Brackish Groundwater Resources in Bahrain: Current Exploitation, Numerical Evaluation and Prospect for Utilization
W A L E E D K. Z U B A R I 1 and A H M E D R. K H A T E R 2 1 Desert and Arid Lands Sciences Program, Arabian Gulf University, PO Box 26671, Manama, Bahrain 2 Scientific Research Department, Bahrain Center for Studies and Research, PO Box 496, Manama, Bahrain
(Received: 16 June 1994; accepted in final form: 20 January 1995)
Abstract. In Bahrain, where water resources available for direct use are finite and the best of its quality has a salinity of over 2.5 g L - 1, utilization of brackish groundwater is an essential part in the management of the country's water resources. Bahrain's brackish water occurs in the Rus-Umm Er Radhuma formations in the form of a lens of a finite lateral extent, with a salinity ranges between 8 and 15 g L-1. Planning for utilization of brackish groundwater for desalination purposes in Bahrain was based on simulation modeling of the aquifer system using a mixing cell model developed originally in 1983. The model was used to predict the aquifer response to pumping from the proposed wellfield in terms of changes of TDS over a period of 20 years. Construction and operation of the wellfield in 1984 was based on the predicted salinity changes. Over the past 9 uears of wellfield operation (l 984-1993), and through continuous monitoring of the aquifer response to pumping, the collected data is used to post-audit the original model by history matching. The calibration process adopted has resulted in a statisfactory agreement between the model output and the observed data. The model is then used to predict the wellfield salinity changes and the aquifer potentiometric levels. The expected life span for the brackish groundwater utilization by the wellfield is redefined through constrained utilization that takes into account salinity deterioration coupled with the effect of head decline on hydraulic interaction between the brackish water and the upper fresh water aquifer. The results suggest that the operation of the wellfield should cease by the year 2007. Construction of a new model that enables testing and evaluating different development scenarios is recommended to aid future management decisions regarding the utilization of brackish groundwater.
Bahra in Is lands (Figure 1), as m o s t o f the Arab ian Peninsula, has an arid to ex t reme-
ly arid env i ronment . It is charac ter ized by irregular, scanty rainfall, and high evap-
ot ranspira t ion rates. The average annual rainfall is less than 80 mm, while the
potent ia l evapot ransp i ra t ion averages about 1,850 m m y - t (Zubari, 1987), leading
to a h igh deficit in the water budget , and therefore creat ing imposs ible condi t ions
for any surface water to exist. Fresh water demands in the count ry have been met
ma in ly by g r o u n d w a t e r abstract ion and desal inated water.
278 W.K. ZUBARI AND A.R. KHATER
Development activities in Bahrain, represented by urban, agricultural, and industrial expansion, have led to a continuous depletion of the small island's lim- ited fresh groundwater resources. A deterioration of more than half of the original groundwater reservoir has occurred due to its over-exploitation mainly by the agri- cultural and domestic sectors (Zubari et al., 1993). As an alternative, desalination has been introduced to reduce domestic groundwater abstraction, and to meet the domestic water quality standards and quantities needed. At present, domestic water demands, about 100 million cubic meters per year (Mm 3 y-l) , are met by desali- nated water produced form three desalination plants and blended with groundwater in the ratio of one to one.
Generally, in the Gulf states, including Bahrain, seawater is the main source to feed desalination plants with raw water. However, in Bahrain, brackish groundwater ranging between 8 and 15 grams/liter (g L -1) has been utilized in feeding a desalination plant. It is sought from this development to lower the potentiometric surface of the brackish water lens in order to reduce its upwards migration and pollution to the upper relatively fresh water aquifer, and to reduce the risk of shutting down desalination plants in the case of oil spills in thee Arabian Gulf waters (as was the case in the first and second Gulf wars). Furthermore, the utilization of the brackish water appears to be more economical compared to the seawater (Dannish and A1-Ansari, 1991). Starting from 1984, about 24 Mm 3 y-1 is being abstracted from the Rus-Umm Er Radhuma (Rus-UER) brackish aquifer system to feed a reverse osmosis (RO) desalination plant, at the Abu-Jarjur area in the east coast of Bahrain (Figure 1), with an output capacity of about 17 Mm 3 y- l .
