Groundwater for Sustainable Development 11 (2020) 100481
2
home to 85% of the country’s population (Djabri et al., 2019).
The alluvial plain of Cherchell, in the north-central region of Algeria,
on the coast of the western Mediterranean, is one of the coastal areas
characterized by agricultural and tourist activities. The choice of this
area comes in response to the hydrological disturbances that the region
is seeing after the construction of the Boukourdene dam in 1992 in the
sub-watershed of the region, about 11 km from the coast and 3 km from
the aquifer. This dam blocks the water that has flowed into this watershed, inhibiting the flow of water and the feeding of the aquifer, thus its
discharges containing fine fractions plug up the soil ports by stopping
the infiltration of water into the soil and thus the replenishment of the
aquifer. This is confirmed by the decline in piezometric levels (Tadrist
et al., 2016) and seawater intrusion. Mapping the vulnerability of the
Cherchell coastal aquifer to marine intrusion is a major concern in terms
of land-use planning and groundwater protection, no similar studies
have been carried out on the Cherchell area aquifer. The methodology
adopted in this work is based on the proven methodology of the GALDIT
model for coastal aquifers (Chachadi and Lobo-Ferreira, 2001a,b) and
modified by the same authors in 2005. This method permits the
assessment of the sensitivity and quality of coastal groundwater by
determining the intrinsic characteristics of the aquifer (Lobo-Ferreira
and Chachadi, 2005). This model requires six parameters: Groundwater
occurrence (G), Aquifer hydraulic conductivity (A), depth to groundwater Level above sea (L), Distance from the shore (D), Impact on
existing status of sea water intrusion (I), Thickness of the aquifer (T).
These parameters are then incorporated into a Geographic Information
System (GIS), which is a very good tool to help the decision. The marine
intrusion vulnerability map obtained for the Cherchell aquifer, serves to
identify the parts of the aquifer most vulnerable to marine intrusion, and
forms a basic tool for local managers and decision-makers, for future
management decision, and for the protection of the Cherchell aquifer
from marine intrusion.
2. Overview of Saline Intrusion vulnerability in coastal aquifers
In general, the term vulnerability refers to the potential degree of
harm that can be expected based on the characteristics of an element at
risk in relation to a certain hazard (Varnes, 1984). In the field of water
resources, the term vulnerability is used in the context of evaluating the
performance of systems as defined by (Hashimoto et al., 1982). These
authors define three concepts of system performance: the probability of
system failure in relation to its reliability; the speed of return of the system
satisfactory state after its failure is expressed by its resilience, and the severity
of the consequences of a failure measured by its vulnerability. This concept
of vulnerability may be used in the context of groundwater intrusion
into groundwater “system failure” through over-exploitation of
groundwater “severity of consequences” affecting water quality as a
resource. In our context, the vulnerability of coastal aquifers to marine
intrusion is that which incorporates the intrinsic characteristics of the
aquifer, and for the most part escapes human factors. It is therefore
proposed to redefine the vulnerability of groundwater to the intrusion of
seawater, in accordance with the conclusions and recommendations of
the International Conference on the Vulnerability of Groundwater to
Saline Intrusion, held in 1987 in the Netherlands (Andersen and Gosk,
1987; Duijvenbooden and Van Waegeningh, 1987): as follows the sensitivity of groundwater quality to imposed groundwater pumping or sea-level
rise, or both, in the coastal belt, which is determined by the intrinsic characteristics of the aquifer.
3. Description of the study area
3.1. Geographical overview
The Cherchell aquifer is located about 20 km west of the chief town
of the wilaya of Tipasa and 76 km from the capital Algiers. It is bordered
to the north by the Mediterranean Sea, to the south by Djebel Boumaad,
to the east by the Chenoua massif and to the west by the town of
Cherchell (Fig. 1a, Fig. 1b).
Latitude: 36
◦
36
′
56.757
′′
N and 36
◦
33
′
23.601
′′
N.
Longitude: 2
◦
15
′
3.069
′′
E and 2
◦
19
′
4.167
′′
E.
3.2. Hydrogeological overviews
The Cherchell aquifer is 8 km long and covers an average area of 13
km
2
(1300 ha), its water reservoir is 44 hm
3
, but its capacity is 200 hm
3
,
the aquifer’s renewal time is 9.48 years (Mancet, 1972), the hydraulic
gradient varies from 10‰ to 22‰ and its permeability varies from 6 ×
10
− 4
to 1.2 × 10
− 2
m/s (Mancet, 1972), transmissivity varies from 10
− 3
and 5*10
− 3
m
2
/s (Mancet, 1972). The exploitable reservoir of the
aquifer (Q) is from 50 m
3
/h to 100 m
3
/h in the south and centre and
from 10 m
3
/h to 50 m
3
/h in the north (Mancet, 1972). Piezometric
levels above sea level increase from North to South, ranging from 0.8 m
in the North and 16 m in the South (Mancet, 1972). Two directions of
flow exist, the most important follows the direction of flow of the wadi in
the SE-NW direction, the other in the S–N direction by feeding the
aquifer of the western sector from the Gouraya carbonate massif
(Mancet, 1972). The piezometric curves converge lightly to the center of
the aquifer (Fig. 2), indicating the replenishment of the aquifer by the
borders (Mancet, 1972). The aquifer’s reservoir thickness varies from 5
m in the north to 20 m in the south, but it exhibits a relatively rapid
response to the different impulses since it is only replenished by rainwater infiltration and runoff water from the piedmonts during rainy
periods (Maamar 2004) and by releases from the Boukerdene dam
during the summer period and by overflow of irrigation water (Tadrist
et al., 2016). After the construction of the Boukerdene dam in the region’s watershed, runoff water is stored in the dam, inhibiting the
replenishment of the aquifer. The aquifer is sometimes free, at other
times confined, under a clay or marl cover of variable thickness not
exceeding 20 m (unconfined in the South between Bou Aroua and Zurich
and confined in the North) (Mancet, 1972).
