SPATIAL AND TEMPORAL TRENDS IN GROUNDWATER RESOURCES IN

Emirates Journal for Engineering Research, 10 (1), 57-67 (2005) (Regular Paper)

57

SPATIAL AND TEMPORAL TRENDS IN GROUNDWATER RESOURCES IN BAHRAIN, 1992-2002

W. K. Zubari

Desert and Arid Zones Sciences Program, College of Graduate Studies, Arabian Gulf University, P.O. Box: 26671, Bahrain, E-mail: [email protected]

(Received February 2005 and accepted June 2005)

تعتبر المياه الجوفية لمكمن الدمام المائي المصدر الطبيعي الوحيد للمياه العذبة لتلبية الطلب المتزايد على المياه في ولقد أدى الاستغلال المتواصل ولفترات طويلة لهذا الخزان المائي إلى انخفاض مستمر لمستوياته . مملكة البحرين

المالحة المحيطة به، مؤدياً - بين مياهه العذبة نسبياً والمياه المالحة وشبهالمائية وانقلاب الانحدارات الهيدروليكيةوفي هذه الدراسة، تم القيام بمسح للمستويات المائية والملحية لمياه الخزان . إلى غزوها له وتدهور نوعية مياهه

لقد بينت التحاليل المكانية و. لتحري عملية التدهور المائي به2002-2001المائي على مستوى المملكة في الفترة لمتغيرات الحالة إلى أن أغلبية السطح البيزومتري للخزان في البحرين قد انحدر إلى مستويات أسفل مستوى سطح البحر، وبأن المياه التي تغذي خزان الدمام من الجانب الشرقي للملكة العربية السعودية تتأثر نوعيتها بشكل آبير

ت ملوحة عالية، مصدرها الرئيسي مياه البحر والمياه شبه المالحة الواقعة أسفل الخزان بسبب اختلاطها بمياه ذاإضافة لذلك، وبالاعتماد على البيانات المتوفرة من الدراسات السابقة، تم تقصي التغيرات الزمانية . المائي

لهذه الفترة إلى حدوث هبوط ، ولقد أشارت التحاليل الزمانية2002-1992لمتغيرات الحالة بخزان الدمام للفترة عام ومستمر في المستويات المائية، وأن مستوى الملوحة بمياه الخزان في عموم مناطق البحرين قد ازداد بنسبة

، وأن عملية تدهور نوعية المياه الجوفية قد أصبحت أآثر انتشارا خلال فترة التقييم هذه عن الفترات 30%ية والزمانية التي أجريت في هذه الدراسة على أن مخططات وجهود الإدارة المائية ودللت التحاليل المكان. السابقة

التي تم اتخاذها في العشر سنوات الماضية بواسطة سلطات المياه في المملكة بهدف السيطرة على تدهور نوعية لمياه الجوفية، وبأنه إذا لم المياه الجوفية، وترآزت على جانب تعظيم المياه المتاحة، لم تكن فعالة في تحسين حالة ا

يتم تخفيض مستويات سحب المياه الجوفية إلى مستويات مقاربة لمعدل تغذيتها فإن نوعية هذه المياه ستستمر في ولإحداث تخفيض هام ومؤثر في معدلات سحب . التدهور مؤدية في النهاية إلى خسارة هذا المورد المائي بالكامل

إنه يقترح أن يتم التوجه نحو التدخلات والإجراءات الإدارية ذات العلاقة بجانب إدارة المياه الجوفية بالمملكة، ف .الطلب، مثل تطبيق الحافز الاقتصادي بفرض تعرفة على الكميات المسحوبة من المياه الجوفية

In Bahrain, groundwater in the Dammam aquifer is the only natural source of fresh water to meet the country's increasing water demands. Prolonged over-exploitation of the aquifer has led to continuous water level decline, reversal of hydraulic gradients between the relatively fresh-water aquifer and adjacent brackish- and saline- water bodies, and their encroachment and salinization of the aquifer. In this study, a countrywide potentiometry and salinity survey has been conducted in the year 2001/2002 to investigate the aquifer deterioration process. Spatial analysis of the aquifer state variables indicated that most of the potentiometric surface of the Dammam aquifer has declined to below sea level, and the quality of the aquifer recharge water, received from eastern Saudi Arabia, has been significantly influenced by inland mixing with higher concentration waters, mainly seawater and underlying brackish water. Furthermore, temporal changes in the aquifer's state variables over the period 1992-2002 are examined using salinity and potentiometry data from a previous survey. Temporal analysis indicated a general water level decline, and that the salinity in most of the aquifer areas has exhibited an increase by about 30%, and the deterioration process has become more widespread over the assessment period. Analysis of the spatial and temporal changes of the Dammam aquifer indicated that management schemes and efforts undertaken in the last 10 years by the water authorities have not been effective in modifying the aquifer's conditions, and that unless the current levels of abstraction from the aquifer are brought back closer to the level of renewable recharge, groundwater quality would be completely ruined. In order to have significant reduction in groundwater abstraction rates, it is suggested that a shift towards demand-side interventions is made, such as employing the economic incentive of groundwater pricing.