The economics of this process, and hence its continuation, depend principally on the resulting increase of the wellfield salinity as compared to the salinity of the seawater (45 g L -1) as well as the associated hydraulic gradients between the brackish groundwater and the relatively fresh aquifers. Construction and operation of the wellfield in 1984 was based on hydrogeological investigations followed by a modeling study to evaluate numerically the response of the aquifer system to pumping from the proposed wellfield (GDC, 1980; 1983a; 1993b). These pre- development investigations and numerical evaluation were made when the aquifer system was in a rather steady state conditions, and naturally the model calibration could not include transient conditions. As a result, the model predictions for the future changes of the wellfield salinity were given within a wide range of uncer- tainty. After 9 years of exploitation and through continuous monitoring of the aquifer response to this development, adequate data on the aquifer behavior have become available.
In the present paper the collected data is used to examine the transient behavior of the aquifer system and to calibrate, or post-audit, the original model via history matching. The calibrated model is then used to predict the future wellfield salinity, corresponding abstraction rates, and associated changes of potentiometric levels. Based on the updated numerical evaluation, the prospect for utilization of the
BRACKISH GROUNDWATER RESOURCES IN BAHRAIN 279
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Rus Formation outcrop area
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Scale, km
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4 50 000 E " 60 70
Fig. 1. Location map of Bahrain showing the Abu-Jarjur wellfield.
280 w. K, ZUBARI AND A.R. KHATER
brackish aquifer system and its life span as a reliable source for desalination purposes is also determined.
2. Hydrogeological Setting
Brackish water resources in Bahrain are present in the form of a large lens located in Bahrain's main island, which is an anticline structure (Figure 2a). The brackish water lens is developed in the Rus and Umm Er Radhuma (UER) formations. The Rus Formation (Lower Eocene) is composed of chalky dolomitic limestone with subsidiary shale and anhydrite intercalations. The Rus Formation section in the central and eastern areas of Bahrain has undergone extensive solution of anhydrite, which led to the collapse of overlying rocks and has produced very large voids. The UER Formation (Paleocene to Lower Eocene) is composed of dolomitic limestones containing enlarged joints, cracks and vugy intergranular porosity. The Rus Formation was conformably deposited over the UER Formation, and the boundary between the two is difficult to detect lithologically; the upper UER section appears hydraulically and chemically continuous with the Rus section. The UER is underlain by the shales of the Aruma Formation (Upper Cretaceous), as shown in Figure 2a.
The brackish aquifer upper boundary is limited by the water table when under unconfined conditions at the eroded core of the anticline, and where the Rus rocks outcrop. On the flanks of the anticline, the aquifer is confined by the shale beds of lower Dammam Formation and upper Rus Formation, in addition to the Rus anhydrite when present (Figure 2a). Its lower boundary i s usually taken at the relatively sharp boundary of a hyper-saline water, of over 15 g L -1 , and occurs at a depth of about 180 m below Bahrain national Level Datum (BNLD).
The saturated thickness of the brackish water zone ranges from 0 m at the flanks of the anticline, where it is intersected by the dip of the overlying aquitard, and reaches a maximum of about 185 m at its crest.
The aquifer transmissivity shows a great heterogeneity; reported aquifer trans- missivities range from 110 to about 50 000 m 2 d -1 (GDC, 1980; 1983a). This extreme variation in the transmissivity reflects the fractured nature of the aquifer, especially the karst zones located in the middle of the UER Formation (Figure 2a). The aquifer storage coefficient averages about 0.0002 (dimensionless) in its con- fined parts (GDC, 1980; 1983a), while the aquifer specific yield is calculated at about 0.2 (dimensionless) using change in head per change in volume analysis for the unconfined parts.
