Natural Discharge: a Key to Sustainable Utilization of Fossil Groundwater

Journal of Hydrology (2007) 335,25– 36

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Natural discharge: A key to sustainable utilization of fossil groundwater

M. Sultan a,*, E. Yan b, N. Sturchio c, A. Wagdy d, K. Abdel Gelil e, R. Becker a, N. Manocha a, A. Milewski a a Department of Geosciences, Western Michigan University, Kalamazoo, MI, USA b Environmental Science Division, Argonne National Laboratory, Chicago, IL, USA c Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, IL, USA d Irrigation and Hydraulics Engineering Department, Cairo University, Giza, Egypt e National Water Resource Center, Ministry of water Resources and irrigation, Cairo, Egypt

Received 10 April 2006; received in revised form 27 October 2006; accepted 31 October 2006

KEYWORDS Summary Rising demands for fresh water supplies in arid lands are leading to excessive Natural discharge; exploitation and unsustainable mining of non-renewable fossil groundwater. Using the Fossil groundwater; Nubian Aquifer of Saharan Africa as a test site, we demonstrate an integrated approach Sustainable to identify areas of discharge that could have gone undetected, and to model extraction development; that is sustained by natural discharge. Using isotopic and geochemical analyses along with Nubian Aquifer field and remote sensing data we show that discharge of the Nubian Aquifer is occurring on a larger scale, primarily through deep-seated fault systems, and that ascending groundwater discharges into relatively thick alluvial aquifers proximal to the fault complex that defines the River Nile and the Gulf of Suez. We develop a hydrologic model to assess the discharge and to constrain sustainable extraction in the Asyuti area along the River Nile and to demonstrate a replicable model for similar reservoirs. A two-dimensional groundwater flow model was constructed and calibrated against head data from 19 wells. Results point to a significant contribution to the Asyuti groundwater system from rising Nubian groundwater (3.19 · 107 m3/yr: 75% of incoming flow) and a modest contribution (1.08 · 107 m3/yr: 25% of incoming flow) from surface runoff. Approximately 2.5 · 107 m3/yr of groundwater could be used in a sustainable manner in Asyuti. Assuming geologic settings and discharge rates similar to those at Asyuti, we estimate that 70 · 107 m3/yr of groundwater may be available for sustainable development in similar settings around the River Nile and the Gulf of Suez. ª 2006 Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +1 269 387 5487. E-mail address: [email protected] (M. Sultan).

Introduction jor basins: (1) the Kufra Basin of eastern Libya, and northern Chad, and (2) the Dakhla Basin of western Egypt and north- Demand for fresh water supplies in arid and semi-arid coun- western Sudan (Fig. 1). The Aquifer consists mainly of con- tries world-wide is on the rise from increasing populations tinental sandstones with intercalated shales of shallow and limited water supplies. These problems are exemplified marine and deltaic origin, unconformably overlying Protero- in countries of Saharan Africa (North Africa) and the Middle zoic basement. The Aquifer reaches a thickness approaching East where scarcity of water resources contributes to polit- 3000 m in the center of the basin (Hesse et al., 1987). North ical instabilities, disputes, and conflicts. Studies have shown of the 25th parallel, the Nubian Aquifer is confined beneath that the number of countries unable to meet their water thick marine shales. North of the 29th parallel, northward needs to be self-reliant in food production were 12 in groundwater flow is limited by saline water. In the north, 1993, rose to 16 in the year 2000, and will be 18 by 2025. where the Aquifer is confined, it is divided into four thick Only Iraq, Lebanon, and Mauritania will have enough water (ten to hundreds of meters thick) horizons of sandstone lay- in this area, and their average availability in 2025 will have ers that are separated by relatively thin (meters to tens of dropped to 535 m3 per capita year, less than one half of meters thick) shale layers. Water in the Dakhla Basin flows what is considered necessary (IN-WARDAM, 1990). Sources from southwest to northeast (gradient 0.5 m/km) and dis- of fresh water supplies in these areas include: (1) surface charges naturally at oases (Sturchio et al., 2004). For a long runoff (e.g., River Nile in Egypt and Sudan) that generally time, it was realized that discharge from the Nubian Aquifer originates from allochthonous precipitation over distant occurs in the lowlands or in the depressions of the Western mountains located in wetter climates; and (2) groundwater Desert of Egypt and of the Libyan Desert. It is now becoming resources that originated as autochthonous precipitation clear that the discharge of the Nubian Aquifer waters is that recharged the aquifers in previous wet climatic periods occurring on a larger scale, primarily through deep seated (e.g., Nubian Aquifer in Egypt, Sudan, Libya). Anthropic fault systems that act as pathways for ascending Nubian controls on hydrologic systems in these areas are largely re- Aquifer groundwater (Sturchio et al., 1996; Yan et al., lated to: (1) environmental and hydrologic consequences of 2004). The most prominent of these systems are those engineering efforts intended to expand and to exploit water bounding the River Nile and the Gulf of Suez grabens as resources, largely involving impoundments of surface shown on the schematic cross-section extending from areas waters; and (2) excessive exploitation of non-renewable west of the Nile, across the Nile graben, the Eastern Desert, fossil groundwater without a clear understanding of these the Gulf of Suez graben, and ending in the Sinai Peninsula hydrologic systems. Nowadays, the Nubian Aquifer is being (A–A0 in Fig. 1). tapped extensively for irrigation in the oasis and lowlands The Eastern Desert extends from the River Nile to the of the Western Desert of Egypt (e.g., Kharga, Dakhla, Fara- Red Sea and covers an area of more than 200,000 km2 in fra), eastern Libya (e.g., Kufra Basin), and northern Chad. areal extent. In the central part of the Eastern Desert is a As a consequence, the naturally flowing springs in many of mountain range cored by Precambrian (550–900 Ma) vol- these oases and lowlands have dried up and groundwater cano-sedimentary arc terrains and interleaving ophiolitic levels have been dropping for decades due to excessive belts. The mountains are dissected by a series of wadis that pumping. It is estimated that by the year 2070, deep draw- drain into the River Nile (west) and the Red Sea (east). The down cones will form, and the extensive and interconnected modern River Nile valley was excavated and developed as a basins that exist nowadays within the aquifer will be dis- subsequent stream to the numerous valleys emanating from sected by interleaving dry areas (Soliman et al., 1998). the elevated Red Sea hills to the east (Said, 1993). The These development scenarios are typical for similar settings cross-section shows a concentration of high angle deep- elsewhere; it is often the case that development efforts are seated fault systems in the proximity of the Nile and the largely localized in areas that are more suited for settle- Gulf of Suez complexes. Should these faults terminate at, ment of communities. In the case of the Nubian Aquifer, or define the distribution of, the relatively thick alluvial development is largely concentrated in the lowlands for aquifers along the Gulf of Suez or the River Nile graben, obvious reasons: (1) natural discharge and/or shallow the rising Nubian Aquifer groundwater might ultimately flow groundwater; (2) rich soils (clay deposits from old playas), through these shallow aquifers before discharging into the and (3) mild climatic conditions. Overdevelopment prob- Nile or the Gulf. Next, we show that the isotopic affinities lems could be alleviated (at least in part) through the iden- of these shallow waters now residing in these alluvial aqui- tification and sustainable development in areas where fers are indicative of their origin, being largely derived from natural discharge occurs. In this manuscript, we demon- the fossil water of the Nubian Aquifer. strate that many of the discharge areas may have gone undetected in the Nubian Aquifer and give examples for sustained development of such systems. Methods

