Analysis of pumping tests, with regard to tectonics, hydrothermal effects and weathering, for fractured Dalha and stratiform basalts, Republic of Djibouti
Jalludin, M.; Razack, M.
Journal of Hydrology 155(1-2): 237-250
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 fault network is particularly well developed and controls the permeability of volcanic aquifers. Pumping test data are analysed for two major basaltic series, the stratiform series (3.4-1Ma) and the Dalha series (9-3.4Ma), which are distinguished mainly by their geodynamic characteristics and geological history although they are both recognized as trap rocks. Drawdown and recovery were studied by Jacob's logarithmic approximation of the Theis equation and by the Theis recovery method. Elsewhere, step drawdown data were interpreted with the Cooper-Jacob graphical 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 transmissivities range from 1.5X10 (super -4) to 5.7X10 (super -1) m (super 2) s (super -1) and from 2.9X10 (super -6) to 1.6X10 (super -2) m (super 2) s (super -1) . Storage coefficients determined from piezometer observations and computer simulations describe the aquifers in the Gulf basalts, a particular type of the stratiform series, as semiconfined, and those in the Dalha basalts as confined or unconfined depending on the area. Thus, the calculated specific capacity demonstrated that the stratiform basalt aquifers have more favourable hydrodynamic characteristics than the Dalha basalt aquifers. Specific capacities vary between 0.372 and 510.01m (super 3) h (super -1) m (super -1) within the stratiform basalts and between 0.008 and 35.2m (super 3) h (super -1) m (super -1) in the Dalha basalts. As they were originally composed of the same type of trap rocks, the stratiform and Dalha basalts should have a similar magnitude of permeability, and this was confirmed by some wells. The lower specific capacities exhibited by the Dalha basalts is explained by a longer period of alteration and greater hydrothermal activity, as has been observed in well cuttings and field studies.