Civil Engineering Department, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan e-mail: fabdulla6just.edu.jo. Tel.: c962-2- ...
Research article
Development of groundwater modeling for the Azraq Basin, Jordan F.A. Abdulla 7 M.A. Al-Khatib 7 Z.D. Al-Ghazzawi
Abstract The three-dimensional groundwater flow model MODFLOW was applied to simulate water level change in the complex multi-aquifer systems (the Upper and Middle Aquifers) of the Azraq basin. The model was calibrated by matching observed and simulated drawdown for steady and transient states over the period 1970–1992. Drawdown data for the period 1993–1997 were used to test the model’s ability to predict the response of the aquifers. The model performed well in representing the water level contours of the Upper and Middle Aquifers for steady state calibration. Agreement between the observed and simulated drawdowns was obtained for transient state calibration. To predict the aquifer system responses for the period of 1997–2025, four different pumping schemes (scenarios) have been investigated. The first scenario (present pumping rate) reveals that there will be approximately a 25 m drop in the water level at the well-field area in 2025. However, the worst scenario (pumping rate at 1.5 times the present rate) reveals an approximate 39 m drop in the water level at the well-field area in 2025. The safe yield for the Upper Aquifer System was found to be about 25 million cubic meters (MCM) yearly. Keywords Calibration and prediction 7 Groundwater modeling 7 Aquifer systems 7 Jordan
Introduction Jordan has very limited water resources. The surface water resources depends on precipitation, insufficient to keep up with the increasing demand for water to meet
Received: 24 June 1999 7 Accepted: 30 November 1999 F.A. Abdulla (Y) 7 M.A. Al-Khatib 7 Z.D. Al-Ghazzawi Civil Engineering Department, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan e-mail: fabdulla6just.edu.jo Tel.: c962-2-295111 Fax: c962-2-295018
domestic, agricultural and industrial needs. Jordan is suffering from a water shortage despite its strategic value. Groundwater resources are the major reliable source of water supply in Jordan. Azraq basin is one of the most important groundwater resources in Jordan. The basin is a major source of drinking water for the cities of Amman and Zarqa as well as the Azraq area itself. There are more than 500 wells operating within the basin. Due to excess abstraction from these public and private wells, the springs and swamps dried up and the water level in the well fields declined significantly. The decrease in the water table in the well field may lead to salt-water intrusion from the Qa’a Azraq. The farms may become covered with the saline water. To avoid the irreversible environmental impacts such as depletion and deterioration of groundwater quality, groundwater modeling is used to simulate the behavior of groundwater systems and introduce good water resources management. The present study is concerned with using a three-dimensional finite-difference model to simulate the groundwater flow and to predict the behavior of groundwater levels affected by excess abstraction.
Methodology The methodology is fairly straightforward and can be summarized as follows: data needed for model application were collected. This includes physical parameters such as hydraulic conductivity, aquifer thickness, recharge and pumping rates. Geologic formations, topographic maps, and a map with well locations were also collected. Then the study area model domain was identified. The size of the grids was irregular, depending upon the locations and intensity of the wells. The MODFLOW (Chinag and Kinzelbach 1993) was applied to simulate three-dimensional groundwater flows for the study area. The model was manually calibrated for both steady and transient states. Then the model was validated using other sets of data. Finally, a wide range of simulation scenarios were carried out to predict the water table level and drawdown under different abstraction levels.
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Fig. 1 Location map of the study area. (WAJ 1996)
Study area The Azraq basin is located between 250 to 4007E and 055 to 2307N (according to Palestine grids) (Fig. 1) and covers an area of about 12,710 km 2, of which 94% is located in Jordan. More than 5% is in Syria and a minor area is inside Saudi Arabia. The model area in this study lies between 289.30 to 342.55 E and 093.30 to 195.30 N. It covers 5432 km 2 of the basin area. The Azraq town and its oasis are located in the central part of the basin, about 100 km east of Amman. The Azraq basin is a depression surrounded by hilly relief. The highest elevation is in the northern part of the basin, adjacent to Jebel Druze, with an elevation of about 1550 m above mean sea level (amsl). The lowest elevation is at the Azraq depression at Qa’a Azraq with an elevation of approximately 500 m amsl. The elevation increases to about 900 m amsl in the south, east and west. The Azraq Depression is comprised roughly of a circular area of mudflat and salt pan. There are many intermittent flowing wadis from all directions of the depression. Water from these wadis remains several months before being lost to evaporation.
