Cases and solutions

Hydrochemistry of aquifers in the southern Dead Sea area, southern Jordan N. S. Abu-Jaber 7 N. A. Wafa

Abstract The demand for water resources in the area south of the Dead Sea due to continued development, especially at the Arab Potash Company (APC) works necessitates that water quality in the area be monitored and evaluated based on the local geology and hydrogeology. The objective of this paper is to provide information on the past and present status of the main aquifers under exploitation or planned for future development. Two main aquifers are discussed: the Safi water field, presently being operated, and the Dhiraa water field, which is being developed. The aquifer developed in the Safi water field is shallow and fed by the Hasa fault system, which drains a significant portion of the Karak mountains. This aquifer seems to be well replenished within the core, where no obvious long-term degradation in water quality can be identified. However, in the low recharge areas within the distal portions of the alluvial fan, there has been a degradation in water quality with time. The degradation is caused by the dissolution of the Lisan Marl, which is present at the outskirts of the fan system, based on hydrochemistry of water in the wells. The Dhiraa field is a deep (800–950 m) aquifer drilled specifically for the extraction of brackish water present in the Kurnub aquifer. Available data indicate that there are at least three distinct water types within this field. These water types are variable in quality, and there may be potential for mixing of these waters, thus affecting the quality of the freshest waters presently available. Tritium and oxygen isotope analysis indicate that the water is old and possibly nonrenewable. Key words Jordan 7 Dead Sea 7 Hydrochemistry 7 Water quality

Received: 24 July 1995 7 Accepted: 26 September 1995 N. S. Abu-Jaber (Y) Department of Earth and Environmental Sciences, Yarmouk University, Irbid, Jordan N. A. Wafa Projects Department, Arab Potash Company, Ghor al Safi, Jordan

Introduction Due to the increasing needs of the Arab Potash Company (APC) for water for production and domestic purposes, and the arid nature of the area of potash production, optimal development of all possible water resources in the area is needed to sustain and increase productivity. Well fields in the area have been developed in order to keep up with increasing demands for water. However, it is very important that these water resources be managed in a way that maintains the quality of water from these fields over the longest period. The large-scale development of the region anticipated in the near future will further strain the water resources of the area. The potential for water-quality degradation in the area is high due to a number of factors, including salt-water intrusion from the Dead Sea, dissolution of evaporite minerals in the playa sediments of the area, the presence and intrusion from other high-salinity waters in the subsurface (Vengosh and Rosenthal 1994), as well as the depletion of the water caused by over exploitation. The Dead Sea occurs at the lowest point of the Jordan Valley rift system (about 405 m below mean sea level). The rift system defines the boundary between the Arabian crustal plate to the east and the African crustal plate to the west. Rifting in the area began during the Eocene– Oligocene and continues (Atallah 1992). The tectonic nature of this rift has been extensively discussed (e.g., Quennell 1958; Freund 1965; Garfunkel 1981; Atallah 1992). The movement along this left-lateral strike-slip fault since its origin has been about 107 km. The thickness of the sediments in the Dead Sea region is as much as 12 km, consisting both of prerift and postrift sediments. The prerift sediments contain the stratigraphic sequence present throughout Jordan (Fig. 1). These include Triassic and Jurassic limestones, sandstones, and evaporites (gypsum); Early Cretaceous quartzose sandstone (probably the main aquifer in the Dhiraa area); Middle Cretaceous limestone; and Late Cretaceous and Early Tertiary silicified limestone, phosphorite, and bituminous limestone (Khalil 1992). Postrift sediments include the late Oligocene to Miocene Dana conglomerate, which consists of about 125 m of conglomerate, sandstone, and detrital calcarenite (Khalil 1992). Overlying the Dana conglomerate is the Lisan Formation that consists of about 25 m of alternating laminations of aragonite and clay minerals

