Geophysical and sedimentological evaluation of quaternary groundwater aquifers, eastern Hafit anticline, Al-Ain, United Arab Emirates (UAE).
United Arab Emirates is located in a semi-arid climate zone shortage of water resources has and will be a vital problem in securing sustainable water management. While desalination plants account for most drinking and household water, groundwater is still the backbone of society uses for agriculture, industry and urbanization. Major natural storage of the groundwater in the UAE varies from recent alluvium and sand dunes to old sedimentary, igneous and metamorphic rocks. Here we focus on Quaternary alluvium aquifer which is located along eastern limb of a major anticline (Jabal Hafit) at the southeast border of Al-Ain city. This city is the largest one at the eastern part of the UAE with a population of about 0.6 million and plans for extensive expansion. Groundwater represents the main source in the city, although a considerable amount of fresh water is delivered through desalination plants in Abu Dhabi. Although several investigations have dealed with the groundwater quality and occurrence in the Quaternary aquifer system, there is no data on the subsurface architecture of the aquifer system with respect to possible existence of preferred permeability channels that can be linked to the paleo-geomorphology of the region. Here, we provide geophysical and Sedimentological data that support the existence of subsurface paleochannels. These features act as specific conduits of groundwater in the area and can be targeted for future exploration and exploitation.
Geological and Hydrological Settings
Al-Ain city is one of the largest and ancient oases of the Arabian Peninsula, due to the underground fresh water supply which derived from the Oman Mountains to the east. The city is situated in the eastern part of Abu Dhabi Emirates near the border with Sultanate of Oman and the western margin of the northern Oman Mountains (Fig. 1). It is located within the arid desert belt of the world; the existing surface drainage net has been formed as a result of the prevalence of humid climate during the Quaternary times. Jabal Hafit is located at the southeast of Al-Ain and considered as one of the most prominent monuments of the area, it is a Tertiary anticlinal structure with approximately 29 km length and 5 km width and maximum elevation of about 1160 m above the sea level, plunging south-easterly in Oman and northwesterly in Emirates (Hunting Geology and Geophysics (1979) and Abou El Enin (1993). The limestone and marls exposed in Jabal Hafit are considered of Lower, Middle and Upper Eocene age. To the east, it is bounded by Al-Jaww plain and Oman Mountains. Al Jaww Plain is 15 km wide and prominent piedmont situated east and southeast of Al-Ain city between the Oman Mountains and Jabal Hafit (Fig. 1). Hunting Geology and Geophysics Ltd. (1979) recognized five sediment types which are: alluvial deposits, desert plain deposits, mixed deposits, sabkha deposits and aeolian sand. Surface drainage on the piedmonts and alluvial fans subdivisions are generally canalized in wadis with variable flow patterns exhibiting complexly braided channel morphologies (Menges and Woodward, 1993). Two gravel plains are terminating the eastern part of Al-Ain area; one fringes the Oman Mountains and the second fringes Jabal Hafit. The first fringe reaches its maximum development at Al Jawwa plain (Hunting Geology and Geophysics, 1979). The drainage basins in Al-Ain area are of two systems; one is related to the northern Oman Mountains and the second belongs to Jabal Hafit, the first system is generally dendritic, as it is typical massive igneous rocks forming these mountains. In Al Jawwa plain, the dendritic pattern usually changes to braided pattern where the slope decreases in Al Jawwa plain. The main reasons for the variation in the drainage pattern are either deformation, or decrease in slope (Al-Shamsei, 1993). The second system of the drainage pattern occurs in the west of Al Jawwa plain and south of Al-Ain area. The pattern ranges from dendritic to braid with some parallel or rectangular pattern especially in the structurally-controlled areas. The plain is transverses by numerous wadis such as wadi Shik, Al Ain and Muraykhat (Fig.2)
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Three alluvial fans within the plain; namely: the Zarub fan in the north, the Moundassah fan in the middle and the Ajran fan in the south (Al-Shamsei, 1993). The main existing aquifers in the UAE area, fractured ophiolite rocks in the east, gravel aquifers flanking the eastern mountain ranges on the east and west and sand dune aquifers in the south and west (Al-Shamsei, 1993). The most important aquifer in the study area is the Quaternary aquifer.