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Hydrogeology of the Azraq basin Three major aquifer systems were identified in the Azraq basin: the Upper Aquifer System, the Middle Aquifer System and the Deep Aquifer System. The first two aquifer systems are investigated in this study. The Upper Aquifer System, also called Shallow Aquifer, represents the major aquifer in the Azraq basin. The groundwater in this aquifer is unconfined. The depth to the water table ranged from a few meters in the central part of the basin at Qa’a Azraq to more than 350 m at the northern end of the basin. This aquifer system essentially consists of four members, partly separated from each other by layers of low permeability. These members are: Quaternary Alluvial Deposits; Basalt; Shallala Formation (B5); and Rijam Formation (B4). Al-Khatib (1999) presents a detailed description of these members. The Middle Aquifer System is locally known as the Amman-Wadi Sir (B2/A7) Aquifer System. It outcrops in the western part of the basin. This aquifer is classified as a confined-unconfined aquifer due to the presence of about 300 m of low permeability bituminous marl of the Muwaqqar Formation B3. The bituminous marl acts as an aquiclude between the Shallow and Middle Aquifer Systems. The Amman Formation (B2) is composed of chert, chalk and limestone. The Wadi Sir Formation (A7) is
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composed of marl, marly limestone and occasionally sandstone. The highest groundwater level is in the southwestern part of the basin. It decreases from 565 m to less than 515 m amsl towards the center of the basin and reaches about 525 m amsl in the northern part of the basin. The Deep Aquifer System was formed in the lower Cretaceous age. Lower parts of this aquifer are called Disi formation. This formation consists of stratified sandstone containing a very high percentage of quartzite. The main characteristic of this formation is its fragmented structure, increasing the permeability of the rocks. The upper part of this aquifer consists of sandstone of the Kurnub and Zarqa formations. These formations are very similar in character but different in age and grain size. The Kurnub formation is a white granular to fine-grained sandstone. The Zarqa formation is of finer grain size. This aquifer is covered by a thick sequence of low permeability materials, mainly alluvial sediments with clay and marl. This sequence acts as an aquitard, preventing direct recharge from penetration.
Input parameters A topographical map and thickness contour maps of the hydrogeological units (BGR-WAJ 1996) were used to determine the top and the bottom of the three layers for each model cell. Hydraulic properties of the aquifer systems were estimated by using pumping tests and previous studies (Humphreys 1982; UNDP-AOCP 1996; Ayed 1996) and were assigned to each of the model cells. The analysis of the pumping tests indicated that transmissivity and hydraulic conductivity have a wide range. In the AWSA well-field area (Basalt and Rijam Aquifers), the transmissivity ranges between 306 to 11,721 m 2/day and hydraulic conductivity ranges from 2.0 to 90 m/day. Transmissivity for the middle aquifer system ranges from 25.8 to 7089 m 2/day. Hydrological analysis shows that the average direct annual recharge into the upper aquifer is about 34 million cubic meters (MCM, which represents approximately 2.9% of the average annual rainfall (Ayed 1996). The discharge from the Azraq basin occurs at springs and pumping wells. There are two major groups of springs and more than 500 wells in the basin, comprising the main discharge outlets for the Upper Aquifer. In 1982, groundwater abstractions from the Upper Aquifer System Model application The Processing MODFLOW (PM version 3.0; Chiang and reached about 21.57 MCM (9.5 from the governmental Kinzelbach 1993) was selected to simulate the behavior of wells, 1.5 from the private wells and 10.57 MCM from the groundwater flow in the Azraq basin aquifer. This model spring’s discharge). This amount of abstraction was increased in 1994 to approximately 50 MCM (25 from govsimulates three-dimensional groundwater flows by using ernmental wells and 25 MCM from the private wells). finite-difference techniques. The conceptual model used here consists of three hydrogeological layers as shown in However, the amount of the groundwater abstraction was Fig. 2. The first model layer represents the Upper Aquifer subsequently reduced to 45.4 MCM in 1997. The amount of groundwater abstracted from the Middle Aquifer inComplex which includes Alluvium, Basalt, Shallala and Rijam Aquifers. It was modeled as an unconfined aquifer. creased from 0.0146 in 1982 to approximately 1.76 MCM in 1997. The Muwaqqar Formation aquiclude represents the second layer in the conceptual model. The lowermost model layer represents the Middle Aquifer Complex (AmmanBoundary and initial conditions