dr. ibrahim elshinnawy_final version
TRANSCRIPT
FOOD AND AGRICULTURE ORGANIZATION
OF THE UNITED NATIONS TECHNICAL COOPERATION PROGRAMME
Arab Republic of Egypt
TCP/EGY/3301 (D)
Monitoring of Climate Change Risk Impacts of Sea Level Rise on Groundwater and Agriculture in the Nile Delta
Final Report
National Consultant: Hydro-geologist
Ibrahim Elshinnawy
(March 2013)
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Table of Contents Background ............................................................................................................................... 4 Introductory remarks ................................................................................................................ 4 Chapter I .................................................................................................................................... 9 Baseline Data of the Nile Delta ................................................................................................ 9 1.1 Study Area ........................................................................................................................... 9 1.2 Climate Conditions ........................................................................................................ 10 1.3 Topography and Geomorphology.................................................................................... 10 1.4 Land Use in the Nile Delta Region ................................................................................... 11 1.5 Geology of the Nile Delta Region ..................................................................................... 12
1.5.1 Geologic Setting of the Nile Delta Region ............................................................ 12 1.6 Hydrogeology of the Nile Delta Region ........................................................................... 15
1.6.1 Aquifer Geometry .................................................................................................. 15
1.6.2 Irrigation – Drainage Network in the Nile Delta Region ...................................... 15 1.6.3 Groundwater Aquifer Systems in the Nile Delta Region ...................................... 17 1.6.4 Recharge-Discharge Sources of the Nile Delta Aquifer ........................................ 18 1.6.5 Groundwater Levels in the Nile Delta Aquifer ..................................................... 19
1.6.6 Groundwater Use in the Nile Delta ....................................................................... 20 1.6.7 Hydro-geochemistry of the Nile Delta Aquifer ..................................................... 22
Chapter II ................................................................................................................................. 25 Sea Water Intrusion in the Nile Delta Aquifer ....................................................................... 25 2.1 Sea Water Intrusion Phenomenon ................................................................................ 25 2.2 Modeling Freshwater-Saltwater Interface ..................................................................... 26
Ghyben-Herzberg Principle ....................................................................................................... 26
Hubbert Theory of the Dynamics of Saltwater-Freshwater Interfaces ...................................... 27 Glover Formula of the Dynamics of Freshwater ....................................................................... 28
Henry’s Model to Describe the Mixing Process between Freshwater and Saltwater in
Coastal Aquifers. ....................................................................................................................... 29 Numerical Models of sea water intrusion .................................................................................. 29
Chapter III ................................................................................................................................ 35 3.1 Introduction: ................................................................................................................... 35 3.2 Statistical Analyses for Tide Gauges Data and Land Subsidence .............................. 35 3.3 Expected Sea Level Rise till 2100 ................................................................................. 35 3.4 Vulnerability Assessment .............................................................................................. 36
VA Methodology ....................................................................................................................... 36 Results 37 CoRI scenario ............................................................................................................................ 37
B1 Scenario ............................................................................................................................... 37 A1F1 Scenario ........................................................................................................................... 37
Chapter IV ................................................................................................................................ 39 4 Impact of Sea Level Rise on the Coastal Aquifer of Nile Delta ................................... 39
4.1 Theory of Sea Water Intrusion ...................................................................................... 39 4.2 Saltwater/Freshwater Interface Dynamics ..................................................................... 39
Ghyben-Herzberg Formula ........................................................................................................ 39 Glover Solution ......................................................................................................................... 41
4.3 Sharp Interface Numerical Model ................................................................................. 41 4.4 Results and analysis for a pilot area within the Study Area .......................................... 42 4.5 Remarks on results: ....................................................................................................... 46
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4.6 Conclusion ..................................................................................................................... 47 Chapter V ................................................................................................................................. 49
5.1 Locations of the observation wells ...................................................................................... 49 5.2 Background Monitoring ..................................................................................................... 49
5.3 Monitoring for Specific Land-Use Impacts on Ground-Water Quality ............................. 50 5.3.1 Selection of Groundwater-Quality Indicators................................................................ 50
5.3.2 Criteria for Indicator Selection .................................................................................. 51
5.3.3 Monitoring chloride to determine the extent of saltwater intrusion...................... 52 5.3.4 Real-time, long-term monitoring of chloride concentration .................................. 53
5.4 Numerical modeling: .......................................................................................................... 54
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Background
Introductory remarks
The project started on October 1st, 2010, with a total duration of 24 months, i.e. project closure date is September 30, 2012. While the effective start of project activities has seen some important delays for several reasons that might first of all be attributed to setting up the rather complex institutional and project management arrangements, there can be no doubt that subsequent implementation of the project’s key activities was severely affected by the January / February 2011 revolution and the ensuing deep political crises and turmoil, generally leading to a climate of insecurity as well as frequent changes at key management and decision-making levels in almost all Government institutions, including Ministry of Agriculture and Land Reclemation (MALR) and Agriculture Research Center (ARC). The project inception workshop was held on December 26, 2011, i.e. almost 15 months after project start, bringing together a total of 58 participants and key resource persons, mostly from MALR and its various entities and few representatives from Ministry of Water Resources and Irrigation (MWRI) and other specialized institutions. However, soil and water monitoring activities carried out by Soil, Water and Environment Research Institute (SWERI) started in June 2011 and so far represent the project’s most important activity with respect to the main project objective of improving the current knowledge base on the potential impacts of Sea Level Rise (SLR) and saltwater intrusion on groundwater tables as well as groundwater and soil salinity in the coastal area of the Nile Delta. Soil and water monitoring by SWERI Carrying out the soil and water monitoring activities has been assigned to SWERI on the basis of 2 letters of agreement (LoA) signed to this effect with the FAO Regional Office: LoA 1: Installation of a total of 75 bathometers (or observation wells) to monitor groundwater levels as well as water and soil quality in the coastal belt extending from Alexandria to Port Said. This activity was successfully completed in June 2011. It should be noted that in the meantime, SWERI using its own budget has added approximately 30 bathometers with a view of increasing the sample size (see figure A.1 on the following page). LoA 2: Monitoring and data collection for the following parameters: groundwater depth / depth-to-water table; ground water salinity, and soil salinity. Data collection started immediately after having completed the installation of the 75 bathometers Groundwater monitoring is done on a bi-weekly basis for 2 parameters: (1) groundwater depth / depth to water table; and (2) water salinity (restricted however to measuring only electrical conductivity (Ec), without analyzing salt composition and especially sodium contents). The first monitoring period extended over 5 months from June to October 2011. Data analysis has been completed, raw data
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sets are available and results have been presented mainly in a map format (see figures A.2 and A.3 as examples for the results observed on June 15th, 2011, for both groundwater salinity and depth-to-water table).
Source: SWERI / Dr. Mohamed Ismail, The relation among sea level rise, groundwater table and salinization of soils and groundwater. Presentation given at the project inception workshop.
Figure A.1: Location of the SWERI soil and water monitoring sites
Source: SWERI / Dr. Mohamed Ismail, The relation among sea level rise, groundwater table and salinization of soils and groundwater. Presentation given at the project inception workshop.
Figure A. 2: Groundwater salinity-SWERI soil and water monitoring sites
(15(6/2012)
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Source: SWERI / Dr. Mohamed Ismail, The relation among sea level rise, groundwater table and salinization of soils and groundwater. Presentation given at the project inception workshop.
Figure A.3: Depth-to-water table - SWERI soil and water monitoring sites
(15/6/2012)
It should be noted that at project start and prior to embarking upon the SWERI soil and water monitoring activities, no comprehensive conceptual framework has been developed on the potential impacts of SLR and salt water intrusion on groundwater tables, water and soil salinity and irrigated agriculture. There is no doubt that a proper understanding of these very complex interactions and its underlying cause-effect- relationships can only be achieved on the basis of an integrated and multi-disciplinary approach. Accordingly, the development of the conceptual framework have required bringing together specialized key resource persons from all relevant disciplines and institutions, both within MALR and from outside, particularly the various research institutes established under the National Water Research Center (NWRC) of MWRI such as Ground Water Research Institute (GWRI), Coastal Research Institute (CoRI), Drainage Research Institute (DRI), and Environmental & Climate Research Institute (ECRI)… etc.
As indicated, the main preparatory project activity concerning the in-depth review of all previous work done on the potential impacts of SLR and saltwater intrusion in the Nile Delta could not be successfully. As a direct consequence, the design of the soil and water monitoring activities assigned to SWERI did not benefit from such a systematic review. For this design, the results and experiences already available, particularly with respect to methodological issues of data collection and analysis, including the appropriate selection of monitoring sites, seem to have been hardly taken into consideration and obviously did not sufficiently guide this research process.
It is also worth mentioning that the national hydro-geologist consultant (Prof. Ibrahim Abd El Magid El Shinnawy, Director of CoRI) and the national CC consultant (Prof. Mohamed El Raey, Institute of Graduate Studies and Research, Centre for Climate Change and Risk Reduction, University of Alexandria) have only been recruited when the soil and water monitoring activities of SWERI had
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already started. Consequently, both specialists have not been consulted and associated for identifying the final selection criteria of the monitoring sites.
