The Geometry, Hydro-geochemistry and Vulnerability of Aquifers to Pollution in Urban and Rural Settings- a Case Study of Kisumu and Mt. Elgon Aquifers

Abstract

Kenya is one of the countries in sub-Saharan Africa considered to have water scarcity, and the national per capita water is below 1,000 cubic meters. The estimated per capita by the Kenya government in 2005 was about 647 cubic meters, and in 2009 it was estimated at 534 cubic meters. Water scarcity in the country is expected to worsen by 2025, when per capita water is estimated to be about 235 cubic meters. This projected low per capita is a serious threat to socio-economic development, the ecosystems, and the efforts to achieve the Government of Kenya blueprint of Vision 2030 and the United Nations sustainable development Goal 6. The low per capita can be mitigated by the proper understanding of the finite freshwater resource in the country. Like in other parts of Sub-Saharan Africa, there is an explicit lack of information on aquifer geometry, water chemistry, groundwater vulnerability to contamination, and the timing of recharge water in both rural and urban settings in Kenya. This study aimed to appraise aquifers’ aquifer geometry, hydrogeochemistry, and vulnerability to pollution in Kenya’s urban and rural settings. The urban aquifer is Kisumu and the rural aquifer is Mt. Elgon. The specific research objectives were; (a) to delineate the aquifer geometry in the selected areas of study and estimation of aquifer properties, including yields, transmissivity, and storativity values through pumping tests data in selected wells, (b) to establish groundwater quality and evaluate how groundwater chemistry was modified by rock-water and surface-groundwater interactions and land uses through analysis of water samples and (c) to determine recharge sources through environmental isotope signatures in groundwater. The existing literature was reviewed and, historical borehole data and current data from selected wells was analysed to determine current knowledge and gaps. The research entailed field surveys to determine geology, in-situ physico-chemical parameters, thermotolerant coliform bacteria, and interviews with the local community to identify the location of existing water points. The study also entailed water sampling and analysis for the determination of water chemistry and isotopic composition. Geological mapping and vertical electrical sounding combined with petrographic, XRD, XRD, and interpretation of VES data using relevant software were used to determine the aquifer geometry. Test drilling for lithological samples, aquifer testing, and analyses of previously available data permitted delineation of aquifer dynamics, generalised transmissivity distribution, and interpretation of the groundwater flow system. Field measurements using relevant field kits were used to measure the physic-chemical parameters of water and bacterial loading in the water. Full chemical analysis of water and isotopic analysis for oxygen-18 and deuterium was done locally and in the United Kingdom. Relevant computer software was used vii to construct the geological maps, statistical plots, chemical, and isotopic plots. The research revealed that the shallow Kisumu aquifer is extremely heterogeneous. Mt. Elgon aquifer is also heterogeneous. In an attempt to understand the hydrogeochemistry of the study areas, the origin and composition of solutes in groundwater sampled from the Kisumu and Mt. Elgon aquifers were charecterised using a range of techniques. Classical graphical methods (i.e., Gibbs, Durov, Piper, Schoeller, Stiff and Ternary plots) were utilised as interpretative tools of the main hydrogeochemical processes. The study identifies three main groundwater geochemical signatures in the Kisumu study area: cation exchange (Ca2+-Na+ and Ca2+-Mg2+) between aqueous and solid phases, the chemistry of recharge water, and groundwater mixing. The concentration of major ions in groundwater varied with geology and also seasonally. The dominant water facies was Na+ - Ca2+ - HCO3- type; other hydrochemical facies include Ca2+-Mg2+-HCO3- and Na+-HCO3-. Hydrochemical plots suggest that the dissolution of carbonates and halite are the other major chemical processes and cation exchange that controls the groundwater chemistry in the Kisumu aquifer. The dominant water type in the Mt. Elgon aquifer is the Na+-HCO3- type. The Kisumua quifer’s mineralisation was more than in the Mt. Elgon, as revealed by the saturation indices. The historical records of δ18O and δ2H in rainfall at Kericho from the IAEA Global Network of Isotopes in Precipitation (GNIP) database were used to plot the local meteoric water line. Stable isotope ratios in groundwater reflect orographic effects on rainfall controlling recharge, a bias to months of heavy rainfall, and evaporative enrichment in heavy isotopes associated with local, convectional rainfall derived from Lake Victoria. The isotopic tracers indicate that shallow groundwater in Kisumu is recharged directly and locally from rainfall, and there is no direct connection between Lake Victoria and groundwater. The study concluded that the current deficiencies in providing adequate water and dignified sanitation to the poor in rural and urban settings could be remedied through improved knowledge of shallow aquifer dynamics, the geological control of groundwater flow isotope hydrology, and innovative water research. Improved scientifically based evidence is the only path to achieving the UN SDG 6 and Kenya’s four development pillars.