Hydrogeophysical Investigations Using DC Resistivity …
267
are horizontally and/or vertically interconnected through faulting system (CEDARE
2002; Foster and Loucks 2006).
The water bearing formations of NSAS are involved Taref (topmost), Sabaya, and
Six Hills formations. The flow direction and water tables in the topmost aquifer zone
of Taref Formation are certainly fluctuated by the unplanned and intensive pumping
that lads to degradation of shallow wells in the Dakhla and other places (Kato et al.
2014). The Taref aquifer (upper zone of NSAS) is characterized by high effective
porosity of 25% in the topmost part and decreases with the depth due to increase
of clay contents and ferruginous cementation as indicated from borehole logs. The
transmissivity values in this zone varies between 400 m
2 /day in the eastern regions
and 550 m
2 /day in the western parts due to the high sand content compared with
the clay content (Borelli and Karanja 1968; Ghoubachi 2001). The Sabaya aquifer
showed an average thickness of 300 m while the deeper Six Hills aquifer attained
more than 900 m with overall increase to the west and south (Korany et al. 2002).
This variation in the aquifer thickness is mainly attributed to the impact of normal
faults which throws towards the south and west directions. The peizometric surface
above the mean sea level in the area is varied between 118 m, 111,and 135 m in Taref,
Sabaya, and Six Hills aquifers, respectively (Gad et al. 2011). In the shallow wells,
the depth of water level from the ground surface varies from 8 m at well no. 58 in
the southern part to 40 m at well no. 30 in the northern part. The piezometric levels
of the aquifer range between 129 m (.a.s.l) in wells no. 58 and 57 at the extreme
southern portion of the area and 84 m in wells no. 26 and 22 at the central part. The
flow direction in this aquifer is mainly from the north and south parts towards the
center of the area (Fig. 5).
In several regions, pumping of groundwater from the NSAS reserves is currently
taking place and increasing yearly. This has forced the water level to decline progressively (Fig. 6). Consequently, over 35% of the free-flowing wells and springs in many
locations had to be deepened or substituted with deeper wells (CEDARE 2002). As
an immediate consequence of the groundwater extraction of the Nubian aquifer,
a lot of environmental impacts are expected as an outcome (Puri et al. 2001; ElRawy and Smedt 2020). In addition, successive lowering of the groundwater level
directly affects the hydraulic parameters of the aquifer like the transmissivity and
the groundwater quality suffers noticeable variation as in the present case study.
3 Methodology
A total of 46 Vertical Electrical Soundings (VES) were carried out using the Terameter of SAS-300C with the Schlumberger array (Fig. 7). The current electrode spacing
(AB) was gradually changed in sequential steps starting from 3 m to 1400 m. This
spreading is long enough to penetrate the Maghrabi clay as maker bed and reach the
second aquifer zone of Sababya Formation as indicated from the available boreholes
information. The measured soundings were scattered to cover the whole area and
were marked using GPS instrument. Some measured soundings were located beside
267
are horizontally and/or vertically interconnected through faulting system (CEDARE
2002; Foster and Loucks 2006).
The water bearing formations of NSAS are involved Taref (topmost), Sabaya, and
Six Hills formations. The flow direction and water tables in the topmost aquifer zone
of Taref Formation are certainly fluctuated by the unplanned and intensive pumping
that lads to degradation of shallow wells in the Dakhla and other places (Kato et al.
2014). The Taref aquifer (upper zone of NSAS) is characterized by high effective
porosity of 25% in the topmost part and decreases with the depth due to increase
of clay contents and ferruginous cementation as indicated from borehole logs. The
transmissivity values in this zone varies between 400 m
2 /day in the eastern regions
and 550 m
2 /day in the western parts due to the high sand content compared with
the clay content (Borelli and Karanja 1968; Ghoubachi 2001). The Sabaya aquifer
showed an average thickness of 300 m while the deeper Six Hills aquifer attained
more than 900 m with overall increase to the west and south (Korany et al. 2002).
This variation in the aquifer thickness is mainly attributed to the impact of normal
faults which throws towards the south and west directions. The peizometric surface
above the mean sea level in the area is varied between 118 m, 111,and 135 m in Taref,
Sabaya, and Six Hills aquifers, respectively (Gad et al. 2011). In the shallow wells,
the depth of water level from the ground surface varies from 8 m at well no. 58 in
the southern part to 40 m at well no. 30 in the northern part. The piezometric levels
of the aquifer range between 129 m (.a.s.l) in wells no. 58 and 57 at the extreme
southern portion of the area and 84 m in wells no. 26 and 22 at the central part. The
flow direction in this aquifer is mainly from the north and south parts towards the
center of the area (Fig. 5).
In several regions, pumping of groundwater from the NSAS reserves is currently
taking place and increasing yearly. This has forced the water level to decline progressively (Fig. 6). Consequently, over 35% of the free-flowing wells and springs in many
locations had to be deepened or substituted with deeper wells (CEDARE 2002). As
an immediate consequence of the groundwater extraction of the Nubian aquifer,
a lot of environmental impacts are expected as an outcome (Puri et al. 2001; ElRawy and Smedt 2020). In addition, successive lowering of the groundwater level
directly affects the hydraulic parameters of the aquifer like the transmissivity and
the groundwater quality suffers noticeable variation as in the present case study.
3 Methodology
A total of 46 Vertical Electrical Soundings (VES) were carried out using the Terameter of SAS-300C with the Schlumberger array (Fig. 7). The current electrode spacing
(AB) was gradually changed in sequential steps starting from 3 m to 1400 m. This
spreading is long enough to penetrate the Maghrabi clay as maker bed and reach the
second aquifer zone of Sababya Formation as indicated from the available boreholes
information. The measured soundings were scattered to cover the whole area and
were marked using GPS instrument. Some measured soundings were located beside