River-aquifer interaction in the Middle Yobe River Basin, North East Nigeria
Development of the shallow alluvial groundwater of the Hadejia-Jama'are-Yobe River valley flood plain (northern Nigeria) has been taking place with increasing intensity over the past decade. However little has previously been known about the nature of the Yobe River-aquifer interaction, including the recharge mechanisms. This thesis reports on a study of the river-aquifer interaction in the middle section of the basin, centred on a field site near Gashua, Yobe State. Detailed field studies were undertaken over a period of 14-months, which involved geoelectical sounding, drilling and water level monitoring. The results of the field study show that the Yobe basin is underlain by a sand and gravel aquifer, which is covered by an average of 1-3m of clay. The Yobe River is in hydraulic continuity with the adjacent alluvial aquifer and variations in aquifer storativity have been recognised as an important factor in understanding the hydraulic behaviour of the Yobe River-alluvial groundwater system. A confinedunconfined groundwater regime exists within the present site and is an inherent characteristic of the alluvial groundwater system. The recognition of this state enables a conceptual flow model of the system to be developed. A multi-layer, spatially distributed model is proposed, in which transitions between confined and unconfined conditions can be realistically represented. A numerical model needs to be designed with these concepts in mind in order to simulate the system. The research techniques employed in the study are appropriate for the assessment of the Yobe basin system where detailed data is currently not available. The combination of detailed geophysical survey, water level monitoring and conceptual modelling has led to a good understanding of the Yobe River-alluvial aquifer interaction. It is for this reason that the techniques employed in this study can be adapted for investigating the remainder of the basin downstream of the present site.