GEOLOGICAL AND STRUCTURAL CONTROLS ON HYDRAULIC AQUIFER
PROPERTIES AND GROUNDWATER DISTRIBUTION IN ISINYA AREA OF
KAJIADO COUNTY, KENYA
ABSTRACT
This research investigated the geological and structural influences on hydraulic aquifer
characteristics and groundwater distribution in Isinya area of Kajiado County, Kenya.
Groundwater is an essential resource in this area and this importance increases in the light of the
rapid increase in human population, industrial expansion, and agricultural activities. The geology
of the area is comprised of Pleistocene volcanic rocks, which include Kapiti phonolite and the
Upper Athi series, and the Precambrian metamorphic rocks. These are overlain by recent
surficial deposits and soil.The soil retains water for an extended period of time after the rains and
this partly recharges aquifers in the area.The influence of faults, joints and other fractures on
groundwater in the study area is two way. They act as drainage channels of groundwater flow
and also as aquifers in the area. The area receives groundwater recharge from Ngong’ hills and
Kikuyu springs which are to the South west and flow is towards the lower areas to the North
East.
The project involved determining these geological and structural controls by analyzing the
available borehole data from the area and producing a ground water potential map with respect to
the same. The different lithologies in this area form aquifers with differing hydraulic
characteristics. In quartz-feldspathic gneiss, the aquifers is has lower permeability as compared
to the volcanic layers. Boreholes drilled in these rocks have very low values of transmissivity
and therefore for regions of low groundwater potential. In the Upper Athi Series, The boreholes
have high transmissivity values indicating high groundwater potential. In the Kapiti Phonolite,
two scenarios are observed. The area around Isinya town where the rocks are highly fractured
show high values of transmissivity. However, in the north-western part of the area where
fracturing is limited, low transmissivity values are observed.
From the groundwater potential map produced during the data analysis part of this research, the
area surrounding Isinya town has high groundwater potential. This property extends northwards
towards essenya. Areas of low groundwater potential include the northwestern region and part of
the southwestern region where the lithology is quartz-feldspathic gneiss.
Generally, groundwater is fairly distributed in Isinya area and it’s rare to site a dry borehole.
However geology and geological structures should be taken into consideration while exploring
for groundwater. This reduces the chances of losses associated with drilling dry boreholes and
helps in proper groundwater and aquifer management for a sustainable supply.