The deep Maastrichtian aquifer belongs to the Senegalese basin and bears considerable groundwater resource. This system outcrops in the Diass horst zone, is composed of sandstone formations with mean thickness of 250-300 m. It is tapped by several hundred boreholes pumping at rates between 50-200 m3/h for human consumption, agriculture, pastoral and industrial needs. It is characterized by a general flow pattern (SE – NW), and the recharge zone is thought in the contact with the basement rock in the SE, in the East with the Senegal River, in the west at the Diass Horst and probably at Bissau Guinea. Since the 70s, the system has been subject to numerous investigations to understand its functioning, recharge processes as well as origin of high Cl and F in the central saline band.
Computed residence times from the detectable 14C range from 10,000 to more 30,000 yr evidencing palaeorecharge in the hydraulic system. Very low 14C (close to detection limit) and few 36Cl data reveal age of more than 250,000 yr. However high 14C values reveal recharged young water in the Northern part of the basement rock contact. These contents rapidly decay to low values at a short distance evidencing low to non-laminar circulation pattern in this region. Toward the West at the Diass horst zone, detectable 14C between 2-66pmc are mainly due to high abstraction for urban water supply which create an imbalance in terms of water budget in the system and continuous lowering of the water table, which is presently at more than 35 m.
Radiocarbon data distributed throughout the system and O/H stable isotopes indicate old to very old water which was replenished under different climate conditions. Rapid decayed 14C in the supposed recharge zone and detectable radiocarbon in the horst zone are an indication of dynamic flow patterns which are thought to be convective and/piston type accompanied by dynamic influenced by high pumping in parts of the system. Despite these findings, some scientific issues need to be further investigated in order to build a conceptual model of the system functioning.
Considering the stable isotopes, distributed statistical values reveal different patterns with high range occurring in the East, saline band and West while lower range characterized the South and the heart of the system. Plotted in the conventional 18O/2H graph, values scatter along the meteoric line with an absence of evaporative effect. The higher range depicted in the eastern part of the aquifer should infer either by different climate recharge period or by circulation pattern which can be through Piston and/or convective flow. In the saline band and the Horst, differentiation in isotopic contents can derive from inflow of high pumping in boreholes operating in these areas.