3.3. Geological overview
All the studies carried out on the Cherchell Plain show that the only
formation of particular interest from a hydrogeological viewpoint is
illustrated by the Plio-Quaternary alluvial deposits of the El Hachem
Wadi Plain. The aquifer consists of pebbles and gravel with red clay with
pebbles at the top and sandy clay with some intercalation of gravel beds
at the base. The whole rests on an impermeable substratum consisting
essentially of Cretaceous marls and Eocene limestones (Maamar 2004).
The formations of the aquifer are covered by highly erodible sedimentary rocks, which are at the origin of the high sensitivity of the soils in
the region. These are mostly limestone, shale, soft marls and shale
(Fig. 3a). Its deposits coming from the slopes of Chenoua and the reliefs
of Djebel Boumaad (Maamar 2004).
Electrical prospecting conducted by the Compagne G´ en´ erale de
G´ eophysique in 1996 in the Cherchell Plain showed the existence of
several electrically distinct terrains (Fig. 3b):
- Pliocene substratum with a resistivity of 3 Ohm.m;
- A approximate resistivity level of 50–60 Ohm.m and a thickness of
3–14 m represented by the aquifer reservoir;
- A fine upper level of resistivity that varies from 5 to 10 Ohm.m
indicating a finer material with a thickness of 17 m, with rude passages to the centre of the plain.
3.4. Climatological overview
Cherchell is characterized by a sub-humid Mediterranean climate
that is relatively cold and humid in winter, hot and dry in summer. The
average temperature fluctuates between 11 and 26
◦
C, but the minimum
and maximum temperatures are 6
◦
C in winter (January and February),
N. Amarni et al.
2
home to 85% of the country’s population (Djabri et al., 2019).
The alluvial plain of Cherchell, in the north-central region of Algeria,
on the coast of the western Mediterranean, is one of the coastal areas
characterized by agricultural and tourist activities. The choice of this
area comes in response to the hydrological disturbances that the region
is seeing after the construction of the Boukourdene dam in 1992 in the
sub-watershed of the region, about 11 km from the coast and 3 km from
the aquifer. This dam blocks the water that has flowed into this watershed, inhibiting the flow of water and the feeding of the aquifer, thus its
discharges containing fine fractions plug up the soil ports by stopping
the infiltration of water into the soil and thus the replenishment of the
aquifer. This is confirmed by the decline in piezometric levels (Tadrist
et al., 2016) and seawater intrusion. Mapping the vulnerability of the
Cherchell coastal aquifer to marine intrusion is a major concern in terms
of land-use planning and groundwater protection, no similar studies
have been carried out on the Cherchell area aquifer. The methodology
adopted in this work is based on the proven methodology of the GALDIT
model for coastal aquifers (Chachadi and Lobo-Ferreira, 2001a,b) and
modified by the same authors in 2005. This method permits the
assessment of the sensitivity and quality of coastal groundwater by
determining the intrinsic characteristics of the aquifer (Lobo-Ferreira
and Chachadi, 2005). This model requires six parameters: Groundwater
occurrence (G), Aquifer hydraulic conductivity (A), depth to groundwater Level above sea (L), Distance from the shore (D), Impact on
existing status of sea water intrusion (I), Thickness of the aquifer (T).
These parameters are then incorporated into a Geographic Information
System (GIS), which is a very good tool to help the decision. The marine
intrusion vulnerability map obtained for the Cherchell aquifer, serves to
identify the parts of the aquifer most vulnerable to marine intrusion, and
forms a basic tool for local managers and decision-makers, for future
management decision, and for the protection of the Cherchell aquifer
from marine intrusion.