W. K. Zubari

58 Emirates Journal for Engineering Research, Vol. 10, No.1, 2005

1. INTRODUCTION Bahrain consists of an archipelago of 33 islands located in the Arabian Gulf, about midway between Saudi Arabia and Qatar (Figure 1). Like most of the Arabian Peninsula, the climate is characterized by high temperatures, erratic often scanty, rainfall of less than 80 mm/yr, and high evapotranspiration rate mounting to more than 1800 mm/yr1. Under such arid conditions, Bahrain has no surface water, and groundwater is the only natural source of freshwater supply.

The country's fresh water demands, amounting to about 310 Mm3/y in 2002, are met mainly by groundwater abstraction (68.4%) and desalination plants (27.1%), and to a lesser extent (4.5%) by treated wastewater. However, heavy reliance on the Dammam aquifer, the principal aquifer in Bahrain, to meet the country's ever-increasing water demands, particularly during the last three decades, have resulted in a rapid decline in its potentiometric levels and a reversal of hydraulic gradients between the relatively freshwater aquifer and adjacent brackish- and saline-water bodies. As a direct consequence, encroachment of these waters into the aquifer has occurred, causing its salinization. The deterioration of groundwater quality has restricted the use of this resource from the invaded parts of the aquifer, which are estimated in 1992 to be more than half of the original pre-development groundwater reservoir in Bahrain2. If this trend continues, the only natural source of water in Bahrain may be lost, and might dictate expensive remedies unless proper aquifer management is considered.

Past and current groundwater management measures undertaken by the water authorities in Bahrain to control and reverse the deterioration of groundwater quality generally aim at reducing groundwater abstraction in the agricultural, municipal, and the industrial sectors. These measures have concentrated mainly on the supply augmentation side, and were manifested by the construction of desalination plants, reuse of treated wastewater, and artificial recharge by storm runoff water. Demand management efforts and measures were also made, however only recently, and were represented in the domestic sector (24% of total groundwater consumption) by escalating tariff system, rationing demand, pressure control, leak detection, installation of household water saving equipment, and public awareness campaigns. In the agricultural sector (main groundwater consumer at 73%), these were manifested by subsidizing modern irrigation systems, metering groundwater wells, programs for changing crop type to less water consuming crops, and agricultural extension services. Furthermore, legislations were issued to restrict groundwater use by the industrial sector.

Figure 1. Location of Bahrain and groundwater sampling sites

The above management measures, undertaken by

the water authorities in Bahrain since the early 1980s, aim primarily at reducing groundwater abstraction from the Dammam aquifer and enhancing its storage. The overall objective is to allow the aquifer's potentiometric levels to recover and modify its water quality, thus increasing its sustainability as a usable water resource as well as a strategic reserve for the country.

In this study, the spatial distribution of the Dammam aquifer state variables (potentiometry and salinity) are investigated through a countrywide potentiometry and hydrochemical survey conducted in 2001/02. Furthermore, a countrywide potentiometry and hydrochemical survey conducted in 1991/923 is used as a baseline data to evaluate the effectiveness of groundwater management efforts and schemes in terms of temporal changes in the aquifer water level and quality over the period 1992-2002. Accordingly, further measures required to control future deterioration in the groundwater quality are suggested.

440000 450000 460000 470000UTM East, meters

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Areas where the aquifer rocks outcrop

Sampling site withfull chemical analysis

Sampling site withEC/TDS analysis

Muharraq Isl.

Sitrah Isl.

Umm Naasan Isl.

Mohamm-adiyah Isl.

Nabih Saleh Isl.

Manama

Budayaa

Hamalah

Zallaq

Wasmiyah

Ras Abu Jarjur

Ras Hayan

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Jiddah Isl.

Qatar

Iran

SaudiArabia

Kuwait

Oman

U. A. Emirates

Bahrain

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Spatial and Temporal Trends in Groundwater Resources in Bahrain, 1992-2002

Emirates Journal for Engineering Research, Vol. 10, No.1, 2005 59

2. MATERIAL AND METHODS In Bahrain, the Water Resources Directorate (WRD), Ministry of Municipalities and Agriculture, has an established groundwater level monitoring network that consists of 21 boreholes in the Dammam aquifer (Figure 1). All of these observation wells have continuous water-level data since 1980, and are reported on a monthly basis. Annual average water level data were used for preparing the water level contour map for the year 2002 for the spatial analysis of the aquifer potentiometry. Furthermore, a previous water level contour map for the period 19923 was used for the preparation of a water-level difference map for the period 1992-2002, to enable the analysis of the temporal trends in the aquifer potentiometry.