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213

Cases and solutions

Fig. 2 Location map of the studied aquifer fields

Fig. 1 General stratigraphic column in the southern Dead sea area (from Powell 1988)

with a number of gypsum horizons (Abed 1985). The Lisan Marl contains a gravel facies at the mouths of the local wadis, such as the Wadi al Hassa. The alluvial deposits of the Lisan Marl probably represent the main source of water in the Ghor al Safi area. The Dead Sea is a large (about 1000 km 2 in 1959, before massive diversion of the headwaters) hypersaline lake (300–350 g l P1 TDS). The area of the Dead Sea is divided into two distinct subbasins: the northern basin, which occupies about 750 km 2, and the southern basin, which occupies about 250 km 2 (Abed 1985). The northern basin reaches depths of about 400 m, whereas the southern basin is now exposed and artificially filled through pumps to the APC and the Israeli Dead Sea Works evaporation ponds in order to extract the valuable dissolved minerals of the Dead Sea. The southern basin became exposed during the 1980s due to the extensive exploitation of the fresh waters that feed the Dead Sea, i.e., the headwaters of the Jordan River, by Israel and Jordan. This diversion has led to an imbalance between input and removal due to evaporation, and thus a decline in the Dead Sea by about 10 m since 1960. This imbalance has also led to the 214

Environmental Geology 28 (4) December 1996 7 Q Springer-Verlag

Fig. 3 Map of the Safi field including well locations and topography. Note contours are meters below mean sea level. Palestine grid used

overturning of the previously stratified Dead Sea water in 1980 (Abed 1985). In this paper two aquifers in the southern Dead Sea area adjacent to the southern basin will be evaluated based on hydrochemical factors and their relationship to potential

Cases and solutions

degradation of water quality in the area. The Ghor el Safi field (Figs. 2 and 3) obtains water from a well-developed shallow aquifer south of the Dead Sea from which a significant amount of potable water is now produced (about 800 m 3 h P1 or about 6 MCM yr P1). The Dhiraa field (Fig. 2) lies farther to the north of the Ghor el Safi field and is in the initial stages of development. The Dhiraa field produces from a deep Cretaceous aquifer with different hydrochemical properties and different potential sources of pollution. Chemical properties of water from the two fields were determined from the records of the APC and are presented within the context of the geology and hydrogeology of the region. The data also include recent analyses of these waters as well as historical data showing variations and long-term trends in water quality.

1. The areal distribution of salinity is shown in Fig. 4. The interpretation of the data indicates a number of trends. The core of the alluvial fan contains the water with the lowest total dissolved solids. The quality of the water quickly changes away from the core of the fan, reaching values of 1200 ppm TDS away from the core both at the distal parts as well as the proximal areas. This degradation of water quality away from the core of the fan seems to indicate that as the amount of fan material decreases, the water in the aquifer is in longer contact with the surrounding sediments, i.e., the main Lisan Marl facies. The Lisan Marl contains soluble evaporite minerals such as aragonite and gypsum, which may easily dissolve into the groundwater in contact with it. When the long-term trends in water quality are considered, two distinct types of water-quality-characteristic wells are seen to be present in the area. In the core of the fan, water quality since the beginning of production has not changed much, with variations reflecting seasonal The Ghor el Safi field changes in input. Figure 5 shows the long-term trend in water quality at well BN309. The second type of well This field lies to the south of the southern basin and co- away from the core of the aquifer shows definite longvers an area of about 12 km 2 (Figs. 2 and 3). This field term degradation in water quality, probably due to the developed water from a shallow aquifer that consists of a pumping of higher salinity older water, which has partialPleistocene alluvial fan deposit as described by Powell ly dissolved the gypsum and anhydrite from the Lisan (1988). The alluvial fan was formed at the mouth of Wadi Formation. Well SPB5 is an example of this type of well el Hassa, which drains a large portion of the Karak high- (Fig. 6). lands and presently runs through the Ghor el Safi alluvial The degradation of water quality in the distal parts of the fan. Wadi el Hassa is a fault-controlled river with an aquifer can be attributed to either intrusion of Dead Sea WSW orientation until it reaches the Safi area, where it water or to the presence of old water with high salinities abruptly changes direction and runs to the NNW until it within or near the aquifer itself. The geochemical signareaches the Dead Sea (Powell 1988; Atallah 1992). tures of the water analyzed indicate the latter possibility. The average annual rainfall in the Ghor el Safi area is Typical Dead Sea water concentrations are shown in Taonly about 73 mm yr P1 (Powell 1988). The recharge area ble 2. A number of inferences can be made when looking includes the Karak highlands, which has about 3 to 4 at these data. The Mg/Ca weight ratio in Dead Sea waters times the average rainfall in the Safi area. The Wadi el is consistently greater than 2. When one considers the Hassa drainage basin covers an area of 2370 km 2 (Parker Mg/Ca weight ratio in a well such as SPB5, which shows 1970), and the annual baseflow of Wadi el Hassa is about degradation with time, this ratio is about 0.4. If Dead Sea 25 MCM, most of which is pumped away before it water were intruding into the Safi aquifer, then the Mg/ reaches the alluvial fan. However, as noted by Parker Ca signature should be reflected in the aquifer waters. (1970), the Hassa fault is a zone of very high permeabili- The low Mg/Ca ratio indicates that the source of solutes ty, and thus the base flow does not reflect the total in the Safi aquifer is the dissolution of soluble minerals amount of water that may be drained from the highlands that contain calcium, such as gypsum, anhydrite, calcite, into the Safi alluvial fan aquifer. Channeling of subsuror aragonite. face water through the Hassa fault system is probably the The contrast in the HCO3/Cl weight ratio between the Dead Sea and the Safi aquifer is even more stark. This major source of recharge to the Safi aquifer. In the Safi aquifer, the top of the water table in the distal ratio is around 2 in the Safi aquifer, whereas in the Dead Sea it is about 1!10 P3. The hardness of the water in the areas of the alluvial fan ranges from 387 to 389 m below Safi aquifer can also be attributed to the dissolution of mean sea level. This range is about 10–15 m higher than calcite and/or aragonite. There is a direct correlation bethe present level of the Dead Sea, which is about 405 m tween the hardness and salinity of the Safi aquifer below mean sea level. At the proximal areas of the fans (Fig. 7). the water table ranges from 364–369 m below mean sea The ratio of sodium and chlorine is mostly clustered level and is 15–20 m higher than the distal areas. This confirms the suggestion that the major source of water is around 0.4–0.5, which is closer to the signature reflected subsurface flow through the Hassa fault system. The wa- by marine aerosols than either the dissolution of halite (expected Na/Cl ratio of 0.66) or the effect of Dead Sea ter table is highest at the end of the fault system, where water, which has a Na/Cl ratio of about 0.17 (Vengosh the course of the wadi turns towards the north (Fig. 3). Chemical data based on results of analyses of water sam- and Rosenthal 1994). ples collected from the entire field are presented in Table