2. Overview of Saline Intrusion vulnerability in coastal aquifers
In general, the term vulnerability refers to the potential degree of
harm that can be expected based on the characteristics of an element at
risk in relation to a certain hazard (Varnes, 1984). In the field of water
resources, the term vulnerability is used in the context of evaluating the
performance of systems as defined by (Hashimoto et al., 1982). These
authors define three concepts of system performance: the probability of
system failure in relation to its reliability; the speed of return of the system
satisfactory state after its failure is expressed by its resilience, and the severity
of the consequences of a failure measured by its vulnerability. This concept
of vulnerability may be used in the context of groundwater intrusion
into groundwater “system failure” through over-exploitation of
groundwater “severity of consequences” affecting water quality as a
resource. In our context, the vulnerability of coastal aquifers to marine
intrusion is that which incorporates the intrinsic characteristics of the
aquifer, and for the most part escapes human factors. It is therefore
proposed to redefine the vulnerability of groundwater to the intrusion of
seawater, in accordance with the conclusions and recommendations of
the International Conference on the Vulnerability of Groundwater to
Saline Intrusion, held in 1987 in the Netherlands (Andersen and Gosk,
1987; Duijvenbooden and Van Waegeningh, 1987): as follows the sensitivity of groundwater quality to imposed groundwater pumping or sea-level
rise, or both, in the coastal belt, which is determined by the intrinsic characteristics of the aquifer.
3. Description of the study area
3.1. Geographical overview
The Cherchell aquifer is located about 20 km west of the chief town
of the wilaya of Tipasa and 76 km from the capital Algiers. It is bordered
to the north by the Mediterranean Sea, to the south by Djebel Boumaad,
to the east by the Chenoua massif and to the west by the town of
Cherchell (Fig. 1a, Fig. 1b).
Latitude: 36
◦
36
′
56.757
′′
N and 36
◦
33
′
23.601
′′
N.
Longitude: 2
◦
15
′
3.069
′′
E and 2
◦
19
′
4.167
′′
E.
3.2. Hydrogeological overviews
The Cherchell aquifer is 8 km long and covers an average area of 13
km
2
(1300 ha), its water reservoir is 44 hm
3
, but its capacity is 200 hm
3
,
the aquifer’s renewal time is 9.48 years (Mancet, 1972), the hydraulic
gradient varies from 10‰ to 22‰ and its permeability varies from 6 ×
10
− 4
to 1.2 × 10
− 2
m/s (Mancet, 1972), transmissivity varies from 10
− 3
and 5*10
− 3
m
2
/s (Mancet, 1972). The exploitable reservoir of the
aquifer (Q) is from 50 m
3
/h to 100 m
3
/h in the south and centre and
from 10 m
3
/h to 50 m
3
/h in the north (Mancet, 1972). Piezometric
levels above sea level increase from North to South, ranging from 0.8 m
in the North and 16 m in the South (Mancet, 1972). Two directions of
flow exist, the most important follows the direction of flow of the wadi in
the SE-NW direction, the other in the S–N direction by feeding the
aquifer of the western sector from the Gouraya carbonate massif
(Mancet, 1972). The piezometric curves converge lightly to the center of
the aquifer (Fig. 2), indicating the replenishment of the aquifer by the
borders (Mancet, 1972). The aquifer’s reservoir thickness varies from 5
m in the north to 20 m in the south, but it exhibits a relatively rapid
response to the different impulses since it is only replenished by rainwater infiltration and runoff water from the piedmonts during rainy
periods (Maamar 2004) and by releases from the Boukerdene dam
during the summer period and by overflow of irrigation water (Tadrist
et al., 2016). After the construction of the Boukerdene dam in the region’s watershed, runoff water is stored in the dam, inhibiting the
replenishment of the aquifer. The aquifer is sometimes free, at other
times confined, under a clay or marl cover of variable thickness not
exceeding 20 m (unconfined in the South between Bou Aroua and Zurich
and confined in the North) (Mancet, 1972).
3.3. Geological overview
All the studies carried out on the Cherchell Plain show that the only
formation of particular interest from a hydrogeological viewpoint is
illustrated by the Plio-Quaternary alluvial deposits of the El Hachem
Wadi Plain. The aquifer consists of pebbles and gravel with red clay with
pebbles at the top and sandy clay with some intercalation of gravel beds
at the base. The whole rests on an impermeable substratum consisting
essentially of Cretaceous marls and Eocene limestones (Maamar 2004).
The formations of the aquifer are covered by highly erodible sedimentary rocks, which are at the origin of the high sensitivity of the soils in
the region. These are mostly limestone, shale, soft marls and shale
(Fig. 3a). Its deposits coming from the slopes of Chenoua and the reliefs
of Djebel Boumaad (Maamar 2004).
Electrical prospecting conducted by the Compagne G´ en´ erale de
G´ eophysique in 1996 in the Cherchell Plain showed the existence of
several electrically distinct terrains (Fig. 3b):
- Pliocene substratum with a resistivity of 3 Ohm.m;
- A approximate resistivity level of 50–60 Ohm.m and a thickness of
3–14 m represented by the aquifer reservoir;
- A fine upper level of resistivity that varies from 5 to 10 Ohm.m
indicating a finer material with a thickness of 17 m, with rude passages to the centre of the plain.
3.4. Climatological overview
Cherchell is characterized by a sub-humid Mediterranean climate
that is relatively cold and humid in winter, hot and dry in summer. The
average temperature fluctuates between 11 and 26
◦
C, but the minimum
and maximum temperatures are 6
◦
C in winter (January and February),
N. Amarni et al.