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Priority aquifer systems
IHP network on groundwater protection in the Arab region Series of Best Procedures in Research and Development
Edited by Fatma A.R. Attia and Abdin Salih
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IHP-V Technical Documents in Hydrology No. 54 UNESCO, Paris, 2002
The designations employed and the presentation of material throughout the publication do not imply the expression of any
opinion whatsoever on the part of UNESCO concerning the legal status of any country, territory, city or of its authorities, or
concerning the delimitation of its frontiers or boundaries.
In the framework of the activities of the "IHP- Network on Groundwater Protection in the Arab Region”, a series of documents are found helpful, namely in the field of "Best Procedures in Research and Development", "Manuals for Field Investigations for
Groundwater Development", "Monitoring and Evaluation" and others. This document is the first in the area of Best Procedures in Research and Development.
This document has been prepared by a number of experts from the Arab Region aiming at disseminating their experiences with respect to research and development, lessons learned and best procedures for selected aquifers in the region.
The contributors are listed below on alphabetical order:
- Abdin Salih UCO
- AbdulAziz S. Al-Turbak Saudi Arabia
- Ahmad R. Khater Egypt
- Ahmad R. Allam Egypt
- Ahmad Memo Tunisia
- Fatma A.R. Attia Egypt
- Jamal Abdu Sudan
- Jean Khouri ACSAD
- Mohamed Al-Eryani Yemen
- Mohamed Shatanawi Jordan
- Waleed Al-Zubari Bahrain
- Yussuf Al-Moji Yemen
ACRONYMS
ACSAD The Arab Center for the Studies of Arid Zones and Dry Zones ALECSO Arab League Educational, Cultural, and Scientific Organization CRTW Cairo Round Table Workshop
IHP International Hydrological Program of UNESCO IHP-V The fifth phase (1996-2001) of IHP
NSAS Nubian Sandstone Aquifer System
RIGW Research Institute for Groundwater, NWRC, MPWWR, Egypt ROSTAS Regional Office for Science and Technology of UNESCO
UCO UNESCO Cairo Office
UNESCO United Nations Educational, Scientific and Cultural Organization WG Working Group involved in the meeting
SUMMARY
General Background
The initiative behind the preparation of this document is vehicled by UCO as a result of recommendations made by the Arab countries. The document is the first in a series of documents aimed at supporting decisions and actions related to groundwater protection in the region.
The main aquifers identified (as first priority) for research and development are: (regional non-renewable aquifers with deep groundwater which are generally subjected to over- exploitation; (ii) coastal aquifers, which are subjected to saline water intrusion and upconing; (iii) wadi aquifers, which consist major resources to the region; and (iv) local aquifer subjected to pollution due to recharge with non-conventional water.
This document contains relevant experiences of some Arab countries in research and development, including lessons learned and proposed procedures.
Non-Renewable Aquifers
1.1 Regional aquifers of large extents are common in the Arab region. They are
characterized generally by deep groundwater which is almost non-renewable due to:
(i) either the small magnitude of the transmissivity of the water bearing formation;
and/or (ii) the change in climatic conditions from humid to extremely arid.
1.2 Due to the large extension of these aquifer systems and the large variations in physical conditions (topography, accessibility, soils, etc.), developments are generally confined to local areas (e.g depressions) where depth to groundwater is minimum.
1.3 Since development on local areas may, in the long-term, affect the regional behavior of the aquifer system, a proper identification of the impacts on the regional aquifer is a must.
1.4 Numerical simulation is considered the most powerful mean to identify the state of groundwater along with predicting the impact of future developments. For the case of regional aquifer systems, two simultaneous steps are carried out, first the regional groundwater flow is simulated, then the local. Interaction between both should take place all over the exercise (Figure 1.10).
1.5 For the regional model, the primary objective would be to identify boundaries and boundary conditions for local models, and create a consciences between the groundwater users (across national boundaries). Accordingly, accurate data on
boundaries are very important. In general terms, the development of regional models may not need detailed field investigations.
1.6 The objectives of the local models include, among others: (i) design of well fields;
(ii) investigate the impact of various development scenarios (depth to groundwater, change in quality, etc.); and (iii) design control measures, based on socio-economic conditions. This dictates a proper knowledge on the hydrodynamic status of the system, including geometry and lithostratigraphy, hydraulic parameters, recharge and discharge, etc; which are generally based on a variety of field tests.
1.7 The following steps are proposed to set up a regional model:
- Elaborate a conceptual model for the aquifer system, using available information.
- Perform sensitivity analyses to identify the important data to be collected to improve accuracy and reliability of the model output. Major parameters subject to this analysis include: (i) boundaries of the model, including both geological (e.g. no-flow) and hydrological boundaries (e.g. water divide, fresh-saline interface); (ii) hydraulic parameters (e.g. transmissivity, storativity); (iii) recharge-discharge rates; and (iv) groundwater heads.
- Collect additional data and information to fill the gaps for sensitive data and information.
- Refine the model and calibrate it, based on available historical data.
1.8 The following steps are proposed to set up a local model:
- Locate static boundaries of development sub-regions/areas from regional model.
- Simulate (initially) expected stresses (withdrawals) to define the dynamic boundaries (extension of future impacts).
- Perform sensitivity analyses to identify the important data to be collected to improve accuracy and reliability of the model output. Major parameters subject to this analysis include: (i) geological and hydrological boundaries;
(ii) hydraulic parameters (e.g. transmissivity, storativity); (iii) natural and artificial recharge (rates, quality); (iv) natural (springs) and artificial (wells, evaporation) discharge rates; and (v) groundwater heads.
- Carry out detailed field investigations to fill the gaps for sensitive data and information within the dynamic boundaries (step 2).
- Develop and simulate strategies with the help of the local models. Estimate, based on economic data and related criteria, the sustainable rate of
groundwater withdrawals.
- Transfer impacts to regional model and refine local boundaries (note that interaction between regional and local models should take place all over the exercise).
- Transfer impacts to regional model and check impacts on neighboring countries (i.e. across national boundaries).
- Finalize strategies, taking into consideration results of step 8.
Groundwater Affected By Saline Intrusion and Upconing
2.1 Salinity is one of the problems affecting the sustainable use of groundwater. It is caused by a variety of reasons, among which: (i) subsurface migration of sea water in coastal aquifers; (ii) upward movement of sea water that has entered the aquifer during deposition or during a high stand of the sea in the past geologic times (connate water); (iii) seepage of highly saline water concentrated by evaporation in tidal lagoons, playas, or other enclosed areas (e.g. sabkhas); (iv) return flow from irrigated lands; and (v) infiltration of saline wastes.
2.2 The Arab region is characterized by long coastal lines. Accordingly, the first two mechanisms affect a much larger portion of the aquifers compared to the effect of the other mechanisms. This does mean that the others are of low importance as agriculture direct salinity problems also represent another important problem, especially under the climatic condition of the Arab region and the large areas under irrigation, especially in the absence of proper drainage means that efficiently evacuate subsurface drainage water.