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Cases and solutions

Table 1 Chemical data from the Safi field Sample

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

216

Well

SPB1 SPB1 SPB1 SPB1 SPB1 SPB1 SPB1 SPB1 SPB1 SPB1 SPB2 SPB2 SPB2 SPB2 SPB2 SPB2 SPB2 SPB2 SPB2 SPB2 SPB3 SPB3 SPB3 SPB3 SPB3 SPB3 SPB3 SPB3 SPB3 SPB3 SPB4 SPB4 SPB4 SPB4 SPB4 SPB4 SPB4 SPB4 SPB4 SPB4 SPB5 SPB5 SPB5 SPB5 SPB5 SPB5 SPB5 SPB5 SPB5 SPB5 SPB6 SPB6 SPB6 SPB6 SPB6 SPB7 SPB7 SPB7 SPB7 SPB7

Date

04. 04. 05. 12. 12. 12. 12. 12. 06. 04. 04. 04. 05. 12. 12. 12. 12. 12. 06. 04. 04. 04. 05. 12. 12. 12. 12. 12. 06. 04. 04. 04. 05. 12. 12. 12. 12. 12. 06. 04. 04. 04. 05. 12. 12. 12. 12. 12. 06. 04. 12. 12. 12. 06. 04. 12. 12. 12. 06. 04.

83 84 84 85 86 90 91 93 94 95 83 84 84 85 86 90 91 93 94 95 83 84 84 85 86 90 91 93 94 95 83 84 84 85 86 90 91 93 94 95 83 84 84 85 86 90 91 93 94 95 90 91 93 94 95 90 91 93 94 95

Conductivity 1060.000 950.000 950.000 1050.000 1150.000 1110.000 1123.000 1277.000 1390.000 1394.000 1100.000 1050.000 1020.000 950.000 1150.000 1120.000 1159.000 1216.000 1228.000 1131.000 1000.000 950.000 950.000 980.000 1050.000 932.000 918.000 986.000 961.000 977.000 1040.000 980.000 980.000 1100.000 1050.000 1055.000 1104.000 1305.000 1361.000 1362.000 1020.000 1000.000 990.000 1200.000 1250.000 1410.000 1424.000 1641.000 1729.000 1766.000 759.000 773.000 810.000 828.000 793.000 987.000 1190.000 1940.000 2066.000 802.000

pH

8.120 7.720 7.340 7.330 7.300 8.120 7.520 7.890 7.360 7.460 8.100 7.600 7.480 7.440 7.300 7.900 7.480 7.560 7.560 7.560 8.050 7.800 7.580 7.580 7.200 8.240 7.720 7.650 7.810 7.530 8.080 7.780 7.500 7.680 7.200 8.150 7.670 7.910 7.510 7.550 8.180 7.600 7.640 7.460 7.400 7.980 7.850 7.880 7.520 7.450 8.360 7.710 7.850 7.950 7.880 8.140 7.420 7.550 7.970 7.500

