite Gypsum CaCO3 CaMg(CO3)2 CaSO4.2H2O
Table 4. Saturation indices of the Kuwait Group aquifer
–3·871 –4·461 –4·182 –4·339 –4·647 –4·671 –4·807 –4·967 –3·928 –4·369 –4·634 –3·944 –4·908 –4·714
–6·331 –7·109 –8·674 –6·601
–6·037 –6·621 –6·034 –7·391
Halite NaCl
2·60E-02 5·10E-03 7·30E-03 4·29E-03 9·51E-04 1·36E-03 1·77E-03 2·00E-03 6·79E-03 6·99E-03 2·17E-03 4·50E-03 3·70E-03 3·84E-03
1·48E-03 1·89E-03 1·27E-03 2·96E-03
3·60E-03 4·47E-03 3·01E-03 4·86E-03
PCO2
WATER QUALITY IN KUWAIT 199
Shamiya
Residential areas Mishref
Well fields
MF-11D MF-16A MF-03A MF-05B SM-10D-2 SM-09G-1 SM11E-4 SM-03A
Well no.
–0·405 –0·202 –0·243 –0·053 –0·287 –0·344 –0·207 –0·216
Anhydrite CaSC4 –1·481 0·778 0·419 0·273 1·182 1·254 0·447 0·384
Aragonite CaCO3 –1·337 0·922 0·562 0·417 1·326 1·398 0·591 0·528
–2·965 1·484 0·367 0·806 2·681 2·704 1·208 1·088
–0·191 0·017 –0·024 –0·238 –0·071 –0·127 0·008 0·001
Phase / Chemical formula Calcite Dolomite Gypsum CaCO3 CaMg(CO3)2 CaSO4.2H2O
Table 4. Continued
–3·319 –4·967 –5·574 –1·924 –3·727 –4·113 –3·539 –3·938
Halite NaCl
1·74E-01 1·59E-02 5·84E-04 1·09E-01 3·70E-04 2·02E-04 3·03E-03 3·10E-03
PCO2
200 F. AL RUWAIH ET AL.
WATER QUALITY IN KUWAIT
201
is undersaturated with respect to anhydrite, gypsum and halite and supersaturated with respect to aragonite, calcite and dolomite. The calculated mean values of PCO2 for the fresh and brackish water fields in the study areas are 4 3 10–3 and 5 3 10–3 atm, respectively, which is considerably less above the PCO2 of the earth atmosphere (10–3.5 atm). This indicates that the groundwater in the Kuwait Group aquifer became charged with CO2 during infiltration trough soil zones (Freeze & Cherry, 1979).
Multivariate statistical analysis Correlation and cluster analysis were carried out for the 167 samples. The standardized m-space Euclidian distance is used as a method of clustering (Davis, 1986). A low distance indicates that two objects are similar or close together, whereas a large distance indicates dissimilarity. The correlation matrix of non-transformed data of the variables and their mean and standard deviation is displayed in Table 5. It is obvious that the TDS is in good correlation with all variables except the ionic concentration of HCO–3 and SO2– 4 , which suggested that water of meteoric origin and the recharge process is not local, and the bicarbonate is precipitated as carbonate along the recharge route. Ca2 + should be of oceanic origin and this is evident from the relatively high correlation coefficient between Cl– and Ca2 + . The elements Cl–, Na + , K + and Mg2 + are strongly associated forming what can be called a cluster of seawater components (Eriksson, 1985). Cluster analysis of the non-transformed data input of 134 cases of the Kuwait Group aquifer using the hydrochemical compositions of nine variables is shown in Fig. 7 which shows fair similarity level between cases, where one independent case and three main cluster are recognized. Cluster I includes cases of the ground-water fields of Wafra, Umm-Gudair, Atraf, and Abdali with one sample from the residential area of Shamiya (case No. 68). The cluster shows a range of total dissolved solids of 3372–7900 mg l–1 and represents a brackish ground-water of NaCl type. Cluster II includes the cases of the fresh ground-water fields of Rawdhatain and Umm Al-Aish where the salinity ranges from 359–1737 mg l–1 and the water types are Na2SO4, Ca(HCO3)2 and NaHCO3 of meteoric origin, and the ratio of SO4/Cl ranges from 0·52 to 4·45. Cluster III includes all cases from Mishref and Sabah Al-Salem residential areas where the total dissolved solids ranged from 10,000–115,000 mg l–1 represents saline– brine ground-water. The ratio of SO4/Cl ranges from 0·02 to 0·54 which is close to the value of seawater. Therefore, it is obvious that the ground-water samples of the Kuwait Group aquifer of different water well fields have been clustered as fresh, brackish, and saline–brine ground-water based on the total mineralization and on the water types.
Conclusions The hydrochemical results indicate that the Kuwait Group aquifer is occupied by a brackish ground-water with TDS ranging from 3372–7900 mg l–1 in Wafra, UmmGadair, Atraf and Abdali and of NaCl water type, and in Sahmiya the TDS ranges from 7800–13,000 mg l–1. A fresh ground-water with TDS ranging from 359–1737 mg l–1 is located in the northern country at Rawdhatain and Umm Al-Aish fields, and is composed of Na2SO4, NaHCO3 and Ca(HCO3)2 water types. In addition, saline–brine ground-water is present in the residential areas of Mishref and Sabah Al-Salem where the TDS ranges from 10,00–115,000 mg l–1 of NaCl water
pH TDS K+ Na+ Mg2+ Ca2+ Cl– SO2– 4 HCO3–
Standard mode
+1·00 –0·37 –0·44 –0·41 –0·39 –0·40 –0·40 –0·13 –0·17
pH
+1·00 +0·97 +0·97 +0·96 +0·93 +0·97 +0·32 +0·39
TDS
+1·00 +0·99 +0·98 +0·95 +0·98 +0·31 +0·41
K+
+1·00 +0·99 +0·95 +0·99 +0·32 +0·39
Na+
+1·00 +0·94 +0·99 +0·32 +0·39
Mg2+
+1·00 +0·96 +0·27 –0·38
Ca2+
+1·00 +0·27 +0·39
Cl–
+1·00 +0·18
SO2– 4
+1·00
HCO3–
7·44 16,548·72 138·20 4183·07 336·34 1019·30 8030·43 1740·07 174·57
Mean
Table 5. Correlation matrix of non-transformed data input of the hydrochemical variables of the Kuwait Group aquifer
