APPlication of Recharge, Aquifer Media, Vadose Zone and Hydraulic Conductivity on Modeling Vulnerability of Groundwater To Pollution In Central Rift Basins, Kenya

Abstract

Four parameters involving Recharge, 6quifer media, Impact of the vadose zone, and hydraulic Qonductivity out of the possible seven required by the DRASTIC model have been used to show variation of intrinsic properties of the subsurface and their effect on vulnerability of groundwater to pollution in the Lakes of Nakuru, Elmentaita and Naivasha basins referred as the Central Rift Valley of Kenya. The purpose is to protect groundwater from pollution and manage susceptible areas from anthropogenic effects that might lead to the deterioration of groundwater quality. Modeling has been done using the DRASTIC model approach by employing Arcview 3.3 and Spatial Analyst 2.0 extension. Results show that the two factors which mainly contribute to recharge variation are the amount of the annual rainfall and the distribution of the faults distribution within the rift. The aquifer is seen to be recharged mainly in the eastern part of the study area due to the high rainfall compared to the centre of the rift. The area is arid to semi arid with higher evapo-transpiration rates than rainfall. The presence of faults lines is mainly in the central northern side of the area. This place provides significant recharge rates during periods of high rainfall especially when moisture potential exceeds the gravitational potential of the geologic materials thereby bringing about the infiltration and subsequent percolation of water into the aquifers. The susceptibility of the aquifers to pollution in the areas is generally high in the eastern and northern parts of the area. The contribution of fault lines to vulnerability to pollution is high to the north east of Lake Elmentaita. " The susceptibility of aquifer to pollution is mainly restricted to localities in the Naivasha region to the western, eastern and the northwestern parts and to the Aberdares near the Kipipri and 01 Kalou. This is mainly due to the aquifer being mainly of sand, basalt and Lake beds and gravels. Clays are the weathering products of the pyroclastic rocks and the tuffs in the Mau, the western and southern parts of lake Nakuru where it makes up the aquifer media in those localities and thereby have low susceptibility to pollution. Vadose zone variation is almost similar to the aquifer media variation except that the clays are absent in all the strata. Lake beds, gravels and the sediments are mainly in the Ndabibi Maiella and the neigbouring localities are surrounded by the sandy and basaltic aquifers. The greatest portion 11 of the area is covered by rocks of igneous origin which range from rhyolite, ignimbrite, phonolite and pyroclastics. The igneous unsaturated layers are mainly in the Mau escarpment, Njoro, Dundori, Gilgil and part of Narok district to the southwest of the locality. The area is dominated by pyroclastic material and, as a result, the hydraulic conductivity is generally low. The area to the north of the study area has higher vulnerability indices due to the predominance of fluvial material. In the areas where the geology is dominated by basalts, the hydraulic conductivity is also high. The hydrogeological zones vulnerable to pollution are few and widespread.