Environmental Geology 28 (4) December 1996 7 Q Springer-Verlag

Concentration (mg l P1) TDS

CaCO3

530.00 571.000 532.00 588.00 678.500 621.00 628.900 753.000 820.000 822.400 550.000 589.000 571.000 532.000 678.500 627.000 649.000 717.000 725.000 667.300 500.000 536.000 532.000 548.000 619.500 521.000 514.100 582.000 567.000 576.400 520.000 563.000 549.000 616.000 619.500 590.000 618.200 770.000 803.000 803.600 510.000 591.000 554.000 672.000 737.500 789.000 797.400 968.000 1020.000 1041.000 425.000 432.900 478.000 489.000 467.900 552.700 666.400 1144.000 1219.000 473.200

381.000 337.000 354.000 366.000 367.100 396.000 330.000 446.000 474.000 486.800 367.000 396.000 390.600 382.800 387.800 412.500 396.000 425.000 433.000 350.600 332.000 333.000 346.000 341.600 342.400 358.900 367.100 330.000 318.000 321.800 342.000 343.000 367.000 341.600 371.300 379.500 420.700 433.000 454.000 449.600 334.000 346.000 343.000 362.600 379.500 474.400 515.600 532.000 553.000 548.600 279.000 313.500 276.000 281.000 247.500 334.100 433.100 577.000 611.000 255.800

Na

Ca

Mg

160.000 174.000 171.000 201.000 200.000

115.500 129.000 128.700

37.800 37.000 40.000

80.500 74.000 77.000 81.000 86.000 83.000 74.000

152.000 174.000 157.000 167.000 140.000

104.000 114.000 87.800

40.000 36.000 32.000

82.200 73.200 69.000 67.000 72.000 85.000 63.000

140.000 162.400 128.000 131.000 124.000

79.000 86.000 79.200

32.000 25.000 30.000

66.800 72.000 76.000 89.000 83.000 94.000

180.000 185.600 215.000 219.000 198.000

102.000 119.000 112.200

43.000 38.000 41.000

86.100 82.000 105.000 109.000 125.000 129.000 128.000 52.000 53.000 56.000 54.000 51.000 69.000 86.000 146.000 158.000 52.000

240.000 266.800 270.000 291.000 306.000 100.000 127.600 102.000 101.000 100.000 168.000 266.800 437.000 498.000 110.000

140.000 149.000 128.700

44.000 44.000 55.000

74.000 76.000 66.000

22.000 22.000 20.000

132.000 144.000 62.700

60.000 61.000 24.000

76.900 71.600 75.000 80.000 87.000 88.000 86.000

Cl

125.600

Cases and solutions

Table 1. (Continued) Sample

61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106

Well

SPB8 SPB8 SPB8 SPB8 SPB8 SPB9 SPB9 SPB10 SPB10 SPB10 SPB11 SPB11 SPB11 SPB12 SPB12 SPB12 SPB13 SPB13 SPB14 SPB14 SPB15 SPB16 SPB16 S1 S1 S1 S1 S1 S1 S2 S2 S2 S2 S2 S2 S2 S2 BN309 BN309 BN309 BN310 BN309 BN309 BN309 BN309 BN309

Date

12. 12. 12. 06. 04. 06. 04. 12. 06. 04. 12. 06. 04. 12. 06. 04. 06. 04. 06. 04. 04. 06. 04. 12. 12. 12. 12. 12. 06. 04. 12. 12. 12. 12. 12. 06. 04. 04. 04. 05. 12. 12. 12. 12. 06. 04.

