Contribution for better understanding the quaternary aquifer of Lake Chad basin in Cameroon.

International audienceIn the Far north of Cameroon, the main source of drinking water is the Quaternary sedimentary aquifer. In this semi-arid region, with high population growth, the groundwater resource is affected by climate and land-use changes but also by the impact of Lake Chad level variation. For better quantifying the role of each of these processes, it is necessary to improve our knowledge about aquifer characteristics. Thus, 13 MR soundings were performed in 2012 and 2103 for characterizing hydrodynamic properties of aquifer in the vicinity of the Limani-Yagoua sand-ridge which is considered by many authors as the limit of the Mega-LakeChad during the last humid period of Holocene (~-6000 years BP). Despite of difficult conditions of measurement (high natural electromagnetic noise in the afternoon and low Lamor frequency), data of acceptable quality were obtained with a fairly efficientvelocity of execution (1 sounding per day). MRS results are in good agreement with available geological logs and with the water levels measured in neighboring wells. They also show high heterogeneity of aquifer characteristics: the water content ranges from less than 2% to 25.5 %, the thickness from 4 to 25 meters and the transmissivity (calibrated with available data in literature) varies from 6×10-5 to 1 ×10-4 m2/s. No water was found within the first 30 m of the Limani-Yagoua sand ridge, confirming the absence of water storage in this formation laid on the regional quaternary aquifer. During this study, the main limitation for MRS was the depth of investigation (~ 30 meters) with the equipment we employed (Numis Lite). Thus a new field survey is scheduled in March 2015 with more powerful equipment (Numis Plus)

Contribution of time domain electromagnetic and magnetic resonance soundings to groundwater assessment at the margin of lake Chad basin, Cameroon

At the edge of a semi-arid sedimentary basin, the Piedmont plain of the Mandara mountains is potentially a key area for the recharge of the Lake Chad Quaternary aquifer. We conducted two geophysical surveys based on magnetic resonance sounding (MRS) and Time-Domain Electromagnetism (TDEM) techniques for a better understanding of the aquifer structure at.the piedmont scale in aim to identify preferential groundwater recharge pathways. MRS water content and electrical resistivity of the medium confirmed the very heterogeneous and clayey nature of the quaternary aquifer, limiting the recharge to the sandiest areas. Thanks to the geophysical methods, these areas have been highlighted near temporary rivers and upstream of the study area near Maroua city. Because of the thickening of the aquifer downstream, the most transmissive zones are in the mid-slope of the piedmont. The geographical distribution of aquifer properties defined by geophysics and the range of MRS water content (from <2% to 26%) may be used in the future to constrain groundwater modelling and specify the recharge of the quaternary aquifer at mountain front