3. Current Exploitation
In the fourth quarter of 1984, a wellfield with 15 production wells located at Abu-Jarjur area on the east coast went into operation to feed the Abu-Jarjur RO desalination plant (Figure 1). The production intake zone was designed to produce
BRACKISH GROUNDWATER RESOURCES IN BAHRAIN
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Fig. 2. (a) Hydrogeological section showing the geometry of the Rus-UER brackish aquifer system (modified after GDC, 1983a). (b) Typical salinity profile of the Rus-UER aquifer (modified after GDC, 1983a).
282 W.K. ZUBARI AND A.R. KHATER
TABLE I. Abu-Jarjur weUfield production rates from 1984-1993, Mm 3 y-i
Source: Water Production Directorate, Ministry of Works, Powers and Water, Bahrain.
from the upper and middle UER zones located at a depth between 75-145 m below BNLD (Figure 2a). The average rate of abstraction from the weUfield is about 24 Mm 3 y-1 (Table I). The production rate is expected to increase due to the anticipated salinity increase in the wellfield, so that the plant production capacity of about 17 Mm 3 y-1 of desalinated water can be maintained.
3.1. PRE-DEVELOPMENT CONDITIONS
Prior to the construction of the wellfield, detailed hydrogeological investigations were carried out during the period 1978-1983 by GDC (1980, 1983a). The pre- development potentiometric surface of the Rus-UER aquifer system was approxi- mating a flat surface; most of the observation wells displayed potentiometric levels ranging between 5.0 and 5.3 m above BNLD. The salinity of the aquifer showed a pronounced and systematic increase with depth. In the central area, the salinity of the brackish water lens is found to increase gradually from less than 8 g L -1 at the water table to about 15 g L -1 at about 180 m below BNLD (Figure 2b). The brackish water lens is underlain by a mixing zone overlying brines with a salinity greater than 100 g L-1 as illustrated in Figure 2b. The reserve of the brackish water (<15 g L -1) in place was estimated to be about 10 000 Mm 3 (GDC, 1983a).
3.2. POST-DEVELOPMENT CONDITIONS
Continuous monitoring of the Rus-UER brackish aquifer in terms of potentiometry and salinity has been carried out from 1984 to present. The potentiometric levels in the aquifer are monitored at 15 observation wells equipped with continuous water level recorders and are reported on monthly basis. Figure 3a is a location map of the observation wells network for the aquifer. Aquifer salinity was measured at Abu-Jarjur wellfield output line, being the main factor in determining the life of the wellfield. A full chemical analysis (TDS and major anions and cations) was determined every month for the wellfield pumped water since its commencement to present.
3.2.1. Potentiometric Level Changes
Figure 3b illustrates a plot for all the observed potentiometric levels in the 15 observation wells of the Rus-UER aquifer. There are two striking features that can be observed from these hydrographs. The first is that although these wells differ in
a yearly data represents a twelve month average. Source: Water Production Directorate, Ministry of Works, Powers and Water, Bahrain.
location and distance from the producing wellfield, their drop in head had occurred almost at the same time with a lag of less than two months from the beginning of production by the wellfield. The second observed features is that the pattern and rate of head drop in most of the wells' hydrographs is the same regardless of their location from the wellfield; the observed drawdowns in most of the wells in 1992 are very close. These two observed features suggest strongly that the flow in the aquifer towards the wellfield occurs in very high transmissivity zones. These are probably the fractured karstic zones present in the middle UER Formation. Furthermore, analysis of the observation wells hydrographs indicates that there are no signs of future stabilization or reduction in the magnitude of the present drawdown rate in the aquifer, calculated at 0.325 m y - l , which further confirms the limited areal extent and closed hydraulic system of the brackish water lens.
Figure 3c is a drawdown map prepared to illustrate the potentiometric levels changes and pattern in the Rus-UER aquifer for the period from 1984 to 1992. The map indicates the absence of major potentiometric head coning under the wellfield, and that the cone of depression in the brackish water lens extends to all areas of Bahrain. In other words, the aquifer water is collected from all areas of Bahrain by the fractured zones. Maximum drop in head in 92 was observed at about 2.7 m under the wellfield, while drawdown values of about 1.5 m were observed at the edges of the brackish water lens in the north-eastern and western areas. The observed drawdown pattern, suggests a uniform depletion of the brackish water lens.
