Résumé
The multilayer aquifer of Morroa (northwest Colombia) is mainly composed of sandstone and is almost under consolidated conglomerates interbedded with clayey layers. It is the main water supply source of the region characterized by a dry tropical climate, and includes many urban centres. During the last 50 years, its intensive use has resulted in a significant decrease of the water table, even if the cessation of pumping seems to show some recovery. In this context, classical and isotopical geochemical measurements ( (super 18) O, (super 2) H, (super 13) C, (super 14) C and tritium) were made between 2002 and 2003 to characterize current recharge conditions. The water chemistry shows an evolution between calcium bicarbonate and sodium bicarbonate in proportion to borehole depth, which can be explained by a cation exchange process with the layers of clay. Oxygen-18 and deuterium give homogeneous values, indicating an important water mixing in this aquifer and thus a large water residence time, which agrees with tritium results (none) and (super 14) C (50-90% of activity). The possibility to have a recent recharge is very limited study and it is very important to consider the current strategy to supply the zone with water.