Groundwater samples were collected in high-density poly- Geologic setting ethylene bottles and tightly capped. Chloride concentra- tions were measured by ion chromatography with a Beneath the surface of the Eastern and Western Deserts of precision of ±2%. Stable hydrogen and oxygen isotope ratios Egypt and adjacent portions of eastern Libya, northeastern were measured by standard methods of equilibration with Chad, and northwestern Sudan (Fig. 1) lies an immense res- hydrogen and carbon dioxide, respectively (Coplen et al., ervoir (>50,000 km3) of freshwater in the Nubian Aquifer 1991; Nelson, 2000). Hydrogen and oxygen isotope data system (Thorweihe, 1990). This resource resides in two ma- are reported in terms of the conventional delta (d) notation, Natural discharge: A key to sustainable utilization of fossil groundwater 27

Figure 1 Location map showing the Asyuti watershed, the boundaries of the Nubian Aquifer in Egypt and the distribution of the outcrops of the Nubian sandstone (recharge areas), the Eocene limestone, and the crystalline bedrock in this area. Inset shows the extension of the Nubian Aquifer in the surrounding countries and the general groundwater ﬂow direction (SW–NE) and local groundwater ﬂow direction (towards the River Nile) in the Eastern Desert. A cross-section along A–A0 is shown in Fig. 2.