3.2.2. Salinity Changes
Table II illustrates the calculated annual average concentrations of TDS, major anions (CI-, SO42-, HCO3-) , and major cations (Na++ K +, Ca 2+, Mg2+), mea- sured in the wellfield pumped water from 1985 to 1993. As can be seen from the
BRACKISH GROUNDWATER RESOURCES IN BAHRAIN
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285
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Fig. 4. Predicted wellfield salinity by the original uncalibrated model (modified after GDC, 1983a).
table, there are no major concentration changes in TDS, anions, and cations for the produced water by Abu-Jarjur wellfield since its commencement in 1984 to present; the average TDS in 1985 and 1993 are about the same. However, within this period, the TDS showed a variation between a low of 12.35 g L -1 in 1988, and a high of 13.08 g 1-1 in 1992, which could be attributed to the fluctuation in the wellfield abstraction rates (Table I). Observed salinity changes are in agreement with the above mentioned observations of uniform depletion of the brackish water aquifer.
4. Numerical Evaluation
Prior to the development of the Rus-UER aquifer system, hydrological investiga- tions were followed by a modeling study to predict the response of the aquifer system to pumping from the proposed wellfield (GDC, 1983b). The objective was to estimate the life span for the utilization of the brackish water defined by the time required for the salinity of the wellfield to reach that of the seawater. The modeling study was carried out when the aquifer system was in a rather undisturbed hydraulic conditions, and naturally the model calibration could not include transient condi- tions. As a result, numerical evaluation in terms of the model predictions for the wellfield salinity were given within a wide range of uncertainty representing the worst and best scenarios (Figure 4). Accordingly, the construction and operation of the wellfield in 1984 was based on the worst case scenario, which indicated that
286 W.K. ZUBARI AND A.R. KHATER
the salinity of the wellfield would increase to reach that of the seawater after about 22 years.
Over the past 9 years (1984-1993) and through continuous monitoring of the aquifer's potentiometry and salinity, adequate data on the aquifer behavior have become available. This data is used to calibrate, or post-audit, the original model by history matching to narrow down the area of uncertainty when the model is used for numerical evaluation of updated predictions.
4.1. MODELING APPROACH
The Multi-Layered Mixing Cell Strip (MLMCS) Model was developed by GDC in 1983 to simulate movement of solutes in the Rus-UER aquifer system in Bahrain and to predict the changes of solute concentrations over a period of 20 years for the water pumped from the wellfield (GDC, 1983b). The aquifer system is subdivided into a number of layers, which allowed aquifer hydraulic parameters to vary. Flow due to both density and potentiometry gradients is simulated in the model and salinity changes are calculated using a mixing cell approach. Thus aquifer system behavior with abstraction from the wellfield, and the movement of waters of different salinities were simulated.
4.1.1. Conceptual Model
Figure 5a displays the conceptual model used in the construction of the MLMCS Model. The system of aquifers and aquitards are conceptualized as follows (GDC, 1983b):
- Aruma Aquifer: assigned a constant pressure and functions as a controlled head reservoir to receive or provide vertical leakage. The salinity varies with time as the brackish water in the top Aruma is used up and replaced by brine.
- Aruma Aquitard: separates the Aruma and aquifer C and has a defined upper and lower limits and vertical permeability, and in which vertical flow is simulated.
- Umm Er Radhuma Aquifer: subdivided into 6 layers to allow accurate rep- resentation of the vertical distribution of salinity, permeability and heads. Horizontal flow is simulated within the layers, which each have defined upper and lower limits and horizontal permeability distribution. Vertical flow is sim- ulated across the leakance interfaces which separates the layers. The energy losses due to vertical flow in the aquifer are assumed to be concentrated at these boundaries which have negligible thickness.
