Hydrogeological Study of the Area North of Menengai Crater Using Electrical Resistivity Survey

Abstract

An integrated study involving geophysical, geological and hydrogeological investigation was undertaken to determine groundwater potential in the areas north of Menengai Crater. Rocks exposed in the study area represent a varied series of lava flows and sheets to superficial sediments and volcanic soils. Lava sheets have their origin from the fissures associated with rift valley faulting and those associated with the eruption of the central volcano (Menengai). Pumicious tuffs and sedimentsowe their origin to the dust and vitric ash which emanated from the explosive episodes of the Menengai caldera. Electrical resistivity sounding studies were conducted using Schlumberger array with a maximum spread of AB/2=400m. to investigate the resistivity structure beneath the study area. The resistivity data analysis was carried out by partial curve matching and fitting prior to use of linear inverse theory in the determination of the most probable physical parameters of the subsurface layers necessaryfor two-dimensional interpretation of the underlying geology. Geophysical and geological data analysis has identified three geoelectrical units with varying thicknesses and depths. These are a top low to high resistive layer, a conductive middle layer and the resistive substratum. From this study it is noted that the geology of the area comprises volcanic rocks and sediments. Sedimentary units intercalated in the volcanic formation are the main source of groundwater. Weathered and fractured trachytes and reworked coarse tuffs provide aquifers in the volcanic formation. Groundwater is also likely to occur at the interfaces (old land surfaces) of the various volcanic flows. The study has also revealed that there is a vast amount of groundwater in the study s; area, particularly in the north, northwest and along the eastern region. However, the abstraction potential is influenced by the swarm of faults and fractures found on the floor of the Rift Valley. Even though the area has been affected by intense faulting, it supports a regional aquifer system. The dominant recharge mechanism is regional groundwater drainage from Bahati forest to the east and Menengai Crater to the south. The flow is supplemented ~ith replenishment from infiltrating river water, Molo and Rongai Rivers to the west, Olobanita.River to the east and Crater streams to t,· , the south. Three groundwater zones have been identified based on aquifer characteristics and the structural set up. These are zone 1, zone 2 and zone 3 in the north-west, east and south-west parts of the area, respectively. It is worth noting that groundwater is distributed over the whole of the area as shown by the zones. However, the best area for future groundwater development is in the north western part (zone 1),where the potential is indicated by high.er average borehole specific capacity.