Hydrogeochemistry of groundwater in Nairobi area

Abstract

The results of hydrogeochemical analysis of Nairobi borehole waters are presented. Seventy-eight boreholes, and two shallow-wells were sampled in this study, and seventeen physico-chemical parameters were determined. Using computer techniques the overall distributions of the physico-chemical parameters were plotted. The distribution patterns have been interpreted in terms of geology, of water types, and of human influences. The chemical data obtained from the study area were subjected to statistical analysis. The results of the physico-chemical parameters of the groundwater analysis of the study area indicate that the dominant cation is Na^ and the dominant anion is HCO3' alkalinity. The results from the descriptive statistics for all the hydrochemical parameters of the study area reveal that the Na+ has the highest mean for the cations whereas; HCO3' has the highest mean for the anions. The results obtained from the statistical correlation coefficients of all physico-chemical parameters of the groundwater of the study area (Nairobi) indicate a positive correlation between Na+, and HCO3' ions The results of the principal component analysis / factor analysis (PCA/FA) of the physico-chemical parameters of the groundwater of the study area indicate at a significant and positive correlation among Na*, and HCO3' ions. The results from a plot of the concentration of the major constituents on the Piper (Trilinear) diagrams have revealed that the Na+, and HCO3' as dominant ions, and thus the water of Nairobi area may be termed as Sodium-bicarbonate waters. Therefore, Na~ and HCO3' alkalinity are the prevailing major components, which are determining the groundwater chemistry in Nairobi. Groundwater flow within the study area has been determined by the use of piezometric water surfaces within the boreholes. The flow direction of groundwater is roughly NW-SE direction and is the same direction as the surface water and is controlled basically by the geology and topography. A lot of groundwater recharge and discharge areas were established within the study area, which could be attributed to faulting activities, which affected the western part, the area. Quality aspects of the water resources have been discussed in relation to use or intended purpose to which the water is to put for. The groundwaters of the study area were generally found to be suitable for most purposes, domestic and industrial, but not good for agricultural activities, when compared to the international standards.