2.3 In the case of deltaic aquifer systems, the most appropriate method to investigate the problem of saline water intrusion and upconing should be in three dimensions.
However, due to the complexity of 3-D models, the aquifer extent, and the lack of data in most regions, it may be difficult to determine the hydraulic and
hydrochemical parameters of the aquifer in three dimensions. Hence, two-
dimensional cross sectional models may be considered more elaborate and can easily solve the problem. Such models can be adapted to simulate a number of cross- sections with different hydrogeological conditions representing the variable conditions.
2.4 Accordingly, for the case of deltaic aquifer systems, the following approach is proposed:
- Selection of a number of representative cross sections representing the
- Collection of spatial and temporal data on wells, water levels, salinity, discharge and recharge, rainfall, evaporation, water channels characteristics.
- Determination of hydraulic parameters and infiltration tests.
- Isotope and chemical analysis to determine water types and sources of salinity.
- Selection of suitable simulation package(s), based on the width of the transition zone.
- Initial simulation and identification of information gaps for further field measurements and investigations (including geophysical measurements, drillings, aquifer tests, sampling and analysis, etc.).
- Incorporation of regional geological history and associated dynamic boundaries.
- Calibration of the model(s).
- Simulation and testing of development/management scenarios.
2.5 In the case of fissured aquifer systems, the problem is somewhat different. The three-dimensional distribution and chemistry of saline groundwater is normally not understood because of the natural reluctance to invest in drilling in such areas.
Geophysical investigations may alleviate the problem partially; however, hydrochemical investigations need to be as comprehensive as possible.
2.6 Accordingly, for the case of fissured aquifer systems, the proposed procedure would be:
- Carry out hydrochemical investigations to determine the origin of the chemical composition of groundwater and relation between water bodies of different compositions and rock chemistry.
- Determine the occurrence and origin of saline groundwater in relation to flow, based on field measurements of heads and quality.
- Carry out geophysical investigations to identify the physical framework within which saline intrusion/upconing occurs.
- Determine all relevant hydraulic parameters.
- Select the simulation method suitable to the physical conditions. Where the zone of diffusion is small, solutions given by piston flow models (interface models) may provide satisfactory results. When the zone of diffusion zone is rather extensive density-dependent models are more suitable.
- Develop appropriate criteria for the development and management of groundwater to ensure sustainability.
- In the case of problems, develop appropriate remedial measures for rehabilitation.
Wadi Aquifers
3.1 The main features of wadi hydrology is the intricate relationship between wadi runoff and wadi aquifers. Such surface-groundwater relationship needs to be elucidated. Groundwater availability and renewability in arid and semi-arid regions should, thus,be based on an integrated approach. Since direct recharge is generally small or negligible, recharge from runoff is the principal factor for ensuring the sustainability of the resource.
3.2 The experiences gained in the area of wadis indicates that research and development in wadi aquifer systems should take into consideration man's influence on the natural system, i.e., wadi development, water use and impact of such human activities. Research needs, however, may vary from one country to another.
3.3 Since aridity is the general condition of the Arab region, the general framework for research and development could be summarized as follows:
- Definition and delineation of wadi flow systems and their relation to regional flow or deep groundwater systems.
- Evaluation of wadi-bed transmission losses by: (i) undertaking infiltration experiments; and (ii) measurement of vertical distribution of moisture content (using neutron probes or TDR technique).
- Survey of lithofacies variations to define spatial heterogeneity, and upscaling point profile measurements.
- Estimation of aquifer recharge by quantifying the relation between transmission losses and recharge. This needs data collection on the following: (i) residual moisture in the unsaturated zone; (ii) evaporation losses; and (iii) subsurface geology.
- Measurement of stage and wadi flow in the upper, middle and lower reaches.
- Periodic measurement of groundwater levels (deep and shallow).
- Water sampling, and chemical and isotopic analyses for runoff, wadi aquifer and deep groundwater.
- Design and installation of a permanent observation network to provide: (i)
rates; and (iv) determination of quality changes.
3.4 Research on wadi aquifer systems often requires data and information to help a proper assessment of recharge, flow, and quality. The use of remote sensing and nuclear techniques are good tools in such studies. Data on the groundwater use can be collected either by the responsible institutions, or by the water users
(stakeholder), or both. Landsat imagery may provide appropriate information on land use and land cover.
3.5 Hydrogeologic mapping and vulnerability maps are good guides in such studies if properly developed from field measurements. The development of such maps should thus be considered part of the systematic approach for investigating and protecting wadi aquifer systems.
3.6 The process of scaling-up from point data to areal estimates of hydrogeologic variable is a complicated issue that needs considerable efforts by hydrogeologists.
However, with the developments in hardware and software (e.g. GIS and numerical techniques), estimates of areal hydrogeologic variables can be improved, providing the existence of well trained staff.
Aquifers Subjected to Recharge with Non-Conventional Water
4.1 The major portion of the Arab Region is already facing water scarcity, which will become more serious in the near future. Several countries have started recycling water either through direct use/application or by storing it in the aquifer for
additional treatment and conditioning. The quality of the recharge water is generally moderate to poor, which may result in several environmental problems that are difficult to recover.
4.2 The water quality requirements for injection of storm water and treated wastewater into aquifers for storage and reuse are based on three objectives: (i) management of clogging; (ii) protection or improvement of groundwater quality; and (iii) ensuring that the quality of recovered water is fit for its intended use.
4.3 Before any decision on recharging groundwater, the following steps are proposed:
- Determination of suitable aquifer availability. This step dictates the performance of hydrogeological investigations, including: (i) geophysical survey; (ii) drilling of few bore holes for checking stratigraphy; (iii)
estimation of hydraulic properties of the aquifer (e.g. hydraulic conductivity, transmissivity, storativity) with the help of aquifer tests; (iv) depth to
groundwater; and (v) groundwater quality.
- Availability of storm water and/or wastewater. The availability of storm
include rainfall intensity, rainfall-runoff routing, evaporation, and hydrography.
- Demand for reclaimed/stored water. The injected water is meant to be reused for various purposes based on its quality. The demand for such water is an important factor that should be investigated prior to deciding on
injection/recharge.
- Feasibility of injection/recharge. This depends to a great extent on the cost of recharge, recovery, and the return from water use. Feasibility studies are generally based on experimental tests that precede the implementation of a full scale project.
- Quality of water used in injection/recharge. Both the injection/recharge water and groundwater should be sampled and analyzed. Analyses include physical, chemical, and biological parameters. Comparison with the standards of intended use is then made. If any parameter in the injection water is exceeded, pretreatment should be carried out.
- Change in water quality. The injected/recharged water will move and mix with the native groundwater. Electrical conductivity (EC) may be used as an indicator for mixing and movement. EC measurements are made at various distances downstream the injection well(s) or recharge facility. Biological parameters should also be checked to determine any attenuation of
contaminants.
