Hydrostratigraphic Characterization of Groundwater Systems in Khatatba Area Using Vertical Electric Sounding and Well Log Data, Sw Nile Delta, Egypt
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ISSN 2321 – 9149 IJAEES (2014) Vol.2, No.1, 39-50 Research Article International Journal of Advancement in Earth and Environmental Sciences HYDROSTRATIGRAPHIC CHARACTERIZATION OF GROUNDWATER SYSTEMS IN KHATATBA AREA USING VERTICAL ELECTRIC SOUNDING AND WELL LOG DATA, SW NILE DELTA, EGYPT Abdulaziz M. Abdulaziz Mining, Petroleum, and Metallurgical Engineering Department, Faculty of Engineering, Cairo University, Giza, Egypt ( [email protected]) --------------------------------------------------------------------------------------------------------------------------------------- ABSTRACT: The study area represents the western extension of the Nile Delta flood plain at the contact of the desert, where the importance of ground water is rising. This requires a thorough understanding to the developed groundwater systems, particularly the shallow aquifers. In this study, well log data was integrated to a Vertical Electric Sounding (VES) survey acquired using Schlumberger configuration at the Khatatba area to characterize the aquifer systems. The results were interpreted using the published stratigraphic data of the study area, and consequently the extension of the identified aquifers can be laterally correlated. Two aquifer systems separated by an aquitard or basaltic layer were mapped and characterized in details. The upper aquifer corresponds to El Khashab formation that produces fresh to brackish water and generally shows unconfined condition that could locally change to partially semi confined. The second aquifer is commonly confined and stratigraphically correlates to Qatrani formation with groundwater quality relatively better than the upper aquifer. Detailed characterization of these aquifers is discussed to improve understanding the groundwater systems of the study area and the adjacent desert. KEYWORDS: Vertical Electric Sounding, Schlumberger configuration, Khatatba aquifers, aquifer characterization, groundwater systems. --------------------------------------------------------------------------------------------------------------------------------------- 1. INTRODUCTION: Availability of groundwater resources is a fundamental parameter in initiation and continuity of desert land development for industrial, urbanization, and agricultural projects [(1), (2)]. Therefore, exploring the quantity and quality of such valuable resources has the prime importance for these projects. Resistivity sounding method has proved popular and useful in investigating hydrological, environmental, and engineering problems [(3), (4)]. In a resistivity survey, electric current is injected to the ground through two current electrodes, conventionally called A-B as shown in Fig:I, and the potential difference is measured between two potential electrodes known to be M-N (Fig:I). The apparent resistivity of a geologic medium depends basically on the lithologic composition and fluid content (5) and therefore, geoelectric units usually define parastratigraphic units with boundaries discordant with the stratigraphic boundaries (6). Fig I: Schlumberger electrode configuration Abdulaziz, A.M./ International Journal of Advancement in Earth and Environmental Sciences, Vol.2, No. 1 40 For a geologic medium the apparent resistivity is a function of the resistance value and a geometric factor as calculated form the following modified Ohm’s equation: Where ρ is the resistivity in Ω.m, K is a geometrical factor that accounts for the separating distance of both current and potential electrodes, is the potential difference in mV, and is the electric current in mA. The value of the geometrical factor can be approximated using parameters presented in Fig:I by the relation; A variety of VES arrays were developed since the first application by Schlumberger in 1934 (7), but the Schlumberger array remained the best configuration for deeper soundings. Each VES configuration adopted data acquisition method and exploit various data processing techniques to convert the resistivity measurements into interpretable 2D and 3D geology information (8). Schlumberger sounding is the most popular sounding method in DC resistivity survey (9). In this method, the mid- point of the survey remains fixed, while the separating distance between the electrodes is gradually increased that enables investigating deeper sections and obtaining continuous vertical data (Fig:I). The interpretation of resistivity data is relatively ambiguous, and sometimes impossible to obtain a unique interpretation (5). But a good control and wise constraints provided by calibrated well logs and modern drilling practices that ensure minimal changes in rock properties greatly diminish ambiguity and facilitate interpretation. In the present study, a preliminary interpreted well log data and VES, provided by Al-Faid Agricultural Development Company, are integrated to the geology of Khatatba and Wadi El Natrun area as observed in field work and literatures. Such integration helps identifying in details the vertical and horizontal distribution of the subsurface sedimentary units and delineating the important water-bearing horizons. Accordingly, the hydrostratigraphic framework influencing groundwater occurrence in the Khatatba and the surrounding area is determined. 