Following the discussions that the international CC consultant had in the course of his first mission with the national hydrology consultant, the national consultant CC, as well as other key resource persons from DRI and ECRI, has concluded that, based on the current research design, the soil and water monitoring activities of SWERI will most probably not provide the data and results that are urgently needed for improving the current knowledge base on the interaction of SLR, saltwater intrusion, water and soil salinity and agricultural production / productivity. Furthermore, these data will definitely not be sufficient for successfully carrying out groundwater modelling and simulation activities as envisaged under the project as long as these are not completed and correlated with other relevant data.
Project Outputs
The expected outcome of the project is to elaborate a comprehensive monitoring system able to generate sound forecasts of SLR impacts resulting from CC on soil and groundwater in the Nile Delta to serve as a decision-support (information) system for mitigating/adapting to such impacts on the environment, agriculture and population. More specifically, the following outputs are expected from the project:
CC impact monitoring sites (for soil and groundwater) identified, observation wells installed, and geo-referenced;
likely impacts of SLR on groundwater table, and soil and groundwater quality known;
action-oriented information system for predicting the likely impacts and adaptation measures of CC along the coastal areas of the Nile Delta established.
The main project inputs to be provided through the contribution of FAO mainly comprise the mobilization of specialized consulting services, through the recruitment of international as well as national short-term consultants.
ToR for National Hydro-geologist Consultant
The consultant will perform the following duties and responsibilities: 1. Advise and assist the experts of the implementing institutions in the knowledge
base study to collect data on changes in SLR, groundwater table, and salinazation of soil and groundwater along the coastal area of the Nile Delta,
2. Assess local conditions in the envisaged sites where the monitoring and data collection activities would be operational,
3. Work closely with GIS/RS expert and provide technical inputs in the selection of representative sampling areas for monitoring groundwater tables, and soil and groundwater salinity levels.
4. Work closely with GIS/RS expert and climate change experts to collect historical data on soil and groundwater and other geophysical/geological data necessary for building groundwater and soil models.
5. Undertake calibration and validation of groundwater and soil models for local conditions
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6. Work closely with the Statistician/Simulation Specialist and CC Experts and conduct the simulation of the likely impacts of SLR on groundwater table and soil and groundwater salinization.
7. Run calibrated and validated groundwater model and soil model with potential SLR scenarios to assess possible future CC impacts (groundwater table and salinity) in the coastal area of the Nile Delta.
8. Provide technical assistance in the selection of sites and network installation points,
9. Check the operation of the observation wells once installed. 10. Act as a resource person for relevant components of national workshops.
Undertake any other duty related to the post that may be assigned by the FAO Representative/Egypt and direct technical supervisor. Comments on the ToR
Regarding responsibilities 1 and 2; According to previous explanation the consultant has been recruited after the implementation of the observation system that does not represent the data needed to achieve the project main objective.
Regarding responsibilities 3; Institutional setup did not work in an integrated approach since the beginning of the project to facilitate the cooperation process needed between the consultant and GIS/RS consultant.
Regarding responsibilities 4; Memorandum of Understanding (MoU) was initiated by the consultant between Agriculture Research Center (ARC) and National Water Research Center (NWRC) to integrate efforts of both institutions for the purpose of achieving the project objective.
Expected Outputs:
Brief inception technical report indicating the workplan, parameters data that will be measured / collected, methodology, etc….and an end of assignment technical report on the activities performed and the results achieved. Note Most of the background material is cited from the first mission report presented by Blanken J., 2012.
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Chapter I
Baseline Data of the Nile Delta 1.1 Study Area
The area under investigation lies between longitudes 300 00’ and 320 30’ East and latitude 300 00’ and 320 00’ North as shown in Figure (1.1). It covers an area of 31,000 km2 in the Northern Region of Egypt. The area is bounded by the Mediterranean Sea in the North, the apex of the Delta in the South, the Suez Canal in the East and Wadi El-Natrun fault in the West. The two branches of the Nile River form a triangle with its base along the Mediterranean Sea in the North and the apex at the Delta Barrage in the South.
Figure (1.1) Location Map of the Study Area
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1.2 Climate Conditions
The study area is located in an arid climate region with an annual rainfall ranging from 25 mm in the South of the Delta to a maximum value of 200 mm in the North along the Mediterranean Sea. The low values of rainfall do not contribute by any means to the aquifer recharge within the study area. The maximum temperature in the Delta area ranges between 26 and 340 C and minimum temperature between 6 and 130 C. The annual mean values for relative humidity in the morning and the evening are between 60 % and 80 % respectively.
1.3 Topography and Geomorphology
The Nile Delta region is a morphotectonic depression open to the Mediterranean Sea in the North. The Nile enters the Delta 20 km north of Cairo, where it is divided into two branches, Rosetta in the West and Damietta in the East. The Delta region is bounded from the East and Southeast by two main watershed areas rising to more than 500 meters above mean sea level. To the West, the watershed is less developed morphologically, being only 100 meters above mean sea level. The geomorphologic features of the Nile Delta region are shown in Figure (1.2) and are discussed below in details: 1.3.1 The Foreshore Plain
The foreshore plain occupies the area determined by the coastal lakes and their inland extension into brackish water lagoons. Landforms existing in that plain include the wetland areas of the main lakes and the sabkhas. 1.3.2 The Old Coastal Plain The old coastal plain occupies a portion of the Northwest corner of the Nile Delta depression and may be a remnant of the offshore plain, which was submerged in early Holocene times. 1.3.3 The Young Alluvial Plains The Young Alluvial Plains dominate the cultivated lands bordering the channel of the Nile River and its two branches. These plains cover most of the Nile Delta Region (Shata and El-Fayoumy 1969). The irrigation canals and drains break through these plains. The ground surface elevation ranges between 18 meters above mean sea level (msl) in the South to about 5 meters near Tanta sloping gently in the Northward direction by an average value of 10 cm/km (Saleh 1980). Furthermore, the Nile Delta slopes also from East to West making Damietta Branch higher in elevation than Rosetta Branch by about three meters. The area is generally flat and covered by Recent and Quaternary sediments that were formed from the disintegrated igneous and metamorphic rocks of the Ethiopian Plateau and South of Sudan. These sediments were transported by the Nile River and its tributaries to the Delta during the flood seasons for more than 10,000 years. The old and traditionally cultivated lands are dominant in these areas.
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Figure (1.2) Geomorphologic Units for the Nile Delta Region
1.3.4 The Old Alluvial Plains The old alluvial plains are present along both sides of the Nile Delta and are exposed at various heights above the young alluvial plains. They occupy the areas on the Eastern and Western fringes of the Nile Delta. The surface of this plain is gently undulating and displays classical examples of landforms by wind deflation. The surface is also incised by the downstream portions of a good number of dry channels (wadis), which acted as active rivers in Middle Pleistocene.
1.4 Land Use in the Nile Delta Region
The Nile Delta Region is divided into three sub-regions with respect to the two Nile Branches, Damietta and Rosetta. These sub regions are mainly the Western Delta, the Middle Delta and the Eastern Delta. As shown in Figure (1.3), the Nile Delta can be distinguished into three main regions with respect to land use: (i) the agricultural land; (ii) the wetland; and (iii) the desert. The agricultural land includes the traditionally cultivated areas and the newly reclaimed areas. The wetland portion includes the coastal lakes and the marshlands. The desert portion borders the Nile Delta from both east and west sides. Geographically, the traditional cultivated lands are predominated in the Middle Delta while, the reclaimed lands are present in the Eastern and Western Fringes of the Nile Delta. Surface water, groundwater or conjunctive use of irrigation water is applied when appropriate. Many towns and villages and some industrial zones are scattered in the Delta with an extensive population.
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Figure (1.3) Land Use Map for the Nile Delta Region
1.5 Geology of the Nile Delta Region
1.5.1 Geologic Setting of the Nile Delta Region Schlumberger (1984) published a generalized litho-stratigraphic column of the Nile Delta as shown in Figure (1.4). Deep drilling in the Nile Delta Region revealed a very thick sedimentary succession. The litho-stratigraphic cross sections in the Nile Delta are shown in Figure (1.5). There are two main geological units in the Nile Delta region which are:
(1) The Quaternary deposits that include the Holocene and Pleistocene sediments. The Holocene comprises sand dunes, coastal deposits, sabkha deposits and silty clay sediments capping the flood plain. The Pleistocene comprises desert crusts, kurkar ridges and the graded sand and gravel that contain the main water bearing formation. The thickness of the Quaternary aquifer is about 100 m at Cairo, reaches to 1000m at the coast, and decreases to zero to the east and west of the Nile Delta fringes. Figure (1.6) shows the contour lines of the base of the Quaternary aquifer relative to the mean sea level, RIGW (1980)
(2) The Tertiary deposits include the Pliocene, the Miocene, the Oligocene,
the Eocene and the Paleocene sediments. The Pliocene forms the lower
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boundary of the main water bearing formation. The Miocene deposits (e.g. the Moghra) exist underneath the surface with thickness that reaches up to 2000 meters. The Oligocene and Eocene are of a little hydrogeological interest due to their small contribution to groundwater.