The spatial analysis of groundwater quality was made through the use of a detailed hydrochemical survey for the Dammam aquifer at 283 wells across the country. The survey was carried out in the period 2001/2002. Groundwater samples were taken from constructed piped wells covering all the Dammam aquifer utilization areas in Bahrain (Figure 1). Groundwater samples from the north-central parts of Bahrain were somewhat limited, because most productive wells in these parts have been abandoned; aquifer salinity had reached about 11,000 mg/L since late 1970s4. The sampling procedure involved collecting water samples in polyethylene bottles after pumping the sampled well for at least 5 minutes and then sampling the recovered water in clean 1.5 liter polythene bottles. This was done in an attempt to remove groundwater stored in well bore and to obtain a representative sample. Collected samples were transported directly to the laboratory for chemical analysis. Out of the total 283 water samples, 132 samples were further analyzed for major cations (Na, Ca, Mg, K) and major anions (Cl, SO4, HCO3), total alkalinity (CaCO3), total hardness (CaCO3), pH, Electrical Conductivity (EC), and Total Dissolved Solids (TDS) . The rest of the water samples (151) were only analyzed for EC, and their TDS was calculated from an established relationship between the TDS and EC measured in the 132 wells (Figure 2).

Standard methods were used for the determination of the chemical characteristics of the water samples5. The pH and EC were measured using portable pH and EC meters. The total alkalinity was measured by the acidimetry method using methyl orange as indicator, while the total hardness was determined by EDTA titrimetric method using black T indicator. Na and K were determined by flame photometry. SO4 was determined using standard gravimetric method using Barium Chromate. Cl was measured by the orgenometric method against Silver Nitrate using Potassium Chromate as indicator, and HCO3 by calculation from total alkalinity. Mg was measured by calculation using the formula: Mg in mg/L = (total hardness ml titrant value-Ca hardness ml titrant value)

0 5 10 15 20 25EC, micromhos/cm

0

4000

8000

12000

16000

TDS

, mg/

L

TDS = 622 x EC - 52.8R2 = 0.98

Figure 2. EC vs TDS relationship for Dammam aquifer

hydrochemical survey, 2001/2002

x Mg atomic weight. Ca hardness used to calculate Mg was determined by EDTA titrimetric method using Murexide indicator. Finally the TDS was measured by adding the total anions and cations values of the water sample.

Bahrain does not have an established regularly monitored groundwater quality network, and groundwater quality monitoring is not carried out on regular basis6. However, in the past, groundwater quality monitoring for the Dammam aquifer in Bahrain has been carried out in the form of major country-wide surveys every about 10 years. Three comprehensive groundwater hydrochemical surveys were conducted in the past. These were made in 19707, 1978/794, and 1991/923. The last hydrochemical survey (1991/92) was used with the current survey to produce the salinity difference maps for the period 1991/92-2001/02. Furthermore, the 1978/79 survey4, which surveyed all producing wells in Bahrain at that time (854 wells), was used for the statistical comparisons with the two other recent surveys as a base-line data for the temporal analysis.

Records of groundwater abstraction from the Dammam aquifer were obtained from the WRD records. Groundwater abstraction was monitored on an annual basis since 1980.

3. HYDROGEOLOGIC SETTING The Dammam aquifer, a confined coastal aquifer, is developed in the Tertiary limestone and dolomite members of the Dammam Formation. The Dammam aquifer in Bahrain forms only a small part of an extensive regional aquifer system, termed the Eastern Arabian Aquifer, which extends from central Saudi Arabia, where the aquifer crops out and its main recharge area is located, to the Arabian Gulf, including Bahrain, Kuwait and southern Qatar. The Dammam aquifer in Bahrain receives its water mainly by under-flow from this regional aquifer, with additional, insignificant amounts by rainfall recharge, averaging about 0.5Mm3/y4. According to many researchers 3,8,4,9,

W. K. Zubari


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Alat MemberOrange Marl Member

Khobar Member

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Rus Formation

Umm Er RadhumaFormation

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Bahrain main island Sitrahisland

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Shale and marl (Aquitard)Anhydrite (Aquitard)

AB Dammam aquifer zones

C Rus-Umm Er Radhuma aquifer

0 10 20 30Distance, km

the steady-state rate of underflow from eastern Saudi Arabia to Bahrain ranges between from 100-112 Mm3/y.

The Dammam aquifer system in Bahrain islands, which is an anticlinal structure, consists of two aquifer zones, designated the ‘A’ and ‘B’ zones, developed in the Alat (15-25 m thick) and Khobar (40-49 m thick) limestone members of the Dammam Formation, respectively (Figure 3). In the core of the anticline, at the center of Bahrain main island, the rocks of the Dammam Formation are completely eroded (Figure 1), exposing the rocks of the underlying Rus Formation (early Eocene). The Dammam aquifer system is confined in most of Bahrain from above by the claystones of the Neogene Formation (10-60 m thick), and from the base by the shale beds of the Sharks Tooth Shale Member (8-20 m thick) of the Dammam Formation, in addition to the anhydrite and shale deposits in the upperpart located in the top of the Rus Formation. The two aquifer zones are separated by the Orange Marl Member (9-15 m thick) of the same formation. In the western part of Bahrain island, at the locality of Hamalah, the Neogene claystones and parts of the Alat limestone are eroded due to a structural high that exists at that locality, and the aquifer is unconfined (Figure 1).

