Distribution of Heavy Metals in Water and Sediments of Lower River Nzoia

Abstract

Heavy metals are major pollutants of aquatic systems due to their bio-accumulative nature. Overreliance on agro based farm inputs to improve agricultural yield, discharge of industrial waste and lack of proper sewerage system within the towns that the lower section of Nzoia River passes through has resulted to an increased amount of pollutants including heavy metals being deposited into the river. With the water from the river being used for irrigation purposes, drinking by human beings and livestock as well as recreational areas, concerns have been raised on the suitability of the river to serve the above purpose as well as the safety of the produce that depend on the river. This study aimed at investigating whether the waters and sediments along lower Nzoia River have been contaminated with heavy metals. In this study, sediment and water samples were collected and analyzed. Sediment samples were collected at depths of 30 cm, 50 cm and 100 cm on either side of the river at thirty sampling sites to give a total of 180 samples. Water samples were also collected at both sides of the river at the exact locations sediments were collected to make a total of 60 water samples. The collected water samples were analyzed using TXRF whiles the sediment samples were analyzed using EDXRF. Trends in variation of concentrations with depth were evaluated as well as observing the trends in the concentration of metals downstream. The mean concentrations (mg kg-1) in sediments ranged: Mn (520 – 2060), Fe (2.7% – 8.3%), Cu (250 – 390), Ni (260 – 510), V (150 – 380), Cr (85 – 320), Zn (40 – 100), Pb (7 – 30) and As (10 – 15). Metal concentration levels in sediment samples increased downstream with a significant variation (p < 0.05) being observed between the sampling points. The mean concentrations of Mn, Fe, Cu, Cr and Ni were above the USEPA guidelines; Mn (30), Fe (30), Cu (16), Cr (25) and Ni (16). Metal concentrations in water samples were largely below the WHO guidelines except for copper that had higher amounts indicating that the waters are polluted with copper; Mn (100), Fe (2000), Cu (20), Zn (20), Pb (100) and Ni (20). The mean water concentrations reported in μg l-1 were Mn (78), Fe (6144), Ni (13.7), Cu (26.5), Zn (161) and Pb (11.2). Lower river Nzoia sediments were determined to be contaminated with Mn, Fe, Cu, Cr and Ni with the concentrations in sediments being higher than those reported for water