Document published in 2011 by OSS (Sahel and Sahara Observatory), focused on the design and implementation of a hydrogeological model of the Iullemeden Aquifer System. The aquifer, extending through Mali, Niger and Nigeria, is one of the two outmost important groundwater systems in the Niger basin, with groundwater flowing mainly from north to the south. Quite interestingly, the aquifer extends up to the Mekrou W park, Niger and Rima (in Nigeria) rivers acting as the regional drainage references. Also the huge alluvial fan, extending in direction N-S from the human settlement of Boumba (on left wing of Niger river, with its cultivated areas), is another relevant draining body. The document, which has been prepared with various contributions, including private consultants in groundwater modelling, UNEP/GEF and UNESCO, is part of a collection of 5 volumes, all focused on the aquifer system. The document has been downloaded from OSS web site: http://www.oss-online.org