Modelling the sustainability of low-cost hand drilling methods of shallow groundwater abstraction: the case of the Upper Benue River, NE Nigeria B.A. Ankidawa 1* , C. Bradley 2 , P. Collins 3 and S.A.G. Leroy 4 1 Department of Agric. & Environmental Engineering, SEET, PMB 2076, Modibbo Adama University of Technology, Yola, Nigeria 2 School of Geography, Earth and Environmental Sciences, B15 2TT, University of Birmingham, UK 3 Department of Civil Engineering, SED, UB8 3PH, Brunel University London, UK 4 Institute for the Environment, UB8 3PH, Brunel University London, UK *Corresponding author: ankidawa03@yahoo.com Telephone: +2348063124156 Abstract This paper: 1. assesses whether current rates of groundwater abstraction from the alluvial aquifer of the River Benue, NE Nigeria, are sustainable; and 2. examines the significance of fluctuations in summer river levels on the floodplain water table. A conceptual groundwater flow model for a floodplain cross-section is developed using MODFLOW with stratigraphic data obtained by hand augering. Floodplain water tables are initially simulated for a 6-month period (January to June 2012); and longer model runs were completed using data collected over >50 years (from 1960 to 2012). Model output is verified using observed data from shallow piezometers installed by hand in the floodplain, with water tables logged automatically and read manually at weekly intervals. Model output is most sensitive to: 1. specific yield, and 2. hydraulic conductivity, but not to river bed conductance. The results confirm that river seepage is the primary inflow to the alluvial aquifer (~75%) and under ‘normal’ conditions, groundwater abstraction from pumping wells constitutes the largest outflow from the aquifer (~56%). The model was used to consider three predictive scenarios: 1. changes in river stage; 2. variable rates of groundwater abstraction from agricultural wells and 3. global climate change. The results suggest that low river water stages (as a consequence of upstream river regulation), continued high groundwater abstraction as a result of population increase and most probably global climate change will lead to a significant and potentially dangerous fall in the floodplain water table to depths beyond which further abstraction is no longer feasible using hand drilling methods. Keywords: MODFLOW, Modelling, Groundwater, Surface water, River - aquifer interaction, River Benue, Alluvial floodplain, low-cost hand drilling. 1 Introduction Alluvial groundwater is the major source of irrigation water for agriculture on the floodplain of the River Benue, North Eastern Nigeria. In common with other arid and semi-arid regions, alluvial deposits are potentially highly productive aquifers that can provide an important International Journal of Scientific & Engineering Research, Volume 6, Issue 8, August-2015 ISSN 2229-5518 2062 IJSER © 2015 http://www.ijser.org IJSER