- Rus Aquifer: has a defined upper and lower limits and horizontal permeability distribution in which lateral flow is simulated. Vertical flow between the Rus and Khobar, through the Rus and Sharks Tooth Shale aquitards, is simulated at the upper boundary. A leakance interface is used to simulate head losses
- - 3 . 7 5 . - P o t e n t i o m e t r i c l e v e l ~ F l o w l i n e 1 7 N o d e o f s t r i p m o d e l
Fig. 5. (a) Conceptual model used in the MLMCS model (after GDC, 1983b). (b) Geometry of the MLMCS model (after GDC, 1983b).
288 W.K. ZUBARI AND A.R. KHATER
due to vertical flow between the Rus and Upper Umm Er Radhuma aquifers. The salinity of water within the Rus is considered during mixing calculation.
- KhobarAquifer (lower Dammam Formation): assigned a fixed head and salin- ity and therefore functions as a reservoir to receive or provide vertical flows. Where the Khobar aquifer is dry or not present, the Rus acts as the upper boundary of the system.
4.1.2. Model Geometry and Boundary Conditions
Prior to the construction of the MLMCS model, a simple one-layer finite differ- ence model was constructed to aid in defining its geometry (GDC, 1983b). In this preliminary model, the aquifer was conceptualized as a single layer, containing water of constant density, with an upper and lower aquitard layers separating it from the Khobar and the Aruma aquifers. The model was designed to cover most of the northern parts of the aquifer by assuming the abstraction pattern and hydro- geology of the aquifer are symmetrical about the west-east line through the center of the wellfield. Thus, the model southern boundary is an axis of symmetry, and was modeled as no-flow boundary (Figure 5b). The northern, eastern, and west- ern boundaries were also assumed to be no-flow boundaries as they approximated the brackish water limits and inflows from these boundaries were assumed to be negligible. The model enabled the variation of potentiometry and weUfield salinity to be predicted, with abstraction from the wellfield. Based on the potentiometry distribution and flow paths resulted from this one layer model, the MLMCS model geometry was constructed. Two strips bounded by flow lines were chosen to sim- ulate flow to the center and end of the wellfield, as shown in Figure 5b. The strip model for the central part of the wellfield (section 1) contains 23 nodes, while the shorter strip near the end of the wellfield (section 2) has 20 nodes.
The strip models lateral boundary conditions were assumed to be coincident with the flow lines, which represent no-flow boundaries. These were assumed to be constant with time, which is confirmed in this paper, as the flow pattern to the wellfield is observed to be established rapidly once abstraction begins. The upper boundary is defined by the water table in the unconfined layers of the central part of the model area. At the east and west sides of the wellfield the overlying Khobar aquifer acts as an upper boundary, and was considered as a constant head boundary. The lower boundary of the model is the Aruma aquifer, which was considered as a constant boundary.
4.1.3. Original Prediction Results
The uncalibrated MLMCS model was used initially in 1983 to predict the wellfield salinity changes due to a constant RO plant production rate of about 17 Mm 3 y-1 (GDC, 1983a). The prediction runs were made for a period of 20 years using a time step of one year. Salinity changes predictions over the period of 21 to 30 years were
BRACKISH GROUNDWATER RESOURCES IN BAHRAIN 289
extrapolated using a parabolic curve fitting method (GDC, 1983b). Figure 4 shows the predicted wellfield salinity as a function of time for the two sections modeled. The figure shows that, from an initial salinity of 12 g L -1, the wellfield average salinity will rise to about 18 g L -1 after 10 years, about 28 g L -1 after 20 years, and about 38 g L -1 after 30 years. In order to assign limis to the model prediction results, the previous modeling study conducted a series of runs by using a range of values for the critical hydraulic parameters (upper and lower aquitards resistance, porosity, etc.) to produce an upper and lower confidence limits.