Figure (1.4) The Litho-Stratigraphic Column in the Nile Delta Region.
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Figure (1.5) Geological Cross Sections in the Nile Delta Region, RIGW (1992)
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Figure (1.6) Contour Map of the Base of the Quaternary Aquifer Relative to the Mean Sea Level, RIGW (1980)
1.6 Hydrogeology of the Nile Delta Region
1.6.1 Aquifer Geometry
The hydrogeologic investigation of the Nile Delta aquifer revealed the existence of well-defined hydraulic boundaries that delimit the aquifer geometry. As shown in Figure (1.7), the aquifer is bounded by the Mediterranean Sea in the North and the Suez Canal in the East. Ismailia Canal in the South East is considered as the limiting boundary of the aquifer while in the West and South West, Wadi-El-Natrun Fault represents another distinct boundary of the aquifer. In the vertical dimension, the Nile Delta aquifer has variable thicknesses as shown earlier in Figure (1.6).
1.6.2 Irrigation – Drainage Network in the Nile Delta Region
The Nile River represents an outstanding feature that has shaped not only the physical characteristics of the area but also its history and the nature of its settlers. The water resources in the Nile Delta consist of complex irrigation
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and drainage networks which are hydraulically connected to the underlying aquifer system. The Damietta and Rosetta branches of the Nile River, the irrigation system and drains are formed through the area. These hydraulic features are generally directed to the natural discharging areas where the Northern lakes and the Mediterranean Sea. The surface water levels in the Nile River and its irrigation canals are controlled by the Delta Barrages and the other hydraulic control structures scattered everywhere. At the tail ends of the two branches, hydraulic dams were constructed to prevent the flow of the salt water from the Mediterranean Sea to the Nile River.
Since the construction of the Aswan High Dam (AHD), the flow rates in the irrigation system vary according to the variable water demand. This water demand does not change significantly from year to year unless new expansion projects are being implemented. During the low flood periods, there is a chance to have less flow in the Nile River and hence the water levels in all the irrigation-drainage system are affected. The great barrages built on the Nile are controlling the surface water levels in the Nile branches and the irrigation canals. The lands of the delta were converted to perennial irrigation at the beginning of the last century.
Figure (1.7): Configuration of Aquifer Systems in the Nile Delta Region.
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Open drains are extensively constructed in the Nile Delta region and used principally to drain excess irrigation water. Nowadays, most of the Nile Delta lands are served by tile drains to improve the agricultural drainage. These tile drains are constructed at a depth of 1.5 meters below ground surface. Near the coastal line, the water levels in the open drains are less than the sea level. Therefore, water has to be discharged mechanically to the Northern lakes or directly to the Mediterranean Sea.
1.6.3 Groundwater Aquifer Systems in the Nile Delta Region On the basis of geomorphology, the hydrogeological and hydrochemical features of the Nile Delta region is divided into three major regions which are: the flood plain (area of 9,126 km2), Eastern Nile Delta fringes (area of 10,220 km2) and the Western Nile Delta fringes (area of 11,042 km2). There are different groundwater aquifers with different importance for exploitation in the Nile Delta region. These aquifers are the semi-confined Quaternary aquifer, phreatic sandy aquifer, Pliocene aquifer, Moghra aquifer, and sand dunes aquifer as shown in Figure (1.7). Previous studies revealed that there an inter-aquifer flow, accordingly this section covers briefly all aquifer systems in the Nile Delta region. The Moghra aquifer is located in the Western Delta region. It has an area of about 50,000 km2. The aquifer consists of Lower Miocene sand and gravel. The aquifer is found in the west of Wadi El-Natrun and extends towards the
Qattara Depression. The aquifer is phreatic south of latitude 30 N, but confined by Pliocene deposits in the northern direction. Oligocene rocks (basalts or shales) underlie the Moghra aquifer. The base of the Moghra aquifer slopes from ground level near Cairo to 100 meters above sea level near Burg El-Arab area, to the west of Alexandria. The saturated thickness ranges from 70 to 700 meter (RIGW, 1990). The groundwater flow in the Moghra aquifer is generally directed westward, towards the Qattara Depression. Inter-aquifer flow is a minor component of recharge, which occurs from the Nile Delta towards the Moghra and Pliocene aquifers. The groundwater salinity is classified as brackish with a maximum value of 7,000 ppm. The salinity values increase from very low in Wadi El-Farigh to high in the North and Western part. West of Wadi El-Natrun, the groundwater quality is brackish. The potentiality of Moghra aquifer varies between low and moderate (RIGW, 1998). The Pliocene aquifer is present in Wadi El-Natrun depression. The aquifer is considered a local low productive aquifer. It is a multi-layered aquifer consisting of an alternation of sand and clayey layers belonging to the Pliocene age. The aquifer is underlain by the Moghra aquifer but separated from it by layers of lower Pliocene age. In this area, the groundwater is discharged through a great number of seepage zones into small lakes,
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ponds. Groundwater is lost by direct evaporation with an annual rate of 70 million m3 (RIGW, 1998). The most important regional aquifer in the Nile Delta is the Quaternary aquifer. This aquifer consists of Pleistocene graded sand and gravel, changing to fine and clayey facies in the North. The aquifer is found along the entire Nile Delta flood plain. The clay cap of the Nile aquifer is a semi-confining layer and has a thickness up to 20 meters. The Nile Delta aquifer is underlain by Pliocene marine clay in the Central Delta and wedges out towards the fringes. Data were collected and interpreted to outline the hydrogeological environment of the Nile Delta groundwater aquifer. Simplified latitudinal and longitudinal hydrogeological cross sections, representing the Nile Delta aquifer were developed as shown in Figure (1.8). According to these cross sections, the hydrogeological units of interest in studying the Nile Delta aquifer are as follows: (a) Top unit of Holocene clay aquitard; (b) Quaternary and late Tertiary gravels and sands unit (aquifer); (c) Basal unit of Pliocene clay aquiclude.
1.6.4 Recharge-Discharge Sources of the Nile Delta Aquifer
1.6.4.1 Aquifer Recharge
The Nile Delta aquifer in the flood plain is continuously recharged by irrigation water in the southern and central portions. Therefore, the aquifer acts as a storage reservoir that can be used conjunctively with the other surface water supplies. Recharge of groundwater is taking place in the Nile Delta area by three processes: (1) Infiltration of rainfall; (2) Downward leakage of the excess irrigation and seepage from the Nile River, irrigation canals and drains and Inter-aquifer flow of groundwater. Recharge of the Nile Delta aquifer occurs mainly through direct seepage from the irrigation canals and drains. In the central and southern parts of the Nile Delta floodplain, the downward leakage towards the aquifer varies between 0.25 and 0.80 mm /day (DRI, 1989). In the desert fringes, high leakage rates of 1-2.5 mm/day are recorded for basin irrigation whereas low rates of 0.1-0.5 mm/day are occurring for drip irrigation (RIGW, 1991).
1.6.4.2 Aquifer Discharge
Discharge of groundwater takes place by four components: outflow into
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the drainage system, direct evaporation, extraction and inter-aquifer flow of groundwater. Groundwater discharge to the drainage system occurs in the northern portions of the Delta through upward seepage with a daily rate of 0.2-0.9 mm/day (DRI, 1989). Discharge of groundwater through evaporation may occur in low-lying areas with a shallow groundwater table. In Wadi El Natrun depression, lakes and sabkhas, groundwater is discharged naturally by evaporation.
Inter-aquifer flow is a minor component of discharge, which occurs between the Nile Delta aquifer on one hand and the Moghra aquifer and the Wadi El Natrun aquifer on the other hand. The total amounts of groundwater flowing from the Nile Delta to the Moghra aquifer is estimated to vary between 50 and 106 million m3/year (RIGW, 1990). Groundwater extraction from the Nile Delta aquifer will be presented in a subsequent section.
Figure (1.8) Hydrogeological Cross Section in the Nile Delta Area.
1.6.5 Groundwater Levels in the Nile Delta Aquifer
1.6.5.1 Depth to Groundwater Surface
The depth to groundwater surface depends mainly on the ground surface elevation at a particular point. The field survey indicates that the depth to the groundwater surface is less than 5 m in most of the Nile Delta region and reaches to less than 1m near the coast, Figure (1.9). It is obvious from the map that the depth to groundwater decreases northward and Northeast. As a
Holoce
ne Quaternary Basal
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result, water logging problems are encountered in such areas. In the Nile Delta Fringes, the depth to groundwater increases to more than 50 meters due to high topography in these areas.
1.6.5.2 Fluctuations of the Piezometric Heads
Groundwater levels in the Nile Delta aquifer fluctuate in response to the stage of the Nile River, aquifer recharge from excess irrigation water and groundwater pumping. Since the construction of the High Aswan Dam, it was observed that the piezometric head increased within the Nile Delta aquifer. The agriculture projects that rely on the groundwater resources caused a severe decline in the piezometric head as noticed in the delta fringes. On the contrary, the agriculture projects that depend on the Nile water caused an increase in the water table and the development of water mounds at some localities. The piezometric head map for year 2002, Figure (1.10), shows that the groundwater level decreases gradually towards the North and Northeast direction with an average gradient of 11 cm/km. The groundwater head decreases from 15 meters at Cairo to less than one meter near the Mediterranean coast.