- Quality of recovered water. This item, together with the percent of water recovered, is also one of the factors that determine the economy of injection.
A study should be made to investigate the percent recoverable water; while quality is carried out through continuous sampling and analysis of pumped water.
- Clogging and development of injection well. Another factor that affects the feasibility of injection is clogging. Clogging is a result of many factors, namely accumulation of zooplankton and particulate, microbial growth, gas binding, mobilization of aquifer fines, and iron and manganese precipitation.
The well can be redeveloped by various means (e.g. airlifting), and facility cleaned by removing the blanket.
4.4 Some criteria are important here to ensure the protection of native groundwater:
- Pretreatment. It is always recommended to remove or reduce contaminants at the surface before injection/recharge, especially those which are resistant to degradation in the aquifer. Pretreatment methods may include passive and engineering systems, or a combination of these. Detention storage, as part of pretreatment, is desirable in reducing the variability of the quality of
- Monitoring. In homogeneous aquifers, at least three monitoring wells are needed, being located downgradient on the flow path (based on the regional groundwater flow). In heterogeneous or fractured aquifers, more wells may be needed. Pumping wells may also be used as monitoring wells if cited appropriately.
- Guidance for maximum concentration of pollutants in injectant/recharged water. Guidelines values for individual parameters should be determined by water quality objectives and by the capacity for treatment within the aquifer.
The information in Table 4.2 may be used as a general guide on several parameters which are commonly regarded as constraints to recharge/injection with reclaimed water.
- Minimum residence time. A minimum residence time for undisinfected injectant of 50 days is recommended to provide an acceptable degree of health protection when recovered water is used for recreation or irrigation.
Shorter residence times may be allowed if source water quality and exposure paths provide an equivalent level of public health protection. Protozoa and viruses may have longer survival times in aquifers, and recovered water should not be used for water supplies unless there is either a field-based assessment of the potential for breakthrough of these species, or the injected or recovered water is suitably treated.
4.5 A well considered environmental management plan presents the opportunity to trade-off sustainable treatment of the aquifer against pretreatment of
injectant/recharge water, providing all objectives can be met. Aquifer treatment effects on recovered water may be increased for some contaminants by lengthening the flow path and residence time in the aquifer by the arrangement of
injection/recharge and recovery wells. The increase in knowledge in the sustainable attenuation capacity of aquifers as a result of information collected at these sites will allow improved design of matching pretreatment systems, and allow costs to be contained, particularly for non-potable uses of recovered waters from aquifers which are initially non-potable.
4.6 There is a strong linkage between environmental sustainability and economic feasibility for proposed recharge/injection sites. Operations which are marginally economic may be unable to meet the required costs of monitoring, and the level of operational management may be compromised. It is important therefore that the recharge/injection facility is economically viable, taking account of monitoring costs.
TABLE OF CONTENTS
Page SUMMARY
GENERAL BACKGROUND
Introduction 2
Specifics of the Arab Region 2
Challenges 3
IHP-Groundwater protection network 4
Preparatory activities 4
Initiation of the network 5
Main activities 5
1. REGIONAL NON-RENEWABLE AQUIFERS
1.1 General background 7
1.2 The Nubian sandstone aquifer 9
1.3 Proposed procedures for research 15
2. GROUNDWATER AFFECTED BY SALINE INTRUSION AND UPCONING
2.1 General background 23
2.2 The Nile delta of Egypt 23
2.3 Saline water intrusion in fissured carbonates in Bahrain 27
3. WADI AQUIFERS
3.1 General background 38
3.2 The Tihama wadi aquifer system in Yemen 41
3.3 Management of wadi aquifers in Saudi Arabia 43
3.4 Proposed procedures for research 46
4. AQUIFERS SUBJECTED TO RECHARGE WITH NONCONVENTIONAL WATER
4.1 General background 49
4.2 Artificial recharge with nonconventional water 49
4.3 Experiment on artificial recharge-Egypt case 52
4.4 Proposed procedure 57
ANNEX I: Investigations and tools related to saline water intrusion
and upconing 62
ANNEX II: An example of a framework for wadi aquifers studies 71 SUMMARY IN ARABIC
GENERAL BACKGROUND
INTRODUCTION
1. Groundwater is an important source of fresh water in arid and semi-arid regions. It is either the main source of fresh water, or a complementary source to surface water. It can be either renewable or fossil. Accordingly, groundwater protection should be one of the top priority in such regions to ensure sustainability of developments.
2. At this point, two questions can be raised: "what is protection?" and why should we put emphasis on groundwater.
3. As to what, i.e. the meaning of protection, a possible definition is "preservation from loss, waste, or harm (damage)". In other words, "prevention of non-beneficial use and prevention of wastage and harm which includes quantitative as well as qualitative wastage and harm".
4. As to why, various reasons can be given, including: (i) surface water has received more attention in the past due to the respective easy way to deal with it as a visible resource, while groundwater studies have always been lacking behind; (ii) for the Arab region, in specific, the majority of the countries are suffering from insufficient (or no) surface water resources, and most rivers in the region originate from outside the borders; and (iii) groundwater is unique in its characteristics which makes it a safe source of water.
5. Inspite of the non-visibility of groundwater, it has many eye-catching functions in close relation to economical, ecological, and public health. To protect groundwater resources, one should first identify its potential, in terms of available quantities and quality, the present water use/allocations, state of degradation (both quantitative and qualitative), future needs, and finally, possible (simple and applicable) conservation means and remediation measures.
SPECIFICS OF THE ARAB REGION
6. The Arab region can be distinguished in four main hydrological zones, the Mediterranean-Atlantic Ocean, the Sub-Sahara, the Saharan, and The Red Sea- Indian Ocean zones.
a) The Mediterranean-Atlantic Ocean Zone consists of coastal watersheds or low- Aquifers have specific characteristics that distinguish them from other water bodies: (i) they can help in removing suspended solids and disease-causing organisms; (ii) they can store water in quantities exceeding those which are or conceivably could be stored in all natural and artificial surface-water bodies; (iii) they can regulate the water temperature and its chemical quality; (iv) they transport water from areas of recharge to areas of need; and (v) they slow-down the natural discharge of water to the surface. As such, aquifers can be utilized as strategic storage reservoirs for water to make up the bulk of the dry-weather flow of streams.
lands in the borders of mountains facing the Mediterranean Sea and Atlantic Ocean where large volumes of fresh water are captured regularly from continental rains.
Countries falling in this category (Morocco and northern part of Algeria, Lebanon, and small portions of Syria and Iraq) have generally renewable water resources (rivers and shallow groundwater).
b) The Sub-Saharan Zone consists mainly of arid to semi-arid low-lands traversed by international rivers originating in other countries having humid climates.
