Hydrogeological mapping of north-central Madagascar using limited data

North-central Madagascar is well endowed with surface-water. Due to soil erosion and pollution, rivers can no longer provide year round clean water supplies. Shallow aquifers are being developed to provide sustainable rural and urban water supplies. A survey of water sources located 2760 boreholes, wells, ponds and springs in the area, but understanding of groundwater occurrence remains poor. The area comprises four hydrogeological zones: the dry Central High Plateau with erosion surfaces underlain by weathered and fractured Basement Complex aquifers ; the wet East Coast Basin underlain by Cretaceous limestone, Neogene volcanic and Recent alluvial aquifers; and the northern Antsiranana and the western Mahajanga Basins underlain by Karoo continental and marine sedimentary and Cretaceous -Tertiary sedimentary and volcanic aquifers . Thick lateritic soils mantle much of the area. As part of the 2005-8 geological mapping of north-central Madagascar, hydrogeological mapping at 1:500 000 scale was undertaken based on hydrogeological, geological and geomorphological data layers within an Arc9 GIS format. Georeferencedgroundwater data were collated with information from a weathered basement aquifer research site north of Antanarivo. Experience from studies of weathered Basement Complex, Karoo, Mezo -Tertiary and Quaternary aquifers on the mainland informed map content. Seven map sheets were produced at a scale of 1:500 000 using the UNESCO hydrogeological legend modified to show aquifer distribution. Insert maps show depth to water-table/DEM and waterquality/ geomorphological unit distributions. These maps were compiled for planning purposes in areas of limited hydrogeological data.