Assessment of Heavy Metal Pollution in Water and Sediment of River Thiba, Kirinyaga County, Kenya

Abstract

Trace elements find their way into humans through ingestion, direct absorption or inhalation. All trace metals are toxic to animals and plants when present in excess amounts. The harmful effects of trace heavy metals in mammals may manifest as growth retardation, decrease in longevity, changes in reproductive cycles, chronic diseases and tumour formation. River Thiba catchment is Mt. Kenya forest and then flows through rocks, soil, farmlands, residential areas and town centres. Due to geological factors and human activities, trace heavy metals may be getting in river Thiba and since the water is used for domestic purposes and irrigation, it was necessary to determine the heavy metal concentration in the river. Sediment and water samples were collected along river Thiba during rainy and dry seasons. The samples were digested then concentrations of eight heavy metals determined using ICP-MS. The mean amounts of Cd, Cr, Ni, Pb, Zn, As, Mn and Se in sediment were 0.0908, 39.8969, 85.1085, 10.3918, 42.5555, 2.3679, 1678.3876 and 5.4907 mg/kg respectively during rainy season and 0.0628, 42.6319, 396.4692, 3.2669, 58.7585, 2.8139, 1766.4009 and 6.1059 mg/kg respectively during dry season. The mean concentration of the same metals in water was 0.0002, 0.0916, 1.0066, 0.0434, BDL, 0.0025, 1.8484 and 0.0038 ppm respectively during wet season and BDL, BDL, BDL, 0.0068, BDL, 0.00002, 0.0070 and BDL ppm respectively during dry season. The mean amount of Cr, Ni and Mn in sediment were found to be above WHO and US EPA permissible limits during both seasons. During rainy season, the mean concentration of Cr, Ni, Pb and Mn in water were above WHO and KEBS/WASREB permissible limits but the concentrations of Cd, Zn, As and Se were below the limits. However, during dry season, all the eight heavy metal concentrations in water were below WHO, KEBS/WASREB permissible limits. As far as the eight heavy metals are concerned, water from river Thiba may not be potable during rainy season but is potable during dry season. However, further investigations should be carried out to determine other water parameters.