The ‘A’ aquifer has limited hydraulic properties, where it possesses an average transmissivity of about 350 m2/d. The ‘B’ aquifer zone, developed in highly fractured limestones and dolomites, the principal aquifer in Bahrain, where it provides more than 70% of the total groundwater abstraction. This is due to the high transmissive properties (average transmissivity is about 10,000 m2/d) of the aquifer. Due to the presence

of a semi-confining layer between the two zones and improper well- completion practices, in which both aquifers are tapped by most wells, little variation of water chemistry exists between the ‘A’ and ‘B’ aquifer zones4. Therefore, in this study, the two are considered together to represent the Dammam aquifer.

A third aquifer zone, termed locally as ‘C’, is developed in the Rus Formation and the upper parts of the Umm Er Radhuma (UER) Formation (Paleocene to early Eocene). The Rus Formation is composed of fractured chalky dolomitic limestone, with subsidiary shale and anhydrite intercalations in its upper section. The Rus Formation in the central and eastern parts of Bahrain has undergone extensive solution of its anhydrite (Figure 3), which has led to the collapse of the overlying rocks, and more importantly, has reduced the effectiveness of its upper confining layer, which causes a relatively easier migration of its water into the Dammam aquifer in those areas10. Groundwater in the 'C' aquifer zone occurs in the form of brackish water lens (8000-15000 mg/L) in Bahrain main island, with total reserves of about 10,000 Mm3. The salinity of groundwater in the aquifer gradually increases with depth. In central Bahrain island, salinity increases from about 8000 mg/L at the water table, at about 5 m elevation above mean sea level, to about 15,000 mg/L at a depth of about 150 m below mean sea level. The brackish water lens is underlain by brine with a salinity of more than 40,000 mg/L. Due to its high salinity, utilization of groundwater from the 'C' aquifer is restricted to industrial purposes in the north-central region and to supplying desalination plants on the eastern coast of Bahrain main island; total abstraction is about 30 Mm3/y.

Figure 3. Hydrogeological cross section, Bahrain



4. GROUNDWATER DEVELOPMENT AND MANAGEMNET

Prior to 1925, Bahrain’s population depended entirely on the naturally flowing land and offshore springs, to meet its domestic and agricultural needs. The estimated natural springs (about 15 land and 20 offshore) discharge from the aquifer was about 95 Mm3/yr 11-13. Mechanized well drilling and abstraction was introduced to Bahrain Islands in 1925 along with oil exploration activities. The oil discovery in 1932 and the sudden increase in the country’s oil revenues in the early 1970s have resulted in a rapid population growth, urban development, industrial and agricultural expansion, and was accompanied by a dramatic increase in water demands and consumption. These demands have been met mainly by abstraction from the Dammam aquifer and hence substituted the natural springs, which experienced a significant reduction in their discharge (53 Mm3 in 195215; 14.7 Mm3 in 197913; 5 Mm3 in 19903). At present, all springs have ceased to flow14.

The total abstraction rate from the aquifer was about 65 Mm3/y in the early 1950s16, increased to about 112 Mm3/y in the mid-1960s12 and reached about 138 Mm3/y in the late 1970s4. In the early 1990s, the total abstraction from the aquifer reached about 205 Mm3/y3, and in 2001 it is estimated at about 218 Mm3/y. Figure 4 displays the abstraction history from the Dammam aquifer in Bahrain. The figure also indicates the Dammam aquifer safe yield, estimated at about 110 Mm3/y (equal to the aquifer’s recharge rate received at Bahrain by lateral under-flow from the up-gradient Saudi Arabia aquifers under steady-state conditions), and it also shows the observed natural springs discharge rates. Total groundwater withdrawal (abstraction + natural springs discharge) in Bahrain have exceeded the suggested safe yield of the aquifer since the early 1960s, and presently is more than twice that rate, indicating that a large proportion of the water abstracted is being taken from the aquifer’s storage. The reduction of the Dammam aquifer storage and drop in its potentiometric surface has led to a marked quality deterioration and salinization of the aquifer’s water mainly by seawater intrusion and brackish water up-flow.

Realizing the deteriorating conditions of groundwater resources and their limited capacity, the general policy of Bahrain water authorities with regard to water use is to allocate groundwater exclusively for the agricultural sector and to reduce groundwater dependency for the municipal sector by constructing additional desalination plants. Furthermore, tertiary treated wastewater is to supplement groundwater used in irrigation, and to meet additional requirements for future agricultural development in the country. These supply augmentation efforts were supplemented rather recentlyby demand management schemes to further reduce water use in general and groundwater in particular.

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Figure 4. Dammam aquifer abstraction history in Bahrain, 1920-2002.

At present, all water management schemes concentrate principally on reducing groundwater abstraction from the Dammam aquifer in the agricultural and municipal sectors, which are the main groundwater consumers, at 73% and 24%, respectively. Reduction of groundwater abstraction in the agricultural sector, currently (2001) at about 160 Mm3/y (73% of total groundwater withdrawal), is conceptualized through the gradual replacement of groundwater used for irrigation by tertiary treated wastewater through an ambitious plan that would provide eventually about 73 Mm3/y of these waters by the year 2010 17-19.

