Characterization of groundwater aquifers in a structurally complex region: - a case study of West of Lake Naivasha Area

Abstract

Climate change, groundwater abstraction, horticulture and geothermal activities have influenced the volcanic aquifers within and around Lake Naivasha area. In the previous studies, the effects of faults on varying groundwater quality in different areas and the relationship between surface and groundwater has not been well established within the study area. The main objective of this study was to characterize groundwater aquifers west of Lake Naivasha based on aquifer properties, structural controls on groundwater flow, as well as understanding the relationship between surface and groundwater interactions in context of water quality. In order to achieve this, hydraulic conductivity of groundwater system and aquifer transmissivity were determined to provide an understanding on the extent of localized aquifers. The aquifer transmissivity values are high in the northern parts of Lake Naivasha (1447-2652 m2/day) due to high rates of groundwater abstraction, moderate within Ndabibi Plain (138-1042 m2/day) and low in the southern and Kiambogo areas (<138 m2/day). Four out of nine faults identified influences groundwater flow while the rest of the faults are permeable to groundwater flow. The general flow of groundwater is toward southwest. From the piezometric heads, there exist a water divide between Lake Sonachi and Lake Naivasha hence the two waters are not hydraulically connected. From electrical resistivity measurements, the aquifers in Ndabibi Plain occur in pools within weathered tuffs/pyroclastics and lake bed sediments. Shallow aquifers occur between 20-60m bgl around Lake Naivasha within lake sediments while deeper aquifers penetrated by the current boreholes range from 167 to 297 bgl. Electrical resistivity measurements identified deeper aquifers around Lake Naivasha between 180 and 240 m bgl which are not penetrated by the current boreholes. The concentration of ions follows the same pattern as the groundwater flow gradient, the northern parts of the study area are less saturated with respect to sodium, calcium and lithium ions compared to the southern parts, where the groundwater gradient is small hence more time of residence. Using lithium ion, two flow direction are realized; one is toward Lake Oloiden and the other around Lake Sonachi in Ndabibi plain. The ratios of Na and Cl of Lake Sonachi (6.7) resembles borehole waters around it (5.97), while River Marmanet, has an average value of 2.79 resembling that of Lake Naivasha (2.91) and possibly may indicate lake water-river water-groundwater interactions. The upgraded groundwater flow map will be useful in tracing the pollutants within the study area while delineated zones of fresh and saline water will enhance sustainable abstraction of fresh water while avoiding areas with deplorable quality.