1.6.6 Groundwater Use in the Nile Delta
Before the construction of the High Aswan Dam, groundwater in the Nile Delta was used to supplement irrigation water at the tail ends of the irrigation canals in the coastal zones. In 1957, the first well inventory was launched and revealed that a total of 0.2 billion cubic meters was pumped annually from the aquifer through 5600 wells, GWRI (1966). It was reported that most of the production wells are 70 meters deep and the screen length ranges between 20 and 30 meters. Shallow wells operated by hand pumps are still extensively used for domestic purposes especially in the rural communities of the Nile Delta. Freshwater outflow to the sea was calculated and it was found that a total of 0.37 billion cubic meters was flowing directly to the sea in 1958 compared to 0.283 billion cubic meters in 1962. The reduction in the outflow of the freshwater was referred to as the illegal drilling of new wells by farmers. For coastal aquifers such as the Nile Delta, such freshwater outflow to the sea is required to keep the balance of the interface between the saltwater and the freshwater. In 1980 an extensive study was conducted to evaluate the safe yield of the Nile Delta aquifer, RIGW (1980). It was reported that the total annual extraction rate from the aquifer was 1.6 billion cubic meters while the net recharge rate to the aquifer from the Nile River and the irrigation canals was 2.645 billion cubic meters. The annual outflow of freshwater to the sea was estimated as 97 million cubic meters. Despite the reduction in the outflow to the sea and the increase in the groundwater extraction, the chemical analysis of the groundwater did not show significant increase in
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the salinity level. This concluded the fact that the saltwater and freshwater were in dynamic equilibrium. The study recommended increasing the annual extraction by 500 million cubic meters to lower the piezometric surface to a level that prevents water logging and soil salinization. This recommendation was based on the results of a two-dimensional finite difference model that did not account for the seawater intrusion phenomenon. In 1991, the total annual groundwater extraction from the Nile Delta aquifer was 2.77 billion cubic meters, RIGW (1992). The total number of wells was 13,000 compared to 5,600 wells in 1958. In 1997, the annual groundwater extraction was reported as 0.86, 1.6 and 0.56 billion cubic meters for the Western, Middle and Eastern Nile Delta region respectively, Hefny (1998). In 2003, the annual extraction rate was 0.9, 2.0 and 0.6 for the three regions of the Delta, RIGW (2004). From the above, it is obvious that since 1981 the groundwater extraction is increasing annually in a linear fashion by 0.1 billion cubic meters.
Figure (1.10) Depth to Groundwater in the Quaternary Aquifer, (RIGW, 2002)
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Figure (1.11) Piezometric Head Map of the Quaternary Aquifer (RIGW, 2002)
1.6.7 Hydro-geochemistry of the Nile Delta Aquifer
1.6.7.1 Groundwater Origins
The main source of groundwater in Nile Delta aquifer is the Nile, which flows from the Ethiopian Plateau. Therefore, the groundwater is of meteoric origin. The meteoric water of Pleistocene sediments covers all Delta except the coastal area up to N 310 00\ which occupied by saline water of marine origin coming from either the Mediterranean Sea or from the old marine deep aquifers. In the Miocene aquifer of the Western Delta, the groundwater is of meteoric origin whereas the same aquifer in eastern Delta is of marine origin as well. The groundwater of Pliocene aquifer in Wadi El Natrun is of marine origin. Groundwater of old marine origin was observed in areas with old active tectonic movements ascending along fault plains in the northern Delta, southeastern fringe and in the northwestern fringe. This type of groundwater is characterized by the considerable presence of CaCl2. Old meteoric groundwater was observed along the old passages of Nile tributaries in the eastern and western fringes. This type of groundwater is characterized by the presence of Na2SO4.
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1.6.7.2 Groundwater Types in the Nile Delta Aquifer The water type is classified according to the chemical composition and the hydrochemical process encountered in the aquifer. Figure (1.11) shows the different water type zones in the Nile Delta aquifer. In the Southern part of the Nile Delta (Pleistocene and Moghra aquifer) fresh Ca (HCO3)2 and Mg (HCO3)2 groundwater types are found. The chemical composition and the hydrochemical process indicate that groundwater replenishment is taking place as a result of the continuous recharge of the aquifer by excess irrigation water.
To the Northwest, North and East part of the Nile Delta, zones of fresh, brackish Na HCO3 and Na-mix water type are present. Refreshing of the aquifer is taking place as a result of the downward seepage of irrigation water. More to the North and in Eastward and West direction a zone with NaCl type groundwater is found. In this zone the groundwater is brackish to saline, but to some extent the aquifer is still flushed by freshwater. Near the coast NaCl to sea water type is found, indicating that the groundwater is invaded by the saline water from the Mediterranean Sea or affected by the saline groundwater flowing upward from the deeper aquifers. The present water type distribution in the Northern Nile Delta has been affected by the encountered shift of shoreline 20,000 years ago. At this time, the shoreline was at 125 meters below its present level and located about 50 km north of the present coast. During this period of time, the coastal region was formed of saline lagoons and depressions. Following this period, around 10,000 years ago, a rapid sea level rise and a southward movement of the shoreline occurred. Later, the rate of sea level rise started to decline and the Holocene clay cap started to accumulate as an overbank deposit of the Nile River system. As a result of the Holocene deposits and its depositional evolution, the salinity distribution in the Northern Delta is highly variable and complex (RIGW, 1999). The following chapter will discuss the sea water intrusion in the Nile Delta aquifer. The discussion will include a brief description for the sea water intrusion phenomenon in the coastal aquifers, a presentation of the baseline data related to the sea water intrusion, and the previous studies and new research for simulating the Nile Delta aquifer system.
24
Figure (1.11) Groundwater Type in the Quaternary Aquifer (RIGW, 2002)
25
Chapter II
Sea Water Intrusion in the Nile Delta Aquifer
2.1 Sea Water Intrusion Phenomenon
In coastal aquifers, the general class of groundwater systems consists of a saturated porous medium containing a miscible fluid of variable density. In such systems, the denser saltwater tends to remain separated from the overlying freshwater. However, a zone of mixing known as the transition or dispersion zone is formed between the two fluids. Cooper 1959 hypothesized that where a zone of dispersion exists between the saltwater and freshwater, the saltwater is not static but flows in a cycle from the floor of the sea to the zone of dispersion and back to the sea, Figure (2.1).
Figure (2.1) Hypothetical Cross Section Showing the Zone of Dispersion and Flow Patterns in a Coastal Aquifer, (Reilly, T. and Goodman, A., 1985)
The shape and movement of the mixing zone between the two fluids is governed by the hydrodynamic balance of the fresh and saltwater. Theoretically, the interface between fresh and saltwater bodies in an intruded aquifer represents a flow line, which implies no flow across the surface. However, reality has shown that the interface normally consists of narrow mixing zones. The zones are resulted from some or from all of the following process: (1) the hydrodynamic dispersion occurring in porous media; (2) the molecular diffusion; (3) seasonal
26
water table fluctuation and (4) fluctuation in the sea level as a result of the tide effects.
2.2 Modeling Freshwater-Saltwater Interface
Ghyben-Herzberg Principle
Two European scientists Badon Ghyben and Herzberg in the late 1800’s discovered that saline water occurred underground at a depth approximately 40 times the height of freshwater above sea level (Reilly and Goodman, 1985). This phenomenon is attributed to a hydrostatic equilibrium existing between two fluids having different densities. The equation derived to explain the phenomenon is generally referred to as the Ghyben-Herzberg relation which is derived from the following basis as shown in Figure (2.2).
Figure (2.2) Idealized Sketch of Freshwater-Saltwater Distribution in an
Unconfined Aquifer to Illustrate the Ghyben-Herzberg Relation
27
In the coastal cross-section of unconfined aquifer depicted in Figure (2.2), the total hydrostatic pressure (P) at points A and B is given by Equation (2.1) and (2.2) respectively.
ss hg PA (2.1)
sfff hghg PB (2.2)
f and s are the freshwater and saltwater density, hf and hs are shown in Figure (2.2) and g is the acceleration of gravity. The hydrostatic balance at the interface leads to the well-known Ghyben-Herzberg formula. PA = PB
sSsfff hghghg
or in a simpler form
f
fs
f
s hh
(2.3)
Taking an average value of f = 1.000 and s = 1.025 g/cm3 will lead to
fs hh 40 (2.4)
From the Ghyben-Herzberg formula it is important to note that the freshwater and saltwater equilibrium requires that the water table or piezometric surface should lie above sea level and the water table should slope towards the sea. If not, saltwater will advance directly inland causing the unintended sea water intrusion. The formula also contains a sever oversimplification because it implies that there is no vertical head gradient (Dupuit assumption). The most obvious erroneous result of this assumption is that the thickness of the freshwater zone is represented as zero at the shoreline where the water table elevation is zero. However, the fact that freshwater is discharging from the system indicates that it must have some finite thickness at the shore to act as an exit window.