Wadi Sir Formations) and was modeled as confined-unUsing the map of the groundwater flow pattern for the confined because layer two disappears in the northern Upper Aquifer, the boundary conditions were identified section of the model area. for the first layer. At the western end of the model area, The model domain is divided into 73 rows and 54 columns, making a total of 3942 cells in each layer and cov- the limit of saturation was defined as no flow boundaries (inactive cell). No flow boundaries were also defined in ering 5432 km 2 of the model area. The width of the cells along rows (DX) varies between 266 to 2130 m and along the southeastern and northwestern parts of the basin bethe columns (DY) between 510 to 4080 m depending on cause groundwater divides were found. Constant head the location and intensity of the wells. cells were defined in the northern, southern and eastern parts of the model area. Head is considered constant at these boundaries because flow comes from these sides and the influence of well pumpage is marginal. At the northeastern side of the model area, no flow boundaries were defined because groundwater divides were found within the Azraq basin. The internal cells were considered variable head cells. The second layer was identified as aquiclude formation (B3). Groundwater flow is zero in this layer, so there are no constant head cells to be considered and inactive cells were defined along all sides of the model area. The third layer, representing the Middle Aquifer (B2/A7), had constant head cells in the northern and southern secFig. 2 tions of the model area. In addition, constant head bounConceptual Model
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Fig. 3 The measured and calculated water levels of the Upper Aquifer System
daries were assigned for some places in the eastern and western sections of the model area. The other external boundaries are streamline and groundwater divides. The internal cells were considered variable head cells. The initial condition in the steady state is the head distribution within the model area at initial time. Figure 3 represents the initial head distribution at the end of the 1960s for the Upper Aquifer. These initial heads were considered as the initial condition for the steady state calibration. The results of the steady state flow were considered as the initial conditions for the transient condition. Steady state calibration Trial and error calibration was used to adjust the hydraulic conductivities during the sequential model runs. Hydraulic conductivities, estimated from pumping tests and some previous studies (Humphreys 1982; UNDPAOCP 1996; Ayed 1996), were used as an initial guess for 14
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the calibration. A comparison of measured and simulated water level contours of the Upper Aquifer is shown in Fig. 3. Agreement between observed and simulated water levels was obtained for the Upper Aquifer. The relative bias between observed and simulated water levels for the Upper Aquifer was found to be approximately 5%. Similar results were obtained for the Middle Aquifer where the relative bias in this case was less than 9%. The results of the calibrated model indicate that the horizontal hydraulic conductivity of the Upper Aquifer ranges between 0.1–100 m/day (Fig. 4). The vertical hydraulic conductivity ranges between 0.01–2.5 m/day. The highest horizontal hydraulic conductivity occurred in the basalt area at the AWSA wellfield area (80–100 m/day). In the Qa’a Azraq, the horizontal hydraulic conductivity ranges from 0.1–10 m/day (Fig. 4) The average horizontal and vertical hydraulic conductivities of the aquiclude formation are 9!10 –5 and 8.6!10 –6 m/day respectively. The horizontal hydraulic conductivity of the Middle Aquifer is less than 0.1 m/day
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Fig. 4 Map of calibrated horizontal hydraulic conductivity of the Upper Aquifer System
in the southern and northern parts, and increases to 10–30 m/day at the central area of the basin, while the average vertical hydraulic conductivity is approximately 8.6!10 –3 m/day (Fig. 5).
year in which water production started. Twelve stress periods were selected to cover a 22-year time period (1970–1992). These stress periods have a length of 365 days and one time step, while the first one has 11 time steps with 365 days for each step. Figure 7 shows the comparison of observed and simulated drawdown in the observation wells (AZ12 and AWSA No.2). As Fig. 7 shows, the model successfully simulates the drawdown where close agreement was obtained between the observed and simulated drawdown in these observation wells. The relative bias between the observed and simulated drawdowns for AZ12 and AWSA No. 2 wells were 7 and 8% respectively. However, the correlation coefficients between the observed and simulated drawdowns for both wells are 0.95 and 0.93 respectively.
