Assessment of saltwater contamination extent in the coastal aquifers of Chaouia (Morocco) using the electric recognition

Abstract

The coastal Chaouia, located on the Moroccan Atlantic coast, comports three aquifers: a first Plio-quaternary constitutes the cover, a second Cretaceous located at SW in the zone of Azemmour-Tnine Chtouka, and a third Primary located at SE in the zone of Tnine Chtouka-Casablanca. These reservoirs constitute the only exploitable water resource for the social and economic development of the region between Azemmour and Casablanca. These aquifers are unconfined, pellicular, and discontinuous. They feed by infiltration of rainwater and discharge at sea. Agricultural irrigation is carried out exclusively from the groundwater, which causes a drop of the piezometric surface and the intrusion of saltwater into the aquifer at the coast. The consequence is the abandonment of some wells contaminated by seawater leading to conflict situations and significant economic losses. Therefore, delineation of the freshwater/saltwater interface is very important in order to build a sustainable groundwater management system and to implement appropriate regulatory policies.The purpose of this study is to determine the extent and the geometric characteristics of the saltwater contamination extent in the coastal aquifers of the coastal Chaouia. To achieve this, 399 vertical electric soundings and 48 electrical resistivity tomography profiles were performed perpendicular to the ocean. Furthermore, the electrical conductivity and the piezometry were measured in 344 wells distributed over the study area.The study demonstrated the effectiveness of electrical methods for mapping seawater intrusion into coastal aquifers. In fact, the results of the interpretation of the VES and ERTs after calibration with the lithological data of the boreholes, as well as the values of the electrical conductivity, have shown that the length of the saltwater wedge penetration inland depends on the lithological nature of the aquifer formations. Thus, in the zone of Azemmour – Tnine Chtouka, characterized by the presence of Cretaceous terrains, the extension of the saltwater wedge exceeds 2 km towards the continent and its depth reaches 45 m. On the other hand, in the zone of Tnine Chtouka- Casablanca, characterized by the rise of the schists surmounted by the altered schists the seawater intrusion remains limited at 700 m from the coast and at 20 m of depth.