Groundwater resources have become a national concern due to several factors, particularly intensive exploitation generated by population growth and agricultural and industrial development. In 2021, about 90% of Algerians live near the Mediterranean coast. As a result, a large part of Algeria's population is dependent on coastal water resources and for the majority on groundwater resources. In addition, high water demands occur during the summer season, when groundwater levels are at their lowest (Comte, 2008; Moulla, 2013; Cocheril, 2019; Inim, 2020).
Algeria's annual water needs by 2030 will reach about 13 billion m3. Currently, about 20% of the needs are provided by groundwater. The level of groundwater renewal is dependent on meteorological conditions, such as rainfall, this later is rare which increase our vulnerability to climate change (Comte, 2008; Priju, 2018). A simple rise in sea level directly affects the water quality of coastal aquifers and accelerates the process of marine intrusion. Water quality degradation is generally expressed as contamination due to salinization of freshwater by Salt water (Bouderbala and Remini, 2014; Senthilkumar, 2019).
The aquifer of Nador (Tipasa, Algeria), located on the Mediterranean coast under a semi-arid climate. The geographical situation of the aquifer and the excessive water pumping, can create an imbalance between the two liquids and favor the migration of seawater inside the aquifer. This is due to a decrease in the piezometric level of the fresh water. According to the principle of density, the seawater below the fresh water extends into the aquifer, forming the salt wedge. The extent of the seawater inside the aquifer causes the salinization of the exploitation wells. This induces their closure and abandonment, creating a deficit in fresh water.
Hydrogeological studies, conducted by Taibi and Hamadache in 1992, Bouderbala (2015a) and Bouderbala et al. (2016a), seem to indicate that in the Oued Nador aquifer the limit of the salt wedge reached a distance of 1700 m far from the coast. The electrical surveys, we carried out during May of 2015, 2016 and 2017, seems to indicate that the origin of the salinity is due to the marine intrusion far inland inside the aquifer.
To avoid the pollution of the freshwater table by salt water requires: a) a better knowledge of the physics of the recharge of the aquifer by freshwater, b) an powerful and robust management and c) a protection of the freshwater resource.
In this article, we will focus on the downstream part of the plain located in the Oued Nador area, Tipaza (Algeria). The objective of this work is to detect the current position of the salt wedge and to estimate its spatio-temporal evolution. To achieve this objective, we have chosen to use jointly electrical soundings and seismic refraction. These two geophysical methods are complementary: the electrical survey is based on the measurement of resistivity, and the seismic refraction measures the velocity of propagation of waves in the medium.
The large resistivity contrast between saturated saltwater and saturated freshwater- formations has been used by many researchers to study saltwater intrusion in coastal areas (Choudhury, 2001; Batayneh, 2006; Sathish et al., 2011; Oyeyemi et al., 2015; Kumar et al., 2016; Adeyemo, 2017; Yusuf et al., 2019; ...). Only, the presence of clay formations makes the resistivity contrast, between clays and saltwater saturated formations, lower. The resistivities of conductive and saltwater saturated aquifer formations are generally between 1 and 10 ohm.meter (Demirci, 2020). It is the same resistivity range, (between 7 and 10 ohm.meter), that characterize the marls and clays of the Nador aquifer (Bouderbala, 2015b; Bechkit et al., 2017, 2018a and 2018b).
The superposition of several effects such as: a) lower contrast in terms of electrical resistivity between salt water and clays, b) presence of barriers (elevation of the bedrock) of clay and c) up-welling towards the surface of the soil, in the form of a salt dome and d) up-welling in the form of an uplift of the interface freshwater - saltwater, makes the interpretation more delicate and make difficult the determination of the interface of separation freshwater - saltwater. In addition, the effect of soil moisture can influence the determination of the position of the freshwater-saltwater interface. The electrical resistivity depends on the state of moisture of the soil, for this we have established a water balance (Tab. 1), this is true in the case of a rainy episode that follows a period of drought, because there is also the effect of dissolution by rainfall, dry salts located near the surface of the soil in the unsaturated zone. The use of seismic refraction allows to separate, through the velocity ranges, the salt water formations from the clay formations and to specify the structure of the formations.
Table 1
Hydric balance (September 2017 - August 2018).
Sep 2017 - Aug 2018
|
Sep
|
Oct
|
Nov
|
Dec
|
Jan
|
Feb
|
Mar
|
Apr
|
May
|
Jun
|
Jul
|
Aug
|
Sum
|
P (mm)
|
14.00
|
33.00
|
68.00
|
78.00
|
39.00
|
99.00
|
121.00
|
160.00
|
67.00
|
66.00
|
2.00
|
0.00
|
747.00
|
T (°C)
|
26.00
|
22.00
|
16.00
|
12.00
|
13.00
|
11.00
|
14.00
|
18.00
|
20.00
|
26.00
|
31.00
|
30.00
|
|
PET (mm)
|
135.41
|
100.71
|
56.94
|
37.04
|
42.96
|
33.54
|
56.47
|
86.02
|
109.81
|
158.80
|
206.70
|
184.65
|
1209.04
|
EUR (mm)
|
0.00
|
0.00
|
11.06
|
52.02
|
48.06
|
100.00
|
100.00
|
100.00
|
57.19
|
0.00
|
0.00
|
0.00
|
|
RET (mm)
|
14.00
|
33.00
|
56.94
|
37.04
|
42.96
|
33.54
|
56.47
|
86.02
|
109.81
|
123.19
|
2.00
|
0.00
|
594.96
|
Excess (mm)
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
13.52
|
64.53
|
73.98
|
0.00
|
0.00
|
0.00
|
0.00
|
152.04
|
AD (mm)
|
121.41
|
67.71
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
35.61
|
204.70
|
184.65
|
614.08
|
In order to delineate the freshwater-saltwater interface, to determine the nature and morphology of the geological formations and to validate the hypothesis of the local elevation of the bedrock, acting as a natural barrier and preventing any marine intrusion, we have completed, in 2018, the electrical measurements by a seismic refraction survey. These measurements enabled to distinguish clays from salt water and to determine the diopter inclination within the prospected depth range.
Indeed, the simultaneous interpretation of the results of the electrical soundings, the seismic profiles and the hydrostatic approach allow to significantly reduce the uncertainties related to each of these methods independently used.