Groundwater Investigation And Characterisation In Marigat Area, Baringo County, Using Vertical Electrical Sounding Resistivity Surveys

ABSTRACT

Marigat area, located in Baringo-Bogoria basin is a semi-arid part of the eastern Rift valley experiencing limited supply of potable water. Groundwater in this region is unexploited due to challenges of undefined nature of fault lines and presence of underground geysers. This study was carried out with the aim of investigating the groundwater potential and to characterise water bearing formation in Marigat area, Baringo County using resistivity method. Vertical electrical sounding (VES) method was applied using Schlumberger electrode configuration to determine the vertical variation of resistivity with depth and to delineate probable aquifers that can be developed into productive boreholes. A total of 28 VES points were probed along five Horizontal Electrical Profiles (HEP) within an area of about 19.2 km2 using an ABEM SAS terrameter 1000/4000. The collected data was analysed using IP2WIN software and Surfer 8 Golden software which revealed the presence of 3-6 interpretable geoelectric layers which were categorized into three inhomogeneous formations corresponding to the existing borehole data within the study area. The first formation is an unsaturated top alluvial deposit with resistivity ranging from 2.49 m to 258 m and thickness ranging from 0.284 m to 44.1 m. The second formation which is slightly weathered and fractured rock has resistivity varying from 0.77 m to 71.5 m while the thickness varies from 4.33 m to 63.2 m. The third formation is characterized by fresh and weathered basement of compact basaltic rock with resistivity values varying from 0.0685 m to 6979 m and thickness ranging from 25.1 m to 52.1 m. Two of the soundings were carried out near existing boreholes in which pumping tests had been carried out. Dar Zarrouk parameters were computed and used to estimate the aquifer hydrologic properties. It was found that the transmissivity values obtained range between 13.569 m2 /day – 1429.052 m2 /day while the geothermal gradient determined using Salabani borehole data (C6362) was found to be 239.730C/km. The results of this study shows that groundwater potentials along the sedimentary basin is good for development at shallow depths ranging between 35 m – 50 m located at the mid-central and the stretch towards south east of the study area while the basement rock with low resistivity values and high geothermal gradient is good for geothermal exploration. Based on the geological setting and the resistivity results of this study, it is highly recommended that chemical analysis for potable groundwater should be carried out after drilling in order to ascertain on its quality.