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Quaternary-age deposits consist of near-surface and surficial sediments of mixed alluvial, aeolian and locally, sabkha (Evaporites) origins. The stratigraphy of Al Ain area, comprises a sedimentary sequence ranging in age from the Cretaceous to the Quaternary (e.g. Hunting Geology and Geophysics, Ltd, 1979; Hamdan and El-Deeb, 1990; Hamdan and Anan, 1993; Whittle and Alsharhan, 1994)..The Upper Cretaceous sequence includes (from base to top): Semail ophiolites (serpentine andserpentinized predotite), the oldest exposed rocks in Al-Ain area, Qahlah Formation (red and yellow Unfossiliferous clast-supported conglomerates of serpentized predotite, derived from the Semail ophiolites, Simsima Formation (marine bioclastics limestones with rudists, corals and echinids, it is disconformably overlies the Qahlah Formation (Hamdan and Anan, 1993).
The Paleocene sequence is separated from the underlying Upper Cretaceous sequence by a regional unconformity with local conglomerate at its base. It is represented by the Muthaymimah Formation. The Eocene sequence includes Rus Formation and Dammam Formation. The Rus Formation (Lower Eocene) is composed of fossiliferous doloitic limestone with thin argillaceous limestone grading upward to well-bedded limestone. (Whittle and Alsharhan, 1994). The Dammam Formation (Middle to Upper Eocene) unconformably overlies the Rus Formation (Hamdan and Bahr, 1992). The Oligocene Asmari Formation ranges in age from Middle to Late)It is composed upwardly of silty marl, bioclastic nodular limestone, and interbeddd bioclastics limestone and marl. The Miocene succession unconformably overlies the Asmari Formation (Whittle and Alsharhan, 1994). It is low-lying and located at the eastern flank of Jabal Hafit as interbeds of Gypsum and Clay and fossiliferous clay. Quaternary age deposits cover most of Al-Ain area and consist of near surface and surficial sediments of mixed alluvial, aeolian, and locally, sabkha (evaporatic origin) Quaternary alluvium constitutes the principle waterbearing litho-stratigraphic unit.
The electrical methods in general include different techniques and instruments depending on the nature of the method used in prospecting. Some of these methods make use of the natural currents and others depend on injection of artificial currents into the earth. For more details about these different techniques references are made to Mussett and Khan, (2000), Reynolds (1997), Parasins (1997), Telford et al., (1990), Robinson and Coruh (1988) and Dobrin (1976). The DC-resistively methods of geophysical exploration are popular and proved to be successful and have many implications in the fields of geo-environment and hydrogeology. These methods have also been used to monitor types of groundwater pollution; in engineering surveys to locate sub-surface cavities, faults and fissures, mineshafts and in archaeology for mapping out the areal extent of remnants of buried foundations of ancient buildings, amongst many other applications. One of the new developments in recent years is the use of 2-D electrical imaging/tomography surveys to map areas with moderately complex geology (Griffiths and Barker 1993). A more accurate mode of the subsurface is a two-dimensional (2-D) model where the resistivity changes in the vertical direction, as well as in the horizontal direction along the survey line. In addition to the VES surveys also helps to investigate the vertical variations in this study. Nine (2-D) electrical tomography profiles using Wenner electrode configuration along some selected profiles crossing of Wadi Muraykhat and Wadi Sa'a, Al Ain area. As well as, Five VES were acquired. (Fig. 1). Eight Profiles extends to 600 m, length while profile 9 extends to 800 m length (Fig. 1). For profile II' and G-G', 10 m electrode spacing has been utilized, while 20 m electrode space has been used for the rest of profiles. The apparent resistivity data were inverted to create a model of the resistivity of the subsurface using Res2dinv, ver. 3.54. Res2dinv uses an iterative smoothness-constrained least-squares method (deGroot-Hedlin and Constable, 1990; Loke and Barker 1996). For each profile, Wenner array was used in resistivity data acquisition. Five vertical electrical sounding (VES) are carried out, using Schlumberger configuration. The electrode separations up to AB = 400 m were used in general and sometimes AB = 600 m especially at soundings carried out nearby the water wells, drilled in the area. This separation could penetrate the ground layers to a depth more than 100 m. The field apparent resistivity values were collected by using the resistivity and self-potential unit (Sting R1 R&IP). The obtained values were analyzed by computer software. These programs include Zohdy (1974), Meju (1992), and others. The use of those programs indicates the similarity of the results. The true resistivity maps were then constructed using SURFER computer software (Golden Software INC, 1990).
