Hydrogeochemical characterisation of the low Mandrare area - South Amboasary District

Geosciences Engineering

The present study is interested in the mode of mineralization of groundwater distributed in the lower Mandrare zone. The latter, which is located in the extreme south of Madagascar, is administratively divided between five municipalities: South Amboasary, Tanandava, Ifotaka, Behara, and Sampoina. However, this zone presents interesting geological characteristics which influence both the underground flow mode, as well as the ionic exchanges which lead to the final mineralization of the studied waters. By adopting a multidisciplinary study such as geology, hydrogeology, and hydrochemistry, the mineralization process is observed in a transversal way, connecting these three elements in order to explain the complexity of water mineralization measured (500 to +3000 µS/cm) on site, using a conductivity meter. However, more in-depth analyzes on major ions such as bicarbonate, chloride, sodium, calcium and also potassium, were carried out in an analytical laboratory before discovering that these are predominant in all the waters inventoried in the lower Mandrare area. The same is true for minor elements like nitrate and sulfate which seem to be in fairly high percentages in a few areas. The variation in the characteristics of the waters is quite different depending on their distance as well as the variation in the hydrostatic level allow them to be located on a grid of assessments in order to bring out the dominant facies in the area (sodium bicarbonate, etc.). These waters are technically easy to access, because even with a fairly variable general topography, the depth of the structures for any operation does not exceed twenty meters to find water. However, their hydrogeochemical characteristics reveal aspects that do not conform to potability, which requires a more complex intervention for their treatments.

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