Countries falling under this category (Egypt, Mauritania, northern Sudan, and most of Syria and Iraq) have renewable rivers supplemented mainly from deep non- renewable groundwater.
c) The Saharan Zone is a flat desert region where extremely arid conditions prevail and rare or no renewable water resources exist. Countries falling under this
category (Libya, Tunisia, Jordan, and a large portion of the Arabian Peninsula) depend mainly on deep non-renewable groundwater existing in extensive sedimentary rocks.
d) The Red Sea-Indian Ocean Zone comprises a variety of formations and
geomorphological features. Countries falling under this category (southern part and coastal areas of Sudan, western part and coastal plains of Yemen, southwestern part of Saudi Arabia, northern part and coastal plains of Oman, and northern Emirates) have either big rivers that provide recharge to wadi deposits (Sudan), or have intermittent short-duration flash floods resulting from heavy summer rains in the mountains as well as perennial flows along the upstream areas of some wadis.
7. The Arab region is suffering at present from water scarcity; which will become more severe in the near future. The prevailing arid to semi-arid climate is generally a common feature. Most rivers originate outside the borders and their water is almost fully utilized. Although extensive aquifer systems are encountered, groundwater contained in such systems is almost non-renewable. Renewable groundwater, on the other hand, is limited to specific regions where aquifers of limited extent are prevailing. The percapita fresh water in some countries is as low as 100 m3/year, or even less. This has already resulted in or will soon dictate recycling of used water (multiple use); thus approaching closed water systems.
8. The present problems related to groundwater management in the Arab region can be summarized as follows: (i) extensive drawdowns that are affecting the sustainability of the resource; (ii) sea water intrusion in coastal aquifers; (iii) pollution from various sources; (iv) vague or poor institutional set-up; (v) lack of proper
management tools and related knowledge; (vi) poor enforcement of legislation; and (vii) lack of public participation and public awareness.
CHALLENGES
9. Groundwater protection is thus a major challenge. It dictates the initiation and implementation of several actions, including: (i) the implementation of monitoring system(s); (ii) development of proper tools to support groundwater protection, e.g.
hydrogeological maps, groundwater potential maps, groundwater vulnerability maps, etc.; (iii) enforcement of legislation, including well licensing, types of agro-
chemicals, proper disposal of effluents, etc.; (iv) special attention to environmental concerns and caution with the use of sewage water in irrigation, which should be limited to regions of low vulnerability; (v) raising public awareness in relation to the protection of the resource; (vi) development of simple technologies to protect
groundwater, which should be based on multi-criteria analysis of their
appropriateness, capability to improve, and adoption by the users; (vii) preparation for significant changes which may present opportunities, but with potential adverse impacts.
IHP-GROUNDWATER PROTECTION NETWORK
10. The main objectives of the network are to promote information exchange among researchers and decision makers within the Arab region and other regions; and to demonstrate the technical, economical, institutional, and environmental feasibility and constraints of groundwater protection means and technologies.
11. The program will thus contribute to rational development and appropriate management of groundwater resources leading ultimately to sustainable socio- economic development through: (i) assisting in strengthening and coordinating research concerning groundwater assessment, planning, and management; (ii) assisting in the transfer of adequate technology from other countries to the Arab region and among the Arab countries; and (iii) assisting in disseminating and exchanging information on the state-of-the-art of groundwater protection though meetings, workshops, etc.
PREPARATORY ACTIVITIES
12. The initiative on groundwater protection in the Arab region has been launched in 1994 by the UNESCO Cairo Office. The first task of the initiative was a regional working group formed of four renowned experts. The working group has prepared a state-of-the-art report on the subject, as well as a priority list and broad documents for extra-budgetary funding. The findings of the working group were endorsed by an expert meeting organized jointly with ACSAD, held in Damascus in the period 16- 18 October 1994.
13. The Damascus meeting had also emphasized the importance of utilizing the group work in preparing and organizing a Training For Trainers Regional Workshop on
"Groundwater Protection". This activity was successfully implemented by
UCO/ACSAD/Tunisian IHP committee within the period 10-20 October 1995 in Tunis.
14. In support of UCO's (1994/95) concentration on "Groundwater Protection", the fifth cycle of the International Hydrological Program of UNESCO (IHP-V) has explicitly devoted a whole theme to "Groundwater Resources at Risk" (theme 3) and implicitly included groundwater (in one way or another) in at least three of its remaining seven themes.
15. The importance of groundwater protection in the Arab region has also been
endorsed as a priority subject by the participants of the sixth regional meeting of the Arab IHP committees, held in Jordan within the period 3-6 December 1995.
Consequently, it has been adopted by UCO as a concentration area of its 1996/97 program in hydrology.
INITIATION OF THE NETWORK
16. In cooperation with ACSAD and the NWRC of Egypt, UCO convened a Regional workshop on Groundwater Protection in Cairo, within the period 5-6 June 1996. At the end of this activity, the participants proposed a regional network on
"Groundwater Protection" and elected the Research Institute for Groundwater (RIGW) as the focal institute and convener of the network.
MAIN ACTIVITIES
17. The main activities of the Network include:
a) Initiation of a "Data Base" for Groundwater protection.
b) Development of "Manuals" in various areas related to groundwater protection (various users categories).
c) Identification of training courses and packages in various areas related to groundwater protection (various types of audience).
d) Development of "Best procedures in research and development" for the priority aquifers).
e) Selection of suitable "Dissemination means" and their preliminary outlines (web site, newsletter, etc.).
f) Decision on types of awareness means.
18. Four priority aquifers have been selected as of importance to the region, as follows:
a) Regional non-renewable aquifers with deep groundwater subjected to overexploitation.
b) Coastal aquifers subjected to saline water intrusion.
c) Wadi aquifers.
d) Local aquifers subjected to pollution due to recharge with non-conventional water (e.g. sewage).
19. A working group (WG) has been formed from experts from the region and resource persons identified for each type of aquifers. It was decided to acquire more than one person for each type of priority aquifer to ensure inclusion of the various
hydrogeological conditions and experiences.
20. This document has been prepared based on the contributions received from the WG members. A chapter has been reserved for each type of aquifers to ensure a better focus.
REGIONAL NON-RENEWABLE AQUIFERS
1.1 GENERAL BACKGROUND Sustainable Groundwater Use
Sustainable development is generally a function of the availability of the natural resource base over time. To attain such a development, management and conservation of natural resources and orientation of technological and institutional change should be planned in such a manner as to ensure the attainment of continued satisfaction of human needs for present and future generations. Scarcity and misuse of fresh water pose a serious and growing threat to sustainable development and protection of the environment.
Identification of aspects that make development unsustainable has been more successful than the development of remedial measures that reduce or eliminate those undesirable effects. For example, if sustainable groundwater resources development is considered, it is known that excessive use of fertilizer and pesticide in agriculture may impair the use of groundwater for drinking purposes. However, responses to eliminate such a threat are usually very slow.