The (2-D) resistivity tomograms and the VES guided with the available borehole information (Fig. 1) are guided with the geoelectric model given by Al Nuaimi (2003) (and US Geological Survey,(1993). Investigations of the inverted resistivity tomograms (Figs. 3 to 6, for examples), lateral variation of lithological units are recognized. The geo-electric section of VES shows a high to moderate anomaly zone extending from southwest to northeast is detected with maximum amplitude of about 150 Ohm.m. This value may reflect the humid sand and gravel deposits surrounded by sediments of low resistivity which include clay and mudstones intercalated with silty sands. This high resistivity horizon is believed to be a buried paleochannels. This paleochannel is appeared clearly on the vertical and lateral section of apparent resistivity, passing through VES 1;2;3;4; and 5 as oval shaped high anomaly between VES (3) and VES (5) (Fig.7).
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The main three identified geoelectric-lithologic units along these tomograms (Figs.3-6) are
Layer-1 is a surficial zone of loose to weakly consolidated sand and gravel. This zone corresponds to the upper part of the very resistive (> 100 ohm-m) layer. The resistive nature of this layer is indicative of dry conditions in the upper part of the alluvium.
Layer-2 is a thick zone of gravel and sand comprising the bulk of the alluvium. The moderate to relatively small resistivity in this middle interval suggests partial saturation and/or the presence of a clay-rich matrix (profiles B-B' and I-I). However, at certain locations along these tomograms, there is a zone of coarse gravel of a varying thickness at the bottom of the alluvial section. This zone appears to represent a basal deposit of the saturated channel gravels. This feature is identified along the mid part of profile B-B' (Fig. 4) and the mid of profile I-I' (Fig.5). The third layer has a resistivity range of less than 10 Ohm-m. This low resistivity layer is composed of bedrock consisting of marl, clay, mudstone, or shale. In some places in the deeper depth this zone would have resistivities of less than 5 Ohm-m probably due to the increase of salinity with depth (profiles I-I', G-G' and F-F' (Figs. 3, 5 & 6). To relate the inverted 2-D resistivity tomograms with lithology and hydrogeological conditions, previous work done by both Al Nuaimi (2003) and US Geological Survey, (1993) have been correlated with 2-D tomograms. Figure (8) shows typical TEM soundings done at Zaroub gab to the north of study area (Al Nuaimi (2003). The model of TEM data in interdune area to the northwest of study area is reviewed and shown in Fig.(9) (US Geological Survey, 1993).
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Petrography and Mineralogical studies
Textural and compositional characteristics of the sediments of Al Jaww aquifer have essential effect on the quality of water. Sixty thin sections were prepared form the selected samples collected at different depth intervals in six wells. These wells are directed in the line nearly East-west direction in Al Jaww Plain. The various rock types constituting the aquifer system in the studied wells have been examined microscopically. The results of this examination revealed presence of the following four facies types: the ophiolite clasts conglomerate, ophiolite brecciated conglomerate facies, calclithite facies and lithic carbonate facies. The Ophiolite clasts conglomerate (Figs. 10A & 10B) was recorded in the upper part of the aquifer particularly eastward the study area (well No. 17, 18, 20). The framework of the sediments consists of silty to coarse pebbly, poorly-sorted clasts which are subangular to subrounded and have low sphericity. These clasts are essentially ophiolitic consisting of serpentine, olivine, pyroxenes, plagioclase and amphiboles with minor of cherty made up of microquartz and mega quartz. Some clasts are rimmed with isopachous calcite crystals. Some clasts are cemented by intergranularmeso-to-coarse sparry calcite cements.
Two phases of cementation were recognized in few cases. The earlier is isopachous calcite cement which followed by intergranular calcite cement. The ophiolitic breacciated conglomerate facies (Figs. 11C &11D) was recognized in the upper part of the western aquifer (well No. 203, 329). The sediments composed medium tocoarse pebble sized, poorly sorted, mainly ophiolitic clasts composed mainly of serpentine, altered olivine and pyroxenes. Stockwork texture was detected due to filling of the network of altered ophiolite by subhedral to euhedral calcite crystals. The calclithite facies (Figs. 10E & 10F) was detected in the lower part of the aquifer particularly westward (well No. 203, 329). The facies is mainly made up of sandsized, moderately-sorted mainly angular to subangular consisting of altered serpentine, chert and subordinate quartz with minor components of pebble-sized clasts ophiolites, micro-quartz, polyquartz and micrite limestone. Scattered silt-sized, angular grains of iron oxides are recorded. Some clasts are cemented by meso-tocoarse calcite crystals and rhombohedral dolomite crystals. The lithic carbonate facies (Figs. 14G &14H) was recognized in several hoizon especially in westward aquifer (well No. 203,309). It consists of ophiolite grains which made up of fine sand-sized, well sorted, subangular to subrounded ophiolite, quartz, chert and micrite limestone grains scattered in groundmass composed of partially dolomitized calcite or micritic matrix. The SEM micrographs (11A-F) showing altered ophiolite clasts partially coated by authigenic serpentine, talc (C), clay minerals (D) and well developed rhombohedral dolomite crystals (E,F). (G,H) pore-filling, pore-lining, pore-bridging of authigenic well developed crystals across lithoclasts grains. Note, the identification of inspected grains and authigenic crystals by SEM as serpentine, dolomite, calcite, talc based on x-ray diffraction and EDX spectrum analyses (12A,B,C,D).
