No abstract

The Central Kenya Rift contains small soda lakes such as Nakuru, Elmenteita and Bogoria, freshwater Lake Naivasha, and the partly (spatially) freshwater Lake Baringo. The hydrology of this area is controlled mainly by climate, tectonically controlled morphological and volcanic barriers, faults, and local water-table variations. Much of the area relies on groundwater for human and industrial use, though there are widespread quality issues particularly in relation to fluoride. Despite the huge demand for the resource, little is known about the highly complex groundwater systems; lacking monitoring data, an assessment is developed on the basis of regional geological, hydrogeological and hydrochemical analyses. Significant hydrological changes have taken place in the region over the last 10 000 years as a result of global, regional and local changes, but the impacts on groundwater resources are still largely unknown. The IPCC projects a 10-15% increase of rainfall in the area, but it may not necessarily result in a proportional increase in groundwater recharge. High groundwater recharge periods appear to be anchored on a decadal cycle.Key words Holocene; hydrology; groundwater; climate change; Central Kenya Rift; lake basins Paléohydrologie Holocène, eaux souterraines et changement climatique dans les bassins lacustres du Rift du Centre du Kenya Résumé Le Rift du Centre du Kenya contient de petits lacs salés tels que les lacs Nakuru, Elmenteita et Bogoria, le Lac Naivasha d'eau douce, et le Lac Baringo partiellement (spatialement) d'eau douce. L'hydrologie de cette zone est principalement contrôlée par le climat, des barrières volcaniques et morphologiques sous contraintes tectoniques, des failles, et des variations piézométriques locales. La plupart de la région compte sur les eaux souterraines pour satisfaire les besoins domestiques et industriels, bien que les problèmes de qualité soient largement présents en particulier en termes de fluorures. En dépit des besoins immenses vis-à-vis de la ressource, la connaissance des systèmes hydrogéologiques hautement complexes est faible. Manquant de données de suivi, une évaluation est développée sur la base d'analyses régionales géologiques, hydrogéologiques et hydrochimiques. Des changements hydrologiques significatifs ont eu lieu dans la région au cours des 10 000 dernières années suite à des changements globaux, régionaux et locaux, mais les impacts sur les ressources en eaux souterraines sont largement inconnus. Le GIEC prévoit une augmentation de 10-15% de la pluviosité dans la région, mais qui ne résultera pas nécessairement en une augmentation proportionnelle de la recharge de nappe. Des périodes de forte recharge en eaux souterraines apparaissent être inscrites dans un cycle décennal.

The straight‐slope technique introduced some years ago by Vacquier et al. (1951) is employed to develop simple empirical procedures that can be used to determine depth to the top/center of anomalous sources on measured aeromagnetic vertical gradient profiles. Five geologic bodies/structures in the form of their magnetic/geometric model equivalents, namely, point pole, point dipole, finite dipole, dipping dike, and dipping contact are considered. From analysis of the normalized theoretical curves due to those models it is observed that the horizontal projection of the straight part of the steepest sections of each curve is insensitive to changes in the inclination of the Earth’s magnetic field and also to the dip angle of dipping models. Further analysis of the curves using this observation leads to the conclusion that, when dealing with the interpretation of observed vertical gradient profiles, the length of the horizontal projection on a given profile must be doubled to obtain depth to the point‐pole, point‐dipole, or finite‐dipole source. For a geologic contact and a wide but shallow (i.e., the width more than twice the depth) dike, the length of the projection gives the depth for either source. However, a thin but deeply buried (i.e., the width less than twice the depth) dike, requires use of characteristic curves such as those developed in this study. Application of the procedures to observed vertical gradient results from the White Lake region of Ontario, Canada, has proven quite successful.

Investigations in many parts of the world have indicated that resistivity of saprolite (a chloritization zone at the base of the weathering profile) depends uniquely on the underlying rock type. A study of electrical properties of the weathered layer was undertaken near Yala in the Nyanzian-Kavirondian greenstone belt in western Kenya. Resistivity soundings were systematically carried out in a 24 x 40 km area. Saprolite formed over basalt had the lowest resistivity (14 Qom), followed by andesite (27 Q*m) and rhyolite (67 Clem). Statistically meaningful data sets conclusively show, for the first time, that saprolite resistivity increases with the silica content of parent volcanic rocks. Plutonic and sedimentary rocks had higher average resistivities (granite 135 Q-m, mudstone 79 Q*m, grit 213 Qom). Thickness of the weathered layer varied between 10 and 40 m, with the largest values observed over grit (sandstone). Compared with other tropical regions, the saprolite layer in western Kenya appears relatively thin, possibly because of the relatively rapid tectonic uplift of the area which leads to fast erosion. As observed in earlier studies, resistivity and electromagnetic surveys can increase the speed and reliability of geologic mapping in tropical terrains.

No abstract