The expansion of the tertiary treated facilities in Bahrain are still under planning and construction phases. Bahrain has a major wastewater treatment plant (Tubli Wastewater Treatment Plant), currently receives 160,000 m3/d (58.4 Mm3/y), but due to treatment capacity limitations only 40,000 (14.6 Mm3/y) of which is tertiary treated and are used for irrigation and landscaping. The rest receives secondary treatment and is disposed of into the sea. It is planned that by the year 2010 the plant will be gradually expanded to receive and tertiary-treat about 200,000 m3/d (73 Mm3/y), all of which all will be used for irrigation.

Furthermore, at present, agricultural development program emphasizing demand management and groundwater conservation has been initiated and is being partially implemented along with the expansion in the utilization of the tertiary treated wastewater in irrigation to substitute groundwater. This program is represented by three major components aimed at reducing groundwater consumption, its efficient utilization, and minimizing groundwater wastage. These are: 1) metering of all groundwater wells; 2) groundwater pricing using an incremental tariff based on volume used; and 3) changing crop types, particularly alfalfa, to less water consuming fodder crops. The first component of the program (metering groundwater wells) has started in 1997 and was implemented successfully, and had a positive impact on groundwater abstraction levels (Figure 4).

W. K. Zubari


However, the second component of the program (groundwater pricing) could not be implemented due to socio-economical and political constraints, and is still pending. The third component of the program (changing crop type) is still in the experimental and extension stage and also receives farmers resistant.

Currently (2001), domestic consumption in Bahrain is about 143 Mm3, with about 90 Mm3/y supplied by desalination, while the rest (53 Mm3) is from groundwater abstraction, and represents about 24% of total groundwater withdrawal. Reduction of groundwater abstraction in the municipal sector is being planned through the construction of a major desalination plant and rehabilitating and upgrading the capacity of its existing three desalination plants. It is expected that desalination capacity increases to about 176 Mm3/y by the year 2010. By this expansion, it is anticipated that groundwater abstraction by the municipal sector would be reduced significantly.

On the demand management side, there have been considerable efforts and major achievements in reducing and/or maintaining water consumption in general and groundwater in particular, through a well-planned schemes in the municipal sector. Unlike the conditions in the agricultural sector, the presence of a conducive environment, represented by a metering/billing system since the early 1980s and the ability to control supply have facilitated the implementation of these schemes. These programs have started initially in 1986 and were enforced in the early 1990s, and were represented by reviewing and applying escalating tariffs system, rationing demand during peak times, pressure control, leak detection, economic incentives, advice and installation of households' water saving equipment, and public awareness campaigns20. The estimated total impact of these programs was in lowering the annual increase in municipal water demand from about 11% in the period before implementing these programs to 4% in 1994 21.

Despite these continuing efforts, groundwater production for municipal purposes has continued to increase, due to the limited capacity of the country’s desalination plants and increasing municipal water requirements resulting from population growth and urban expansion. This has prompted the issuance of a prime ministerial order to maintain a ceiling for groundwater production by the municipal sector to no more than 54 Mm3/y in May 1994, until the commissioning of a new desalination plant with a capacity of about 50 Mm3/y. In April 2000, Al-Hidd desalination plant was commissioned, and groundwater production ceiling was left.

Another management measure was aimed at reducing groundwater utilization by the industrial sector, currently at about 5 Mm3/y (3% of total groundwater withdrawal). Beginning from 1980, the water authorities have issued a legislative order that consisted of prohibiting the industrial sector from using the Dammam aquifer water22 and to meet its

water demands from the underlying brackish water zones (Rus-Umm Er Radhuma aquifer; TDS = 8,000 - 15,000 mg/L). The objectives of this scheme are to control and decrease the abstraction rates from the Dammam aquifer by the expanding industrial sector, and to lower the potentiometric levels in the brackish water zones and decrease the vertical hydraulic gradients between the Dammam aquifer and its underlying brackish water zones in order to reduce their upward migration and salinization of the aquifer4,10.

Moreover, in the early 1990s, artificial recharge was considered by the water authorities in Bahrain to augment depleting groundwater resources and to improve their deteriorating quality. Two pilot projects for rainfall collection into retention basins and its recharge into the Dammam aquifer via gravity recharge wells are being carried out and are still under investigation and evaluation. However, rainfall variability in space and time constrains the proper evaluation of the effectiveness of these schemes23.

In the following section, the current potentiometry and salinity surveys were used to assess the effectiveness of the above management schemes in modifying the deteriorating groundwater resources in Bahrain in terms of spatial and temporal changes in the aquifer's state variables.

