ANALYSIS OF PUMPING TESTS, WITH REGARD TO TECTONICS, HYDROTHERMAL EFFECTS AND WEATHERING, FOR FRACTURED DALHA AND STRATIFORM BASALTS, REPUBLIC-OF-DJIBOUTI

The Republic of Djibouti is the location of an exceptional geodynamic situation, the Afar Depression, which is an emerged triple junction of the Red Sea, the Gulf of Aden and the East African rifts, where a fau lt network is particularly well developed and controls the permeabilit y of volcanic aquifers. Pumping test data are analysed for two major b asaltic series, the stratiform series (3.4-1 Ma) and the Dalha series (9-3.4 Ma), which are distinguished mainly by their geodynamic charact eristics and geological history although they are both recognized as t rap rocks. Drawdown and recovery were studied by Jacob's logarithmic a pproximation of the Theis equation and by the Theis recovery method. E lsewhere, step drawdown data were interpreted with the Cooper-Jacob gr aphical solution and Rorabough's method. The results describe aquifers in the stratiform and Dalha basalts which are identified by different hydrodynamic characteristics. Stratiform and Dalha aquifer transmissi vities range from 1.5 x 10(-4) to 5.7 x 10(-1) m2 s-1 and from 2.9 x 1 0(-6) to 1.6 x 10(-2) m2 s-1. Storage coefficients determined from pie zometer observations and computer simulations describe the aquifers in the Gulf basalts, a particular type of the stratiform series, as semi confined, and those in the Dalha basalts as confined or unconfined dep ending on the area. Thus, the calculated specific capacity demonstrate d that the stratiform basalt aquifers have more favourable hydrodynami c characteristics than the Dalha basalt aquifers. Specific capacities vary between 0.372 and 510.01 m3 h-1 m-1 within the stratiform basalts and between 0.008 and 35.2 m3 h-1 m-1 in the Dalha basalts. As they w ere originally composed of the same type of trap rocks, the stratiform and Dalha basalts should have a similar magnitude of permeability, an d this was confirmed by some wells. The lower specific capacities exhi bited by the Dalha basalts is explained by a longer period of alterati on and greater hydrothermal activity, as has been observed in well cut tings and field studies.