Subsurface Characterization for Groundwater Management nearby the Unfinished Obelisk Archeological Site, Aswan Governorate, Egypt

Recently, the area located within the Unfinished Obelisk (UO) archeological site showed numerous seepages and accumulations of groundwater in a small pond located a few meters from the Unfinished Obelisk. The Supreme Council of Antiques sponsored integrated geological, geophysical, and hydrogeological studies to identify the possible sources of groundwater and the optimum technique to manage the groundwater flow system that may jeopardize this invaluable sculpture. The geological units and the prevailing structure have been studied in detail using Landsat imagery and field work over two consecutive seasons. The field studies indicated the development of several fault/joint systems oriented mainly ENE-WSW with clear indications of mineralization and intensive weathering effects along these fabrics. Several resistivity (vertical seismic profile and resistivity imaging) measurements extending down to at least 20 m depth and Radar imaging down to 10 m depth are gathered to investigate the extension of outcrop units and the dominant structures prevailing the near subsurface. Geophysical data indicated the development of at least three hydrostratigraphic units arranged from top to bottom as valley fill, fractured/weathered granite, and slightly fractured to massive granitic unit. In addition, the major faults mapped by resistivity images helped to locate several observation wells and a production well to test the transmissivity across the groundwater system. The results of a pumping test indicated very low aquifer conductivity and the development of an aquitard with preferential vertical flow at the study area. This enforces a local in-terference through a shallow underground drainage system with sump and pump to maintain low groundwater level at the UO-archeological site.


Introduction
Fractured crystalline basement typically develops extremely heterogeneous aquifer systems with significant anisotropy, especially where weathering effect prevails [1] [2] [3]. The capacity of fractures to groundwater flow changes over several orders of magnitude [4] [5] with flow paths normally vary between less than a meter and several kilometers long, while some fractured aquifers could be controlled by few fractures or faults [6] [7]. Such anisotropy results in a complex flow pattern that may involve a combination of preferential flow and diffuse piston flow [8] [9].
These aquifers are, therefore, known with vertical flow uncertainty [10], connectivity dependent to water level [9], and horizontal/vertical preferential flow paths [2]. Such a complexity naturally associates a rapid decrease in groundwater yield and storage with depth [11] [12]. Many studies have investigated fractured basement aquifers to appraise groundwater structure and functionality [13] [14] [15] [16], understand groundwater recharge [17] [18], evaluate the hydrogeochemical evolution of groundwater [19] [20], and place production water wells [21]. Regional scale hydraulic properties are preferably interpreted using numerical simulation of long-term groundwater fluxes and records of hydraulic heads distributed throughout the aquifer [22] [23]. Alternatively, well tests at small scale and upscaling the interpreted hydraulic properties are commonly applied as a substitute to the simulation studies, e.g. [24], but the consistency of the results is strongly dependent on the aquifer complexity [5]. Several hydraulic testing techniques have been developed to determine the hydraulic properties of fractured reservoirs using the responses induced by the precise perturbations within the aquifer [25]. These tests can assess a particular zone to determine the transmissivity of individual/closely spaced fractures [26] or evaluate the entire penetrated aquifer section to measure the effective transmissivity of all fractures [27]. In addition, well tests may resolve the vertical hydraulic properties testing the entire aquifer zone [28], or identifying the spatial connectivity of fractures in three dimensions if a particular zone within the aquifer is isolated by packers [6].
The Unfinished Obelisk (UO) archeological site represents a genuine quarry that enables invaluable scientific opportunity to know the techniques and fracture mechanics applied for obelisks production in ancient Egypt [29]. Obelisks represent the ancient skyscrapers that were created as a glory for the sun [30] and the majority of obelisks are made of red granite, especially the larger pieces.
The UO has failed to complete because of the development of an unexpected fissures, not related to human mistakes, and therefore remained at the Aswan quarry connected only at its lower side [29]. It extends over 41

Methods
The present study encompasses integration of geological, geophysical, and hydrogeological investigations to evaluate the near-surface groundwater system at the proximity of the UO.

Geological Study
The geological studies involve a detailed fieldwork to the granitic bodies exposed tures concealed below the soil zone. All geophysical measurements are located at the proximity of the UO and involve Vertical Electrical Sounding (VES), Resistivity imaging, and Radar imaging survey. Figure 1 shows the location of the geophysical measurements at the investigated site. The data of three VESs are acquired using Schlumberger configuration with SYSCAL-PRO unit (IRIS) and a maximum cable length of 100 m to enable a depth of investigation within 20 -30 m. Datum levels are determined using a series of consecutive increase to electrode separation [43], starting with a small electrode spacing (2, 3, 5, and 10 m), followed by gradual increase in spacing for the subsequent measurements. The measurement geometry is designed similar to the configuration described by reference [41] and [44]. To maintain an acceptable contact with minimal soil effect that keep the contact resistance below 2000 Ω, a saline solution is prepared for dispense around individual electrodes. A lithium battery reserved the power supply to the measurement system and field data acquisition is accomplished using armored insulated cables. been applied to the raw field data to remove noisy signals, recover signal deterioration with depth, and apply several data averaging to improve reflectors continuity and the overall quality of the radar image. Figure 3 presents the Radar section at line 5 ( Figure 1) before (left) and after (right) data processing, as an example for data quality improvement using REFLEXW package.