90 91 93 94 95 94 95 93 94 95 93 94 95 93 94 95 94 95 94 95 95 94 95 85 86 90 91 93 94 83 86 86 90 91 93 94 95 83 84 84 86 90 91 93 94 95

Conductivity 1530.000 1463.000 1415.000 1502.000 1592.000 905.000 1000.000 1188.000 1308.000 1324.000 927.000 1127.000 1240.000 918.000 954.000 979.000 1165.000 1367.000 1935.000 1816.000 1069.000 1514.000 1486.000 1000.000 950.000 903.000 927.000 995.000 1197.000 840.000 1050.000 850.000 751.000 786.000 800.000 872.000 858.000 980.000 950.000 980.000 1600.000 850.000 907.000 961.000 711.000 1003.000

pH

8.060 7.500 7.500 7.990 7.430 8.240 7.810 7.770 7.730 7.600 8.020 8.050 7.350 8.010 8.000 7.920 7.520 7.520 7.790 7.400 7.800 7.720 7.680 7.450 7.600 8.150 7.760 7.760 7.990 8.060 7.420 7.500 8.290 7.880 8.050 7.790 7.940 7.640 7.600 7.600 7.000 8.150 7.700 7.640 8.280 8.130

Concentration (mg l P1) TDS

CaCO3

Na

Cl

Ca

Mg

856.800 819.300 835.000 886.000 939.300 534.000 590.000 701.000 772.000 781.200 547.000 665.000 731.600 542.000 563.000 577.600 687.350 806.500 1141.650 1071.440 630.700 892.000 876.700 560.000 560.000 505.700 519.120 587.000 706.000 420.000 588.000 501.500 420.600 440.160 472.000 514.000 506.200 490.000 597.000 544.000 944.000 476.000 507.900 567.000 419.000 591.800

482.600 482.600 417.000 454.000 466.100 309.000 321.800 363.000 408.000 391.900 289.000 367.000 614.600 305.000 309.000 330.000 392.000 470.300 548.000 486.800 317.600 478.000 416.600 300.500 321.800 309.400 367.100 330.000 388.000 284.000 296.400 272.300 268.100 313.500 272.000 281.000 280.500 345.000 363.000 357.700 561.000 309.400 379.500 413.000 182.000 301.100

138.000 125.000 120.000 121.000 124.000 53.000 62.000 95.000 100.000 99.000 76.000 79.000 82.000 64.000 68.000 68.000 79.000 89.000 170.000 158.000 82.000 117.000 114.000 73.200 63.600 66.000 69.000 66.000 80.000

280.000 290.000 244.000 267.000 278.000 113.000 152.000 182.000 211.000 226.000 117.000 155.000 170.000 120.000 131.000 146.000 161.000 178.000 414.000 374.000 170.000 241.000 250.000

96.000 124.000 108.900 86.000 72.600 78.000 107.000 94.100 64.000 97.000 95.700 66.000 79.000 74.300 99.000 110.600 139.000 115.500 70.700 132.000 99.000

43.000 35.000 47.000 23.000 34.000 41.000 34.000 38.000 31.000 34.000 43.000 34.000 27.000 35.000 35.000 47.000 49.000 48.000 34.000 36.000 41.000

140.000 150.800 138.000 197.000

76.000 99.000

34.000 34.000

60.500 50.000 53.000 53.000 56.000 55.000 54.000

104.000 116.000 102.000 123.000 120.000

66.000 74.000 69.300

26.000 23.000 26.000

103.600 59.000 63.000 65.000 61.000 69.000

112.000 139.200 120.000 143.000 140.000

78.000 36.000 72.600

53.000 22.000 29.000

The al Dhiraa field The al Dhiraa water field underlies a large plain of the same name. The Dhiraa plain lies at the southeastern corner of the northern basin of the Dead Sea (Fig. 8). It is bounded on the north by Wadi Ibn Hammad, on the south by Wadi el Karak, and on the east by a prominent highland of Upper Cretaceous sedimentary rocks in the form of a monocline in the north and a splay fault in the

south (Khalil 1992). The Amman silicified limestone and the Wadi Ghudran chalk unit outcrop in the north central part of the plain, whereas the rest of the area is covered by the Pleistocene Lisan Marl gravel facies (Khalil 1992). The Dhiraa is affected by the presence of three major fault trends. These are the major N–S trend mainly manifested by the Dead Sea fault; the NE–SW trend represented by the Siwaqa fault, and a number of NW–SE faults that directly affect the wells that have been drilled

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Cases and solutions

Fig. 5 Time series trend of salinity variation in well BN309

Fig. 4 Isosalinity map of the Safi field in June 1994

in the area (Khalil 1992). The presence of these faults has caused a number of problems, including abandonment of a well before completion due to circulation loss. Other problems occurring during drilling included the presence of pockets of overpressurization as well as stuck drill strings. Another potential problem during drilling was the potential mixing of waters from various sources. There is chemical evidence of water mixing in this area, as will be shown.