Figure 2 Schematic cross-section extending from areas west of the Nile, across the Nile graben, the Eastern Desert, the Gulf of Suez graben, and ending in the Sinai Peninsula (A–A0 in Fig. 1). The grabens are defined by high angle deep-seated fault systems. The cross-sections are based on our field data and on published data (RIGW, 1993). 28 M. Sultan et al. in units of per mil deviation relative to a standard refer- The Eastern Desert groundwater samples have a wide ence, whereby range in hydrogen and oxygen isotope ratios, from a relatively depleted composition of dD=58.7, d18O=7.73 in d; ‰ R =R 1 103 ¼½ð sample standardÞ Bir Laquita to a relatively enriched composition of and R = 2H/1Hor18O/16O and the standard is Vienna Stan- dD = +2.85, d18O=0.47 at Wadi El Ashrafia (Fig. 3). Two dard Mean Ocean Water (Coplen, 1996). Precision of d2H groups of samples could be identified. The first group values is ±1.3& and that of d18O values is ±0.2&. Tritium (Group I) includes samples that have isotopic compositions activities were measured following electrolytic concentra- similar to those of present meteoric precipitation or evapo- tion by gas-proportional counting and are reported in terms rated precipitation. Examples of these include groundwater of tritium units (TU), where 1 TU = 1 tritium atom per 1018 samples in fractured basement rocks and the overlying allu- hydrogen atoms. Precision of measured tritium activities is vium aquifers (e.g., Sheikh El Shazly, Bir Um Ghanam, Bir ±0.4 TU. Hafafit, and Bir Fawakhir) and in the alluvium aquifers in the proximity of the River Nile (e.g., Wadi El Ashrafia). Geochemical affinities of the Nubian Aquifer We have shown (Sultan et al., 2000) that the groundwater groundwater samples of the Wadi Tarfa area and surroundings are mostly evaporated flash flood waters, with relatively short under- ground residence times indicated by the presence of live tri- We collected groundwater samples for isotopic and geo- tium (i.e., <45 yr). The most enriched sample in the Wadi chemical analyses (O, H, tritium, Cl) from the shallow allu- Tarfa area, from Wadi Sheikh El Fadl, was shown by Sultan vial aquifers adjacent to the Nile Valley, in the Precambrian et al. to be evaporated River Nile water. The second group terrane, and adjacent to the Gulf of Suez (Figs. 3 and 4). We (Group II) of analyzed samples have relatively depleted compared data for our shallow groundwater samples to (dD=32 to 58.7) isotopic compositions compared to those from wells penetrating the underlying Eocene lime- Group I samples. Their isotopic compositions could be ac- stone, and those from artesian wells tapping the Nubian counted for by mixtures of modern precipitation (i.e., flash Aquifer at depth. Additional insights into the origin of the flood waters) and Nubian Aquifer water of more depleted Eastern Desert waters were made through comparisons to compositions. The depleted isotopic composition of the Nu- groundwater from the Western Desert and Sinai. bian Aquifer water is best explained by recharge of precip- The stable isotope ratios of hydrogen and oxygen are gi- itation showing the ‘‘continental effect’’ (Dansgaard, ven in Table 1 and are shown in Fig. 3, which includes data 1964), i.e., progressive condensation of water vapor from for samples from the present study as well as: paleowaters paleowesterly wet oceanic air masses that traveled across from the Gulf of Suez, also from the Nubian Aquifer (Stur- North Africa during the wet climatic periods at least as far chio et al., 1996); shallow aquifers in the Wadi Tarfa area back as 450,000 yr before the present (Sonntag et al., (Sultan et al., 2000); and data for modern precipitation 1978; Sultan et al., 1997). Available data for tritium activi- from Sidi Barrani (IAEA and WMO, 1998). ties for these samples (Table 1) reveals no measurable tritium, indicating residence times in excess of 50 years for these waters. The group II samples of the Eastern Desert waters are 60.00 apparently intermediate between the Western Desert Meteoric water line paleowaters and the Gulf of Suez area paleowaters, possibly Sidi Barrani rain reflecting a geographic trend in the isotopic composition of Gulf of Suez paleoprecipitation stored in the Nubian Aquifer. Alterna- 0.00 paleowaters tively, the apparent progressive enrichment in the isotopic Nile River δ D Bir Laquita composition from west (Western Desert) to east (Sinai) Wadi Tarfa groundwater could reflect variable degrees of mixing between fossil water that precipitated during wet climatic periods and -60.00 Group I El Sheikh Fadl meteoric precipitation that is deposited during the inter- Group II leaving dry climatic periods (e.g., nowadays). This hypothe- Western Desert Nubian aquifer paleowater sis is supported by the patterns of modern precipitation. Currently, rainfall over the Nubian sandstone outcrops (re- -120.00 -12 -6 0 6 charge areas) in southern Sinai is considerable (100 mm/ δ18O yr) compared to their counterparts in the Western Desert that hardly receive any precipitation (0–5 mm/yr) (EMA, Figure 3 dD vs. d18O for groundwater samples (solid symbols; 1996; Legates and Wilmott, 1997; Nicholson, 1997). Precip- this study) from shallow alluvial aquifers, from wells penetrat- itation in the Eastern Desert is intermediate between that ing the underlying Eocene limestone, and from artesian wells reported for the Western Desert and for Sinai. Regardless tapping the Nubian Aquifer at depth. Also shown are Nubian of which hypothesis is adopted, shallow groundwater sam- Aquifer paleowaters from the Western Desert of Egypt (open ples in alluvial aquifers that have depleted isotopic compo- diamonds; Sultan et al., 1997), thermal waters from the Gulf of sitions must have been largely derived from the fossil water Suez area (open circles; Sturchio et al., 1996), modern rain of the Nubian Aquifer. Locating these aquifers is an impor- water from Sidi Barrani, Egypt (open square; IAEA and WMO, tant step towards the identification and the assessment of 1998), Nile surface water analysis (open star). Also shown is the discharge. We use the isotopic signatures of the investi- global meteoric water line, dD=8d18O + 10 (Craig, 1961). gated groundwater to identify shallow alluvial aquifers that Natural discharge: A key to sustainable utilization of fossil groundwater 29