5. RESULTS AND DISCUSSION 5.1. Spatial Trend Analysis

Potentiometry

Average potentiometric level contour map for the Dammam aquifer for the year 2002 is displayed in Figure 5. The contour map indicates that most of the potentiometric surface of the Dammam aquifer in Bahrain is below sea level, with two major cones of depression (-2.0 m below sea level) in the western area, where intensive agricultural activities and groundwater abstraction are located. In the north-central region, the potentiometric surface is anomalously high, reaching about +1.2 m above sea level. This high represents the areas where active brackish water up-flow from the underlying Rus Formation is taking place. The highest potentiometric surface in Bahrain is located in north and northwest (-0.4 m at north Muharraq island and -0.2 m at Umm Al-Na'asan island), where the aquifer in Bahrain receives its water by under-flow from Saudi Arabia. The results of the countrywide hydrochemical survey are statistically summarized in Table 1. The Dammam aquifer in Bahrain receives its water by lateral under-flow from the equivalent aquifers in Eastern Saudi Arabia. Groundwater received at Bahrain generally has a background quality limits characterized by: TDS <3000 mg/L, Cl- <1100 mg/L, SO4

2- <470 mg/L,

HCO3- <220 mg/L, Na+ <600 mg/L, Ca2+ <245 mg/L,

Mg2+ <80 mg/L, and K+ <35 mg/L4.24.25.



Figure 5. Potentiometric surface of the Dammam aquifer in Bahrain, 2002

Salinity

A comparison between these background concentrations limits and the means of concentrations obtained from the conducted survey (Table 1), shows an overall increase in the concentration of chemical constituents of the groundwater in Bahrain: 63% of the samples have a TDS >3000 mg/L, t 67% of the samples have Cl- >1100 mg/L, about 68% of the samples have SO4

2- >470 mg/L, 30% of the samples

have HCO3- >220 mg/L, about 66% of the samples

have Na+ >600 mg/L, about 75% of the samples have Mg2+ >80 mg/L, and about 65% of the samples have K+ >35 mg/L. The majority of the sampled groundwater from the Dammam aquifer in Bahrain suggests a widespread inland mixing with higher concentration waters.

Figure 6 shows the spatial distribution of TDS of groundwater in the Dammam aquifer in Bahrain in 2001/02. The figure indicates that better quality water (TDS <3000 mg/L) in the Dammam aquifer flows toward Bahrain from the northwest, where the aquifer is recharged by lateral underflow from its extension in eastern Saudi Arabia. According to the TDS isolines distribution, considerable quality deterioration takes

Figure 6. Spatial distribution of salinity (TDS) in the Dammam aquifer in Bahrain, 2001/2002

Table 1. Statistical Summary of TDS and Major Ions Concentrations in the Dammam Aquifer in 1991/1992.

Constituent N Mean Standard Deviation Mode Median Min Max

TDS, mg/L 283 6051 5046 5482 5125 2097 53562

Cl- , mg/L 132 3119 3831 1888 2078 826 29780

SO42- , mg/L 132 981 543 1251 903 372 3946

HCO3- , mg/L 132 223 31 205 214 132 345

CO32- , mg/L 132 0.0 0.0 0.0 0.0 0.0 0.0

Na+ , mg/L 132 1600 2102 600 1052 417 16480

Ca2+ , mg/L 132 440 168 501 431 168 1093

Mg2+ , mg/L 132 222 235 134 168 61 1885

K+ , mg/L 132 70 76 40 50 20 610

place as the water moves from the northwest to the southeast in the aquifer.

In Bahrain there are four active sources of pollution for the Dammam aquifer that have been identified by several researchers. The areas where these sources are active are marked A, B, C, and D on the salinity map (Figure 6). Since the aquifer in the south-east locality is in hydraulic contact with the sea9, where a seawater - fresh water interface exists at Sitrah Island (TDS ≈16000 mg/L), the salinization process in zone A is essentially attributed to sea water encroachment 26-29,1,2,4,9. Zone B is located over much

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of the north-central region with a salinity ranging between 7000-10,000 mg/L, represents the upward flow from the underlying upper Rus Formations, which has a salinity of 8000-15000 mg/L 9,7,4,10,1,2,29. The zone seems to be extending towards the major municipal and agricultural abstraction areas of the aquifer located north and west of Bahrain, respectively.

In zone C southwest Bahrain, the aquifer lies in the vicinity of an extensive sabkha containing saline water with a TDS of over 10000 mg/L. Therefore, salinization of the aquifer with a TDS of more than 9000 mg/L is most probably caused by the flow of sabkha’s water into the aquifer 30,4,1,2,29. In western Bahrain, and somewhat masked by the brackish water up-flow from the underlying formations, a relatively small salinity anomaly in the TDS of more than 5000 mg/L can be observed (zone D). This salinity anomaly is caused by irrigation return flows in this locality due to the erosion of the aquifer's confining layer, massive agricultural activities manifested by heavy flood irrigation and frequent washing of soil, and inefficient irrigation drainage system 1,29.

Analysis of the chemical data set and relations among various chemical components indicated that the relation between Ca and Mg can be used as a good indicator of the origin of salinization in the Dammam aquifer in Bahrain29. Figure 7 is a scatter plot of the relation between Ca and Mg (mg/L) in groundwater, with the magnitude of the TDS of the samples added as a third dimension to the plot. Uncontaminated groundwater in the Dammam aquifer evolves mainly in the direction of the Rus Formation brackish waters and towards the seawater, indicating that these two mechanisms are currently dominating the salinization process of the Dammam aquifer.