Fig. 6 Time series trend of salinity variation in well SPB5

The Dhiraa water field has been tapped for deep brackish waters emanating from the Lower Cretaceous Kurnub sandstone. Every effort was made to preclude exploitation of the fresher waters in the overlying Upper Cretaceous aquifers due to the specific needs of the APC as well as the fear of depletion and contamination of the overlying aquifers. The possibility of cross-contamination among

Table 2 Chemical composition of the Dead Sea (Abed and others 1990) Depth

0 10 20 25 40 43 70 100 150 200 300 390

218

Concentration (g l P1) Mg

Na

Ca

K

Cl

SO4

HCO3

TDS

38.49 39.51 40.71 42.00 41.68 41.00 42.60 43.48 43.25 45.90 46.35 46.90

37.34 38.20 38.75 39.00 39.10 40.00 39.00 39.11 38.00 39.10 40.12 41.25

17.86 18.05 18.06 17.50 17.93 18.55 18.75 17.23 17.40 17.60 17.45 17.65

6.50 6.61 6.74 7.00 6.34 7.00 6.05 6.10 6.10 6.55 6.55 6.10

202.2 207.1 212.1 217.5 216.7 218.0 218.5 219.3 219.4 222.7 227.8 227.9

0.75 0.79 0.78 0.69 0.82 0.75 0.90 0.83 0.68 0.88 0.78 0.80

0.29 0.30 0.35 0.30 0.29 0.29 0.29 0.30 0.30 0.29 0.30 0.29

310 317 324 330 329 331 332 333 326 336 345 352

Environmental Geology 28 (4) December 1996 7 Q Springer-Verlag

Cases and solutions

Fig. 7. Correlation of bicarbonate with salinity in the entire Safi field

data from the Safi field because the wells are relatively recent and are as yet unexploited. However, it is necessary to evaluate the data from two distinct perspectives, i.e., the long-term potential of this aquifer in terms of chemical degradation, and the possibility of cross-contamination of the brackish water aimed for and the overlying fresh waters. The threat of quality degradation may be due to the presence of the hypersaline Dead Sea and its associated sediments only about 3 km away. Unidentified deeper waters of poorer quality have not been documented in the area, although heterogeneity of the water quality in the Kurnub aquifer is probable, as is seen even within the Dhiraa field itself. When examining the hydrochemical data in Table 3, a number of features appear. The most outstanding feature is the variation in water quality between well TS1-D, which has TDS ranging between 7500 and 9000 mg l P1, and the other wells, which have TDS values ranging between 500 and 600 mg l P1. Specific signatures in the waters of this well (TS1-D) include a Mg/Ca weight ratio of 0.5–0.6, which is significantly lower than the ratio found in Dead Sea waters (F2), an HCO3/Cl weight ratio of 0.9–1.1, which is significantly higher than the Dead Sea ratio of 1!10 P3, and a Na/Cl weight ratio of 0.26, which is half that found in the Dead Sea and even less than that expected due to either salt dissolution or marine aerosols. It is interesting to note here that the weight ratio of Mg/ Ca in dolomite is about 0.6, and so both the Mg/Ca ratio and the high bicarbonate concentration may be due to the dissolution of dolomite in the subsurface. Considering the reaction for the dissolution of dolomite: 2c cCa 2c CaMg(CO3)2c2H c a 2HCOP 3 cMg

Fig. 8 Location map of the Dhiraa field, including topography, structure, and well locations. Palestine grid.

the various aquifers is considered here. Four wells have been successfully drilled in the area, ranging in depth from 800 to 950 m (Powell 1993). Well TS1-D was drilled in the late 1980s by the Jordanian Natural Resources Authority, whereas wells TA-1, TA-2, and TA-6 were drillled by the APC between 1992 and 1994. The Kurnub sandstone crops out along Wadi Ibn Hammad and along Wadi el Karak 3–7 km east of the Dhiraa field. The waters from these aquifers are under artesian pressure and flow naturally. Shut in pressure tests indicate the water head at well TA-6 is about 40 m above ground level (agl), whereas TA-2 has a head of 57 m agl. The pressure heads are about 200 m below the outcrop elevations of the aquifers. Hydrochemical data from the wells in question are presented in Table 3. These data are not as extensive as the