Figure 4 Landsat TM image (false color composite) showing our groundwater sample locations (yellow circles) and the inferred distribution of alluvial aquifers (outlined by striped blue lines) that are likely to be recharged by ascending Nubian Aquifer groundwater. Also shown are the locations of the Asyuti Watershed, and the Tafa Watershed.

host Nubian Aquifer groundwater and search for features cases, e.g. Hammam Faroun (Sturchio et al., 1996). Cooling that are being shared by these aquifers and which could of Group II samples could occur as the ascending hot ground be used in the identification of similar reservoirs elsewhere. water flows towards the River Nile or the Gulf of Suez. The distribution of alluvial aquifers could be inferred in en- hanced Landsat TM images from the distribution of rela- Locating Nubian Aquifer groundwater residing tively bright wadi-fill sediments (Fig. 4). Drilling and in alluvial aquifers geophysical investigations have shown that in general, the thickness of these aquifer sediments increases progressively We classified either as Group I or Group II the following sam- towards the River Nile and towards the Gulf of Suez. The ples on the basis of their stable isotopic compositions (1) depth to the Nubian sandstone also increases with proximity samples collected from the shallow alluvial aquifers in this to the River Nile and Gulf of Suez, reaching depths of up to study; (2) samples from similar aquifers in Wadi Tarfa and 3 km in the Gulf area and up to 2 km in the vicinity of the surroundings (Sultan et al., 2000), and (3) thermal waters Nile Valley (Fig. 2). It has been suggested that many of from springs and shallow artesian wells that are being dis- the deep seated sub-vertical faults in the area are NW- charged along the coastal plains of the Gulf of Suez. The trending Neoproterozoic faults that have been reactivated majority of Group II samples are thermal waters that dis- in the Neogene during the rifting events associated with charge in the alluvial deposits proximal to the River Nile the opening of the Red Sea. These NW-trending faults and and the Gulf of Suez (Fig. 4); their discharge temperatures shear zones are the most prominent structural elements in exceed ambient temperatures reaching up to 70 C in some the Neoproterozoic Red Sea hills terranes. They are part 30 M. Sultan et al.

Table 1 Isotopic and geochemical characteristics of groundwater samples from the Eastern Desert Name Lat Long Type Depth*(m) Cl (mg/L) d18O dDTU El Reshrash1 29.461 31.341 Production well in alluvial 60 3000 5.19 28.69 <1 El Reshrash2 29.454 31.342 Production well in alluvial 55 1400 5.47 29.53 <1 Arab El Ashraﬁa 29.408 31.273 Production well in alluvial 50 840 0.47 2.85 PW-Bir-10 Asyuti 27.258 31.306 Production well in alluvial 44 480 4.95 34.68 PW-16 Asyuti 27.256 31.316 Production well in alluvial 45 300 5.49 38.54 PW-15 Asyuti 27.271 31.374 Production well in alluvial 41 3100 6.78 48.99 PW-36 Asyuti 27.237 31.376 Production well in alluvial 46 830 6.13 46.95 PW-1 Asyuti 2000 27.218 31.336 Production well in alluvial 46 400 7.42 51.96 PW-5 Asyuti 2000 27.193 31.342 Artesian well in alluvial 370 7.34 51.67 Wadi Matully E0005 – 25.895 33.142 Artesian well tapping Nubian 700 7.73 58.71 <1 Bir Laquita Wadi Matully E0006 – 25.880 33.077 Open pit in alluvial 8 1500 6.26 52.14 MrX farm Wadi Matully E0008 – 25.996 32.889 Production well in alluvial 20 550 1.4 18.34 Abdel Maksoud Wadi Qena E0009 26.267 32.786 Artesian well tapping Nubian 1200 590 6.99 50.74 <1 Wadi Qena E0010 26.271 32.787 Well in alluvial 46 2090 5.08 38.60 Wadi Darah Abu Shar El 27.976 33.216 Open pit in limestone 8 1300 7.13 52.00 Bahary Wadi Darah – Darah 4 27.976 33.205 Production well in sandstone 16 1500 7.11 53.53 <1 Wadi Darah – Mohamed 27.989 33.228 Production well in limestone 94 7400 7.14 53.90 Abdul Aziz Field 2 Production Well 27.220 31.331 Production well in alluvial 41 460 6.64 46.81 Wadi Asyuti Field 1 PZ-1 27.258 31.306 Monitoring well in alluvial 34 490 5.53 39.77 <1 deep Wadi Asyuti Field 1 PZ-2 27.258 31.306 Monitoring well in alluvial 34 510 5.26 36.64 <1 medium Wadi Asyuti Field 1 PZ-3 27.258 31.307 Monitoring well in alluvial 31 830 7.35 54.15 <1 shallow Field 3 PZ-1 shallow 27.254 31.369 Monitoring well in alluvial 61 770 5.82 42.30 <1 Field 3 PZ-2 medium 27.270 31.376 Monitoring well in alluvial 65 1230 6.46 47.38 <1 Field 3 PZ-3 deep 27.272 31.378 Monitoring well in alluvial 60 5910 6.45 43.98 <1 Sheikh Shazly 24.200 34.635 Open well in fractured 7 1223 2.72 14.24 basement Bir Hafaﬁt 24.768 34.517 Open well in fractured 30 60 1.99 1.92 basement Bir Um Ghanam 24.724 34.562 Open well in fractured 30 693 1.26 5.65 basement Bir Fawakhir 26.012 33.600 Open well in fractured 12 170 1.92 8.27 basement * Depth (in m) to water table.