Transient calibration Two observation wells are available (F1043-AZ-12, F1280AWSA No.2) in the model area to monitor the water level of the Upper Aquifer (Fig. 6). The F1043-AZ-12 well is located in the center of the AWSA well field. The depletion rate between 1985 and 1992 was 0.6 m annually. The F1280-AWSA No.2 well is located about 2 km southeast of the ASWA well-field area. The depletion rates in this well were approximately 0.4 m annually. The available data on these wells from 1985 to 1992 were used in the transient calibration. The specific yield and specific storage values were manually selected and adjusted until reaModel verification and prediction sonable matches were obtained between the observed and The abstraction rates from 1993–1997 were used for modsimulated drawdown (Fig. 7). 1970 was taken to be the el validation. Reasonable agreement between the ob-
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Fig. 5 Map of calibrated horizontal hydraulic conductivity of the Middle Aquifer System
served and simulated drawdown in the observation wells was obtained (Fig. 7). The average relative bias between the observed and simulated drawdown was less than 8% for both wells. The correlation coefficients between the observed and simulated drawdowns were 0.93 for AZ-12 well and 0.88 for AWSA No. 2 well. These results indicated that the calibrated parameters (such as hydraulic conductivity and specific storage) are acceptable. By using the calibrated parameters, a drawdown of the Upper Aquifer in 1997 was simulated for the whole model domain and the maximum drawdown appeared in the wellfield area, reaching about 17.5 m. Four different scenarios were carried out to predict the drawdown for the Shallow Aquifer of the Azraq basin during the period 1997–2025, focusing on the years 2005, 2015 and 2025. In the first, second and third scenarios the pumping rates were assumed to be 45.4, 22.7 and 68.1 MCM respectively. These scenarios are 1, 0.5 and 1.5 times the abstraction of 1997 (45.4 MCM). The fourth 16
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scenario was taken to determine the safe yield of the Upper Aquifer System by reducing the present abstraction with different ratios. Table 1 shows the predicted maximum drawdown in the well-field area under the suggested scenarios. For example, under the first scenario (SCE1), the maximum drawdown in the well-field area will be 25.32 m in 2025. Under the second and the third scenarios, the maximum drawdown in the well-field area will be approximately 12.5 and 39 m respectively. The fourth scenario (SCE4) reveals that the safe yield for the upper aquifer is 25 MCM with a maximum drawdown of 13.68 m in 2025.
Summary and conclusion The Upper and Middle Aquifer Systems of the Azraq basin are investigated in this study. The Upper Aquifer is
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Fig. 6 Location of the observation wells
an unconfined aquifer while the Middle Aquifer System is classified as confined-unconfined aquifer. Processing Modflow (version 3.0) was applied to simulate the threedimensional groundwater flow for the Upper and Middle Aquifer Systems under both steady and transient conditions. The results of the model calibration and verification showed reasonable agreement between observed and calculated drawdown for the observation wells. The model is also used to predict the drawdown for the period
from 1997 to 2025 under four different scenarios. The first three scenarios assumed that the abstraction rates will be 45.4, 22.7 and 68.1 MCM respectively. Under these scenarios, the maximum predicted drawdown at the wellfield area in 2025 would be 25, 12.5 and 39 m respectively. The fourth scenario showed that the safe yield for the Upper Aquifer System was about 25 MCM per year.
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Acknowledgements This work was supported by the Jordan University of Science and Technology as an assistantship for the second named author in his MSc Thesis. The authors wish to thank the anonymous reviewers for their reading of the manuscript, and for their suggestions and critical comments. The assistance and support of Professor Omar Rimawi from the University of Jordan, and Dr. R. Ayed from the Ministry of Water and Irrigation are highly appreciated.
References
Fig. 7 Observed and simulated drawdown in the observation wells (calibration and validation periods)
Table 1 Predicted maximum drawdown in the well-field area for 2005, 2015 and 2025 under the suggested scenarios Scenario a
SCE1 SCE2 SCE3 SCE4
Maximum drawdown (m) 2005
2015
2025
20.10 12.9 27.74 13.6
22.85 12.53 33.82 13.53
25.32 12.46 39.2 13.68
a
SCE1 : first scenario where the pumping rate is 45.4 MCM(present rate); SCE2 : second scenario where the pumping rate is 22.7 MCM (half the present rate): SCE 3 : third scenario where the pumping rate is 68.1 MCM (1.5 times present rate); SCE4 : safe yield scenario
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Al-Khatib M (1999) Development of a groundwater model for the two upper aquifers (Shallow and Middle) of the Azraq basin. MSc Thesis. Civil Eng Dept, Jordan Univ Sci and Technol, Irbid, Jordan, pp 37–40 Ayed R (1996) Hydrological and hydrogeological study of the Azraq basin. PhD Thesis. Univ Baghdad, Baghdad, Iraq, pp 85–96 Chiang W, Kinzelbach W (1993) Processing Modflow, preand post processors for simulation of flow and contaminants transport in groundwater system with MODFLOW and MODPATH and MT3D. Version 3.0, Hamburg Humphreys H (1982) Azraq wellfield evaluation hydrochemistry and monitoring. Amman Water and Sewage Authority, Amman, Jordan, pp 33–58 UNDP-Azraq Oasis Conservation Project (UNDP-AOCP) (1996) Simulation of the groundwater flow in the Azraq basin, final report prepared for the government of Jordan. Amman, Jordan, pp 27–34 BGR-WAJ (Bundesanstalt fur Geowissenschaften und RohstoffeWater Authority of Jordan) (1996) The North Jordan Resources Investigation Project. Amman, Jordan, pp 7–10