The model results, as illustrated in Figure 4, do not provide a definite answer to the expected life span of the desalination plant, based on the deterioration of the brackish water. This is because of the wide range of the predicted lower and upper limits, where the salinity could be as low as 15 g L -1 or as high as 40 g L -1 after 20 years of pumping. Hence, the water supply authority has chosen to operate the plant under the worst case of lower limit conditions predicted, where the brackish water would deteriorate to the salinity of seawater by the year 2006. After 9 years of pumping, the observed salinity levels (Figure 4) compared to the predicted ones emphasize the need to calibrate the model and update the model predictions, as will be illustrated in the following section.
4.2. MODEL CALIBRATION
The MLMCS model was calibrated under transient conditions to simulate the aquifer system behavior as close as possible. It included, in general, calibration by history matching and performance of sensitivity analysis. The approach adopted was to first diagnose the original uncalibrated model and to define and investigate areas of discrepancy between the hydrological variables (potentiometric heads and salinity) observed in the field at certain time intervals and those calculated by the model for the same time intervals. Based on the results of these comparisons, mod- ification of the model initial conditions was first made and followed by calibration of hydraulic parameters with the aid of sensitivity analysis until satisfactory results for the model's two strips are achieved.
The choice of the hydraulic parameters for the two strip models was based on the best attainable match between the simulated and observed heads and salinities. Differences or lack of match between the simulated and observed data may be attributed to: (a) the original setup of the model boundaries and cells discretization, where the no-flow boundary locations in the modeled area are not perfect except for the southern one. Observed aquifer drawdown contour map (Figure 3c) indicates that water flow paths are different from those envisaged during the initial setup of the model, and that observed water flow paths are not in full agreement with the conceptualized ones. This is clear in the northern, western and eastern areas, where waters from these areas is moving towards the wellfield; and (b) the rigid nature of the model in terms on hydraulic parameters spatial heterogeneity; although the model allows for vertical heterogeneity for the hydraulic parameters, it assumes
290 W. K. ZUBARI AND A.R. KHATER
TABLE III. Predicted wellfield salinity levels for the period 1993-2012, g L - l
these parameters, except for the transmissivity, are homogeneous within the same zone, which constrains the calibration process in that one average representative value has to be assigned for all the cells for a given aquifer zone.
Figure 6 shows the potentiometric heads obtained by the model at the end of the transient conditions calibration stage as compared to those observed in the wells. Figure 7 displays the wellfield salinity and major chemical species (Na++ K +, CI-, SO42-, Ca 2+, Mg 2+, and HCO3-) simulated by the model's two strips as compared to those observed in the waters produced by Abu Jarjur wellfield.
4.3. UPDATED MODEL PREDICTIONS
Once the model has been calibrated, it is used to predict the temporal performance of the aquifer system. Prediction runs are made subject to the constraints that the salinity of the wellfield should not exceed the salinity of seawater (45 g L- 1), and the associated potentiometric heads in the Rus aquifer zone should be above the zero level everywhere north of the wellfield. The first constraint is in accordance with desalination requirement and the economics of this process. The second constraint limits the decline of heads in the Rus aquifer zone to prevent reversal of hydraulic gradient and consequent loss of fresh water from the upper aquifer.
In this case, the life span for utilization of the brackish water is defined by the prediction period over which the two constraints are jointly satisfied. The output of prediction runs has shown that the imposed constraints will continue to be satisfied jointly throughout the period 1993-2007. Though beyond the year 2007, the first constraint remains satisfied, the second constraint has been violated. This is explained by the results of the model predictions over the period 1993-2012, as illustrated below.
BRACKISH GROUNDWATER RESOURCES 1N BAHRAIN 291
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Fig. 6. Simulated vs. observed potentiometric heads at the end of the model calibration stage.