Hydrogeological Study
The hydrogeological study involves detailed site investigation and changes in landuse/landcover at the nearby regions, particularly those located up dip. Several seepage sites and the water body located at the study area are visited and evaluated using Landsat data and local inhabitants' interview. Based on geological and geophysical investigations, six test holes are drilled using rotary drilling techniques and completed as piezometers. These boreholes helped confirming the subsurface architecture delineated by geological and geophysical investigations and also monitoring the groundwater system. In addition, a production well (Asn-P) is drilled at the main fault gouge to sample groundwater for geochemical analysis and test a possible local drainage to the exposed seepage through pumping and aquifer stress. A pumping test over more than 7 hours is completed with a pumping rate of 10 m 3 /h and the aquifer stress is monitored at the nearby piezometers Asn-1, Asn-2, and Asn-3. Groundwater sampling for laboratory analyses is accomplished after pumping the stagnant water out and the produced water is filtered through 0.45 µm filters and subsequently preserved in Figure 3. An example to Radar data (line 5 before "left" and after "right") processing to improve the data quality using REFLEXW package.

Structural Analysis
The UO-area is affected by a group of faults and joints cutting each other and

Geophysical Data Interpretation
Three VESs have been acquired using Schlumberger configuration to target a total depth of 25 -30 m, and the inversion result of the apparent data to true resistivity subsurface model is presented in Table 1   water-saturated section detected by VES between 6.6 and 7.4 m thick.
A 2D resistivity profile of 72 m length and extending SE-NW, is acquired using dipole-dipole configuration with 3m spacing to explore for groundwater accumulation and the prevailing geologic structures down to 20 m depth. Figure 7 presents   profiles are gathered at the central part of the preserved site and appear predominated with the sedimentary-fill over fractured granite that appears highly weathered in GPR-4 as indicated by the severe signal attenuation. Finally, the GPR-6 profile presents a thin shallow sedimentary cover that hardly exceeds 1 m thick followed by highly weathered granitic section with important attenuations attributed to the municipal utilities running parallel to this profile (Figure 8).

Hydrogeological Setting
Geological  Figure 9(a)), and the seldom rainfall that rarely formulates a storm ( Figure   9(b)). Another important, but indirect, source is the Nile River water, as the water pond at the trough disappears and reappears with low and high Nile seasons respectively, indicating a direct relationship. Based on geological and geophysical studies, six observation wells (Asn-1 to Asn-6) and a production well (Asn-P) are drilled, and the well details presented in Table 2 with the hydrostratigraphic units of these wells compiled in Figure 10.     (Table 2). Alternatively, at the south western part the groundwater level reported 81.34 m (Asn-2) and increases northward to 82.5 m as measured at (Asn-3), Table 2. Based on the data interpretation of the resistivity image (Figure 7), a water production well (Asn-P) is drilled to target the main fault gouge-oriented ENE-WSW to a total depth of 19.5 m. The site selection was successful to penetrate the fault gouge down to approximately 17.0 m depth as confirmed by the highly pulverized clay mixed with the highly weathered granitic catalysts recovered from drilling cuttings. The pumping test continued over more than 7 hours at well Asn-P with a discontinuous discharge rate of 10 m 3 /h to keep the groundwater level above the submersible pump and the stress rhythms are shown in Figure 11. Over 2 minutes' time from the start of pumping, the groundwater level has fallen from a depth of 2.83 m to 18.00 m. Monitoring the aquifer perturbations at Asn-1, Asn-2, and Asn-3 piezometers showed no response at all indicating that the main ENE-WSW fault acts as a perfect impermeable zone to groundwater as well as the other faults cutting across it ( Figure 11). The chemical analysis of the water sample collected from well Asn-P (  Figure 11. The aquifer stress and perturbation monitoring in piezometers Asn-1, Asn-2, and Asn-3 reported in a pumping test completed over seven hours period in the study area. This aquitard is characterized by fair to medium vertical hydraulic conductivity but dominantly lacks horizontal conductivity due to the well-developed weathering and mineralization along the fault planes and/or fractures (Figure 9(b)).
This may explain the preferential vertical flow and the relationship of water level in the pond to the seasonal variations in the Nile water. In addition, the dead response to the pumping stress observed in the neighboring piezometers ( Figure 11) of the pumping test and the presence of separate spots of dispersed seepage also support the negligible horizontal conductivity in the fractured/weather granite aquitard. Figure 9(c) shows a good clue to this interpretation as the salt crystals are developed around a perched water accumulation within the valley fill unit overlying the massive granite at a graveyard located up dip of the UO-site from the sewage system of the neighboring communities. In addition, the large water ponds developed over years to the north of the Aswan reservoir where large quantities of sand are quarried for the High Dam construction support this interpretation as well. Accordingly, the complex pattern of impermeable fault and fracture system hinders the use of pumping scenarios to manage the developed surface water pond or seepage, and mandates using local interactions to individual problems. Therefore, the developed water pond at the UO-site can be managed using a French trench with sump and pump to maintain the groundwater level below the ground surface and prevent further surface water accumulations or the development of seepage.

Conclusion
The present study indicated the development of a fractured granite aquitard that is dominated with ENE-WSW, NW-SE, N-S, and E-W fault/fracture systems.