Combined geophysical methods to investigate seawater intrusion in the Souss-Massa coastal area, Morocco

Abstract

Groundwater are strategic water resources for irrigation and drinking purposes in the Souss-Massa semi-arid region (Morocco). These resources have been threatened during the last decades by different factors such as drought, over-pumping and marine intrusion. The latter is responsible for the degradation of the groundwater quality along the coastal area, which is reducing its usability. Therefore, the localisation of the freshwater/saline water interface is essential for better sustainable management of groundwater resources in the area. The objective of the present study is to assess the temporal changes, the extent, and the depth of the seawater intrusion in the Souss-Massa coastal area. To fulfil this objective, two monitoring sites were equipped with water conductivity measurement devices (Subsurface Monitoring Device – SMD) to observe the temporal changes of the salinity at a local scale, as well as to support the interpretation of a transient electromagnetic (TEM) survey. This survey consisted of 83 TEM soundings, along thirteen profiles located perpendicularly to the ocean and the two rivers delineating the study area. The data from the SMD stations highlight the temporal changes of electrical conductivity (EC) with depth, clearly illustrating the freshwater/saltwater interface location in the central part of the study area (−35.55 m NGM at the Tifnit location, ca. 1 km from the coast). In addition, the results of the TEM measurements show that the marine intrusion is already advanced towards the land, over a maximum distance of 2500 m. Finally, some new monitoring sites have been proposed to improve the current monitoring network for seawater intrusion and assess whether mitigation solutions lead to a slower progression of the seawater intrusion. Decision-makers can use these results as a basis for the development and implementation of effective policies to ensure a sustainable and integrated water resources management in this region.