5.2. TEMPORAL TRENDS Potentiometry

Figure 8 displays drawdown contour map between the current survey (2002) and the previous survey conducted in 1992 in the Dammam aquifer. Examination of the spatial distribution of the difference map indicates that there is a general water level drop in the aquifer, and the aquifer did not have any recovery in its water levels in the period from 1992-2002. The maximum drop in the water level, reaching about 2.8 m, is observed at the northwest region of Bahrain, where major groundwater abstraction is taking place and where the aquifer has better water quality.

Salinity A comparison between the TDS levels surveyed in 2001/02 and those of the 1991/923 and 1978/79 (GDC, 1980a) surveys are statistically summarized in Table 2. The table indicates that the mean TDS values for the Dammam aquifer in Bahrain have increased by about 25% over the period 1979-1991, and by about 30%

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Seawater off the east coast of Bahrain

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Upper Rus aquifer brackish water in bahrain (GDC, 1980;1983)

Agricultural drainage water in Hamalaarea, west Bahrain (Raveendran and Madany, 1991)

Typical Dammam aquifer at eastern Saudi Arabia (Hassan and Cagatay, 1994)

Figure 7. Ca vs. Mg plot in the Dammam aquifer in Bahrain, 2001/02

Figure 8. Drawdown contour map for the Dammam aquifer in Bahrain, 1992-2002

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over the period 1992-2002 (with an overall increase of about 60% for the 1979-2002 period), indicating even more spread of salinity deterioration in the period under investigation.

Figure 9 shows a contour map of the salinity difference in the Dammam aquifer for the period 1991/92-2001/02. Examination of the spatial distribution of the TDS temporal changes signifies the deterioration of groundwater quality in most of the aquifer areas in Bahrain; zero difference contour line marks the increase and decrease in the TDS. The map indicates that the upward migration of brackish groundwater from the underlying Rus formation, previously restricted to the north-central parts of Bahrain1, has extended to most of the northwestern areas of Bahrain, where most of the agricultural activities are located and extensive pumping is taking place. Furthermore, at the east coast of Bahrain seawater intrusion has clearly advanced more inland, with salinity increases reaching 9000 mg/L in certain locations (Manama and Muharraq).

Modification in the aquifer TDS levels, reaching about 4000 mg/L, can also be observed at certain locations in Bahrain. However, this reduction is considered insignificant since the TDS of the groundwater at these locations is still high and reaches more than 6000 mg/L. This slight modification is caused by the abandonment of most of the abstraction wells allowing slight recovery in the aquifer in these vicinities.

6. EFFECTIVENESS OF GROUNDWATER MANAGEMENT SCHEMES

Analysis of the spatial and temporal changes of the Dammam aquifer indicates that management schemes and efforts undertaken in the last 10 years have not been effective in modifying aquifer's state variables of potentiometry and salinity. This could be attributed to that the impact of these schemes is still insignificant and/or has been counter-acted by the continuous increase in abstraction rates from the Dammam aquifer by other users, mainly agricultural wells.

As discussed previously, total groundwater withdrawal in Bahrain has exceeded the suggested safe yield of the aquifer since the early 1960s (Figure 4), indicating that a large proportion of the water abstracted is being taken from the aquifer’s storage. It can be observed that groundwater abstraction rates have been intensified during the investigation period (1992-2002); analysis of the abstraction rate of the aquifer in the period of investigation indicates that in 1991/92 groundwater abstraction rate was about 187 Mm3/y, increased to more than 250 Mm3 in 1998, before it could be brought down by the implementa-tion of management schemes to about 218 Mm3/y in 2001/2002 (an increase of about 17% in the period 1991/92-2001/02). In this period, the total abstracted volume from the aquifer was calculated at about 2230

Figure 9. Salinity difference map for the Dammam aquifer in

Bahrain, 1992-2002 Mm3, about 36% increase compared to the total volume abstracted volume (1628 Mm3) from the aquifer in the period 1978/79-1991/92. Obviously the reduction in groundwater abstraction did not have a major impact on groundwater levels and salinity due to the large volumes taken from the aquifer storage during this period, and due to the fact that total abstraction rates are still above the aquifer safe yield (110 Mm3/y).

Table 2. Statistical Comparison for TDS and Abstraction Rates Between the current survey 2001/2002, and the 1978/79 Survey4, and the 1991/92 Survey 3.

1978/79 (N=854)

1991/92 (N=254)

2001/2002 (N=283)

Mean, mg/L 3751 4679 6051 Standard Deviation, mg/L 2385 2723 5046 Mode, mg/L 2432 2240 5482 Median, mg/L 3000 3455 5125 Minimum, mg/L 2016 2120 2086 Maximum, mg/L 29000 16640 53562 Abstraction rate, Mm3/y 138 187 218

7. CONCLUSION AND RECOMMENDATIONS

A countrywide potentiometry and salinity surveys in the Dammam aquifer in Bahrain has been conducted in the year 2001/2002 to investigate the aquifer deterioration process. Spatial analysis of the

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potentiometry and salinity indicated that most of the potentiometric surface of the aquifer is below sea level and that widespread salinization of the aquifer water is occurring. Spatial distribution of the sampled TDS concentrations indicates that the quality of the recharge flow received in Bahrain from the equivalent aquifers in Eastern Saudi Arabia has been significantly influenced by inland mixing with higher concentration waters. Salinization of the aquifer water is caused by: 1) sea water intrusion in eastern Bahrain; 2) brackish water upward flow from the underlying formations in north-central and western Bahrain; 3) migration of sabkha’s water in the south-west; and 4) agricultural drainage water in local areas west of Bahrain.