0·51 24,639·29 226·39 6719·82 439·03 1254·11 13,835·17 1159·96 151·82
Standard deviation
202 F. AL RUWAIH ET AL.
WATER QUALITY IN KUWAIT Hierarchical Tree (Dlink/Dmax) ´ 100 Case no. 1–10–20–30–40–50–60–70–80–90–100
Abdali
13–25
Rawdhatain
26–33
Umm Al-Aish
34–37
North wells
38–54
Wafra
55–67
Atraf
68–71
Shamiya
72–77
Mishref
78–115
Umm-Gudair
116–134
Sabah Al-Salem
Cluster I
1–12
Cluster III
80 73 89 63 64 98 91 60 78 84 93 48 105 34 40 10 67 3 46 11 54 12 36 73 13 27 15 17 23 29 30 33 24 29 13 22 16 19 25 21 31 25 22 20 14 41 69 70 123 52 71 134 75 117 119 124 129 133 72 120 121 127 132 116 74 130 125 131 122 123 76 118 124 77
Case No.
Cluster II
1 44 38 45 49 37 39 51 43 42 47 50 53 68 7 8 6 2 4 9 5 35 55 56 66 57 83 115 110 114 54 61 85 82 104 101 107 111 100 103 109 112 113 97 59 62 81 105 85 100 99 92 58 89 60 87 90 95 108 99
Figure 7. Cluster analysis of the ground-water composition of the Kuwait Group aquifer.
203
204
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type. Salinity ranges from 3000–10,000 mg l–1 for these water fields located before the Ahmadi anticline. The TDS increases remarkably for the residential areas which are located after the Ahmadi ridge, and to the east and near the seashore and ranged from 15,000–150,000 mg l–1. The concentration of strong acids exceeds weak acids, and the alkaline exceeds the alkaline earth’s, while in the fresh water fields the alkaline earth’s is the dominant. The ground-water exhibited simple mixing, dissolution and reverse ion exchange processes. The aquifer is almost supersaturated with respect to calcite, aragonite and dolomite, and undersaturated with respect to anhydrite, and gypsum and halite. The calculated PCO2 of the Kuwait Group aquifer indicated that the ground-water became charged with CO2 during infiltration processes. Multivariate statistical analyses show that the ground-water samples have been clustered according to the TDS and the water types. The authors wish to express the appreciation to the Ministry of Electricity and Water for providing the chemical data. Thanks are also extended to the Kuwait University, Scientific Research Unit for the financial support as Grant No. SG036.
References Ball, J.W. & Nordstorm, D.K. (1992). Geochemical model to calculate speciation of major, trace and redox elements in natural waters. U.S. Geological Survey. International Ground Water Modelling Centre. 189 pp. Chebotarev, I.I. (1955). Metamorphism of natural waters in the crust of Weathering. Geochemical Cosmochimal Acta, 8: 22–48, 137–170, 198–212. Davis, J.C. (1986). Statistics and Data Analysis in Geology (2nd Edn). Canada: John Wiley and Sons. 635 pp. Eriksson, E. (1985). Principles and Applications of Hydrochemistry. U.S.A.: Chapman and Hall. 187 pp. Fetter, C.W. (1994). Applied Hydrogeology. U.S.A.: Macmillan College Publishing Company. 691 pp. Freeze, R. & Cherry, J. (1979). Groundwater. Englewood Cliffs, NJ: Prentice-Hall. 604 pp. Fuchs, W., Gattinger, T.E. & Holzer, H.F. (1968). Explanatory test to the synoptic geologic map of Kuwait. A surface Geology of Kuwait and the Neutral zone. Vienna: Geological Survey of Austria. 87 pp. Kruseman, G.P. & De Ridder, N.A. (1970). Analysis and evaluation of pumping test data. Wageningen, Netherlands: International Institute for Land Reclamation and Imprisonment, Bill. 11. 200 pp. Lloyd, J.W. & Heathcote, J.A. (1985). Natural Inorganic Hydrochemistry in Relation to Groundwater. Oxford: Clarendon Press. 296 pp. Milton, D.I. (1967). Geology of the Arabian Peninsula-Kuwait. United States Geological Survey of Professional paper, 560-F. Washington: United States Government Printing Office. 7 pp. Mitchell, R.C. (1958). Notes on the Geology of Western Iraq and Northern Saudi Arabia. Geol. Rundschau, 46: 467–493. Owen, R.M. S. & Nasr, S.N. (1958). Stratigraphy of Kuwait-Basra area, in Habit of Oil. American Association of Petroleum Geologists Bulletin, 42: 1252–1278. Piper, A.M. (1944). A graphic procedure in the geochemical interpretation of water analysis. Transactions of the American Geophysical Union, 25: 914–923.