2. LOCATION AND GEOMORPHOLOGY: Khatatba area constitutes the western boundary of the Nile Delta encountered 15 Km away from the Rossetta branch of the River Nile. It falls within the northeast territory of Sadat City (Fig:II) and Wadi El Natrun (located 25 Km southwest, Fig:III). Geomorphologically, the study area is covered with hard siliceous Neogen-sediments which constitute undifferentiated chains of hills arrange along E-W fault as groups of bulges and hills alternated with small depressions filled with playa or sand (Fig:III). To the south there are small hills scattered throughout large area that was lumped by hydrothermal solutions and volcanic activity. The climate of the study area falls within arid-semi arid zone with average temperatures (maximum of 28˚C and minimum 14˚C), average humidity 54%, precipitation depth 41 mm/year, and evaporation 9.5 mm/day. This indicates minimal natural recharge to the shallow aquifer but return irrigation water constitutes a major share to aquifer recharge. 3. HYDROGEOLOGY: The hydrogeological map of Menouf area provides preliminary clues to the dominant aquifers in Khatatba area and the general groundwater flow system (10). It indicates, based on potentiometric groundwater head distribution, that the Rossetta branch of the Nile recharges the aquifers in Khatatba through hydraulic connection and the estimated total abstraction of the Quaternary aquifer is about 25*106 m3/year. Recently, several works studied the Quaternary aquifer of the study area at El Sadat City and discussed in details the hydrogeology (e.g. [(11), (12)]), Hydrogeochemical characterization [(13), (14)], and subsurface geophysical investigations (15). The study area entails a relatively shallow aquifer consisting of recent - Pleistocene sand and gravels with intervening discontinuous clay lenses (14). This stratigraphic architecture results in intermediate productivity due to the effect of the intervening clay and silty- clay lenses. Well data confirmed the presence of these two aquifers, Quaternary and Miocene, separated by a basaltic or shale layer usually encountered at 170-200 m depth (Fig:IV). The Quaternary aquifer provides the main groundwater supply for domestic, irrigation and industrial purposes in the study area. It attains extensive areal distribution to the east of Cairo– Alexandria Desert Road and fuse northward with Nile Delta Quaternary aquifer and southeast with Miocene Moghra aquifer of Wadi El-Farigh, but towards the southwest merge with the Pliocene aquifer of Wadi El-Natrun depression, Fig:III and Fig:IV (14). Lithologically, it showed a marked increase in coarse components eastward and is composed mainly of fluviatile sand and gravels occasionally intercalated with clay and usually exhibit variable thickness due to NW- SE step faults of downthrown to east and north (Fig:III). The clay content increases southward to record 20-25% while the northern part only shows up to 15% and the major part of the aquifer shows phreatic conditions but locally may change to Abdulaziz, A.M./ International Journal of Advancement in Earth and Environmental Sciences, Vol.2, No. 1 41 semi-confined aquifer. Likewise, The saturated zone in the Quaternary aquifer decreases southward direction away of the Rossetta branch to record approximately 40 m with variable water salinity (TDS: 400-1200 ppm) due to argillaceous components (11). The depth to water below ground-surface ranged between 14 to 25 m in the northern part and 45 to 56 m at the southern part and east of the Cairo–Alexandria Desert Road. The deeper water table at the southern parts is primarily attributed to excessive groundwater abstractions at agricultural development zones associating inadequate aquifer recharge and replenishment (14). Fig II: Location map of study area, showing locations of the VES profiles and well log, upper right Abdulaziz, A.M./ International Journal of Advancement in Earth and Environmental Sciences, Vol.2, No. 1 42 Fig III: Geologic map to the area around Khatatba modified from Abu Zeid, 1984 (16) 4. METHODS: 4.1 VERTICAL ELECTRIC SOUNDING (VES): To image the dominant subsurface architecture in the study area, three VES of 600-1000 m length are conducted using Schlumberger configuration. The Schlumberger array provides a satisfactory resolution in