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4.3.1. Predicted Salinity Levels
Table III and F igure 8 show the predicted wellfield salinity levels as s imula ted by the cal ibrated model . In general , the present predict ions o f salinity levels are less than those pred ic ted by the original uncal ibra ted model . The previous wide
BRACKISH GROUNDWATER RESOURCES IN BAHRAIN 293
40.0 ~Calibration period---~ 4 Prediction period ~,,
range of uncertainty expressed in terms of lower and upper limits of salinity predictions is now eliminated and replaced by the predicted salinity for strip 1 and strip 2, respectively. The results indicate that the wellfield salinity will reach a maximum of about 28 g L-1 bu the year 2007. Based on the worst case conditions, the uncalibrated model predicted the same level of 28 g L -1 by the year 1999. Accordingly, utilization of the brackish aquifer to feed the RO plant is expected to last 8 years longer than what was predicted originally by the uncalibrated model.
294 w.K. ZUBARI AND A.R. K_HATER
4.3.2. Predicted Abstraction Rates
Abstraction rates from the wellfield are a function of the RO plant recovery and the salinity of the pumped water. This is controlled by a linear relationship embedded in the model. Obviously, the increase of feed water salinity would lead to an increase in the abstraction rate to maintain the same quality standard of water produced by the RO plant. Table IV displays the expected abstraction rates corresponding to the predicted salinity levels in the weUfield. By the year 2007, the predicted abstraction rate would reach a maximum of about 36 Mm 3 y-1. Since the existing wellfield has a maximum capacity of about 29 Mm 3 y- l , the predicted abstraction rates suggest that the RO plant authority should expand the wellfield's capacity starting from the year 1997 to cope with the increased abstraction rates.
4.3.3. Predicted Potentiometric Levels
The changes of potentiometric heads in the main fractured producing zones (UER5 and UER6) and in the upper zone boundary (Rus) are predicted for the period 1993- 2012. The predicted potentiometric levels are displayed in the form of contour maps for the years 1997, 2002, 2007, and 2012 as shown in Figure 9. The predicted heads show that the contour line of zero potentiometric level in UER5 zone will advance to the wellfield area by the year 1997. This will take place in UER6 and Rus zones by the year 2002, as illustrated in Figure 9. However, the zero potentiometric level will completely invade the wellfield area in the three aquifer zones by the year 2007. Any further abstraction beyond the year 2007 will cause the zero line to move north of the wellfield area as shown in the contour maps for the year 2012. Such movement of the zero line violates the constraint imposed to prevent leakage of fresh water from the upper aquifer.
5. Prospect for Utilization
The brackish water aquifer in Bahrain is a nonrenewable resource and its utilization is based on mining a one-time reserve stored in the aquifer. This reserve is defined by the brackish water lens which has a salinity varying from 8 to 15 g L - 1. Mining is usually based on economical considerations expressed in our case by hydrologic constaints on the system's state variables as surrogate economical factors.
The prospect for utilization of the brackish water resource essentially depends on satisfying two main hydrological constraints. The first one is that the drop of head in the brackish aquifer should not exceed a specified value in order not to induce the upward movement of the highly saline water and causes salinization of the brackish water lens. Secondly, the allowable drop of head in the brackish aquifer should not lead to a reversal in the hydraulic gradients between the brackish water and the relatively fresh upper Dammam aquifer and cause loosing of freshwater into the lower brackish aquifer. Accordingly, control of drawdown is a key element in the management of the brackish water resource.
B R A C K I S H G R O U N D W A T E R R E S O U R C E S I N B A H R A I N 295
a)
b)
c)
1997 2002
~o. ~-~- - -~
2007
,
e
¢
, 9/ ~.a-.-.._)
2012
H
;(,,]
Fig. 9. Predicted potentiometric surface for (a) UER5, (b) UER6, and (c) Rus aquifer zone.
The drop of head in the brackish water lens to the zero level can be considered as a critical value for these two hydrological constaints that would limit further utilization and cause to shut down the desalination plant. Numerical evaluation of the aquifer response to the utilization of brackish water suggests that the zero potentiometric level will completely invade the wellfield area by the year 2007. This implies that unless measures are taken to control the drawdown in the aquifer zones, the utilization process would cease by the year 2007.
