Characterization of the aquifer suite within Nairobi County and its environs with respect to the chemical composition and depth.

Abstract

Chemical characteristics of natural waters depend on several factors such as the lithology of the geological strata in which groundwater is flowing, time of residence of water and environmental conditions. Normally, small number of substances constitute the chemical composition of water (major ions), but other ions (minor ions) can also be found in low concentrations. Due to interactions with the atmosphere, soils, bedrocks and the surfacial environment, groundwater can dissolve a wide range of elements that expose it to a widely dynamic environment. With regards to geochemistry, it tends to dissolve more of the mineral elements than surface water. The study area is geologically sandwiched between a massive Neoproterozoic basement, which outcrops, to the East and the highly elevated volcanic formation to the West, representing Lukenya and Ngong’ hills respectively. It forms a huge depression right at the eastern edge of the Great Rift Valley. This depression was filled with Cainozoic volcanics and sediments (Old land surfaces and lake sediments).This dissertation gives a detailed account of analyzed different sets of data from various sources including aerial photographs, borehole data and, most importantly, the chemical analysis. Fluoride, Sodium, Chloride, Iron, Nitrates, Sulphates and Manganese are considered ideal for discussion. Since the scope of this study is confined to the critical chemical analysis with direct influence to human health, other factors/chemicals are considered non-significant having no maximum permissible limits. These include Free Carbon dioxide, Permanganate Value, Total Dissolved Solids, Total Alkalinity, Potassium, Magnesium and Calcium. Generally, extreme fluoride levels as analyzed, tabulated and represented in graphical format in the results section was sampled from The Basement – Kapiti phonolite series aquifer characterized by intercalated lake sediments near Mozambiquan basement dome around Syokimau area. Apart from the Kapiti lake sediments, Fluoride is also detected above the acceptable limit in both the Upper and Middle Athi series. The presence of sodium in a number of these aquifers: Athi series and Kapiti series gave a new angle to the source of the highly concentrated fluoride within these specific aquifers. Iron and Manganese detected in both the Athi series and the Kapiti aquifers increases towards western part of the study area. Safer regions as delineated by this study can be towards the North Eastern areas though minerals that were detected above the WHO guideline are Fluoride, Sulphate and Iron in a number of aquifers.