Geological controls on water quality in arid Kuwait
F. Al Ruwaih*, S. Sayed† & M. Al-Rashed† *Kuwait University, Science College, Geology Department, P.O. Box 5969 13060, Safat, Kuwait †Water Resources Division, Kuwait Institute for Scientific Research, P.O. Box 24885 13109, Safat, Kuwait (Received 27 July 1997, accepted 3 November 1997) The objective of the study was to assess water quality of the Kuwait Group aquifer and outline the role of geological factors and processes and the subsurface structure on controlling the trend in water quality as discharge– recharge relations. Field pH, temperature and content of Ca2 + , Mg2 + , K + , – Na + , Cl–, SO2– 4 and HCO3 have been determined for 167 ground-water samples of the Kuwait Group aquifer. Hydrochemical characteristics of the study fields were investigated by means of extensive data processing in the form of equivalent per million percentage, equilibrium studies, as well as multivariate statistical analyses. It is found that the Kuwait Group aquifer is mainly occupied by brackish, NaCl and CaSO4 water types. Fresh water of Na2SO4, Ca(HCO3)2 and NaHCO3 water types occurred in the upper part of the Dibdibba Formation of the Kuwait Group. In addition, saline–brine ground-water is found in residential areas located east of Ahmadi anticline structure, where the total dissolved solids ranges from 10,000–115,000 mg l–1. A computer program, WATEQ, was utilized to compute the saturation indices of minerals with respect to a given water composition. It is found that the ground-water is supersaturated with respect to calcite, aragonite and dolomite, and undersaturated with respect to anhydrite, gypsum and halite. Calculated mean values of PCO2 for the fresh and brackish water fields are 4 3 10–3 and 5 3 10–3 (atm), respectively, indicating that the ground-water becomes charged with CO2 during infiltration processes. Generally, the salinity increases from 3000–100,000 mg l–1 in the direction of regional flow. ©1998 Academic Press Limited Keywords: multivariate statistical analyses; saline–brine ground-water; saturation indices
Introduction The state of Kuwait is located at the northern side of the Arabian Gulf and occupies an area of about 18,000 km2 of sandy and gravelly desert. The stratigraphic sequence of Kuwait (after Owen & Nasr, 1958) is listed in Table 1. Kuwait is a typical arid region, where rainfall is scarce and variable. Mean annual rainfall is 115 mm year–1, and potential evaporation is very high (2608 mm year–1). 0140–1963/98/020187 + 18 $25.00/0/ae970345
© 1998 Academic Press Limited
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Accordingly, Kuwait is short of natural water resources. Most water resources are brackish ground-water. The Eocene Dammam Formation and the Kuwait Group are the main sources of brackish water in Kuwait. Topography and geology Topography of Kuwait is generally flat, broken only by occasional low hills and shallow depressions. Elevations range from sea level in the east to nearly 300 m in the southwestern corner of the country. Generally, land surface of Kuwait slopes northeastward. The Jal Az-Zor escarpment forms one of the main topographic features in Kuwait (Fuchs, 1968). Another elevation is the Ahmadi Ridge of approximately 137 m separating the plain of Burgan from the Gulf coast, in addition to Wara hill. The major depression is Wadi Al-Batin, a broad shallow wadi making the western boundary of Kuwait. The drainage pattern becomes obscure in the south-western section of Kuwait, and the north-eastern drainage trend tends to be recognized in poorly developed wadis, as shown in Fig. 1. The surface of Kuwait is formed by sedimentary rocks and sediments ranging from Middle Eocene to Recent. The Dammam Formation represents the oldest exposed sedimentary rocks. The recent deposits of fine-grained beach sands covered the southern coast of Kuwait and the Neutral zone. General structure Kuwait is located on the Arabian Plate between the precambrian Arabian shield to the west and the Zagros fold belt to the east. According to Mitchell (1958), Kuwait and the Neutral zone are situated on the unstable shelf of the Arabian Shield. There was a gradual facies change from continental through mixed continental-marine to a marine continental lagoonal facies outward from the shield. Increasing subsidence in Table 1. Stratigraphic sequence of Kuwait (after Owen & Nasr, 1958)
Age
Group
Recent Pleistocene Pliocene Miocene Oligocene Eocene
Paleocene
K u w a i t Group
Formation Marine, aeolian and fluvial deposits Dibdibba Formation Lower Fars Formation Ghar Formation
H a s a
Dammam Formation
Group
Radhuma Formation
Rus Formation
Dominant lithology
Range of thickness
Sands, silts, clays and gravels Gravels and sands, mainly 120 m conglomerate, sandstone, siltstone, shale Calcareous sandstone, 100 m fossiliferous limestone, gypsiferous Quartzose sandstone, some 250 m shale in lower part Discontinuous chert cap, 185–217 m chalky and siliceous limestone, dolomite Anhydrite, limestone, 76–124 m marl Marly limestone, 185–434 m dolomite, anhydrite
WATER QUALITY IN KUWAIT
189
the direction of geosyncline is indicated by the general thickening of the sedimentary beds in this direction. Within Kuwait, only very gentle structures have been found. According to Milton (1967), these structures (Manageesh, Rawdhatain, Umm Gudair, Ahmadi Ridge) have been growing almost steadily since at least Middle Cretaceous times and are possibly as old as upper Jurassic. An exception of this is the Ahmadi structure which appears to be the result of tangential movements in post Eocene times, probably related to the Zagros Orogeny. The conspicuous Jal-Az-Zor escarpment has been considered as an erosional surface because there is no indication of a tectonic origin. Ahmadi Ridge is considered as one of the five structural trends recognized in Kuwait. It is a rare north–north-west contraction trend probably related to the Zagros Orogeny mountain building, where it overprints the Kuwait Arch trend and is considered as the youngest fold structure in Kuwait, and has high structural amplitude at the Rus Formation. Figure 2 shows the structure contour on top of the Dammam Formation. Neogene–Quaternary system Kuwait Group (Neogene) The Kuwait Group consist of sand, gravels, sandstone, clay, silts and limestone or marls covering the entire surface of Kuwait and extending down to the top of the underlying Dammam limestone Formation. The thickness of the Kuwait Group
IRAQ Rawdatain
30 AZ
-Z
O
R
Bubiyan
JA L
18 0
WA D
IA
IR
AQ
l-B AT I
N
Basra Road
90
Umm Al-Aish
Failaka Kuwait Bay
18
N BIA
0
24
ARA
0
Kuwait City
LF GU
30
Shagaya
0
27
Sulibiya AHMADI RIDGE
Wara Hill
SAUDI ARAB IA
30 90
18 0
Drainage system: Contour interval: 30 m 0
25 km
Figure 1. The topographic and dominant north-east drainage patterns of Kuwait.
