Determination of the Aquifer and Its Hydraulic Parameters Using Vertical Electrical Sounding, Borehole Log Data and Borehole Water Conductivity: A Case Study of Olbanita Menengai Area, Nakuru, Kenya

摘要

Development of groundwater needs the capabilities to distinguish the different aquifer layers found in a region, and thereafter the parameters which can be used expressly to define the aquifer type. The past studies in the Olbanita sub-basin have accorded the area as having one aquifer, which has resulted into generalization of the aquifer parameters. The objective in this study is to map the main aquifer layer and determine its parameters. The use of modeled geoelectric layers from Vertical Electrical Sounding (VES) data has been used in the study area to distinguish the major aquifer from the minor ones. There is noted an excellent correlation between the geoelectric layers and the lithologies as outlined by the driller’s log clearly delineating four aquifer stratums. The main aquifer is identified to be geoelectric layer 11 and 12, defined by a thickness of 30.18 m mainly of tuffs, and 17.39 m mainly of weathered phonolites. Hydraulic conductivity of the main aquifer averages value of 17.16389314 m/day, in consideration of the ranges 0.248690465 m/day to 74.62681942 m/day for the 31 VES points. For the aquifer breadth of 30.18 m, the Transmissivity values vary from a minimum of 57.32119 Ωm2 to 53365.49 Ωm2 and for 47.57 m breadth, the range is between 11.83021 Ωm2 and 1390.921 Ωm2. The variance of longitudinal unit conductance shows that 63.15 percent of the aquifer represented by one lithology is having lowest values of S (-1), an indication that the resistivity values of these points are relatively high when compared to their corresponding breadths. Notably, where the geoelectric layer is represented by more than one lithologic layer, the longitudinal unit conductance has high values of S (~1.1 - 5.3 Ω-1) at about 83.33 percent of the aquifer, thus giving a manifestation that a change in lithology has an implication in the aquifer characteristics. The transverse resistance values have a direct proportionality to both the aquifer layer thickness and the geoelectric layer resistivities. Evidently, using the close range of resistivities record indicates that indeed transverse resistance increases with increase in aquifer thickness. For example, for resistivities 52.677 Ω, 54.78 Ω, 54.297 Ω, 57.819 Ω, and 51.85 Ω, for 30.18 m, 47.57 m, 136.35 m, 190.84 m, 277.93 m thicknesses respectively, have their correlated transverse resistances values notably rising incrementally, from 1589.7919 Ωm2, 2605.8846 Ωm2, 7403.396 Ωm2, and 11034.178 Ωm2 correspondingly. There is confirmation that the modeled VES data can help map aquifers despite the limited resources of borehole logs that can used as control points.