Hubbert Theory of the Dynamics of Saltwater-Freshwater Interfaces
During the 1930’s Muskat and Hubbert studied the dynamics of saltwater interfaces. Both showed that the continuity of pressure in the flow field must be maintained across the assumed interface. Thus, the interface can be treated as a boundary surface that couples two separate fields. Hubbert showed that if a potential function (head) is defined for each fluid, the equation governing the interface could be derived in the following manner.
28
By defining the freshwater head, hf, as
Zgρ
P h
f
f
and the saltwater head hs, as
Zgρ
P h
s
s
where hf and hs are measured from the same datum and Z is the elevation (above the datum) of the point at which the head is measured; P is the fluid pressure at the point of measurement and g is the acceleration of gravity. Equating these two expressions gives the well-known Hubbert dynamic formula:
f
f
s
f
hh
s
f
s
s Z (2.5)
This formula defines the position of the interface under equilibrium conditions, that is when the saltwater is stationary or when both fluids are in motion.
Glover Formula of the Dynamics of Freshwater
With the theory of Hubbert as a foundation and with accurate field description of the freshwater and saltwater behavior in coastal aquifers, several analytical solutions were developed to estimate the saltwater-freshwater interface under different hydrologic conditions. In 1959, Glover developed a formula to describe the sharp interface in a coastal aquifer that accounts for the movement and discharge of freshwater toward the sea (Glover 1959). When dynamic factors are considered, it is found that freshwater flows through a narrow gap between fresh/saltwater interfaces and the water table outcrops at the
coast as shown in Figure (2.3), where G = f / (s - f) and Z is the interface elevation. The mathematical formula that determines the location of the interface under the effect of the freshwater flow to the sea is given as:
29
02
2
222
K
QX
K
QZ
(2.6)
Where X represents a distance measured horizontally landward from the shoreline [L]; Z represents distance measured vertically downward from the sea level [L]; Q represents the freshwater flow per unit length of the shoreline [L2T-1]; K is the hydraulic conductivity of the freshwater zone
[LT-1] and = f / (s - f) 40 for ordinary saltwater. It should be noted that the pattern of flow described by Glover is based on the assumption that saltwater is essentially static.
Henry’s Model to Describe the Mixing Process between Freshwater and Saltwater in Coastal Aquifers.
All the aforementioned solutions accounts for the advection dominated systems where a distinct interface between the saltwater and freshwater exists. In 1960, Henry made the first attempt to quantitatively determine the effects of dispersion and density differences on the seawater encroachment. The main contribution of Henry’s work was that it cooperated Cooper’s hypothesis of considering the mixing process of freshwater and saltwater and open up a new approach which uses the advection-dispersion equation (miscible fluids) instead of the sharp interface (immiscible fluids) approach. Results of Henry’s analysis for his idealized mathematical model are depicted in Figure (2.4). Henry’s solution required the simultaneous solution of the flow equation and the transport equation which are linked through the fluid density and concentration state.
Numerical Models of sea water intrusion
With the exception of Henry 1960, all of the available analytical solutions take the sharp interface approach. In addition, the models are based on simple assumptions about geometry (infinitely deep or of constant thickness), properties (homogeneous and isotropic) and simple boundary conditions. if these assumptions are not made it would be difficult, if not impossible to solve the governing equations analytically. In real situations where both freshwater and saltwater are in motion and the aquifer characteristics are complicated, numerical solutions are strongly required. In seawater intrusion problems, numerical models are defined as FULLY MIXED and SHARP INTERFACE models. 2.2.5.1 Sharp Interface Models
Simulation of the freshwater-saltwater interface in coastal aquifers requires a simultaneous solution of the equations that describe the
30
freshwater flow and the saltwater flow. These two equations are coupled by an interface boundary, which satisfies the Hubbert equilibrium theory. The governing equations are solved using the numerical methods such as finite difference, finite element and boundary integral equation methods. A schematic diagram and nomenclature of the sharp interface in an unconfined coastal aquifer are shown in Figure (2.5). The vertically averaged continuity equations for transient flow condition with fresh groundwater pumping are given by: For saltwater domain:
t
h
t
hs)1(BS
y
hBK
yx
hBK
x
fss
ssy
ssx ss
(2.7)
Figure (2.3) Flow Pattern in Coastal Aquifer (Modified from Glover, 1959)
Land Surface Sea
Level
Salt Water
Fresh Water
Interface
Shore Line
Stream Line
Equipotential line
0.1
0.2
0.3
0.4 0.5
0.6
0.7 0.8 0.9
1.0
Xo
Zo
31
Figure (2.4) Flow and Salt Concentration Patterns in an Idealized Mathematical
Model of Henry: (A) Streamlines and (B) Isochlors (Modified from Reilly and Goodman, 1985)
For freshwater domain:
fwsf
fff
fyf
fx )Q(t
h)1(
t
hBS
y
hBK
yx
hBK
x ff
(2.8)
where K is the hydraulic conductivity [L/T]; B is the fresh or the saltwater
thickness [L]; h is the vertically averaged head [L]; is the aquifer
porosity; S is the aquifer specific storage [L-1]; is the density difference
ratio
fs
f
; is the fluid density [M/L3]; (Qw)f is the freshwater
pumping/recharge rate [T-1]; t is time [T] and (f),(s) subscripts refer to
32
freshwater and saltwater respectively and x and y subscripts refer to directions. The two equations presented above are coupled by equating the pressure across the interface such that:
fs hh)1(Z (2.9)
where Z is the interface elevation measured below the mean sea level. To determine the position of the sharp interface at different time intervals, Equations (2.7) and (2.8) are solved simultaneously using either the finite element or finite difference methods. In order to obtain a unique solution for the governing equations described earlier, both boundary and initial conditions should be specified at the model boundaries.
Figure (2.5) Schematic Diagram of a Typical Cross Section of an Unconfined Coastal Aquifer, Sakr (1992).
2.2.5.2 Freshwater-Saltwater Interface in the Nile Delta Aquifer
To describe the geometry and position of the mixing zone between the freshwater and the saltwater in the Nile Delta aquifer, salinity data is required in the three dimensions to represent both the lateral and vertical extent of this zone. It is worth to mention that before 1996, there was not any monitoring system for seawater intrusion except for some shallow wells to monitor the top portion of the aquifer. The collected data from such shallow wells indicated that the salinity distribution in the Quaternary aquifer is very complicated to describe and raised a lot of concerns. The existing drainage and irrigation systems, the return flow from irrigation, the dissolution of the salt in the clay layer overlaying
33
the aquifer and the historical geologic evolution have direct impact on the salinity distribution. To better understand the nature of the mixing zone between the freshwater and the saltwater, three deep observation wells were drilled at Qtour (located 50 km south of the Mediterranean Sea) to a depth of 225, 400 and 600 meters. Respectively, the field measurements in the three wells revealed that the water salinity is 2000, 15,000 and 60,000 ppm while the water level is 2.6, 3.1 and –11.5 meters measured from the sea level (Sakr 2004). The deeper well is fully penetrating the Quaternary aquifer and tapping the lower Basal aquifer. The objective of this well was to investigate the hydrochemical and the hydraulic characteristics of the underneath formation. The increase in the hydraulic head with depth in the Quaternary aquifer indicates that there is a saltwater-freshwater circulation and the freshwater is pushing the saltwater seaward. Using the available data from the deep oil boreholes and after calibrating these data, the salinity distribution along the entire thickness of the aquifer was constructed. Figure (2.6) represents the salinity distribution across the middle region of the delta. The salinity distribution indicates that the transition zone of brackish water (concentration from 2000 to 10,000 ppm) and saltwater (concentration greater than 10,000 ppm) forms a wedge extending into the aquifer to a distance of 90 km from the coast. Using the Ghyben-Herzberg assumption, the historical records of the measured water levels revealed a freshwater-saltwater interface as shown in Figure (2.6). The proximity of the sharp interface position with the brackish water zone validates the use of the sharp interface model to simulate the flow pattern in the Nile Delta aquifer and to estimate the movement of the assumed sharp interface under any proposed scenarios.
0 20,000 40,000 60,000 80,000 100,000 120,000
Distance in meter
Dep
th b
elow
sea
lev
el, m
Qto
ur
Medit
erra
nean
Sea
Del
ta B
arr
age
Base of Quaternary
Assumed sharp interface position
Salinity, ppm
0
-200
-400
-600
-800
-1000
mean sea level
Figure (2.6) Salinity Distribution along a Vertical Cross Section through the Middle Delta (Sakr, 2004).