190
F. AL RUWAIH ET AL.
increases from about 150 m in the south-west to about 400 m in the north-east. The Kuwait Group is completely dry in the extreme south-west, and is almost saturated with water along the coast of the Arabian Gulf. In the north of Kuwait it is possible to divide Kuwait Group into three formations, based on the presence of an intermediate evaporite development. These divisions are the Dibdibba, Lower Fars, and Ghar Formations. The undivided Kuwait Group extends under all of Kuwait state with an extension eastwards beneath the Arabian Gulf where it may have an outcrop. A notable feature
0.5 0 –18165 –
50 –1 35 –1 0
–12 5 0 –10 –9
–7
32°50'
5 –60
0.6 0.4
–45 –3
–105 –90
0
5
–1
–75
32°30'
0.2
0
–6
2.4 5
–4
–30
–15 –45
32°10'
–30 –15
STRUCTURAL CONTOUR ON TOP OF DAMMAM FORMATION (DATUM M S.L) 0
Dip gradient in %
0
0
5
10 km 770
780
790
800
Figure 2. Structural contour map for the top of Dammam Formation.
WATER QUALITY IN KUWAIT
191
is the presence of a green, sandy-clay layer known as the Basal clay which rests unconformably on the eroded surface of the underlying Dammam Formation.
Kuwait Group aquifers Two aquifers, separated by an aquitard, have been identified in the Kuwait Group. An aquitard formation of clay and silty sand alternation separates the lower aquifer from the upper aquifer of the Kuwait Group, which is mostly made of gravelly sand. The sandy facies overlying the basal clayey zone forms the lower aquifer of the Kuwait Group. The initial piezometric heads of Kuwait Group aquifer indicates that water levels were about 120 m a.m.s.l. in the south-western corner of Kuwait. The piezometric heads slope downwards with a relatively uniform gradient in the southwest–north-east direction towards Kuwait Bay. Ground-water moves convergently towards Kuwait Bay to be discharged by evapo-transpiration at marsh lands along the shoreline and ultimately into the Bay and Gulf by seepage. The transmissivity of the Kuwait Group aquifer has been determined for the different water well fields utilizing the analytical pumping test methods (Kruseman & De Ridder, 1970). The resultant values and statistical summary of transmissivity are shown in Table 2. It is found that the transmissivity ranges from 295–3456 m2 day–1 and the transmissivity mostly decreases eastward.
Eocene system (Hasa Group) A marked erosional unconformity separates the Kuwait Group from the underlying Hasa Group which has a distinctive and widespread extent from Saudi Arabia via Kuwait and Iraq to Iran. A triple division of the Hasa Group has been made on the basis of lithology, from top to bottom to Dammam, Rus, and Radhouma Formations. The Dammam Formation underlies the whole of the state of Kuwait and forms the main aquifer in Kuwait. It consists of over 200 m of whitish grey, porous, dolomitized limestone, nummulitic limestone and soft chalky limestone. The top of the formation is generally marked by the presence of a hard, cherty layer with a persistent grey-green waxy shale horizon providing a distinctive base. The initial piezometric heads of the Dammam aquifer indicates that water levels were about 140 m a.m.s.l. in the south-western corner of Kuwait sloping towards the north-east. The head in the Dammam aquifer was 3 to 20 m higher than that in the aquifer of the Kuwait Group. Thus, vertical leakage upward between aquifers is expected.
Hydrochemical data One hundred and sixty-seven ground-water samples collected from different water well fields have been considered for chemical analyses (Fig. 3). In the selection of representative data, different lithologies, and uniform areal distribution of data locations are considered as a basic criteria. Water samples were analysed for major ions –1 such as Ca2 + , Mg2 + , Na + , K + , HCO–3, Cl– and SO2– 4 , expressed in mg 1 . Field measurements of electrical conductivity (EC), pH and laboratory determinations of total dissolved solids (TDS) are also included in the hydrochemical assessment of the
Shagaya A Shagaya B Shagaya E South-western wells Sulaibiya Umm-Gudair Wafra Residential areas Kabd Atraf Abdali All Kuwait Group wells
Well fields 17 2 18 6 4 151 4 6 6 13 3 230
Count 60·48 172·80 34·56 34·56 69·12 60·48 150·00 108·00 69·12 86·40 750·00 34·56
Minimum 345·60 777·60 777·60 864·00 259·20 3456·00 400·00 348·36 518·40 345·60 2400·00 3456·00
Maximum 285·12 604·80 743·04 829·44 190·08 3395·52 250·00 240·36 449·28 259·20 1650·00 3421·44
Range 139·26 475·20 304·32 299·52 211·68 618·36 275·00 200·51 285·12 225·97 1716·67 511·66
Mean
Table 2. Kuwait Group aquifer transmissivities (m2 day–1)
86·40 475·20 259·20 259·20 259·20 432·00 275·00 172·67 259·20 259·20 2000·00 345·00
Median
77·21 427·66 189·58 309·63 95·04 563·83 104·08 94·95 165·29 75·14 860·72 529·84
Standard deviation
192 F. AL RUWAIH ET AL.
WATER QUALITY IN KUWAIT
193
Kuwait Group aquifer. Results of some of these determination are presented in Table 3. All data were carefully checked for recording errors. The resulting ionic imbalances (cations–anions)/(cations + anions) generally did not exceed 5%. The areal distribution of the isosalinity of the aquifer shows an increase from 3000 mg l–1 in south-western fields to 10,000 mg l–1 eastward in Sulaibiya field. Total dissolved solids increased gradually to > 100,000 mg l–1 towards the east of the Kuwait Bay, north-east along the coastal line, and in the residential areas are located east of the Ahmadi anticline (Fig. 4). It is suggested that the presence of the structural element of Ahmadi anticline diverted the ground-water movement towards the north-east, making the ground-water in the residential areas stagnant and of low salinity. Therefore, the increase in TDS of the aquifer from south-west toward north and north-east may be attributed to the increased saturated thickness of the aquifer in this direction, to the presence of the Ahmadi structure, to the potentiometric level which reached near sea level, and to the nature of the marine environmental deposits of the aquifer sediments. In addition, and according to Chebotarev (1955), the salinity increases with time, and recharge–discharge area relations.