In arid zones, which is the case of the major part of the Arab Region, sustainability of groundwater is a major concern due to the large variability of groundwater recharge.
Replenishable groundwater resources may be available in regions where present day recharge is potentially very low or nil (e.g. Nubian Sandstone in North Africa). In such regions, it may not be possible to correlate directly the presence of higher rainfall belts in certain areas in recent years with the size of the local groundwater storage (see Figure 1.1).
Issues of Sustainable Groundwater Use in Arid Zones
The main aim of groundwater development and management is to ensure the sustainability of the resource
and developments based on it. This requires, Figure 1.1 Recharge-Discharge of the NSAS among others, a good knowledge of the system
configuration and its present state which are the bases for predicting the system response to future stresses.
Geophysical investigations, which are generally considered cheap tools in defining the configuration of aquifer systems, meet several limitations in arid zones. Main reasons are:
(i) the relatively dry medium in the shallow horizons which make them very resistant
resulting in false resistivity; and (ii) existence of saline water in the deep horizons making the penetration of the signal difficult and its discrimination low.
A good understanding of the present state of the system is generally based on clear identification of boundaries, flow rates, and hydraulic characteristics. In arid zones,
recharge is very limited and recovery time of pumping (in aquifer tests) is very long which result in a poor estimation of flow (water balance) and hydraulic properties.
Sustainable Use of Non-Renewable Groundwater
Figure 1.1 is a possible illustration of non-renewable groundwater. Another possible
condition is the case when the magnitude of the horizontal transmissivity of the aquifer is so small for a timely replenishment of well withdrawals. This results
For non-renewable water resources, the definition of sustainability is not a straightforward one. Non-renewable (fossil) water can not be treated like minerals and petrol.
Water is life. This does not mean that fossil groundwater should be left under the ground.
Sustainable rates could be defined as "the rate of withdrawal that ensures the availability of the resource for present and future generations at an economical cost", taking into
consideration poverty alleviation and protection of the environment. The time horizon is a major factor that should be determined prior to any development. For example, since such aquifer systems are generally located in desert remote areas, the question to be answered first is whether people would move and settle in such areas or water be transferred to people. When the policy is to move people, other factors of importance include: (i) the period needed to settle people and start initial developmental activities, namely agricultural;
(ii) the period needed to introduce other types of economic developments (e.g. agro- industries, mining, tourism, etc.); and the period needed for full production and recovery of investments.
Normally, the early stages would involve higher rates of groundwater withdrawal, which are expected to reduce after other types of development are introduced. Another factor that can be considered is the possible climatic change, or in other words, the possibility for a returned pluvial cycle.
Regional Non-Renewable Aquifer Systems
A number of regional non-renewable aquifer systems in the Arab region have been
identified (ACSAD, 1990). The most important are the North Sahara in the western region (north-west Africa), and the Nubian sandstone in the middle region (north-east Africa), as shown in Figure 1.2.
Efforts have been made by the countries sharing those aquifer systems to assess the volume of water in storage, recharge, and the sustainable rates of groundwater withdrawal. In this chapter, a summary on the state of knowledge concerning the Nubian sandstone system is made as an example to illustrate the main issues facing groundwater development and management in the research approach. This is followed by a recommended systematic approach for the study of regional aquifer systems.
Figure 1.2 Non-Renewable Aquifer Systems in North Africa 1.2 THE NUBIAN SANDSTONE AQUIFER
Extension
The Nubian sandstone aquifer system (Figure 1.3) covers SE Libya, Egypt, NE Chad, and North Sudan, with a total area of about two million square km. To the east, the border is formed by basement outcrops of the Nubian Plate; to the south and west by the basement outcrops of the Kordofan Block and the Ennedi or Tibesti Mountains. The northern boundary is formed by the saline-fresh water zone which is either due to recent sea water intrusion or old marine water that has not been flushed from the system (needs further investigations). The Nubian sandstone system is thus considered an almost closed system, except in the south where inflow from the Nile sandstone system (in Sudan) is possible.
Hydrogeology
The dominant geological units of the Nubian sandstone aquifer system (NSAS), including Kufra and Dakhla basins, have undergone different geological developments. The formation of the Kufra basin began in the Early Paleozoic and was complete at the end of the Lower Cretaceous. The Dakhla basin was presumably formed at the beginning of the Cretaceous (at least its southern portion). North of the Dakhla Oasis, Paleozoic are found.
The NSAS is subdivided by uplifts into sub-basins (Figure 1.3). However, no evidences of hydrologic separations exist. The different development history of the sub-basins is
extremely critical for the hydrogeological interpretation, concerning thicknesses and extensions of lithological units.
Based on available subsurface information, some parameters have been estimated as
follows: (i) the thickness of the NSAS varies from 500 to 3,500 m (see Figure 1.4); (ii) the groundwater volume in storage is about 150,000 km3; and (iii) the aquifer hydraulic
conductivities vary from 10-4 to 10-8 m/sec (see Figure 1.5).
Note: Northern boundary is tentative
Figure 1.3 Extension of the Nubian Sandstone Aquifer System (after Klitzsch, 1972)
Groundwater Flow
Attempts have been made to estimate possible recharge to the NSAS. Based on available hydrologic and isotopic analyses, the following means of recharge are assumed:
1. South of the mouth of Atbara river in the Dongola area and along Lake Nasser.
2. Along the SW boundary in Chad.
3. Local infiltration during wet periods (based on isotope analysis including D, 18O, and 14C), as shown in Figure 1.5.
Figure 1.4 Thickness of the NSAS Figure 1.5 Fence Diagram Of Hydraulic (after Klitzsch, 1972) Conductivities (NSAS)
Figure 1.6 Frequency Distribution of Apparent 14C Ages of NSA Groundwater Modelling trials have been made using available data, resulting in: (i) a water balance for the NSAS (Figure 1.1); and (ii) steady-state and transient calibration. This effort resulted in the following: (i) contour map for the spatial variability of the transmissivity (Figure 1.7);
(ii) groundwater heads distribution for steady-state filling conditions (Figure 1.8); and (iii) groundwater heads distribution for transient conditions (1980), as shown in Figure 1.9.
It could be concluded (initially) that:
1. There has always been a change between humid and arid phases, each lasting for several thousand years. After an arid depletion of the aquifer, groundwater is replenished over large areas in the entire unconfined part as soon as humid climatic conditions prevail allowing for a hydrodynamic balance (8,000 years ago).
2. The natural discharge does not directly depend on climatic conditions, but on the distribution of groundwater heads.
3. After the groundwater heads decline, natural discharge starts to diminish, approaching actual recharge.
4. The regional flow within the system is very small compared with flow occurring within sub-regions.
5. Deficits prevail in the confined part of the aquifer, but is compensated by regional flow S-N due to the higher topographic elevation in the unconfined part.