34
For the groundwater wells located within the Quaternary aquifer, the temporal variation of the water salinity is an indicator for the impact of the seawater intrusion. Historical records showed that salinity is changing as a result of the water levels in the Nile and the irrigation canals. Between 1957 and 1984, the water quality analysis indicated that the groundwater salinity was improving with time and the freshwater is pushing the saline water seaward. It was also reported that the groundwater heads increased as a result of the construction of the Aswan High Dam. In 1985, the groundwater salinity increased in some shallow wells as a result of the groundwater over pumping and reducing the Nile water flow during the 1980’s drought. In 1990, the groundwater salinity decreased again when the Nile flow was back to its normal rate and due to the excess irrigation water especially in the fringes. In 2000, groundwater salinity increased again in the eastern and western fringes due to over pumping. To simulate the Nile Delta aquifer system taking into consideration all the hydrologic complications is not a straightforward task. Therefore, assumptions should be made according to the objective of the simulation. The first attempt to simulate the Nile Delta aquifer and to account for the seawater intrusion was carried out by Farid, 1985. In this research he assumed a sharp interface that exists between the two fluids and this interface is static. Nevertheless, this model is considered the platform for managing the Nile Delta aquifer. Darwish (1994) and Gaame (2000) simulated the mixing zone between the two fluids using the USGS 2-dimensional cross sectional model (SUTRA). Using this model they predicted the aquifer response for different operational scenarios. Lack of salinity data, which is required for model calibration and verification, made the results questionable. However, the results are still acceptable for the planning purposes. When there is no historical record or enough data to describe the aquifer system, analytical models provide an extremely useful first order approximation of a field problem. An example is the impact of sea level rise on the movement and geometry of the mixing zone between the two fluids in costal aquifers. Since there are no historical records for such phenomenon, analytical solutions are ideal for such problem. Again, there is no such analytical solution that accounts for the mixing phenomenon between the two fluids. Therefore the sharp interface analytical models are the only available models that can be used to simulate the impact of the sea level rise on the interface position and movement.
35
Chapter III
3 Coastal Zones ulnerability to Sea Level Rise (SLR)
3.1 Introduction: Similar to other deltas worldwide, the Nile Delta is presently subjected to changes; including shoreline changes either erosion or accretion, subsidence of the delta, and sea level rise, due to climate changes. The impacts of climate changes on the Nile Delta have been addressed on local and international level as the Nile Delta coastal zones are vulnerable to sea level rise. Accordingly, the main objective of the current work is to assess the vulnerability of the Nile Delta coastal zone to climate change/sea level rise. To achieve these objectives, elevation maps through GIS process were developed. After developing the elevation maps, three scenarios (CoRI, B1, and A1F1) were introduced to assess the vulnerability for the three Nile Delta regions. Aerial photos and about two hundred hydrographic profiles were used to illustrate the vulnerable areas associated with the three scenarios chosen for the study till the end of the current century.
3.2 Statistical Analyses for Tide Gauges Data and Land Subsidence Tide gauges measurements at Alexandria, Al-Burullus, and Port Said have been collected and statistically analyzed to estimate sea level rise (SLR) over the last three decades at each of these regions. Results indicate that SLR varies from region to another because of the land subsidence effect. Land subsidence studies revealed that land subsidence rate is about 0.4 mm/y at Alexandria, 1.2 mm/y at Al-Burullus, while at Port Said the value is 4.0 mm/y. Estimated average rates for SLR at Alexandria, Al-Burullus, and Port Said are 1.6, 2.3, 5.3 mm/year respectively. These values associate the effect of both SLR and land subsidence, Table (3.1).
Table (3.1) Sea Level Rise and Subsidence Rates at the Nile Delta Coastal zone
Region Alexandria Al-Burullus Port Said
Tidal rate (mmly) 1.6 2.3 5.3
Subsidence (mm/y) 0.4 1.1 4.0
SLR (mm/y) 1.2 1.2 1.3
3.3 Expected Sea Level Rise till 2100
The projected values of the mean surface air temperature, 2000-2100, for the low scenario (B1) and high scenario A1F1 of Special Report on Emission Scenario (IPCC-2007) are given in table (3.2).
36
Table (3.2) Projected Values of Mean Air Temperature
Temperature Change for Years 2025, 2050, 2075 and 2100 (ºC)
2000 Scenario 2025 2050 2075 2100
0.6 (ºC) B1 0.9 1.3 1.8 1.8
0.6 (ºC) A1F1 1.2 2.2 3.2 4.0
The average increase in global warming at the end of the twenty century was about 0.6 ºC (IPCC- 2007). Assuming linear variation of global warming with time during the last two decades, the value of 0.006 ºC/y could be accepted as the average annual temperature change during the last century. Assuming that the land subsidence will occur with the same rates, the projected average sea level rise at the end of years 2025, 2050, 2075, and 2100 has been estimated by applying the projected average surface warming given in table (3.2).
3.4 Vulnerability Assessment VA Methodology
After creating elevation maps for the Nile Delta, three scenarios have been introduced to assess the vulnerability for the three Nile Delta regions. The three scenarios named; CoRI scenario which assumes the same increase rate of SLR till 2100; B1 and A1F1 scenarios of IPCC. In these three scenarios, Alexandria represents the western region of the Nile Delta, Al-Burullus represents the middle Delta region, and Port Said represents the eastern Nile Delta region. Nile River branches have been considered as the natural divide between the three regions. In order to present the actual situation in the coastal zones, natural and man-made supporting systems have been considered in the modeling processes. Simulated SLR rates for Nile Delta regions by the three scenarios are presented in Table (3.3)
Table (3.3) Projected Sea Level Rise (cm), (Base year is 2000). (After Elshinnawy I. A. 2008)
City Scenario 2025 2050 2075 2100
Port Said Eastern Region
CoRI 13.25 26.5 39.75 53
B1 18.12 39.5 64.3 72.5
A1F1 27.9 68.8 109.6 144
Al-Burullus Middle Region
CoRI 5.75 11.5 17.25 23
B1 8.75 19.5 32.25 35
A1F1 14.75 37.5 60.3 79
Alezandria Western Region
CoRI 4 8 12 16
B1 7 16 27 28
A1F1 13 34 55 72
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Results
CoRI scenario
Results of CoRI scenario indicate that the sandbar located between Al-Manzalla Lake and the Mediterranean in the east region is vulnerable to SLR. Regarding the middle region of the Delta, the area located between Gamasa City and New Damietta is also vulnerable as well as Al-Burullus sandbar. Estimated areas expected to be flooded by sea level rise till the end of the current century in the Nile Delta coasts are illustrated in Figure (3.1) and presented in table (3.4).
Table (3.4) Total flooded area and its percentage to the Nile Delta area According to CoRI measurements till 2100
Year 2025 2050 2075 2100
Total Area Flooded (km2) 93.68 134.0 139.2 183.8
% of the Nile Delta Area 0.37 0.54 0.56 0.74
B1 Scenario
Results of B1 scenario presented in table (3.5) and figure (3.1) indicate the same trend of vulnerability in the coastal zone of the Nile Delta as mentioned in CoRI scenario with some increase in the vulnerable areas in the middle region. Table (3.5) Total flooded area and its percentage to the Nile Delta area (B1 scenario)
Year 2025 2050 2075 2100
Total Area Affected (km2) 118.5 169.45 221.83 243.1
% of the Nile Delta Area 0.45 0.68 0.89 0.97
A1F1 Scenario
Results of the this scenario presented in table (3.65) and figure (3.1) represent the actual situation that the Nile Delta could face in terms of flooding considering the fact that the boundaries of the lakes are above zero level and the low lands at Abu-Quir Bay are protected by Mohammed Ali sea wall constructed in 1830.
Table (3.6) Total affected area and its percentage to the Nile Delta area (A1F1 scenario)
Year 2025 2050 2075 2100
Total Area Affected (km2) 152.86 256.27 450 761.4
Total % of the Nile Delta Area
0.61 1.03 1.8 3.01
38
According to previous analysis, it's concluded that the vulnerable areas in Nile Delta coastal zones vary from scenario to another. Results of CoRI scenario that depend on actual measurements are compatible with results obtained by B1 scenario. The vulnerable areas account for less than 1% of the Nile Delta area for CoRI and B1 scenario and reaches up to 3% for the worst case represented by A1F1 scenario of IPCC. These results are based on the assumption that the natural and man-made protection systems are functioning well till the end of the century. Although the vulnerable areas (>1% - 3%) seem small, the consequences of SLR could be sever on sea water intrusion, soil salinity, water quality, waves & current pattern, shore line retreat and erosion pattern.
Figure (3.1) Simulated flooded areas due to SLR till 2011 by CoRI, B1, and A1F1 Scenarios
39
Chapter IV
4 Impact of Sea Level Rise on the Coastal Aquifer of Nile Delta
4.1 Theory of Sea Water Intrusion Being hydraulically connected with the sea, groundwater in coastal aquifer has special characteristics and flow conditions. Basically the denser saltwater tends to lay under the less in density freshwater with a zone of mixing known as the transition or dispersion zone formed between the two fluids. Equilibrium of the fluids dynamic pattern is principally governed by, given certain medium/soil characteristics, rates of groundwater recharge/withdrawal and sea level. However, under normal conditions, fresh water flows from inland aquifers to the sea. Early researches have commonly dealt with position movement of the saltwater/freshwater interface in relation to freshwater recharge into the aquifer versus withdrawals and resulting advance or retreat in position. Recently, however, climate change and potential sea level rise added a new dimension to consider in this arena. Climate change is likely to have impressive effects on the hydrologic cycle through altered precipitation, evapotranspiration, runoff and soil moisture patterns…etc. These changes will lead to altered groundwater recharge and flow regime as well as rate of saltwater intrusion in coastal aquifers.