of the Najd Shear System, the largest pre-Mesozoic trans- the Western Desert and undetectable tritium; (2) dis- current fault system on Earth (Stern, 1985), extending in a charge temperatures exceeding ambient temperatures; NW–SE direction over 1200 km in outcrop, with a width of (3) thick alluvial deposits that are found proximal to the approximately 300 km; the system terminates at the east- Nile Graben and Gulf of Suez, and (4) intersecting high an- ern margin of the Red Sea and extends into the Eastern Des- gle fault systems (generally, NW-trending and NE-trending) ert of Egypt (Abuzied, 1984; Moore, 1979; Stern, 1985; bounding alluvial sediments. Intersecting faults will induce Sultan et al., 1993). porosity and facilitate the rise of groundwater. Using All of these features suggest that the Nubian Aquifer these criteria, we have identified a coverage that shows water flows upward along sub-vertical faults in the prox- the distribution of the areas encompassing alluvial aqui- imity of the River Nile and Gulf of Suez graben and dis- fers that are likely to be recharged by ascending Nubian charges into the thick alluvial aquifers that are Aquifer groundwater (areas enclosed by striped blue lines delimited by these faults (Fig. 2). Consequently, we have in Fig. 4). Next we use one of the better studied discharge identified four main criteria to locate alluvial aquifers areas (Wadi Asyuti; Fig. 4) to validate the reliability of that host Nubian Aquifer water: (1) isotopic compositions the criteria we selected for locating alluvial aquifers host- similar to those of known Nubian Aquifer paleowaters in ing Nubian Aquifer groundwater. Natural discharge: A key to sustainable utilization of fossil groundwater 31

A major agricultural development project is underway in tion of the image shows that topographic expressions (val- Wadi Asyuti, where over 70 production wells were drilled leys and grabens) mark the projected extensions of the over the past two decades, each well pumping 50– traces of two of the three NW-trending faults (identified 100 m3/h. All of the analyzed samples that were collected by drilling and geophysical methods) into the adjacent from Wadi Asyuti (8 samples) show depleted isotopic com- mountains. Because of the high cost involved in the acquisi- positions (dD: 34.7 to 52.0). These depleted composition of geophysical or drilling data, we rely on the analysis tions, undetectable tritium (<1 TU), and the relatively of remote sensing data and published subsurface data (com- high (compared to ambient temperatures) discharge tem- position, thickness, structure) where available to infer the peratures (27–32 C) indicate that these are paleowaters, presence of reactivated NW-trending faults. As described largely derived from the underlying Nubian Aquifer. In this earlier, these fault systems are one of the key criteria for area, the Nubian Aquifer is found at depths exceeding identifying alluvial aquifers that host Nubian Aquifer 1 km and thus, the groundwater must have ascended via groundwater. deep-seated faults. Three major subvertical, NW-trending Following the identification of the targeted alluvial aqui- faults were identified along cross-section B–B0 (upper right fers, we developed a hydrologic model to assess the dis- inset, Fig. 5) using geophysical methods and utilizing infor- charge and to constrain sustainable extraction in Asyuti mation extracted from drilling (RIGW, 1993). Along the area and to demonstrate a replicable model for similar res- same traverse, the thickness of the alluvium aquifer was ervoirs along the Nile and the Gulf. The rising Nubian found to increase significantly (to more than 300 m) towards groundwater ascends to the near surface, discharges into the River Nile. The locations of the inferred sub-vertical the alluvial aquifers and ultimately into the River Nile or faults plot along NW-trending structural discontinuities that the Gulf of Suez. If we were to intercept these waters be- could be readily identified on landsat TM images and on dig- fore they make it to the River Nile and at a rate that does ital elevation datasets. Fig. 5 is an overlay of a Landsat TM not disrupt the existing steady state groundwater flow re- image over digital elevation, an image that integrates and gime, we would probably attain sustainable exploitation of displays information contained in the two images. Inspec- this ground water resource.

Figure 5 Landsat TM image (false color image) for the Wadi Asyuti area and surroundings draped on digital elevation data. Upper right inset shows: (a) the model domain and the extent of the Asyuti watershed, and (b) cross-section along traverse B–B0 in Wadi Asyuti (simpliﬁed from RIGW, 1993). Topographic expressions (valleys and grabens) mark the projected extensions of the traces of NW-trending faults identiﬁed by drilling and by geophysical methods into the adjacent mountains. 32 M. Sultan et al.