Accordingly, under arid conditions water comes mainly from unconfined storage.
6. Recent recharge and discharge can be neglected, being very limited; and the Nile acts as a drainage channel.
Figure 1.7 Contour Lines of Calibrated Transmissivity (1,000 m2/day)
7. Sub-regional and local discharges are found of little effect on the balance of the regional model. Thus abstractions have to be limited to few representative centers.
It was thus recommended that for a realistic estimation of sustainable groundwater potential, local models should be developed for development areas (e.g. Egyptian oases, Kufra) with boundaries taken from the regional model.
Figure 1.8 Groundwater Contour Lines for Steady-State Figure 1.9 Groundwater Contour Lines at the end of the Filling-up Conditions Short-Term Simulation (1980)
1.3 PROPOSED PROCEDURES FOR RESEARCH General
Due to the large extension of regional aquifer systems and the large variations in physical conditions (topography, accessibility, soils, etc.), developments are generally confined to local areas (e.g depressions) where depth to groundwater is minimum. However, any development on local areas requires a proper identification of the impacts on the regional aquifer.
Numerical simulation is considered the most powerful mean to identify the state of groundwater along with predicting the impact of future developments. For the case of regional aquifer systems, two simultaneous steps are carried out, first the regional
groundwater flow is simulated, then the local. Interaction between both should take place all over the exercise (Figure 1.10).
The quality of the model(s) depends, to a large extent, on the quality of the input data.
REGIONAL MODEL
Internal Boundaries
LOCAL MODELS
Figure 1.10 Interaction Between Regional and Local Models Data and Information
A variety of data and information are generally needed to develop a model. However, the accuracy of such data depends to a large extent on the scale of the model, the mesh size, and the objective(s) of the model. For the regional model, the primary objective would be to identify boundaries and boundary conditions for local models, and create a consciences between the groundwater users (across national boundaries). Accordingly, accurate data on specific hydraulic parameters may not be of importance; while external and internal
boundaries are very important. In general terms, the development of regional models may not need detailed field investigations.
On the other hand, the objectives of a local model include, among others: (i) design of well fields; (ii) investigate the impact of various development scenarios (depth to groundwater, change in quality, etc.); and (iii) design control measures, based on socio-economic
conditions. This dictates a proper knowledge on the hydrodynamic status of the system, including geometry and lithostratigraphy, hydraulic parameters, recharge and discharge, etc; which are generally based on a variety of field tests. Tables 1.1 and 1.2 summarize the main data and information needed for research and development of regional and local aquifer systems, respectively.
Table 1.1 Data and Maps Needed for the Study of Regional Aquifer Systems
TYPE IDENTIFICATION
1. Topography - contour lines of land elevation (5-10 m interval).
2. Surface
Hydrology - location and extent of surface bodies, including streams and other natural or man-made water courses.
3. Geomorphology - topographical high lands and low lands (based on areal photographs); and - drainage systems.
4. Subsurface geology (see Boxes 1 and 2)
- main geological structures;
- water-bearing formations, their depth, and outcrop patterns; and - cross-sections showing vertical and horizontal relationships between sediments, geological structures, and basement.
5. Aquifers - confined, unconfined, and semi-confining formations.
6. Aquifer extent - lateral extent based on geologic evolution.
7. Boundaries (see
box no. 3) - Geologic and hydrologic boundaries, including outcrop of basement rocks, faults, water divides, large water bodies, main rivers, saline-fresh water interface, etc..
8. Aquifer
characteristics - Tentative magnitude and spatial variation of aquifer characteristics, including transmissivity, hydraulic conductivity, and storativity.
9. Water levels - Maps showing surface water and groundwater levels.
10. Recharge/
Discharge - Recharge and discharge areas can be delineated from areal photographs and topographic maps.
Setting Up Regional Models
The development and management of groundwater in regional aquifer systems is better carried by simulating groundwater flow (dynamics) of the system. Simulation models are very helpful tools that support the understanding of the system behavior to stresses.
The following steps are proposed to set up a regional model:
1. Elaborate a conceptual model for the aquifer system, using available information.
2. Perform sensitivity analyses to identify the important data to be collected to improve accuracy and reliability of the model output. Major parameters subject to this analysis include: (i) boundaries of the model, including both geological (e.g.
no-flow) and hydrological boundaries (e.g. water divide, fresh-saline interface); (ii) hydraulic parameters (e.g. transmissivity, storativity); (iii) recharge-discharge rates;
and (iv) groundwater heads.
Table 1.2 Data and Maps Needed for the Study of Local Areas Within
Regional Aquifer Systems
TYPE IDENTIFICATION
1. Topography and surface features
- contour lines of land elevation (0.5-1 m interval); and - location of production, observation, and exploration wells.
2. Surface
Hydrology - all surface bodies; and
- streams and other natural or man-made water courses.
3.
Geomorphology - topographical high lands and low lands (based on areal photographs and land surveys); and
- drainage systems.
4. Subsurface geology (see Boxes 1 and 2)
- geological structures;
- water-bearing formations, their depth, and outcrop patterns; and
- cross-sections showing vertical and horizontal relationships between sediments, geological structures, and basement.
5. Types of
aquifers - water-bearing formations, confining beds (layers), and semi-confining formations;
and
- types of aquifers (e.g. unconfined, leaky, confined).
6. Aquifer thickness and lateral extent
- lateral extent based on geology and geophysical surveys; and
- a map showing the variation in aquifer(s) thickness based on geophysical measurements/logs, in the form of contours.
7. Boundaries
(see box no. 3) - No-flow boundaries represented by: (i) outcrop of basement rocks; (ii) a fault isolating permeable rocks; and (iii) a water divide. These can be either internal or external.
- Head-controlled boundaries (also known as potential or hydraulic head), represented by: (i) large water bodies where water levels can be considered constant; and (ii) water courses and irrigation canals where water levels may change considerably with time. They can also be internal or external (saline-fresh water interface).
- Flow-controlled boundaries, also known as recharge or discharge boundaries.
8. Aquifer
characteristics - The magnitude and spatial variation of aquifer characteristics, including: (i) transmissivity for confined aquifers; (ii) hydraulic conductivity for all types of aquifers; (iii) storativity; (iv) and hydraulic resistance of semi-confining layers and water bodies.
9. Water levels - Maps showing the (initial) distribution in: (i) depth to groundwater; (ii) groundwater head; and (iii) water levels in water bodies.
10. Recharge - Recharge areas can be delineated from areal photographs and topographic maps.
- In arid regions where the aquifers occur, estimation of recharge is very difficult.
Isotope hydrology may represent a promising tool in this case.
- For confined aquifers, recharge may occur along streams and water courses if hydraulic properties allow, or along faults from adjacent aquifers.
- In the case of unconfined aquifers, surface activities (e.g. irrigation, disposal of domestic/industrial effluent) and rainfall represent the major sources of recharge.