In order to prolong the brackish aquifer utilization, a number of aquifer man- agement options are proposed, and need to be studied prior to their implementa- tion. These options can be used, either individually or combined, to achieve this objective. These are subdivided into two major groups: (a) modification of the present wellfield design, i.e. modifying individual well spacing, location, pumping schedule, raising well intake intervals, in addition to the total number of wells in the wellfield; and (b) augmenting the available water in the aquifer by artificial recharge, i.e. injecting other sources of water like unused treated sewage effluent (TSE), rain water, and agricultural drainage water (ADW) being discharged to the sea. Technical and economic feasibility for each option, and in the case of the TSE and ADW health risk assessment, should be researched and compared to the final option, which is the seawater.
296 W.K. ZUBARI AND A.R. KHATER
6. Conclusion and Recommendations
The Rus-UER aquifer in Brahrain is a limited, nonrenewable source of brackish water which forms a lens of finite lateral extent and with continued exploitation its salinity will eventually reach that of the sea water. Analysis of the aquifer poten- tiometry in the observation wells has indicated that the response to the wellfield pumping has occurred almost at the same time and that the rate and pattern of head drop in most of the wells are approximately the same regardless of their location and distance from the wellfield. Moreover, the absence of major potnetiometric coning within the vicinity of the wellfield strongly suggests that the flow in the aquifer occurs in the fractured zone located in middle UER formation.
This flow system represents ideal conditions for the development of the brackish water resource as major salt up-coning is unlikely to occur under such flow con- ditions. Rather a relatively uniform, gradual salt upward migration resulting from a uniform drop in the potentiometric surface is expected. Analysis of the salinity changes in the wellfield produced water are in accordance with this conclusion; chemical analysis has shown that there are no major concentration changes in the TDS and major anions and cations of the produced waters by Abu-Jarjur well- field since its commencement in 1984 to present (1993). Since management of the aquifer is based on mining principles, it has been demonstrated that the prospect for utilization is governed by to what extent the allowable drop of head in the aquifer is controlled. Drop of head to the zero level can be considered as a critical value that would limit further utilization and cause to shut down the desalination plant.
The present model calibration under transient conditions has resulted in making a better representation of the Rus-UER brackish aquifer with a set of hydraulic parameters for the aquifer zones that provided the best attainable match between the model output and observed data. As experienced in most of numerical models during validation and post-auditing, these models possess some inaccuracy in their conceptualization of real aquifer systems and limitations in their application. However, the performance of the MLMCS model has been satisfactory for the objective of this study.
The calibrated model is used to predict the response of the aquifer system to continuous wellfield pumping over a 20 year prediction period. Based on con- strained utilization, output of the prediction runs indicates that the process of brackish groundwater desalination at Abu-Jarjur RO plant would continue till the year 2007. By that time the wellfield salinity will reach a maximum of 28 g L -1 . Predicted potentiometric surface maps of the main aquifer zones indicate that fur- ther abstraction beyond the year 2007 will cause the zero potentiometric head to move inward north of the wellfield area. This implies that the hydraulic gradient between the Rus aquifer zone and the upper fresh aquifer will be reversed and loss of fresh water downward into the brackish aquifer will take place. Therefore, operation of the wellfield should cease by the year 2007 in order to conserve the limited natural fresh water resources.
BRACKISH GROUNDWATER RESOURCES IN BAHRAIN 297
It is recommended to construct a new numerical model which represents the whole areal extent of the Rus-UER aquifer and resolve the rigidity of the present model. Such model can be used to test and evaluate the aquifer performance under the proposed different development scenarios such as modification of the wellfield design and the effect of artificial recharge on salinity and potentiometric heads. This will greatly help in making future management decisions regarding the utilization of brackish groundwater in Bahrain.
Acknowledgements
This study was funded by the Bahrain Center for Studies and Research (BCSR), Bahrain. The authors would like to thank the Water Production Directorate, Bahrain Ministry of Works, Power and Water, for providing all data relating to the wellfield salinity and abstraction, and the Water Resources Directorate, Bahrain Ministry of Commerce and Agriculture for providing the observation wells data. Permission for publication of this article from BCSR is gratefully acknowledged.
References
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