Assessment of Nubian Aquifer groundwater cally depleted water that recharged the Nubian Aquifer residing in alluvial aquifers: Wadi Asyuti during the wet climatic periods of the Pleistocene. Support for this conceptual model comes from: (1) presence of exten- The Asyuti hydrologic system comprises two flow compo- sive drainage networks that channel rain precipitating over nents: (1) the surface water system of the Asyuti watershed, large segments of the Eocene limestone platforms through and (2) the Asyuti groundwater flow system. The Asyuti wa- Wadi Asyuti (Fig. 5 inset); (2) reported recent flash flooding tershed collects rainfall over an area of 5589 km2 in the Red events in Wadi Asyuti (Naim, 1995), and (3) analysis of the iso- Sea Hills and adjacent mountains and is channeled towards topic affinities of the Eastern Desert groundwater. Assuming the River Nile (Fig. 5). The Quaternary alluvium aquifer the endmembers have isotopic compositions similar to those floors the main channels within the Asyuti watershed; the of the Western Desert and present day precipitation, mass surrounding outcrops are mainly Tertiary Limestones balance calculations indicate mixtures of approximately (Fig. 5 inset). Gheith and Sultan (2002) estimated that the 80% Nubian Aquifer depleted water with 20% flash flood en- infiltration to the limestone is limited and transmission loss riched water. In the next sections, we show that the inferred to the alluvium aquifer along the stream channels is signif- proportions of the two water sources are consistent with our icant at an average annual rate of 2 · 107 m3. The flow to findings from the results of the hydrologic model. the outlet of the watershed is also minimal, amounting to only 7% of the total rainfall. In this section, we discuss our Model construction conceptual model, the construction of the groundwater flow model, and finally we present simulations using the cali- The geographic limits of the model were chosen to coincide brated model for the sustainable development of the Asyuti. with natural boundaries of the alluvium aquifer on the north, east, and south. The River Nile is considered as the Conceptual model western boundary for the model (Figs. 5 and 6). The model grid is constant with spacing of 200 m by 200 m. The model We constructed a ground water flow model for steady-state was simplified to accommodate a single alluvium layer; the and transient simulation under representative flow condi- bottom of the layer is defined by the depth of the underlying tions and calibrated the model against well data and pumping clay or limestone layer. The top of the alluvial layer was ex- test data reported by RIGW (RIGW, 2003). The mathematical tracted from the digital elevation data for the area and the model is based on the following conceptual model: The bottom of the layer was delineated from depth information groundwater flow system in the area includes two possible extracted from well log data and from geologic cross-sec- sources: (1) surface water infiltration from the runoff (trans- tions (RIGW, 2003). mission loss along the ephemeral streams), and (2) groundwa- Three types of boundaries were adopted: river, general ter discharge from the Nubian Aquifer through deep-seated head, and no flow boundaries (Fig. 6b). The river and general subvertical faults. The former is isotopically enriched water, head boundaries allow the simulation of flow in and out of a most likely recharged by flash-floods during dry climatic peri- surface water body (e.g., river) or an external water source ods (such as the present climate), and the latter is isotopi- (e.g., rising groundwater from a deep aquifer) to a ground-