in terms of thermodynamic considerations (Stumm and Morgan 1981), we find that these waters are extremely undersaturated with respect to dolomite. Two other water types can be distinguished when we examine the waters of TA-1. There is a distinct negative correlation between the Ca and Mg contents of the water extracted from this well (Fig. 9). Figure 9 suggests that there are two distinct water types that show varying degrees of mixing: one is relatively calcium-rich and the other is relatively magnesium-rich. When the time series of the chemistry variation is considered, we can see that the samples where taken in a short time period ranging from 4 April to 4 May 1993. The pumping tests were conducted during that time period. The initial pulse of water during this test period was Mg-rich, whereas later waters were more Ca-rich (Fig. 10). This suggests that initially the Mg-rich waters probably represent contamination caused by water transfer through the drill hole or through faults in the vicinity. Many of the hydrochemical signatures previously described are similar among the different water types in the TA-1 well. The Na/Cl ratio is 0.5–0.7 in both the Mg- and Ca-rich waters. These ratios are different from both the ratios seen in the TS1-D well and from the ratios seen in the Dead Sea water. The HCO3/Cl ratio is 1.5–1.7 in both Environmental Geology 28 (4) December 1996 7 Q Springer-Verlag

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Cases and solutions

Table 3 Chemical composition of the Dhiraa field Sample

Well

Date

Concentration (mg l P1) TDS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

TA1

TA2

TA6

TS1-D

06. 09. 09. 09. 09. 10. 10. 10. 10. 11. 11. 11. 11. 11. 11. 11. 11. 12. 12. 12. 12. 13. 25. 26. 29. 30. 01. 02. 04. 06. 26. 27. 30. 16. 08. 10. 13. 16.

04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 93 04. 95 05. 93 05. 93 05. 93 09. 93 10. 93 10. 93 10. 93 11. 93 08. 94 08. 94 08. 94 08. 94 12. 87 12. 89 05. 90 05. 91 23. 05. 94

609.800 616.000 601.000 597.000 597.000 601.000 603.000 635.000 675.000 627.200 629.500 617.100 618.900 611.800 622.500 605.900 606.500 605.300 602.400 599.400 598.300 603.600 589.400 632.500 697.400 556.900 582.900 582.900 576.400 611.800 599.000 594.000 607.000 546.000 672.000 645.500 685.600 607.700 9000.000 7429.000

HCO3 305.250 286.000 272.000 260.000 264.000 268.000 272.000 260.000 268.000 268.100 255.800 268.100 260.000 260.000 260.000 251.600 251.600 251.600 255.800 251.600 260.000 260.000 268.100 206.000 255.800 259.900 255.800 255.900 259.900 255.800 264.000 248.000 268.000 268.000 267.400 260.000 251.630 4372.000 3978.000 597.000

7626.000

Na

Cl

SiO2

Ca

Mg

SO4

130.500 116.000 110.000 107.000 106.000 106.000 104.000 105.000 104.000 111.000 110.000 109.000 107.000 108.000 105.000 105.000 106.000 107.000 105.000 105.000 105.000 106.000 104.000 104.000 102.000 89.000 91.000 88.000 103.000 111.000 96.000 96.000 101.000 99.000 91.000 97.000 96.000 85.000 1385.000 1235.000 212.000 940.000 1085.000

181.180 177.000 202.000 202.000 177.000 202.000 202.000 202.000 202.000 199.900 181.700 199.900 199.900 181.700 218.100 199.900 199.900 181.700 199.900 199.900 181.700 218.100 181.000 181.800 181.800 181.800 199.900 181.800 174.480 156.000 157.000 157.000 153.000 153.000 160.000 161.200 153.600 157.610 4970.000 4520.000 535.000 5087.000 4397.000

24.720 16.450 15.300 15.850 15.800 15.050 15.100 15.200 16.100 6.560 7.260 16.020 7.600 6.650 6.600 8.620 13.840 13.590 7.190 6.050 2.770 2.300 13.130 13.160 13.500 14.510 13.690 12.340 13.600 20.860 15.600 14.160 4.550 14.330 16.700 17.670 16.900 15.900

69.300 16.500 6.600 3.300 4.950 19.800 4.950 4.950 8.250 62.700 66.000 62.700 61.050 51.150 70.950 70.950 67.650 66.000 62.700 70.950 69.300 66.000 75.900 72.600 62.700 67.600 66.000 67.600 69.300 71.000 66.000 66.000 66.000 69.000 70.950 72.600 70.950 67.650 723.000 789.000 142.000 808.000 795.000