11. Discharge - Similar to recharge areas, discharge areas can also be delineated from areal and topographic maps.
- Discharge can occur naturally through faults, from springs, base flow, or low topographic areas. It can occur artificially through groundwater pumpage.
Example of an isopach map showing the net thickness of an aquifer
Structural map showing the impermeable base of an aquifer
3. Collect additional data and information to fill the gaps for sensitive data and information.
4. Refine the model and calibrate it, based on available historical data.
Setting Up Local Models
The following steps are proposed to set up a local model:
1. Locate static boundaries of development sub-regions/areas from regional model.
2. Simulate (initially) expected stresses (withdrawals) to define the dynamic boundaries (extension of future impacts).
3. Perform sensitivity analyses to identify the important data to be collected to Different types of boundaries
1. No-flow boundary;
2. External no-flow boundary;
3. Internal no-flow boundary;
4 and 5. Internal head-controlled boundaries;
6. External head-controlled boundary; and 7. Free surface boundary.
improve accuracy and reliability of the model output. Major parameters subject to this analysis include: (i) geological and hydrological boundaries; (ii) hydraulic parameters (e.g. transmissivity, storativity); (iii) natural and artificial recharge (rates, quality); (iv) natural (springs) and artificial (wells, evaporation) discharge rates; and (v) groundwater heads.
4. Carry out detailed field investigations to fill the gaps for sensitive data and information within the dynamic boundaries (step 2).
5. Refine (smaller mesh) and calibrate the model.
6. Develop and simulate strategies with the help of the local models. Estimate, based on economic data and related criteria, the sustainable rate of groundwater withdrawals.
7. Transfer impacts to regional model and refine local boundaries (note that interaction between regional and local models should take place all over the exercise).
8. Transfer impacts to regional model and check impacts on neighboring countries (i.e. across national boundaries).
9. Finalize strategies, taking into consideration results of step 8.
GROUNDWATER AFFECTED BY SALINE INTRUSION
AND UPCONING
2.1 GENERAL BACKGROUND
Salinity is one of the problems affecting the sustainable use of groundwater. It is caused by a variety of reasons, among which: (i) subsurface migration of sea water in coastal
aquifers; (ii) upward movement of sea water that has entered the aquifer during deposition or during a high stand of the sea in the past geologic times (connate water); (iii) seepage of highly saline water concentrated by evaporation in tidal lagoons, playas, or other enclosed areas (e.g. sabkhas); (iv) return flow from irrigated lands; and (v) infiltration of saline wastes.
The Arab region is characterized by long coastal lines. Accordingly, the first two
mechanisms affect a much larger portion of the aquifers compared to the effect of the other mechanisms. This does mean that the others are of low importance as agriculture direct salinity problems also represent another important problem, especially under the climatic condition of the Arab region and the large areas under irrigation, especially in the absence of proper drainage means that efficiently evacuate subsurface drainage water.
In this chapter, two cases will be presented and discussed, the Egyptian case concerning the Nile delta, and the Barainian case for fissured carbonate aquifer systems. At the end, a systematic approach for research and implementation will be presented together with a brief explanation of field investigations and research tools involved in the study of saline
intrusion and upconing.
2.2 THE NILE DELTA OF EGYPT Coastal Aquifer Systems in Egypt
Egypt has a relatively long coast line (Figure 2.1), including: (i) more than 950 km along the Mediterranean sea in the north; (ii) 300 km along the Gulf of Aqaba; (iii) 400 km along the Gulf of Suez; and (iv) 1,200 km along the Red sea in the east. The coastal area of the Nile Delta is one of the most populated regions in Egypt and is still attracting more
population and a variety of developmental activities. One of the major constraints facing the development is the availability of water. Although groundwater may represent a suitable fresh water resource, it is believed that any further groundwater development may result in saline water intrusion and upconing, thus rendering the development not sustainable.
To properly manage groundwater, it is important to understand the patterns of sea water movement and mixing between fresh and saline groundwater. Although mathematical ability to describe such phenomena has progressed steadily since 1959, the mechanism is not completely understood due to the complexity of the problem.
This specific research is thus considered one of the priorities of the country with respect to:
1. The decision on future development in the region.
2. The decision on the locations and rice cultivation areas to be cultivated rice.
3. The decision on safe groundwater development.
Figure 2.1 General Map of Egypt Hydrogeology of the Deltaic Aquifer System
The Nile Delta extends from Cairo to the Mediterranean sea. It covers an area of
approximately 15,000 km2. The deltaic aquifer system is composed of Quaternary and late Tertiary graded sand and gravel (intercalated with lay lenses) underlain by Pliocene clay.
The aquifer thickness increases from 70 m (at Cairo) in the south, to about 1,000 m along the Mediterranean coast, as shown in Figure 2.2.
Over the last three decades, a great number of studies and investigations have been implemented, resulting in a better understanding of the flow regime and hydrochemistry.
However, the major portion of the observation wells range in depth between 40 and 100 m.
Accordingly, much of the focus has been on the upper part of the aquifer system. Few observations, however, helped with the identification of some features, as follows:
Figure 2.2 A Typical Geologic Cross-Section In the Middle of the Delta 1. The aquifer is a multi-layer aquifer with a possible degree of heterogeneity and anisotropy.
2. The toe of the transition zone is estimated by few researchers as being located at a distance of 100 km south of the coast line.
3. There exists a possibility of mixing between fresh and saline water in a zone 400 m thick, which is considered as the transition zone.
These findings pointed the importance of taking the vertical direction into consideration in analyzing the fresh/saline water distribution within the Delta.
Present Approach in Analyzing Saline Water Intrusion
The early studies on saline water intrusion in the Nile Delta aquifer were based on assumptions to simplify the problem. The main assumptions included:
1. A sharp interface approach using the Ghyben-Herzberg relationship. However, the limited available data indicated that the width of the transition zone is relatively large compared to the aquifer thickness. Thus rendering the assumption of a sharp interface unrealistic.
2. The aquifer is a one-layer homogeneous and isotropic aquifer system.
3. The transition zone between the two fluids was simulated using density dependent models with hydrodynamic dispersion of salts. However, the calibration of the models was very poor due to the lack of data.
4. The routine hydrochemical analyses of the shallow groundwater is considered the only measure of the saline water intrusion and upconing.
Problems Related to The Present Approach
1. The simulation models developed so far assumed a steady-state distribution of salt in groundwater, which is difficult to maintain in the light of the geological evolution of the Delta. These models can only be used as preliminary tools to describe the flow regime.
2. The hydraulic interaction between surface water and groundwater is not taken into consideration in the present approach.