Figure 6 Sketch map showing elements of the constructed groundwater flow model. (a) distribution of all wells in Wadi Asyuti, and clusters of wells used for steady state calibrations; (b) model boundaries and external stress through faults; (c) spatial variations in hydraulic conductivities and (d) postulated recharge areas. Also shown are the locations (a) of two wells (PW10 and PW14) where pumping test data were collected and the locations (b) for five hypothetical pumping wells (P1–P5). Area covered by the model domain is shown on Fig. 5a inset. Natural discharge: A key to sustainable utilization of fossil groundwater 33 water system within the modeled domain. The no-flow production rates, the calibration had to be performed at boundary is used to represent a condition, where there is both steady state and at transient state conditions. negligible or no flow across a boundary of interest. The River Water-level data cover an area extending from the cen- Nile was considered as a river boundary with river stages tral part of Wadi Asyuti to the River Nile (Fig. 6a). Records ranging from 41 m AMSL (upper gradient) to 35 m AMSL (low- for ground water levels prior to pumping are not available er gradient). General head boundaries in the study area are posing difficulties to conducting simulations for ground lumped into two groups: firstly, boundaries that are perpen- water levels under steady state conditions. To minimize po- dicular to the general flow direction and are here presumed tential impacts of pumping on groundwater levels, 77 pro- to intercept the bulk of the flow from fractured limestone duction well data and nine observation well data were and from the channels (three orange segments on Fig. 6b). examined and grouped into groups, each encompassing a Secondly, deep-seated faults that represent discharge zones cluster of wells that lie in the same approximate location. for Nubian Aquifer groundwater. Four major faults were lo- For each group, we selected the wells with the highest cated using information portrayed in cross-section B–B0 water-level measurements. We assumed that these wells (Fig. 6b), well data, and inferences from remote sensing approximate the water levels at steady state conditions data (refer to previous section). Boundaries of the alluvial and are least affected by the subsequent pumping associated sediments that are sub-parallel to the general flow direction with the agricultural development of Wadi Asyuti. Out of the (i.e., perpendicular to River Nile) receive minimal ground- investigated 87 wells, 19 were selected for calibration pur- water flow from adjacent units and intercepting channels poses (Fig. 6a). Due to the paucity of temporal water-level and were considered as no-flow boundaries (Fig. 6b). and pumping records, we resorted to the use of two 24-h The direct surface water recharge is mainly through pumping test data to refine our model calibration under transmission loss along the stream network length to the transient conditions. Fig. 6a shows the locations of two underlying alluvial aquifer. The recharge area, the area cov- pumping tests, which were conducted in areas with rela- ered by the ephemeral streams, was identified using mor- tively low (PW14) and high (PW10) hydraulic conductivities. phological and spectral characteristics of these streams In the four-step calibration process, the initial two cali- that are portrayed in satellite imagery. The outlines of brations were conducted under the steady state conditions the ephemeral streams appear as spectrally bright areas and the third calibration was conducted under transient compared to their surroundings (Fig. 5). The longer the tra- conditions. The final calibration is a fine tune for steady vel distance for sediments in a watershed, the finer the state designed to account for any changes that might be grains that are being transported and deposited in the introduced by the adopted transient calibrations. Most of underlying stretch of the channel, and the brighter the the calibrations were performed through an automated pro- spectral reflectance of the deposited sediment on satellite cess to estimate specific model parameters that produce imagery (Sultan et al., 1987). Our field investigations and the best possible match to measured water levels. The in- inspection of Fig. 5 show that to be the case with the depos- verse model, a parameter-estimation process (PES) incorpo- its outlining the extent of the ephemeral streams in Wadi rated with MODFLOW 2000 (Hill et al., 2000), was used in Asyuti; they are fine-grained sediments that are spectrally this automated process. bright. We assumed that recharge of the alluvial aquifers The detailed descriptions for the four step calibration in Wadi Asyuti could be approximated by the transmission are as follows: losses arising from infiltration throughout the stream reaches and adopted these losses as recharge rates. The (1) Calibration for conductance for the three inflow average transmission loss per unit length is approximately boundaries, hereafter referred to as lithologic 4 · 104 m3/yr/km based on the estimated total transmission boundaries, and the four faults were conducted loss (2 · 107 m3/yr) over the total length (500 km) of the under steady state conditions. Inflow from the three Wadi Asyuti stream reaches (Gheith and Sultan, 2002). lithologic boundaries and the four faults (Fig. 6b) Hydraulic conductivity of the alluvium aquifer is hetero- were calibrated first since they account for the geneous over the study area. Spatial variations in the majority of water flux in the system. All of the seven hydraulic conductivity of the alluvium aquifer were repre- boundaries were considered as general head bound- sented by individual zones of constant hydraulic conductiv- aries; the head of the inflow were estimated using ities (Fig. 6c). The values for each of the individual zones ground surface or near surface elevations as upper ranged from 0.5 m/d to 12 m/d and were estimated by con- limits and water levels observed in the shallow and touring (Kriging method; contour interval 2 m/d) measured deep wells as lower limits. Estimated initial conduc- hydraulic conductivities for 77 production wells (RIGW, tance is dependant on the hydraulic conductivities of 2003). The flow model was constructed using the Groundwa- the limestone and faults, the geometry of the bound- ter Modeling System (GMS), an interface developed by the aries, and the approximated depths of Nubian Aqui- Environmental Modeling Research Laboratory (EMRL) of fer. The calibration was conducted in steps with Brigham Young University around the USGS flow code (Harb- the initial steps involving the three lateral lithologic augh et al., 2000; Hill et al., 2000). boundaries. Simulated groundwater levels were found to be consistently lower than the observed lev- Model calibration els suggesting that additional inflow along the remaining four faults is required. A suggestion fur- The calibration was accomplished via a four-step process ther corroborated by examination of the isotopic and specific model parameters were adjusted at each step. compositions of the groundwater samples. Including Because one of our main goals was to estimate sustainable additional inflow from the four faults, significantly 34 M. Sultan et al.

reduced (by 50%) deviations between observed and (3) Calibration for aquifer properties were conducted simulated heads. All calibrations were conducted under transient state conditions using two 24-h pump- using inverse modeling through automatic adjust- ing test data (RIGW, 2003). The parameters of hydrau- ment within given ranges. lic conductivity and storativity were adjusted to fit (2) Calibration for recharge rate was conducted under the observed drawdown in water level in response to steady state conditions. Because inflow from recharge pumping. The computed drawdown matches well with rate is secondary compared to that from the faults the observed drawdown from the pumping tests and from the inflow boundaries, the calibration for (Fig. 7a). recharge rates followed that for the faults and inflow (4) A fine calibration (tuning) for the seven inflows (faults boundaries. The initial recharge rate (4 · 104 m3/yr/ and lithologic boundaries) was performed under km) was adjusted using the inverse model within a steady state conditions to account for any changes range of recharge rates that are an order of magni- in the flow system resulting from earlier (step 3) tude above and below the initial recharge, respec- adjustments of aquifer properties. tively. This assumption was made to ensure that the estimated optimum recharge rates are realistic Comparisons between the simulated water levels (from values. the calibrated model: steps 1 through 4) and the observed

