Geophysical Approach for Assessment of Seawater Intrusion in the Coastal Aquifer of Wadi Nador (Tipaza, Algeria) (original) (raw)

Abstract

The aquifer of Nador has suffered significant salinization due to seawater intrusion. It was strongly exploited during the 1980s and 1990s. A piezometric analysis in April 2012 showed the piezometric level to lie at 0 m a.s.l. over the plain; as a result, this aquifer is highly sensitive to the marine intrusion with an electrical conductivity of the groundwater in of exceeds 2500 μS/cm and so there are no abstractions for irrigation or drinking purpose from these sectors. The geoelectric study also showed the lateral variation in the electrical resistivity for two moments separated in time by more than 45 years. The fall in resistivity may be due to the encroachment of seawater into previously freshwater zones and/or infiltration during the era of pumped abstractions downstream. The resistivity surveys reveal two distinct sectors: the saturated aquifer in brackish and saltwater having resistivity values to 36-10 Ωm, which extends nearly 1600 m inland.

Figures (13)

Fig. 1. Geological map of the study area. Colour version of this figure is available in electronic edition only.

Fig. 1. Geological map of the study area. Colour version of this figure is available in electronic edition only.

Explanation: LDW — large-diameter wells, DW&I — drinking water and irrigation. Source: National Agency of Water Resources and Department of Hydraulics of the wilaya of Tipaza. Characteristic of measurement points in the Nador plain

Explanation: LDW — large-diameter wells, DW&I — drinking water and irrigation. Source: National Agency of Water Resources and Department of Hydraulics of the wilaya of Tipaza. Characteristic of measurement points in the Nador plain

SEAWATER INTRUSION IN THE COASTAL AQUIFER OF NADOR

SEAWATER INTRUSION IN THE COASTAL AQUIFER OF NADOR

Results for hydro-geochemical analysis of groundwater (April 2012)

Results for hydro-geochemical analysis of groundwater (April 2012)

SEAWATER INTRUSION IN THE COASTAL AQUIFER OF NADOR

SEAWATER INTRUSION IN THE COASTAL AQUIFER OF NADOR

Fig. 6. Map of the apparent resistivity values, AB = 200 m. SEAWATER INTRUSION IN THE COASTAL AQUIFER OF NADOR served close to the Nador city, and are explained by the fact that the highly conductive Plaisancian marly clays rise closer to the surface. Meanwhile, the highest resistivities in 1967 were found in the Plio-Quaternary aquifer close to the coast. By contrast, the map of apparent resistivity for AB = 200 m in 2012 (Fig. 6b) shows a variation from 7 to 18 Qm. On this occasion, the highest values are observed inland, in the middle of the plain, with lower values both up- and downstream. The lowest resistivity value, about 7 Qm, occurs downstream, close to the sea, and can be related to the high salinity into the Plio-Quaternary aquifer. High salinity gradually decreases upstream, towards the middle of the plain, beyond which it is negligible or null. The low apparent resistivity observed further upstream (between profiles H and N) is explained by the presence of the Plaisancian marly clay substratum that rises towards the ground surface. The map of the apparent resistivity for AB = 300 m, generally shows the

Fig. 6. Map of the apparent resistivity values, AB = 200 m. SEAWATER INTRUSION IN THE COASTAL AQUIFER OF NADOR served close to the Nador city, and are explained by the fact that the highly conductive Plaisancian marly clays rise closer to the surface. Meanwhile, the highest resistivities in 1967 were found in the Plio-Quaternary aquifer close to the coast. By contrast, the map of apparent resistivity for AB = 200 m in 2012 (Fig. 6b) shows a variation from 7 to 18 Qm. On this occasion, the highest values are observed inland, in the middle of the plain, with lower values both up- and downstream. The lowest resistivity value, about 7 Qm, occurs downstream, close to the sea, and can be related to the high salinity into the Plio-Quaternary aquifer. High salinity gradually decreases upstream, towards the middle of the plain, beyond which it is negligible or null. The low apparent resistivity observed further upstream (between profiles H and N) is explained by the presence of the Plaisancian marly clay substratum that rises towards the ground surface. The map of the apparent resistivity for AB = 300 m, generally shows the

Fig. 7. Geoelectrical cross-sections (comparison between values of resistivity in 1967 and 2012).

Fig. 7. Geoelectrical cross-sections (comparison between values of resistivity in 1967 and 2012).

Geophysical and hydrogeochemical data used for obtaining the empirical relationship Geophysical and hydrogeochemical data used for obtaining the empirical relationshir This empirical relationship may be also used to estimate local EC values from the difference of transverse resistance (AR7). Based on the empirical re- lationship between the EC and AR; (Fig. 10), we can delimit the water salini- ty of EC=2500 uS/em (Kirsch 2009) saline water begins at EC= 3000 wS/cm (Saxena et al. 2003, Mondal et al. 2008) which corresponds to AR; = 600 Qm?.

Geophysical and hydrogeochemical data used for obtaining the empirical relationship Geophysical and hydrogeochemical data used for obtaining the empirical relationshir This empirical relationship may be also used to estimate local EC values from the difference of transverse resistance (AR7). Based on the empirical re- lationship between the EC and AR; (Fig. 10), we can delimit the water salini- ty of EC=2500 uS/em (Kirsch 2009) saline water begins at EC= 3000 wS/cm (Saxena et al. 2003, Mondal et al. 2008) which corresponds to AR; = 600 Qm?.

The use of geophysical and hydrogeochemical techniques for the evaluation of water quality is very promising because of their ability to map the subsur- face water quality in the aquifers. The geoelectrical and geochemical proper- ties are used to delineate the contaminated water zones. The measured apparent resistivity data was interpreted qualitatively and quantitatively to represent the resistivity distribution of subsurface in the study area. The as- sumption of the hypothesis of seawater intrusion in this coastal aquifer of Nador is justified by the existence of a piezometric level of 0 m a.s.1. inland, and by the increasing salinity of the groundwater when approaching the sea. Three main causes this phenomenon in this aquifer: the intensive pumping, the hydraulic connection with the sea, and the high permeability of the Plio- Quaternary aquifer material. The geannhyveoical nracnertinn carried ant in 9019 haced nn AZ vertical

The use of geophysical and hydrogeochemical techniques for the evaluation of water quality is very promising because of their ability to map the subsur- face water quality in the aquifers. The geoelectrical and geochemical proper- ties are used to delineate the contaminated water zones. The measured apparent resistivity data was interpreted qualitatively and quantitatively to represent the resistivity distribution of subsurface in the study area. The as- sumption of the hypothesis of seawater intrusion in this coastal aquifer of Nador is justified by the existence of a piezometric level of 0 m a.s.1. inland, and by the increasing salinity of the groundwater when approaching the sea. Three main causes this phenomenon in this aquifer: the intensive pumping, the hydraulic connection with the sea, and the high permeability of the Plio- Quaternary aquifer material. The geannhyveoical nracnertinn carried ant in 9019 haced nn AZ vertical

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