4.2 Saltwater/Freshwater Interface Dynamics Early studies on salt water intrusion were raised from concerns of over extraction of fresh ground water near coastal region. Main objective has been to reach safe withdrawal level that balances the recharge rates so that saltwater/freshwater interface advancement inland, along with accompanied threats of increased salinity levels, can be avoided. Among the reviewed approaches, following is a brief of selective ones considered in this report.
Ghyben-Herzberg Formula
Despite the oversimplifications encountered in the Ghyben-Herzberg formula, this formula could be used to understand the interface behavior under different hydrologic conditions. This Equation estimates the interface elevation for a given freshwater head at any point in the aquifer. The equation is written as:
fhZ (4.1)
fs
f
(4.2)
40
where Z is the interface elevation measured downward from the mean sea
level, hf is the freshwater head measured above the mean sea level and is given by Equation (4.2). The Ghyben-Herzberg equation is only valid when both freshwater and saltwater are stationary which unlikely to occur in nature.
To evaluate the impact of the sea level rise (hs) on the movement of the
interface Z, the hydrostatic equilibrium between the two fluids is assumed, Figure (4.1). Applying the Ghyben-Herzberg formula for each state will lead to the following expressions: For the present situation
ofo hZ
For the new situation with the sea level rise
nfn hZ .
Then
)()( sffno hhhZZZno
(3.3)
Figure (4.1): Applying the Ghyben-Herzberg Formula to the sea level rise.
41
From the above, it is obvious that using the Ghyben-Herzberg formula allows predicting the position of the sharp interface between the two fluids at any given point. The major assumption in the above analysis is that the freshwater domain remains static without any change in its hydraulic head values. Although this assumption is not true but still provides a preliminary
assessment to what could happen when sea level rises by a value of (hs).
Glover Solution
In 1959, Glover developed an analytical model that describes the sharp interface position in coastal aquifers. This model accounts for the movement and discharge of freshwater towards the sea. When flow dynamics are considered in such systems, it is found that freshwater flows through a narrow gap between the interfaces and the water table at the coast. The flow pattern described by Glover is based upon the assumption that saltwater is essentially static. The Glover model is expressed as follows:
02
2
222
K
QX
K
QZ
(4.4)
From the previous analysis it is worth mentioning that the Ghyben-Herzberg model assumes that both freshwater and saltwater are static. On the other hand, the Glover model accounts for the freshwater movement towards the sea while the saltwater remains static. Also, the boundary conditions at the coast for the Glover model are more realistic than the Ghyben-Herzberg model. The two analytical models indicated that there is an impact on the Nile Delta aquifer system if sea level rises by one meter. The sea level rise or the inland movement of the shoreline will cause the freshwater-saltwater interface to move inland by approximately 5 km. Except for the East Delta cross section, the two models showed little differences in the interface position. The high discrepancies for the Eastern Delta cross section are due to the existence of two saltwater boundaries, which are the Mediterranean Sea from the North and the Suez Canal from the East. These two boundaries were not considered in the analytical models. Therefore, numerical models are needed to simulate such complexity.
4.3 Sharp Interface Numerical Model
The above mentioned analytical solutions assume sharp interface with steady state flow conditions and fairly simplified aquifer characteristics, geometry, heterogeneity, anisotropy and boundary condition. However, in real situations both freshwater and saltwater are in motion and the aquifer characteristics are complicated (Sakr, 2005). While many approaches follow the assumed sharp
42
interface for simplifying computations, yet considering that the boundary between saltwater and freshwater is not sharply distinct and a zone of dispersion or transition zone exist have been covered in certain researches.
A number of studies have presented numerical modeling approaches for seawater intrusion, while considering either fully mixed or sharp saltwater/freshwater interface condition (details can be found in Sakr, 1992 and Sakr, 2005). Governing equations are customary simultaneously solved using numerical methods such as finite difference, finite element and boundary integral equation methods (Sakr, 2005). In 1992, another numerical model for saltwater/freshwater interface predictions (GWCH2O) has been introduced by Sakr, using the Finite Element method. Applications of GWCH2O model in Nile Delta aquifer have been presented in studies by Sakr et al. (Sakr et al., 2004 and Sakr 2005).
The simplified analytical solutions are widely-applied and found satisfactory in many early researches. Therefore, and with the limitation in historical field data, this research study is presenting an application of potential saltwater/freshwater interface movement and groundwater level elevation resulting from predicted sea level rise, using Ghyben-Herzberg and Glover approaches.
4.4 Results and analysis for a pilot area within the Study Area
Studies curried out by Coastal Research Institute (CoRI, 2012) with the cooperation of Drainage Research Institute (DRI) and Research Institute of Ground Water (RIGW) revealed that the Groundwater flow is directed from south to north where the Piezometric level decreases gradually from more than 15m + MSL at Cairo City to less than 1m + MSL near the Mediterranean coast (Sakr, 2005 and RIGW, 2002). To investigate the potential impact of Sea Level Rise (SLR) on shifting saltwater/freshwater interface position and consequent elevation of groundwater levels, predicted rise in sea levels for years 2025, 2050, 2075 and 2100 have been considered. Additionally, to investigate the salinity distribution and to calibrate the model, water salinity samples were taken from locations across the shore line as presented in figure (4.2). According to the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC-AR4, 2007), predictions of Sea Level Rise by A1F1 scenario have been used in applying the analytical methods and concluding inland/upward movement of the saltwater/freshwater interface. With the predicted Sea Level Rise, shoreline is expected to move inland at variable rates in relation to the topography and, accordingly, new boundary conditions are formed.
43
Figure (4.2) Location of salinity measurement points
Expected water heads in different time periods for shallow and deep ground water have been simulated as shown in figures (4.3) and (4.4). Simulations indicate that water table of the shallow aquifer that affects plants root zone will raise to less than 50 cm from ground level. To simplify modeling process, simulations have been carried out with SLR increase of 0.5 and 1.0 meter as average values for the three regions of Nile Delta. Simulations also have been modeled for one of the most vulnerable areas to SLR located between Gamassa Ciry and Ras-El Bar city as a pilot area. The pilot area chosen has been extensively investigated by CoRI for such studies as well as the coastal hydro-dynamic processes. Simulations of salinity distribution presented in figures (4.5) and (4.6) indicate high water salinity for shallow aquifer in the coastal strip that would affect the root zone. Knowing that water depth will be less than 50 cm from ground level, it is worth mentioning that the case will be more serious as the area is subject to up-coining pressure of groundwater.
44
Figure (4.3) Calculated heads for shallow groundwater (CoRI, 2012)
Figure (4.4) Simulated Head of Deep Groundwater (CoRI, 2012)
45
Figure (4.5) Simulated salinity concentration of shallow groundwater (CoRI, 20012)
Figure (4.6) Simulated salinity concentration of deep groundwater (CoRI, 2012)
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4.5 Remarks on results:
Results illustrated in figures (4.7) & (4.8) conclude that the effect of sea level rise will affect the groundwater aquifer system to a certain limit. In this respect, the applied mechanism of sea level rise is assigned to the model as a vertical component only as data collected within the project do not present the actual situation of the aquifer system. In addition, the selected simulated time of the model was chosen by trial and error methods till the model reached the steady state. In general this output is satisfied at the present time and can be taken as an indication for the predicted impact of sea level rise on both quality and quantity groundwater system.
As the process of sea water intrusion is a dynamic process, the expected variation of heads after 30 years due to sea level rise will lead to a change in head ranging from 0.1 to 0.5 meters from land levels. The change expected in groundwater salinity will be marked only at a distance of 7 km from the shore line (southwards) and will affect the groundwater aquifer system to a certain limit.
Figure (4.7) change in salinity for the shallow aquifer for 0. 5m SLR (CoRI, 2012)
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Figure (4.8) change in salinity for shallow aquifer for 1 m SLR (CoRI, 2012)
4.6 Conclusion
The mixing zone between freshwater and saltwater in the Nile Delta aquifer is not accurately represented due to the lack of water quality data. To simulate this mixing zone an assumption was made that there is a distinct sharp interface between the two fluids. Using the available field data and some theoretical hypothesis, this assumption was proved to be valid. Not too many analytical models were developed to present the position of the interface and its movement. This is due to the fact that the sea water intrusion is not a straight forward phenomenon and has its complexity in terms of the hydraulic properties, boundary conditions ...etc. Therefore, numerical models were developed instead for better simulation of the aquifer conditions.
In this study, three analytical models and one numerical model were used to evaluate the sharp interface between the freshwater and saltwater. The evaluation includes the position of the interface and its movement under present conditions and for the effect of the possible sea level rise. The model results indicated that the predicted maximum sea level rise of 1.0 meter will definitely has an impact on the movement of the sharp interface, water salinity and shallow water table.