Figure 7 Model calibration. (a) Calibration of the model under transient state conditions using two 24-h pumping test data. Hydraulic conductivity and storativity were adjusted to fit the observed drawdown in water level in response to pumping. (b) Comparisons between the simulated water levels (from the calibrated model: steps 1–4) and the observed water levels. Natural discharge: A key to sustainable utilization of fossil groundwater 35 water levels show a good agreement between the two data determine the potential sustainable production, the model sets (Fig. 7b). Deviations between observed and simulated was used to identify the following: (1) the maximum sus- head values were observed in two locations where the local tainable rate and (2) the minimum well spacing at the se- water-levels were inconsistently lower than the general lected production rate. A simulation experiment was flow pattern, most likely due to excessive pumping in these conducted using the calibrated model. Five hypothetical areas. Two quantitative measures were used to examine the pumping wells (P1–P5; Fig. 6b) were allocated within do- correlation between the observed and simulated water lev- mains of varying aquifer properties, and distant from the els: (1) the root mean square error (RMS) was found to be faults (high inflow areas) to ensure that obtained estimates low (3.12 m), and (2) the mean error was found to be close for sustainable production rates are conservative. The to zero (0.012 m), indicating insignificant deviations be- adopted daily pumping schedule (8 h/day) was based on tween observed and simulated heads. reported irrigation practices (morning to early afternoon) in the area and was verified by examination of the recorded Simulation results hourly fluctuations in water level in three monitoring wells located in the proximity of pumping wells (Abdelgelil, The calibrated flow model for the groundwater flow system 2006; personal communication). Pumping rates were ob- in Wadi Asyuti provides quantitative estimates for each of tained at each of the five locations through multiple trials the flow components entering the Asyuti shallow groundwa- to satisfy the full recovery of water levels. This was con- ter system (Table 2). Results highlight the importance of the ducted to ensure no decline in water levels with time. The Nubian Aquifer as the primary source (75% by volume of the area of influence due to pumping at each test location investigated groundwater), whereas the groundwater inflow was estimated based on the area where drawdown in the through the three lithologic boundaries is estimated at a groundwater system is more than 0.1 m. The computed ra- maximum of 22%. The latter is largely related to the initial dius of the significant influence area provides an estimate and transmission losses within the drainage network up gra- for adequate well spacing within the individual zones. The dient from the Asyuti valley. Direct surface water recharge results are summarized in Table 3. For a well in the area (transmission losses within the Asyuti valley) was found to P1, we estimate a sustainable production rate of 50 m3/h be small (3%) raising the total estimated recharge from for 8 h/day, a daily production of 400 m3, and a radius of surface water to 25%. significant influence of about 800 m. For a well in the areas P2 and P3, the daily production is relatively less (P2: Potential sustainable groundwater production 240 m3; P3: 160 m3) and the well spacing is 1100 m and 800 m for P2 and P3, respectively. The estimated sustain- The calibrated model for Asyuti alluvium aquifer provides a able annual production for all wells in the areas P1, P2, quantitative tool to estimate the maximum water produc- and P3 is approximately 9.6 · 104 m3/km2. For areas P4 tion that could be sustained over long time periods. To and P5, the daily production is considerably lower (32 m3or less) and the well spacing is relatively larger Table 2 Estimation of annual flow rates entering the Asyuti (>2000 m). The annual sustainable production in these two 3 2 shallow groundwater system areas is very low (1300 m /km ); only a few production wells can be allocated within these two domains. P1, P2, Flow component Annual flux Percentage 3 and P3 are the major production areas for Wadi Asuyti. (m /yr) (%) We estimate a total sustainable annual production of Recharge from channel 9.36 · 106 22 2.5 · 107 m3 for the entire Asuyti alluvium aquifer. Assuming alluvium and surrounding that the alluvial aquifers defined on Fig. 4 have geologic set- fractured limestone tings and discharge rates similar to those observed in Wadi (general head boundaries) Asyuti, we estimate that 70 · 107 m3/yr of groundwater Recharge from surface 1.47 · 106 3 may be available for sustainable development in similar set- water runoff tings around the River Nile and the Gulf of Suez. Detailed Recharge from Nubian 3.19 · 107 75 investigations similar to those conducted in Wadi Asyuti Aquifer via faults are needed to verify and to refine these first order estimates.

Table 3 Predicted sustainable production rates for the Asyuti shallow groundwater system Model Production Daily Area of Estimated Daily production test well rate (m3/h) production (m3) influence (km2)* well spacing (m) per unit area (m3/km2) P1 50 400 0.6 800 700 P2 30 240 1.0 1100 250 P3 20 160 0.5 800 300 P4 4 32 4.0 2200 8 P5 1 8 3.6 2100 2 * The area of influence is defined by the area that has drawdown more than 0.1 m due to pumping at P1, P2, P3, P4 or P5 (Fig. 6b). 36 M. Sultan et al.

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