Use Of Depth To Watertable,soil And Topography In Modelling Vulnerability Of Groundwater To Polllution In Naivasha- Elementaita- Nakuru Watershed Kenya Rift

Abstract

Abstract Vulnerability of groundwater to pollution using DRASTIC model in the Lakes Nakuru- Elementaita and Naivasha basins has been investigated. From the results, depth to water table parameter indicates that 30% of the basin is vulnerable to pollution when considered alone. The areas with the highest index which means shallow aquifers are limited around at the centre of basin and a small portion in the north western part. The general topography conforms to the rift morphology, i.e. with fault planes indicating high slope percentages. The south western part of the basin is highly vulnerable to pollution from this parameter. However, topography is less weighted in DRASTIC with a value of 1. The most vulnerable areas are the immediate north of Lakes Naivasha and Nakuru. Though least weighted, the whole of rift floor is vulnerable. The soil parameter shows varying degrees of vulnerability. Cambisols which are thin soils arc the most vulnerable and are distributed in symmetry with the rift floor on both eastern and western fault plane. The second vulnerable group of soils rated at 9 are the solonetz but are only limited to the western part of the basin. Though andosols cover over 50% of the basin, they are less vulnerable to pollution when compared to the former. The fmal composite model derived from the three parameters of depth to water table, soil and topography depicts the basin to be highly vulnerable to pollution. The rift floor and the eastern Kinangop plateau are some of the areasthat arc highly vulnerable. The solid '·r basaltic rocks ofEburru ridge and Gilgil are however lowly rated. Both Mau and reworked unconfined aquifers around Lakes Naivasha and Nakuru are equally vulnerable and are rated second to the above areas. The composite vulnerability pattern defy the rift morphology and is diagonally symmetrical. The area is highly rated. The north western part of the basin and the south eastern are least rated. Over 50% of the total basin area is highly vulnerable. Physico-chemical results indicate that the pH values are within WHO drinking-water standard of 6.5 to 8.5 for pH. However, trace element manganese, chloride and total coliform bacteria results are beyond the WHO standards. Equally high in concentration is fluoride across the basin.