Hydrochemical Characteristics of Aquifers in Mwingi North - Kenya

Abstract

The arid to semi-arid area of Mwingi North, Kenya, suffers from low access to potable groundwater in part due to salinity, but also due to uneconomical development of groundwater because of the lack of understanding of the types of aquifers and knowledge of their distribution. The main objective of the study was to characterize the groundwater hydro-geochemical composition and also assess the problems caused by high salinity and fluorides in crystalline basement aquifers of Mwingi North. Groundwater sampling was carried out within the environment of Mwingi North for chemical analysis. 42 water samples collected from shallow depth wells (0-20 m), hand pumps (20 -100 m), and deep boreholes (Below 100 m and motorized) were analyzed using atomic absorption spectroscopy (AAS), ion-selective electrode, argentometric, turbidimetric, and titration methods. Parameters analyzed include cations such as potassium, sodium, calcium, magnesium, manganese and iron and anions such as carbonates, bicarbonates, nitrates, nitrites, sulfates, chlorides, and fluorides. Physical parameters that were analyzed included electrical conductivity, temperature, pH, turbidity, hardness, total alkalinity, color, free carbon dioxide and total dissolved solids. Electrical conductivity ranges from 362 μS/cm to 9379 μS/cm, temperatures ranges from 24ºC to 28ºC, and pH varies from 7.55 - 8.76. The concentration of Na+ ranges from 3 - 800 mg/l distributed along E to W region while Mg2+ and Ca2+ have a spatial distribution pattern on an N to S region along the central areas with their concentrations ranging from 11.61 - 641 mg/l and 5.6 -592 mg/l respectively. Cl- ranges from 3 -2320 mg/l with its spatial distribution pattern following N-S direction along the central areas while F- registered minimal highs ranging from 0.26 - 4.65 mg/l. Using the median, concentrations of cations in groundwater increases in the order Na+> Mg+>Ca2+>K+>Fe2+> Mn2+ while concentration of anions increases in the order Cl-> HCO3-> SO42-> CO3->NO3->F-. EC, Na, Mg, Ca, Cl, and F are above WHO’s maximum acceptable level in hand pumps and boreholes. Shallow wells have the least mineralized groundwaters while boreholes have highly mineralized groundwaters. Cation-exchange is the dominant rock-water interaction process. The relationship between Na+ versus Cl- for the majority of the groundwaters, suggests similar hydrochemical processes. Mivukoni and Ngomeni areas have poor quality groundwaters while Mumoni and south of Kyuso have potable groundwaters.