The interface movement is eventually reducing the groundwater potential in the coastal zones of Nile Delta. It was found from the models’ results that the interface will move vertically by an average value of 0.5 meter. For the inland
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movement, the interface will move for a distance that varies between 5 km to 9 km (7 km on average) from its present position.
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Chapter V
Monitoring of Sea Water Intrusion and Groundwater Quality
5.1 Locations of the observation wells
A detailed investigation was done by RIGW to determine the location of observation gaps in the groundwater monitoring network. According to existing observation wells in the Nile Delta, RIGW proposed drilling of 10 wells with different depths located in the gaps. The exact location of these wells needs a detailed Geo-electric and Geo-physical survey. In addition, the assessment of the stratigraphy of the well drilling, exact depths and location of screens will be identified. Table (5.1) summarizes the preliminary locations and depths for the observation wells needed. Figure (5.1) shows the location of the existing and proposed observation wells used to monitor the sea water intrusion in the Nile Delta Aquifer 5.2 Background Monitoring According to the objectives of the project, water quality monitoring would cover the water column to indicate source of pollution that could happen from land use or as a result of sea water intrusion. A wide variety of chemical, physical, and biological contaminants may affect ground-water quality. As a result, background and ambient-ground-water-monitoring programs are designed to establish baseline water-quality characteristics and to investigate long-term trends in coastal groundwater conditions. The parameters measured in baseline-monitoring programs provide a set of descriptive data on general ground-water conditions.
Table (5.1) preliminary locations and depths for the observation wells:
Number Well Location Governorate Proposed well depth
1 Faraskour Dommita 100
2 Sun El Hagar Elsharkia 150
3 Faqous Elsharkia 150
4 Alstamony Eldaqahlia 150
5 Alhamoul Kafr Elsheik 150
6 Elmahala Elgharbia 150
7 Tanta Elgharbia 150
8 Altawfeqia Elbehera 150
9 Etay Albaroud Elbehera 150
10 Abou Keer Alexandria 150
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Proposed wells in Red Existed wells in Green
Figure (5.1) Location of observation wells
5.3 Monitoring for Specific Land-Use Impacts on Ground-Water Quality Monitoring programs also typically focus on assessing the impact from contaminant sources that are related to specific land uses to indicate the impact of irrigation application on water level and water quality. For these regional or localized monitoring efforts, monitoring parameters are identified on the basis of a thorough understanding of the resource to be evaluated and the sources of contamination.
5.3.1 Selection of Groundwater-Quality Indicators One of the key elements in the design of a water-quality-monitoring program, whether the program is focused on background conditions or land use impacts, is the selection of the properties, elements, and compounds (indicators) to be measured. Ground- and surface-water quality may be characterized by literally thousands of indicators. Selection of indicators for monitoring programs should be based on their
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relevance to important water-quality issues, such as human health protection, the monitoring objectives, and the existence of appropriate analytical methodologies. For some water-quality issues, the choice of indicators to be monitored is a simple task; for example, the substances relevant to the issues of nutrient enrichment and salinity are of limited number, and their chemical analysis is inexpensive. .
5.3.2 Criteria for Indicator Selection Indicators appropriate for ground-water-quality monitoring should meet two general criteria. First, a parameter should be a candidate for monitoring because it fulfills any of or all the following:
Impairs the suitability of the water for general use; for example, hardness, iron, manganese, taste, odor, and color.
Is of interest in surface water and may be transported from ground- to surface-water systems; for example, nitrogen species ammonia, nitrite, and nitrate.
Is an important "support variable" for interpreting the results of physical and chemical measurements; for example, temperature, specific conductance, major ion balance, depth to the water table, and selected isotopes.
Second, analysis of the candidate indicator should be affordable by using well-established analytical methods at appropriate minimum-detection and reporting levels necessary to achieve the objectives of study. Based on these criteria, the following general groups of indicators should be considered for ground-water-monitoring programs to indicate land use and irrigation application.
Field measurements (temperature, specific conductance, pH, dissolved oxygen, alkalinity, depth to water).
Major inorganic ions and dissolved.
Nutrients.
Dissolved organic carbon.
Pesticides.
Volatile organic chemicals.
Metals and trace elements.
Bacteria. The main indicator for sea water intrusion is the chloride. Sea water intrusion into a coastal aquifer depends on many factors, including:
Total rate of groundwater withdrawals compared to recharge rates
Presence of freshwater drainage canals that lack salinity control structures
Distance of stresses, such as wells and drainage canals, from the source(s) of saltwater intrusion
The length of time that aquifer levels are lowered.
Fluctuations in tide stages
Sea level rise
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Seasonal and annual variations in groundwater recharge and evapotranspiration rates
5.3.3 Monitoring chloride to determine the extent of saltwater intrusion
According to the U.S. Geological Survey (USGS), no standard practice exists for defining the transition zone. However, the USGS typically characterizes the transition zone as having total dissolved solid (TDS) concentrations ranging from about 1,000 to 35,000 mg/L and chloride concentrations ranging from about 250 to 19,000 mg/L. Chloride is the main constituent of seawater with an average concentration of 19,000 mg/L (Hem 1992). Due to the high concentration of chloride in seawater, the chloride concentration of groundwater samples is the most commonly used indicator of saltwater occurrence and intrusion into coastal aquifers. Specific conductance (SC) of groundwater also can be used to identify saltwater intrusion because it is a direct measure of TDS and salinity. SC also has been shown to be strongly correlated to chloride concentrations. Chloride, one of the main constituents of seawater and a charged ionic species, makes water conductive. As chloride concentrations increase, so does the solution’s conductivity. A rather well-defined relationship exists for SC and chloride (Hem 1992; Christensen et al. 1999), which implies a linear relationship between these two parameters.
The USGS notes that SC and chloride usually have a long and reliable analytical record, and are useful for evaluating long-term trends. Using chloride concentration as an indicator of saltwater intrusion offers several advantages:
Both SC and chloride are chemically and biologically conservative, or stable, water quality indicators and tracers.
Chloride moves at about the same rate as the intruding seawater and is not retarded by the aquifer matrix (Roberts et al. 1986).
Circulation of the chloride ion occurs largely through physical processes rather than chemical or biochemical processes (Hem 1992).
Chloride is least affected by movement away from the source and provides a true representation of contamination levels
Several analytical methods are available for determining chloride concentration. Chloride ion-selective electrodes (ISEs) can be used to continuously monitor chloride in the field. Figure (5.2) illustrates vertical up-coning of saltwater that results in chloride increase in water abstracted from coastal well.
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Figure (5.2): Vertical movement or up-coning of saltwater.
5.3.4 Real-time, long-term monitoring of chloride concentration Chloride-concentration profiles of several monitoring sites distributed along a coastline can determine changes in the transition zone and the degree of saltwater intrusion. Continuous, long-term monitoring of SC can alert water managers to a potential saltwater intrusion problem and upconing of saltwater resulting from aquifer withdrawals. Continuous chloride monitoring via SC can be used to:
• Understand the extent and degree of saltwater intrusion
• Evaluate the relative quality or potability of water
• Monitor acute and chronic exposure levels in freshwater supplies
• Evaluate aquifer recharge and recovery operations
• Trace chloride concentrations in groundwater studies
• Conduct chloride profiling
To monitor saltwater conditions in the coastal zone, some of the activities of the Sound Science Initiative are:
Construction of temporary monitoring wells offshore where saltwater enters the
Nile Delta aquifer system as mentioned earlier.
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Development and expansion of a network of ground-water monitoring wells
and an associated information database to measure and report changes in
ground-water levels and chloride concentrations;
Development of ground-water flow and solute-transport models to investigate
the paths and rates of ground-water flow and saltwater intrusion in the coastal
aquifer
Evaluation of alternative and supplemental sources of water such as seepage
ponds, rivers and streams, reclaimed water, and ground water withdrawn from
aquifers other than the Nile Delta coastal aquifer; and
Feasibility studies and assessments of engineered and non-engineered
methods that might be used to prevent saltwater intrusion.
5.4 Numerical modeling:
The data that will be collected from the sea water intrusion monitoring network will be used to determine accurately the interface between the sea water and the fresh water in the groundwater aquifer using numerical models. Numerical models are, generally, required to solve complex geometry, boundary conditions and equations describing coupled and uncoupled processes in heterogeneous and anisotropic formations under various initial and boundary conditions. In most numerical models the governing equations are formulated to simultaneous equations relating unknown variables at discrete nodal and different times. Many powerful methods are available for this purpose. The various numerical models differ mainly in the method of numerically formulating the problem. Numerical models for groundwater flow and solute transport are important tools for helping decision makers in planning for future management and development policies taking the environmental aspects into considerations. Visual MODFLOW Pro.V.4.2 is an appropriate numerical model to simulate the problem as the most complete and user friendly, modeling environment for practical applications in three-dimensional groundwater flow and contaminant transport simulation. This software developed by Waterloo Hydro-geologic Inc. The SEAWAT engine is used to allow modeling of variable density flow such as seawater intrusion modeling problems. SEAWAT combines a flow code (MODFLOW) with a solute- transport code (MT3DMS) to form a single program that solves the coupled flow and solute-transport equations. It formulates flow equations using mass conservation instead of volume conservation.
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