32.000 55.000 62.000 61.000 61.000 53.000 63.000 60.000 60.000 27.000 22.000 27.000 25.000 32.000 20.000 18.000 20.000 21.000 24.000 18.000 21.000 23.000 19.000 18.000 24.000 21.000 21.000 21.000 21.000 19.000 24.000 20.000 25.000 23.000 22.000 23.000 20.000 20.000 448.000 488.000 59.000 490.000 446.000

150.000 172.000 160.000 167.000 157.000 161.000 163.000 166.000 170.000 197.000

the Mg- and Ca-rich waters. This signature is similar to that seen both the TS1-D well but different from that seen in Dead Sea water. The different water types occurring in the field are represented in Fig. 11. Taken in its totality, we can see that the higher quality waters present in TA-1, TA-2, and TA-6 are probably partially mixed with water of similar quality but with different Mg/Ca signatures. There is little evidence that these waters are at present being mixed with the lowerquality waters of TS1-D (Fig. 11). However, the fact that the hydraulic head in all of the wells is the same suggests that these wells are not hydraulically separated from each 220

Environmental Geology 28 (4) December 1996 7 Q Springer-Verlag

18.800 19.700 22.300

95.000 86.000 103.000

other, and thus we may assume that the water in TS1-D, which may be deeper water from the same aquifer system, may migrate to the other wells if overpumping occurs. Fresher waters present in the overlying aquifers may be transferred downwards due to the piston effect, as described by Kronfeld and others (1993), which may also transfer lower-quality water from the deeper parts of the aquifers such as the TS1-D water. Three samples of water from TA-1, TA-2, and TA-6 have been examined for their oxygen isotope and tritium content at the Jordanian Water Authority laboratories. The oxygen isotopes were analyzed using a Finnigan Matt del-

Cases and solutions

Fig. 9 Calcium vs magnesium in well TA-1, showing strong evidence of mixing

ta model mass spectrometer. These analyses show that the water in the area contains isotopic signatures ranging from P7.2 to P7.6 vs SMOW. These values are depleted in oxygen-18 relative to modern rainwater (Kronfeld and others 1992) and reflect fossil water signatures suggesting an age of 30 000–40 000 years (Gat and Galai 1982). For tritium the samples were first isotopically enriched through electrolysis and subsequently counted using liquid scintillation. The tritium content of these samples was found to be below the detection limit of the technique, suggesting that the waters from these wells are over 50 years old. This should also be considered when developing the Dhiraa field and monitored as the field matures. Given the data available, it is unclear whether the field is capable of natural replenishment or not.

Conclusions Safi field The Safi field is an alluvial fan aquifer fed by subsurface flow from the Wadi el Hassa drainage basin. The water quality in this field is best at the core of the alluvial fan and degrades as one moves away from the core. Wells drilled in distal portions of the fan show distinct degradation through time, whereas the water from wells at the core show no such degradation. However, the degradation seen in the distal wells may in time move to the core of the aquifer if pumping exceeds recharge to the aquifer. There is no evidence of migration of Dead Sea water into the aquifer at the present time.

Fig. 10 Calcium and magnesium trends in well TA-1 during April and May 1993

Dhiraa field The Dhiraa field develops water from a deep aquifer containing three types of brackish water. Variability in the field calls for monitoring of migration of the lowest quality waters occurring in well TS1-D towards the other wells. The old age of the water from the wells, as determined by analysis for oxygen isotopes and tritium, calls for care in order to determine whether the field is capable of replenishing itself and at what rate. There is no evidence of intrusion of Dead Sea water into the aquifer. Acknowledgments We would like to thank the Arab Potash Company for data as well as financial and logistical support for this project. We also thank Hassan Amr for conducting the tritium analyses and Suzanne Keilani for conducting the oxygen isotope analyses at the Water Authority laboratories.

References Fig. 11 The various water types identified in the Dhiraa field

Abed A (1985) Geology of the Dead Sea: Waters, salts and evolution. Amman: Dar Al Arqam Publishing. 233 pp (in Arabic) Abed A, Sbaeay I, and Khbeis S (1990) The Dead Sea: A recent survey of its waters. In: Saffarini and others (Eds), Geology of Jordan and adjacent areas, Proceedings of the third Jordanian geological conference, April 1988. Amman: Jordanien Geologists Association. pp 446–472

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