Hydrogeochemical Analysis And Modelling Of Ground Water In Mbeere South Sub-County, Embu County, Kenya

ABSTRACT

Clean water is an essential commodity for a healthy nation. A fifth of all water used in the world is obtained from groundwater. In arid and semi-arid areas, groundwater is the main source of water for domestic and irrigation purposes. The semi-arid climate, few perennial water bodies and growing population in Mbeere South, Embu County has resulted into increased use of groundwater for both domestic and irrigation necessities. This has put pressure on these water sources leading to over-pumping. Excessive pumping may draw waters with different composition towards the aquifers, resulting to elevated levels of some minerals. Excess minerals in groundwater makes it unsuitable for domestic and agricultural purposes. For instance, high levels of nitrate in drinking water can cause ‘blue baby syndrome’ and gastric cancer while excess fluoride can cause dental and skeletal fluorosis. To investigate the hydrogeochemical characteristics of groundwater in Mbeere South, a total of 10 boreholes drilled at different years from different administrative locations were sampled. The samples were analysed for physico-chemical characteristics such as pH, temperature, totaldissolved Solids and major ions using standard analytical procedures. The chemical analysis results were then subjected to PHREEQC, Origin pro and AqQA computersoftware’s for modelling to obtain saturation indices, water types and hydrogeochemical processes respectively. The results showed that the dominant cations were Ca2+ ˃ Na+ ˃ Mg2+ ˃ K+ > Fe2+ while the dominant anions were HCO3-˃Cl->SO42->NO3-˃F. The hydrogeochemical facies indicated that 40% of the samplesbelong to Ca-HCO3 water type and 80 % to mixed water type. The facies further illustrated simple dissolution, mixing of the waters, anthropogenic activities and ion exchange as the major processes controlling ground water chemistry in the region. The Saturation Indices indicated that the samples were slightly saturated with aragonite, calcite and dolomite; slightly under-saturated with anhydrite and gypsum, moderately under-saturated with halite and under-saturated with hydromagnesite, trona and portlandite. This shows that the water chemistry in the area is significantly affected by carbonate minerals, i.e. calcite, dolomite and aragonite. The physicalchemical results were also compared with the WHO and East Africa standards (EAS 459-1:2007), in order to assess the suitability of the groundwater for domestic use. The findings showed that Kiambere H. C. (site A), Kinyaga (site F) and Kaseve (site I)boreholes had concentrations of Ca2+, Mg2+, Na+, F-, Cl-, Mg2+, `K+, Na+, Cl-, NO3-and Mg2+ and NO3- respectively much higher than the WHO and EAS standards hence  not suitable for domestic purposes with respect to these parameters.