Report on geophysical studies relating to the coastal aquifer of the Mombasa district, Kenya.

Airborne electromagnetic coverage of the Coastal Plain within 110 km of Mombasa provides information on conductivity variations to depths of 50-100 m. Conductivity levels are generally high due to saline water occuring at shallow depth within the fossil reef by the coast, and to the presence of Jurassic shales which outcrop to the west. Zones of higher resistivity are found over the sandy facies of the Plio-Pleistocene reef complex and one of these coincides with the site of the Tiwi aquifer. However, resistivity sounding data indicate that the reduced INPUT amplitudes relate to their intermediate position between formations of high conductivity and not directly to the aquifer: the back-reef deposits comprise a mixture of sands and clays underlain at least in part by (?)Tertiary clays which have resistivities in the range 20-50 ohm.m. The results of geophysical surveys by Austromineral to the south of Mombasa have been reinterpreted to show the ambiguity inherent to the data: their resistivity sections give more detail at depth than is justified by the quality of the data. Resistivity results provide an indication of the major lithological variations but quite different conditions can give identical resistivities and it is not possible to identify positively the freshwater producing zones. Seismic refraction methods also failed to resolve the back-reef sequence. A combination of induced polarization and resistivity methods might provide more information on clay content and water salinity but this approach has not been tested here. Additional resistivity results in the less-developed region north of Mombasa supported the view that aquifer conditions deteriorate towards Malindi. Conductivity values are generally higher in response to saline water at shallow depth within less premeable, finer-grained sediments. Resistivity values of 30-40 ohm.m obtained over Baraumu deposits near Malindi were similar to those given by the claystone south of Mombasa and by the material causing the INPUT low near Msambweni. Similar values could be obtained from a water-saturated sand/clay sequence but it seems likely that Tertiary deposits have been detected below the reef complex in much of the region. Further geophysical investigations are not recommended at this stage for siting boreholes although reference should be made to the INPUT maps: borehole logging data would be useful for correlation with surface results and for defining the aquifer in more detail. Additional work should be considered if drilling results prove unsatisfactory due to salinity problems or to the occurrence of lateral variations in lithology.

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