48°
47° Abdali
30° Rawdhatain
N
Bubiyan Island
IR
AQ
Umm Al-Aish
KUWAIT BAY
f tra
A
Residential Areas
Shagaya
A ab d
Fields
nm
Ur
SAUDI ARAB IA
K
B 29°
10 20 30 40 50 km
r
dai
Gu
0
Wafra
Legend Fresh ground-water well fields Brackish ground-water well fields South-western wells Northern wells Residential area of Kuwait
Figure 3. Location map of the study water fields.
ARABIAN GULF
Sulaibiya
E
Failaka Is.
W3-14 W5-4 W7-4994 WS11-46
AT-16 AT-19 AT-37 AT-40
Atraf
UG-1 UG-7 UG-15 UG-24
AB-9 AB-20 AB-35 AB-36
U-4 U-6 U-48 U-54
R-3 R-6 R-27 R-58
Well no.
Wafra
Umm-Gudair
Brackish water Abdali
Umm Al-Aish
Fresh water Rawdhatain
Well fields
7·50 7·35 7·45 7·50
7·20 7·20 7·70 7·60
7·70 7·60 7·50 7·50
7·00 7·40 7·20 7·40
7·90 8·00 8·00 7·80
7·50 7·60 7·70 7·60
pH
3753 4178 4442 3686
10,200 6600 4300 12,900
4790 4674 3637 3642
8320 4480 6464 5120
1086 403 1140 640
1737 773 1165 560
TDS
408·0 461·0 456·0 424·0
782·4 730·5 541·1 990·8
533·0 593·0 480·0 435·0
650·0 550·0 770·0 490·0
134·0 50·6 88·4 103·6
252·0 101·0 90·0 91·0
Ca2+
128·0 128·0 120·0 122·0
202·7 185·8 141·1 364·5
113·0 143·0 108·0 113·0
175·0 80·0 200·0 170·0
17·6 7·6 12·8 21·2
31·2 13·0 14·4 12·6
Mg2+
600·0 715·0 700·0 630·0
2163·2 1083·7 785·1 2402·8
710·0 640·0 625·0 510·0
2125·0 1145·0 1870·0 1460·0
140·0 65·0 230·0 65·0
190·0 120·0 255·0 30·0
Na+
15·2 15·0 14·3 14·5
96·3 63·5 44·9 162·4
16·0 16·5 15·0 16·0
40·0 25·0 21·0 20·0
4·0 3·0 6·5 4·0
6·5 5·0 5·0 4·0
K+
Table 3. Chemical analyses of the Kuwait Group aquifer (mg l –1)
144·0 107·0 103·0 132·0
143·0 144·0 140·0 250·0
61·0 70·0 70·0 78·0
339·0 165·0 122·0 122·0
131·3 157·0 133·2 203·0
129·8 191·7 165·4 204·9
HCO3–
1394 957 957 1218
2215 1682 1422 2706
1230 1065 1010 1020
2207 1544 3461 3461
380 86 375 108
713 260 375 108
SO2– 4
Water type
1026 1361 1361 1025
3120 2127 1538 4765
1617 1700 1238 1032
3578 1547 1994 1692
144 45 180 152
Na2SO4 NaCl NaCl NaCl
NaCl NaCl NaCl NaCl
NaCl CaCl2 NaCl Na2SO4
NaCl NaCl Na2SO4 Na2SO4
CaSO4 NaHCO3 Na2SO4 CaCL2
212 CaSO4 80 Na2SO4 152 Na2SO4 52 Ca(HCO3)2
Cl–
194 F. AL RUWAIH ET AL.
SM-10D-2 SM-09G-1 SM-11E-4 SM-03A
8·38 8·60 7·56 7·55
5·64 7·40 7·90 6·40
Residential areas Mishref MF-11D MF-16A MF-03A MF-05B
Shamiya
pH
Well no.
Well fields
12,200 8400 13,000 10,500
18,000 5800 3600 76,000
TDS
778·0 579·0 834·0 707·0
1057·0 627·0 614·0 3862·0
Ca2+
369·0 212·0 393·0 345·0
231·0 126·0 50·0 1340·0
Mg2+
Na+
2581·0 1458·0 2862·0 2174·0
3401·0 815·0 315·0 18660·0
Table 3. Continued
64·0 46·0 71·0 46·0
188·0 38·0 15·0 635·0
K+
148·0 135·0 153·0 149·0
104·0 499·0 59·0 414·0
HCO3–
2559 2290 3240 3200
1540 2559 1750 3178
SO2– 4
4291 2948 6183 3098
8665 700 418 40,416
Cl–
NaCl NaCl NaCl NaCl
NaCl Na2SO4 CaSO4 NnCl
Water type
WATER QUALITY IN KUWAIT 195
196
F. AL RUWAIH ET AL.
Water types relationships For each ground-water sample the dominant cations and anions expressed in meq 1–1 are determined for classification purposes. The water composition of the Kuwait Group aquifer can be categorized into four main groups namely: NaCl, CaSO4, Na2SO4, and CaCl2. CaSO4 and NaCl are the most abundant water types in the brackish ground-water fields, while Ca(HCO3)2 and NaHCO3 are the water types representing fresh water fields. The presence of these different water types are mainly due to the cation exchange process, where water quality changes from NaCl to CaCl2 water type. The reverse process takes place when fresh water flushes marine aquifer water, the Ca2 + is taken up from the water and Na + is retained forming NaHCO3 and Na2SO4 water types. The extension of chloride from NNE to the SW of the study area is due to the farthest distance from the naturally recharged area, and to the increased saturated thickness of aquifer along this direction. Trilinear diagram The Piper trilinear diagram (1944) constitutes a useful tool in water analysis interpretation. The diagram is used as a means of generally indicating similarities and
47°
48°
400
,0
0 ,00 0 0
50
00
40
6000
00
2000
2 30 000 00 0
N
3000
30°
4000 500 60 0 0 70 0 00
IR
AQ
20
10
10,000
6000
30
00
4000
500
0
ARABIAN GULF
00
10,0
KINGDOM O
29°
F SAUDI AR
ABIA 400
0
00
Contour interval (mg l )
30
1000
–1
Well location 0
10
20
30
40 km 5000
Figure 4. Areal distribution map of isosalinity of the Kuwait Group aquifer.