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Mineralogically, the of x-ray diffraction analysis of the Quaternary aquifer sediments and the vertical variations of mineralogy of bulk aquifer (Fig. 13 & 14) revealed remarkable variations in their mineral composition throughout each of the studied wells. Sediments of the well No. 18 consist of serpentine and calcite with minor concentrations of dolomite and plagioclase and traces of quartz, pyroxene and amphibole in some stratigraphic levels. The percentages of serpentine, dolomite and pyroxene increase while those of calcite, quartz and plagioclase decrease upward in the aquifer. In well No. 17, the aquifer sediments are dominated by serpentine and calcite and contain smaller proportions of dolomite, plagioclase and traces of pyroxene, quartz, amphibole, clay minerals and talc in some stratigraphic levels. The proportions of serpentine and dolomite increase while those of calcite decrease upward of the aquifer but with increasing percentages of calcite in the upper part. The proportion of plagioclase oscillates throughout the aquifer. In well No. 466, the sediments essentially made up of serpentine followed by calcite and minor concentration of dolomite and plagioclase with traces of quartz, pyroxene and amphibole. The proportions of serpentine oscillate throughout the succession but increase markedly at its top to the expense of those of calcite. Dolomite, plagioclase and pyroxene increase in the middle part of the sequence while quartz is recorded at its top. On the other hand, the sediments of well No. 203 consist of serpentine and calcite with minor of plagioclase, dolomite, quartz and traces of pyroxene, talc and amphibole. The concentration of serpentine, dolomite and plagioclase increase while those of calcite, quartz decrease upward in the aquifer sequence. The remaining minerals detected in some stratigraphic levels. The lateral variation in the average mineral composition percentages of the aquifer sediments is shown in Figure 15 shows that the major minerals which represent by serpentine decrease towards the west except in well No. 466 it increase, while calcite increase particularly in well No. 203 where it closed of Jebel Hafit. Dolomite and plagioclase exist in subordinate show increasing eastward where it closed from some ophiolites. With respect to the minor minerals, quartz increases westward while the pyroxene in the center. Amphibole occurs nearly the same in all wells while the clay mineral occurs as traces only in well No. 17. Traces of talc were recorded only in the western and central part of the studied wells.
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2D resistivity tomograms of the nine profiles and five VES guided with borehole data indicate remarkably the different hydrostratigraphic units of Quaternary aquifer along the eastern margin of Eastern Hafit anticline (Al Jaww plain). Erosional unconformities at the base of the Quaternary alluvium are traced along some of the (2D) profiles. These unconformities represent the paleochannels in the bed rock that were formed in the geological past by the ancient wadies. These paleochannels are promising targets for fresh groundwater, as they contain appreciable thickness of water bearing formations that are recharged from the surrounding mountain region. The results of X-Ray Diffraction clearly showed the variations of mineral compositions of aquifer materials, and this conclusion is supported by the vertical distribution of the mineralogy of the aquifer. Four different facies were determined using the microscopic examinations and these facies are Ophiolite clasts conglomerate, Ophiolite brachiated conglomerate, Calclithite facies and Lithic carbonate facies. The percentages of major cations of groundwater increased from the east to the west of the Quaternary aquifer of Al Jaww Plain towards Jabal Hafit. It is clear that the lithology of Jabal Hafit affected the water quality.
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Ayman El Saiy, Amir Gabr and Ala Aldahan
Geology Department, Faculty of Science, UAEU, P.O.Box-17551
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|Author:||Saiy, Ayman El; Gabr, Amir; Aldahan, Ala|
|Publication:||International Journal of Applied Environmental Sciences|
|Date:||Sep 1, 2012|
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