The hydrodynamic functioning of the deep aquifer of the Senegalese basin: An isotopic approach

Full text: This work represents a synthesis of the main results obtained from the hydrochemical and isotopic study run within the IAEA project framework registered under SEN/8/005 'Hydrogeological study of the deep aquifer of the Maastrichtian'. The study has allowed a better understanding of the structure of the aquifer with the help of the new hydrochemical and isotopic data; a good understanding of the motion of ground water inside the aquifers and their impact on the mineralisation which is important for the water quality assessment. The geographical distribution analysis of the chemical and isotopic concentrations is done from 725 samples collected all over the aquifer through the project referenced as followed SGPRE/COWI, 2002; FAYE, 1994; SEN/8/005,2002; this shows a very neat separation of different areas all oriented from the north to the south. It is noticed, from East to WEST, an increasing of the total dissolved solid (200mg/l to 700mg/l at the meridian line of 15 deg. 30'). Then, it is recorded a sudden increase of concentrations reaching maximum values of 1500 to 3000mg/l a long a meridian line centred on a north-south axis through the towns of Louga - Kaolack. At the West, the other side of the so-called 'salted central zone', the concentrations decrease again to 350-650mg/l in the area of the Horst de Ndiass. The spatial distribution analysis of the geochemistry characteristics, has shown 3 main geochemical zones oriented along a meridian line. 1. The 'East Zone', at the east of the meridian line of 15 deg. 30', characterised by bicarbonate water usually with a very low concentration of dissolved solids; the concentration increases slightly to the west et sometime becoming locally more sulphated. 2. The 'salted central zone', characterised by the presence of chlorite of sodium in the water at a very high concentration, from the estuary of the Sine Saloum up to the lake of Guiers, along the North-South line joining the towns of Louga and Kaolack as mentioned above. 3. The 'Horst of Ndiass zone': it is situated at the west of the hill of the town of Thies, within a very narrow band to the shallow Maastrichtian until the Cap Vert peninsula, characterised by bicarbonated water with very low mineralisation. This distribution is related to the important structural units defined within the basin (see document on geological structures); the east zone and the 'salted central zone' represents the 'eastern block' defined previously; it is separated to the other structural unit ('extreme west'- the only part of the Maastrichtian aquifer considered as partially shallow) with the help of the geological accidents recorded on the eastern side of the Horst; this type of characterisation by zone has been confirmed by the isotopic geochemistry; On 'the east and south east zone, the concentration of oxygen 18 show values of δ lower than -6.2 per mille; in average more negative than the concentrations found in the actual rainfall of the region; At the central zone, the concentration of the oxygen-18 are lightly contaminated with heavy isotopes compared to the concentrations found in the eastern and southern area (-6.4< δ18O<-5.5 per mille), proving the 'paleoclimatic effect' detected in the aquifer; In the extreme west zone (Horst of Ndiass), the concentrations of oxygen within the aquifers correspond exactly to the mean values of the concentrations recorded from the local rainfall; this confirms the assumption of a recent recharge in the area; The other geo dating elements (14C, 36Cl, Uranium group and the Thorium, 4He) and the Bore 11 have given the opportunity to discuss types and regimes of circulation within the different zones of the aquifer and to design a ground water circulation model; The recharge zones located in the Horst de Ndiass, next to the hard rock crystal aquifer, the North east border surrounding the town of Matam and along the Senegal river; The directions of groundwater circulation; The velocity of water within the aquifer are quite low except in the Casamance region; The time variation of the recharge confirms that the ground water circulation does not comply with a permanent regime. (author)