WATER QUALITY IN KUWAIT
197
differences in the composition of water from certain geologic and hydrologic units. The trilinear diagram of the water chemistry of the study fields is shown in Fig. 5. It is clear from this diagram that calcium sulphate and sodium chloride are the most dominant water types. All the brackish ground-water fields show a concentration of strong acid exceeding weak acid, and alkalies exceed alkaline earth’s. While the fresh water samples exhibited a concentration of strong acids exceeding weak acids, but alkaline earth’s exceed alkalies. While the residential areas showed that the alkalis (Na + + K + ) exceed the alkaline earth’s and the non-carbonate alkali exceed 50%, that is, the chemical properties are dominated by alkalis and strong acids, usually ocean water and brines are related to this area of plot. The Durov diagram has been constructed in Fig. 6 to show the chemical water composition and prevailing chemical processess of the Kuwait Group aquifer. Most of the brackish water fields exhibited a dissolution or mixing process and water is related to the reverse ion exchange of Na + –Cl– water. The dominant of Cl– and Na + indicated an end-point water (Lloyd & Heathcote, 1985). In the residential areas Na + and Cl– are the dominant ions. However, the fresh water fields show a high concentration of SO2– 4 indicating a probable mixing influence of the aquifer material with water of meteoric origin, and some samples show a dominance of 2+ SO2– which may be due to simple mixing or dissolution process. 4 and Ca
50
Mixing
2+
Fresh
g
50
–M
SO 2– 4 – C
2+
Ca
l ––
3
NO –
Fresh water fields Brackish water fields Residential areas
Sea
2+
O–
3
HC
50
Ca2+
Figure 5. Ground-water composition using Piper diagram.
50
(Na+ + K+)
50
Cl– – NO–3
2–
+
50
SO 4
+K
M
+
50
50
Ca2+
Na
g 2+
Mg
198
F. AL RUWAIH ET AL.
Chemical activities The chemical activity of an ion is equal to the molal concentrations times the activity coefficient, α = γM, where α is the chemical activity, M is the molal concentration and γ is the activity coefficient. Once the ionic strength of a solution of electrolysis is known, the activity coefficient of the individual ion can be computed from the Debye-Huckel equation (Fetter, 1994) provided that some parameters in this equation are taken from tables for any specific ions. The ion activity product (Kiap), which is the product of the measured activities, is calculated for the water samples to test for saturation. The value of Kiap for a mineral may be compared with the value of the solubility product of that mineral (Ksp). In order to avoid this long procedure, a computer program WATEQ4F (Ball & Nordstorm, 1992) was utilized in this study to calculate the ionic strength, chemical activity, activity coefficients Ksp and Kiap and saturation index (SI) for about 167 samples of different locations of the Kuwait Group aquifer. Table 4 indicates the logarithms of ion activity product for calcite, anhydrite, gypsum, halite, aragonite and dolomite. Comparison of the activity products with respective logarithmic solubility products (Ksp) of the ions provides an indication of equilibrium or non-equilibrium as undersaturation or oversaturation. If Kiap/Ksp is greater than 1, the solution is supersaturated, if less than 1, it is undersaturated and if equal to 1, the solution is in equilibrium. Furthermore, the ionic activity products have been computed for aragonite, dolomite and halite. Accordingly, it is found that the Kuwait Group aquifer Mg2+ 50%
Ca2+ 25% Mg2+ 25%
Ca2+ 50%
1
Ca2+ 25% + Na 25%
Na+ 25%
2
3
M
HCO–3 25% – Cl 25%
Ca2+ 25% + Na 25%
6
E
PL LU SO
IS D
HCO–3 25% Cl– 25%
N O
5
G
IN
IX
4
R
Cl– 25% SO42– 25%
O
TI
SO42– 50%
Na+ 25% Mg2+ 25%
M SI
HCO–3 25% SO42– 25%
– HCO3 50%
Fresh water fields Brackish water fields Residential areas
REVERSE ION EXCHANGE
8
9
Cl– 50%
7
Figure 6. Durov diagram showing the results of chemical analyses of ground-water of the Kuwait Group aquifer.
Atraf
Wafra
Umm-Gudair
Brackish water Abdali
Umm Al-Aish
Fresh water Rawdhatain
Well fields
AB-9 AB-20 AB-35 AB-36 UG-1 UG-7 UG-15 UG-24 W3-14 W5-4 W7-4994 WS11-46 AT-16 AT-19
U-4 U-6 U-48 U-54
R-3 R-6 R-27 R-58
Well no.
