Abstract
On the northern coast of the Cape Verde peninsula, Lake Retba is one of the few depressions that have preserved its water body due to the drought that plagued the Sahel in the 1960s. The aim of this article is to understand the resilience of the lake to the drought. For this purpose, a deep knowledge of interactions between the different components of the hydro-system, namely: lake, dune tablecloth, ponds and ocean is necessary. The use of electrical resistivity tomography (ETR) and chemical and isotopic tools yielded conclusive results. Electrical tomography of resistivity (ETR) shows throughout the coastal dune a superposition between fresh and salt water, with even a predominance of salt water at the level of the old canal. The chemistry of the major ions and isotopes confirms this contact between the saltwater bison of the sea and the lake by showing a superposition of the fresh/brackish waters on the salty waters of the bevel, which, in turn, are found in the very salty waters influenced by the lake. This translates a feeding of the lake by the sea through the bevel. At the canal level, this feeding is done, on the surface, by means of ponds whose water characteristics show that they constitute an outcrop of the bevel. The freshwater body of the coastal dune that floats on salt water in the form of a lens also flows into the lake. The feeding of the lake, by the sea through the bevel and the ponds, and by the water table, is therefore effective and attested by all the methods used in this work. It allowed the resilience of the lake against the drought. This inflow of seawater into the lake is at the origin of its salinization and therefore of the salt that settles on the bottom. This study shows that the survival of the lake also depends on safeguarding the ponds located in the north of the lake.
Highlights
Lake Retba is located 35 km east of Senegal’s capital, Dakar (Figure 1)
The freshwater body of the coastal dune that floats on salt water in the form of a lens flows into the lake
This study shows that the survival of the lake depends on safeguarding the ponds located in the north of the lake
Summary
Lake Retba is located 35 km east of Senegal’s capital, Dakar (Figure 1). Its watershed covers an area of 160 km (Carn et al, 1976). The waters are over salted and deposit a salty crust at the bottom of the lake (Garnier, 1978), which has been exploited by the population since the 1970s This mining activity produces 110,000 tons of salt annually and generates profit margins ranging from CFAF 1.5 to 3 billion (Sarr, 1997). In addition to this mining activity, there are other activities such as tourism, market gardening and fruit arboriculture. As a result, it plays a significant socio-economic role for the country. In recent years, the level of the lake has continued to fall to alarming proportions, threatening all socio-economic activities
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