Hydrogeological Characteristics and Geophysical Study of Base Aquifers in the Island of Grande Comore: Implications for Water Resources and Saline Intrusion

The Electrical Resistivity Tomography (ERT) and Vertical Electrical Sounding (VES) methods surveys were employed to conduct hydrogeological investigations within the three massifs of Grande Comore. The geometric arrangement of Grande Comore can be subdivided into three distinct segments: A layer exhibiting very high resistivity values ranging from 1000 to over 6000 Ω.m, characterising historical, recent, sub-recent, and even ancient phases; Structures with resistivity values ranging from 100 to over 200 Ω.m, depicting freshwater aquifers; Layers with lower resistivity values ranging from 1 to 100 Ω.m, indicating interfaces between freshwater and saltwater. The island has heterogeneous sub-surface lava formations, with compact, relatively fractured layers that can channel preferential flows. These aspects are visualised through ERT panels, highlighting strong fault zones along the profiles. The perched aquifers, whose resistivity varies from 20 to 90 Ω.m, are exploited in the form of gallerys, mainly in the two massifs located at the extremities of the island. These analyses are supported by various data, including lithological profiles, groundwater table levels, groundwater quality, as well as data from VES. The aquifer porosity (φ) shows a variation ranging from 0.01 to 0.34, with an average of 0.124, and is closely related to hydraulic conductivity (Ks), fluctuating from 5.68 × 10-8 to 8 × 10-6 m/s following the application of the Kozeny equation. The Badjini massif stands out as a unique area with extensive alteration of volcanic aquifers and intercalated slag basalts. The coastal zones show low electrical resistivity values 20 to 100 Ω.m, indicating a substantial contribution from saltwater intrusion.