–0·345 –0·414 –0·114 –0·35 –0·483 –0·512 –0·568 –0·584 –0·275 –0·331 –0·445 –0·156 –0·516 –0·62
–1·165 –2·004 –1·529 –1·717
–0·771 –1·382 –1·334 –1·719
Anhydrite CaSO4
0·207 0·299 –0·116 0·007 0·194 0·211 0·049 0·063 0·114 0·149 0·544 0·809 0·264 0·066
0·364 0·166 –0·608 0·433
0·148 0·156 0·091 0·211
Aragonite CaCO3
0·351 0·443 0·027 0·154 0·338 0·354 0·192 0·207 0·258 0·293 0·683 0·953 0·404 0·209
0·508 0·314 –0·465 0·577
0·292 0·344 0·235 0·355
0·484 0·391 –0·193 0·179 0·346 0·438 0·081 0·171 0·279 0·337 1·126 1·822 0·643 0·208
0·473 0·142 –6·534 0·813
0·013 0·051 0·014 0·198
–0·129 –0·196 0·103 –0·132 –0·265 –0·293 –0·349 –0·365 –0·058 –0·113 –0·226 0·064 –0·297 –0·402
–0·945 –1·784 –1·309 –1·497
–0·551 –1·162 –1·115 –1·499
Phase / Chemical formula Calcite Dolomite Gypsum CaCO3 CaMg(CO3)2 CaSO4.2H2O
Table 4. Saturation indices of the Kuwait Group aquifer
–3·871 –4·461 –4·182 –4·339 –4·647 –4·671 –4·807 –4·967 –3·928 –4·369 –4·634 –3·944 –4·908 –4·714
–6·331 –7·109 –8·674 –6·601
–6·037 –6·621 –6·034 –7·391
Halite NaCl
2·60E-02 5·10E-03 7·30E-03 4·29E-03 9·51E-04 1·36E-03 1·77E-03 2·00E-03 6·79E-03 6·99E-03 2·17E-03 4·50E-03 3·70E-03 3·84E-03
1·48E-03 1·89E-03 1·27E-03 2·96E-03
3·60E-03 4·47E-03 3·01E-03 4·86E-03
PCO2
WATER QUALITY IN KUWAIT 199
Shamiya
Residential areas Mishref
Well fields
MF-11D MF-16A MF-03A MF-05B SM-10D-2 SM-09G-1 SM11E-4 SM-03A
Well no.
–0·405 –0·202 –0·243 –0·053 –0·287 –0·344 –0·207 –0·216
Anhydrite CaSC4 –1·481 0·778 0·419 0·273 1·182 1·254 0·447 0·384
Aragonite CaCO3 –1·337 0·922 0·562 0·417 1·326 1·398 0·591 0·528
–2·965 1·484 0·367 0·806 2·681 2·704 1·208 1·088
–0·191 0·017 –0·024 –0·238 –0·071 –0·127 0·008 0·001
Phase / Chemical formula Calcite Dolomite Gypsum CaCO3 CaMg(CO3)2 CaSO4.2H2O
Table 4. Continued
–3·319 –4·967 –5·574 –1·924 –3·727 –4·113 –3·539 –3·938
Halite NaCl
1·74E-01 1·59E-02 5·84E-04 1·09E-01 3·70E-04 2·02E-04 3·03E-03 3·10E-03
PCO2
200 F. AL RUWAIH ET AL.
WATER QUALITY IN KUWAIT
201
is undersaturated with respect to anhydrite, gypsum and halite and supersaturated with respect to aragonite, calcite and dolomite. The calculated mean values of PCO2 for the fresh and brackish water fields in the study areas are 4 3 10–3 and 5 3 10–3 atm, respectively, which is considerably less above the PCO2 of the earth atmosphere (10–3.5 atm). This indicates that the groundwater in the Kuwait Group aquifer became charged with CO2 during infiltration trough soil zones (Freeze & Cherry, 1979).
Multivariate statistical analysis Correlation and cluster analysis were carried out for the 167 samples. The standardized m-space Euclidian distance is used as a method of clustering (Davis, 1986). A low distance indicates that two objects are similar or close together, whereas a large distance indicates dissimilarity. The correlation matrix of non-transformed data of the variables and their mean and standard deviation is displayed in Table 5. It is obvious that the TDS is in good correlation with all variables except the ionic concentration of HCO–3 and SO2– 4 , which suggested that water of meteoric origin and the recharge process is not local, and the bicarbonate is precipitated as carbonate along the recharge route. Ca2 + should be of oceanic origin and this is evident from the relatively high correlation coefficient between Cl– and Ca2 + . The elements Cl–, Na + , K + and Mg2 + are strongly associated forming what can be called a cluster of seawater components (Eriksson, 1985). Cluster analysis of the non-transformed data input of 134 cases of the Kuwait Group aquifer using the hydrochemical compositions of nine variables is shown in Fig. 7 which shows fair similarity level between cases, where one independent case and three main cluster are recognized. Cluster I includes cases of the ground-water fields of Wafra, Umm-Gudair, Atraf, and Abdali with one sample from the residential area of Shamiya (case No. 68). The cluster shows a range of total dissolved solids of 3372–7900 mg l–1 and represents a brackish ground-water of NaCl type. Cluster II includes the cases of the fresh ground-water fields of Rawdhatain and Umm Al-Aish where the salinity ranges from 359–1737 mg l–1 and the water types are Na2SO4, Ca(HCO3)2 and NaHCO3 of meteoric origin, and the ratio of SO4/Cl ranges from 0·52 to 4·45. Cluster III includes all cases from Mishref and Sabah Al-Salem residential areas where the total dissolved solids ranged from 10,000–115,000 mg l–1 represents saline– brine ground-water. The ratio of SO4/Cl ranges from 0·02 to 0·54 which is close to the value of seawater. Therefore, it is obvious that the ground-water samples of the Kuwait Group aquifer of different water well fields have been clustered as fresh, brackish, and saline–brine ground-water based on the total mineralization and on the water types.
