TEMPERATURES AND THERMAL-GRADIENTS IN THE SENEGALESE MAASTRICHTIAN AQUIFER - SIMULATED TEST ON THEIR EFFECT ON FLOW DISCHARGE

The Maastrichtian aquifer in the Senegalese sedimentary basin appears completely confined and is characterised by a relatively high geotherm al gradient (0.03-0.06 degrees C m(-1)). In the upper part of the aqui fer, temperatures are strongly related to the depth of the roof. At th e basin scale, this could mask possible thermal transfer. However, in the SE, thermal anomalies have been observed in some boreholes; moreov er, radiocarbon investigations have indicated abnormally long groundwa ter residence times. Recharge occurs mainly in this zone, near the bas ement margin, by leakage from the upper aquifer through semi-permeable beds. The natural vertical discharge of the Maastrichtian aquifer occ urs beyond the line of equal piezometric level between the upper and l ower aquifer. By comparing the aquifer to a porous U-tube and imposing the same input and output, the flow patterns have been studied numeri cally using the finite element method. Different numerical simulations have involved fluctuations of three quantities: the geothermal gradie nt, the intrinsic permeability and the input rate. They indicate that, for the mean values observed in the aquifer, natural convection could occur. When the input rate increases, the convective cells have a ten dency to break up but significant mixing remains. In such cases, inter pretation of residence times with natural isotopes is more difficult.