3. The interaction between the alluvium and the sea is not quite proven from the available data and information.
Best Procedures
Based on the natural boundaries of deltaic aquifer systems, the most appropriate method to investigate the problem of saline water intrusion and upconing should be in three
dimensions. However, due to the complexity of 3-D models, the aquifer extent, and the lack of data in most regions, it may be difficult to determine the hydraulic and
hydrochemical parameters of the aquifer in three dimensions. Hence, two-dimensional cross sectional models may be considered more elaborate and can easily solve the problem. Such models can be adapted to simulate a number of cross-sections with different
hydrogeological conditions representing the variable conditions.
Accordingly, the following approach is proposed:
1. Selection of a number of representative cross sections representing the systems, based on available data.
2. Collection of spatial and temporal data on wells, water levels, salinity, discharge and recharge, rainfall, evaporation, water channels characteristics.
3. Determination of hydraulic parameters and infiltration tests.
4. Isotope and chemical analysis to determine water types and sources of salinity.
5. Selection of suitable simulation package(s), based on the width of the transition zone.
6. Initial simulation and identification of information gaps for further field measurements and investigations (including geophysical measurements, drillings, aquifer tests, sampling and analysis, etc.).
7. Incorporation of regional geological history and associated dynamic boundaries.
8. Calibration of the model(s).
9. Simulation and testing of development/management scenarios.
2.3 SALINE WATER INTRUSION IN FISSURED CARBONATE AQUIFERS IN BAHRAIN
Physical Setting
The State of Bahrain consists of an archipelago of 36 islands located in the Arabian Gulf between Saudi Arabia in the west and Qatar in the east (Figure 2.3), with a total area of 700 km2, and a population of about half a million. Bahrain islands, like most of the Arabian Peninsulas, have an arid to extremely arid environment. It is characterized by irregular, scanty rainfall, and high evaporation rates. The average annual rainfall is less than 80 mm, while the annual potential evapotranspiration amounts to 1850 mm. Bahrain has no surface water and groundwater is the only natural resource of fresh water.
Groundwater
The Dammam aquifer, a limestone dolomite confined coastal aquifer, is the only aquifer containing natural relatively fresh water resource available for Bahrain. Abstraction from the aquifer is about 218 million m3/year (1995/1996) and accounts for more than 75% of the country's total consumption (the rest satisfied from desalination plants).
The Dammam aquifer in Bahrain forms only a small part of the extensive regional aquifer system known as the Eastern Arabian Aquifer, which extends from central Saudi Arabia (where the aquifer rocks outcrop and its main recharge area is located) to the Arabian Gulf Water, including Bahrain, Kuwait, and southern Qatar. It is this regional aquifer that provides Bahrain with its water.
Within Bahrain Islands, the Dammam aquifer consists of two zones, A and B, developed in the Alat (15-25 m) and Khobar (40-49 m), limestone members of the Dammam
Figure 2.3 Location Map of Bahrain
formation, respectively (Figure 2.4). The aquifer is confined in most of Bahrain from the top by the Neogene claystones (10-60 m), and from the bottom by the shale beds of Sharks Tooth shale member (8-20 m) of the Dammam formation, in addition to the anhydride and shale deposits located on the top of the Rus formation. The two aquifer zones are separated by a semi-confining layer of the Orange Marl member (9-15 m) of the same formation.
However, the separation or confinement between the two is weak due to improper well completion practices, where the two aquifers are being opened in most wells in Bahrain. In west Bahrain Island in the locality of Hamalah, the Alat limestone and the Orange Marl are eroded due to structural elevation at that locality, and the B zone occurs under unconfined conditions.
The A zone has limited hydraulic properties, and is basically a granular formation with average hydraulic conductivity of about 14 m/day (average transmissivity about 350 m2/day). The B zone, developed in highly fractured limestones and dolomite, is the principal aquifer in Bahrain, as it provides more than 70% of the total groundwater withdrawals. True hydraulic conductivity of this zone is difficult to estimate due to its fissured nature. However, it is believed that it is at least one order of magnitude higher than that of A (average of 350 to 520 m/day, with extreme values of up to 1500 m/day for the fissured upper 5-10 m).
Figure 2.4 A Cross Section Showing the Dammam Aquifer Zonation in Bahrain A third zone (C), is developed in the Rus formation and the upper parts of Umm Er Radhuma formation. The Rus formation is composed of fractured chalky dolomitic limestone with subsidiary shale and anhydride intercalation in its upper section. In the central and eastern areas of Bahrain, the Rus formation has undergone extensive solution of its anhydride, which led to the collapse of the overlying rocks and, more importantly, reducing the effectiveness of the upper confining layer which causes a relatively easier migration of its water into the Dammam aquifer in those areas (GDC, 1983). The C-zone contains brackish to saline water with a typical salinity of about 12000 mg/l, and occurs in the form of large stagnant lens in Bahrain main island. The groundwater salinity increases downward reaching about 40000 mg/l. Due to its high salinity, C-zone water utilization is restricted to industrial purposes in the north central island (Zubari and Khater, 1995).
Dammam Aquifer Development in Bahrain Prior to 1925, Bahrain's population
depended on the naturally flowing fresh water, land and offshore springs, to meet its water needs. The estimated natural springs (about 15 land and 20 offshore) discharge from the aquifer was about 90 Mm3/year.
However, development activities have significantly increased the abstraction rate from the Dammam aquifer which substituted the natural springs
discharge with time. Figure 2.5 Groundwater Abstraction in Bahrain (1920-1997)
The total abstraction from the aquifer was approximately 65 Mm3/year in the early fifties, increased to about 112 Mm3/year in the mid sixties, and reached about 145 in the mid eighties. The present total abstraction (1997) from the aquifer is about 218 Mm3/year.
According to many researchers, the steady-state rate of underflow from Eastern Saudi Arabia to Bahrain ranges between 100 to 112 Mm3/year, and has been recommended to be the Dammam aquifer safe yield. Comparison between the present abstraction and recharge rates indicates that the Dammam aquifer is being over-drafted by a rate of about 100 Mm3/year, i.e., about twice its suggested safe yield.
Changes in Groundwater Heads
Comparison between the groundwater head map observed in 1991 with that reported for the period prior to 1925 (Figure 2.6), which represents the steady-state conditions of the
system, it can be noted that the groundwater heads have dropped, on the average, by about 4 m. The maximum drop is observed at the up-gradient western areas of Bahrain reaching about 5 m; while the minimum drop is observed at the down-gradient area of the aquifer in the east coast reaching about 2 m, where groundwater is in direct contact with sea water.
Figure 2.6 Changes in Groundwater Heads in Bahrain (1925-1992) Changes in Groundwater Salinity
The reduction of groundwater storage in the Dammam aquifer and the drop in groundwater heads have led to the marked deterioration of groundwater quality mainly due to salt water intrusion (Figure 2.7). It can be observed that more than half of the aquifer has experienced increase in groundwater salinity due to its over-exploitation during the development
process. This leaves a small portion for future developments in the competing major water user sectors (agriculture and domestic). If the past schemes and trends continue,
groundwater will soon become totally contaminated with salt.