Conclusions The hydrochemical results indicate that the Kuwait Group aquifer is occupied by a brackish ground-water with TDS ranging from 3372–7900 mg l–1 in Wafra, UmmGadair, Atraf and Abdali and of NaCl water type, and in Sahmiya the TDS ranges from 7800–13,000 mg l–1. A fresh ground-water with TDS ranging from 359–1737 mg l–1 is located in the northern country at Rawdhatain and Umm Al-Aish fields, and is composed of Na2SO4, NaHCO3 and Ca(HCO3)2 water types. In addition, saline–brine ground-water is present in the residential areas of Mishref and Sabah Al-Salem where the TDS ranges from 10,00–115,000 mg l–1 of NaCl water
pH TDS K+ Na+ Mg2+ Ca2+ Cl– SO2– 4 HCO3–
Standard mode
+1·00 –0·37 –0·44 –0·41 –0·39 –0·40 –0·40 –0·13 –0·17
pH
+1·00 +0·97 +0·97 +0·96 +0·93 +0·97 +0·32 +0·39
TDS
+1·00 +0·99 +0·98 +0·95 +0·98 +0·31 +0·41
K+
+1·00 +0·99 +0·95 +0·99 +0·32 +0·39
Na+
+1·00 +0·94 +0·99 +0·32 +0·39
Mg2+
+1·00 +0·96 +0·27 –0·38
Ca2+
+1·00 +0·27 +0·39
Cl–
+1·00 +0·18
SO2– 4
+1·00
HCO3–
7·44 16,548·72 138·20 4183·07 336·34 1019·30 8030·43 1740·07 174·57
Mean
Table 5. Correlation matrix of non-transformed data input of the hydrochemical variables of the Kuwait Group aquifer
0·51 24,639·29 226·39 6719·82 439·03 1254·11 13,835·17 1159·96 151·82
Standard deviation
202 F. AL RUWAIH ET AL.
WATER QUALITY IN KUWAIT Hierarchical Tree (Dlink/Dmax) ´ 100 Case no. 1–10–20–30–40–50–60–70–80–90–100
Abdali
13–25
Rawdhatain
26–33
Umm Al-Aish
34–37
North wells
38–54
Wafra
55–67
Atraf
68–71
Shamiya
72–77
Mishref
78–115
Umm-Gudair
116–134
Sabah Al-Salem
Cluster I
1–12
Cluster III
80 73 89 63 64 98 91 60 78 84 93 48 105 34 40 10 67 3 46 11 54 12 36 73 13 27 15 17 23 29 30 33 24 29 13 22 16 19 25 21 31 25 22 20 14 41 69 70 123 52 71 134 75 117 119 124 129 133 72 120 121 127 132 116 74 130 125 131 122 123 76 118 124 77
Case No.
Cluster II
1 44 38 45 49 37 39 51 43 42 47 50 53 68 7 8 6 2 4 9 5 35 55 56 66 57 83 115 110 114 54 61 85 82 104 101 107 111 100 103 109 112 113 97 59 62 81 105 85 100 99 92 58 89 60 87 90 95 108 99
Figure 7. Cluster analysis of the ground-water composition of the Kuwait Group aquifer.
203
204
F. AL RUWAIH ET AL.
type. Salinity ranges from 3000–10,000 mg l–1 for these water fields located before the Ahmadi anticline. The TDS increases remarkably for the residential areas which are located after the Ahmadi ridge, and to the east and near the seashore and ranged from 15,000–150,000 mg l–1. The concentration of strong acids exceeds weak acids, and the alkaline exceeds the alkaline earth’s, while in the fresh water fields the alkaline earth’s is the dominant. The ground-water exhibited simple mixing, dissolution and reverse ion exchange processes. The aquifer is almost supersaturated with respect to calcite, aragonite and dolomite, and undersaturated with respect to anhydrite, and gypsum and halite. The calculated PCO2 of the Kuwait Group aquifer indicated that the ground-water became charged with CO2 during infiltration processes. Multivariate statistical analyses show that the ground-water samples have been clustered according to the TDS and the water types. The authors wish to express the appreciation to the Ministry of Electricity and Water for providing the chemical data. Thanks are also extended to the Kuwait University, Scientific Research Unit for the financial support as Grant No. SG036.
References Ball, J.W. & Nordstorm, D.K. (1992). Geochemical model to calculate speciation of major, trace and redox elements in natural waters. U.S. Geological Survey. International Ground Water Modelling Centre. 189 pp. Chebotarev, I.I. (1955). Metamorphism of natural waters in the crust of Weathering. Geochemical Cosmochimal Acta, 8: 22–48, 137–170, 198–212. Davis, J.C. (1986). Statistics and Data Analysis in Geology (2nd Edn). Canada: John Wiley and Sons. 635 pp. Eriksson, E. (1985). Principles and Applications of Hydrochemistry. U.S.A.: Chapman and Hall. 187 pp. Fetter, C.W. (1994). Applied Hydrogeology. U.S.A.: Macmillan College Publishing Company. 691 pp. Freeze, R. & Cherry, J. (1979). Groundwater. Englewood Cliffs, NJ: Prentice-Hall. 604 pp. Fuchs, W., Gattinger, T.E. & Holzer, H.F. (1968). Explanatory test to the synoptic geologic map of Kuwait. A surface Geology of Kuwait and the Neutral zone. Vienna: Geological Survey of Austria. 87 pp. Kruseman, G.P. & De Ridder, N.A. (1970). Analysis and evaluation of pumping test data. Wageningen, Netherlands: International Institute for Land Reclamation and Imprisonment, Bill. 11. 200 pp. Lloyd, J.W. & Heathcote, J.A. (1985). Natural Inorganic Hydrochemistry in Relation to Groundwater. Oxford: Clarendon Press. 296 pp. Milton, D.I. (1967). Geology of the Arabian Peninsula-Kuwait. United States Geological Survey of Professional paper, 560-F. Washington: United States Government Printing Office. 7 pp. Mitchell, R.C. (1958). Notes on the Geology of Western Iraq and Northern Saudi Arabia. Geol. Rundschau, 46: 467–493. Owen, R.M. S. & Nasr, S.N. (1958). Stratigraphy of Kuwait-Basra area, in Habit of Oil. American Association of Petroleum Geologists Bulletin, 42: 1252–1278. Piper, A.M. (1944). A graphic procedure in the geochemical interpretation of water analysis. Transactions of the American Geophysical Union, 25: 914–923.