GIS integration of electrical resistivity, remote sensing and hydrogeological data to determine groundwater potential in Kiambu Area, Kenya

Abstract

Groundwater is inextricably linked to surface water in the water cycle as it maintains wetlands and river flow during dry spells and is vital to the maintenance of their rich ecology and biodiversity. If we fail to protect our groundwater quality or quantity, our rivers and springs will also be impacted as these are natural groundwater discharge points. In Kiambu area herein referred to as the "study area" there is a sharp rise in demand for groundwater due to increased establishment of floricultural, horticultural and coffee farms. This has resulted in drilling of additional borehole leading to over abstraction of the aquifers as evidenced from lowering of water table. In addition, human encroachment to catchment areas (forested highlands of Tinganga and Ndumberi) most likely has affected the recharge mechanism thus inhibiting replenishment of the groundwater resources. These myriad problems called for integrated study to understand the groundwater potential. The results will form a basis for formulating policies and guidelines on protection and conservation of groundwater resources. The geology of the study area mainly comprises a succession of lavas and pyroclastics of Cainozoic age overlying a foundation of folded Precambrian schists and gneisses of the Mozambique Belt. An integrated approach of using Geographical Information System (GIS), Remote Sensing, Geophysical survey and the DRASTIC (Depth, Recharge, Aquifer, Soil, Topography, Impact of vadose zone and Conductivity) model procedures as tools were applied to determine distribution of ground water potential areas. Six steps were adopted in the production of the groundwater potential zone map: satellite and ancillary data acquisition, processing and analysis of satellite data, analysis of geophysical (vertical electrical sounding) data, preparing spatial baseline database in GIS, spatial data analysis and integration of results. Using Arc View 3.2 software, different thematic layers of lithology, land use cover, drainage patterns, slope, topography, transmissivity, resistivity measurements and rainfall were input parameters into the model. From the results of the study four groundwater potential zones were delineated ranging from low to very high zones. The very high groundwater potential zone covers the southeast side and grades through high, medium, low potential in the northwestern side. The zones were as follows: (i) very high potential zone covers spatially about 13% of the total area and characterized discharge zones. It occurs as isolated perches within high potential zones (ii) high potential zones covers about 70% of the entire area i.e Kiambu Township (iii) medium zone covers about 15% and characterizes the catchment area (iv) low potential zone are less than 2% and occur as small isolated parts to the northwest of the study area. Therefore, the method adopted for this study proved very successfully in meeting the objectives set and is an improvement to the methods previously used in delineating groundwater potential that focused on single parameter independently.