Temporal analysis of the potentiometric surface (drawdown map) between the current survey and a previous survey conducted in 1992 for the aquifer has indicated a general water level drop in the aquifer, and the aquifer did not have any recovery in its water levels in this period. Furthermore, statistical comparison between the measured TDS in the present survey and a previous survey conducted in 1992 illustrated that the mean TDS values for the Dammam aquifer in Bahrain has increased by about 30% over this period, demonstrating an increase in the deterioration process. This was confirmed by examining the spatial distribution of the TDS temporal changes, where groundwater quality deterioration has become more widespread in the period 1992-2002, caused mainly by brackish water up-flow and seawater intrusion.

Analysis of the spatial and temporal changes of the Dammam aquifer indicated that management schemes and efforts undertaken in the last 10 years by the water authorities in Bahrain, manifested by reducing groundwater abstraction by about 50 Mm3, have not been effective in modifying the aquifer's state variables; the current abstraction rates still exceed the suggested aquifer's safe yield by one fold.

Conclusions drawn from the present study point out that unless the current levels of abstraction from the Dammam aquifer are brought back closer to the level of renewable recharge, groundwater quality would be completely ruined. The potentially ruinous trend, associated with the continuous and prolonged over-abstraction of the aquifer system, could be reversed via implementation of proper management plans designed to restore groundwater quality, with its key factor is the control of abstraction rates.

Moreover, analysis of the implemented groundwater management schemes indicates that most of these schemes lie within the supply-side engineering measures rather than on the demand-side management interventions. Worldwide experiences have shown that significant reductions in abstraction rates can be achieved in the latter, particularly in the agricultural sector, the major groundwater consumer in the country, and where major wastage and major savings can be made. This could be effectively

achieved by introducing economic incentives coupled with enforced legislation, such as groundwater tariffs based on the volume abstracted, especially that metering of most of groundwater wells in Bahrain has been achieved.

This study provides information that may be useful for the management of groundwater resources in Bahrain, especially with respect to sources of salinization and their influence areas. Future artificial recharge areas to the Dammam aquifer by available sources of water (e.g. rainwater, excess tertiary treated wastewater) should be chosen at the north-central and the eastern areas where brackish water up-flow and seawater are active, respectively. Additionally, treated wastewater utilization should be given the priority to be applied in the western agricultural areas. However, it is suggested that irrigation water augmentation by treated wastewater should be used to replace groundwater wells to lower groundwater abstraction and not be used in the expansion of agriculture.

It is recommended that aquifer salinity be monitored on continuous basis, and similar periodic temporal analysis for the aquifer state variables be made against the current survey to measure the effectiveness of the future implemented schemes in Bahrain.

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19. ACE (1997). Production and Utilization of Treated Sewage Effluent – Phase II, Executive Summary. Bahrain Ministry of Works and Agriculture, 14 p.

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24. Sen, Z. and A. Al-Dakheel (1986). Hydrochemical facies evaluation in Umm Er Radhuma Limestone, eastern Saudi Arabia. Ground Water, vol. 24(5), pp. 626-635.

25. Hassan, M. H. and M. N. Cagatay (1994). Hydrogeochemistry of Khobar Aquifer in Eastern Saudi Arabia. Proceedings of The Second Gulf Water Conference, Bahrain, Water Science and Technology Association, Bahrain, pp. 163-178.

26. Hurry, P. (1940). Water Resources of Bahrain Island. Bahrain National Oil Company, Kingdom of Bahrain, 7p.

27. Gulmon, G. W. (1941). Water Resources of Bahrain Island. Bahrain National Oil Company, Kingdom of Bahrain, 24 p.

28. Steineke, J. W. (1942). Groundwater Extraction, Bahrain Island and Coastal Hasa. Bahrain National Oil Company, Kingdom of Bahrain, 13 p.

29. Zubari, W. K. (1999a). The Dammam aquifer in Bahrain-Hydrochemical characterization and alternatives for management of groundwater quality. Hydrogeology Journal, vol. 7, pp. 197-208.

30. Hamilton, N. J. (1965). Water Resources of Bahrain Island. Bahrain National Oil Company, Kingdom of Bahrain, 13p.

31. Hill, W. G. (1953). Submarine Springs Survey. Bahrain National Oil Company, Kingdom of Bahrain, 10p.

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33. Zubari, W. K., Al-Junaid, S. S., and M. N. Alaa El-Din (1996). Impact of deteriorating groundwater quality on desertification of soil in Bahrain (1956-1992) – A case study. Proceedings of the International Conference on Desert Development in the Arab Gulf Countries, Sustainable Development in Arid Zones, Kuwait, KISR, pp. 581-593.

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