Determination of Heavy Metal Levels in Surface Waters and Sediments of River Riana, Kisii County, Kenya

Abstract

In this paper, some trace metals were seasonally determined for six months between January -June 2021 (to cover both the wet and dry seasons) in surface water and sediments from River Riana in Kisii County.  The river is an important source of drinking water and fisheries and receives an array of wastes from agricultural, industrial, commercial and sewage treatment. Consequently, this study evaluated the levels or concentrations of heavy metals (Pb, Cr. Ni, Mn, Cu and Zn) in surface water and surface sediments and the effect of seasonal variations on their concentrations. Samples were collected monthly from three sampling sites along the River and the results of heavy metal concentrations were compared with national and international guidelines including the Kenya Bureau of Standards, World Health Organization (KEBS/WHO) and WHO sediment quality guidelines (SQGs) respectively to establish pollution level. Metal concentrations in water and sediments were analyzed using inductively coupled plasma atomic emission spectroscopy (Shimadzu ICPE-9000) after nitric acid digestion. The results obtained showed that the mean range of heavy metals in water (ppm) were: Pb (0.031-0.196), Cr (<0.001-0.009), Ni (0.014-0.321), Mn (0.131-0.351). Cu (0.114-0.370) and Zn (0.132-0.326) while the mean range of metals in surface sediments (mg/kg) was; Pb (0.041-8.74), Cr (0.001-1.31), Mn (276-692), Cu (1.35-9.74) and Zn (6.37-15.67) respectively. The heavy metals Cr, Cu and Zn complied with the WHO and KEBS drinking water recommended safe limits while the Pb and Mn concentrations exceed the recommended safe limits but Ni concentrations exceeded KEBS recommend safe limit for drinking water. The mean monthly concentrations for the heavy metals in surface water and surface sediments for the River decreased in the order Mn>Zn>Cu>Pb>Ni.>Cr and Mn>Zn>Cu>Ni>Pb.>Cr respectively. The heavy metal concentrations were generally higher during the dry season than the wet season in both surface water and sediments however, there were no significant seasonal differences (p>0.05) except for Cr and Mn in water. All the heavy metals complied with WHO fresh water SQGs and exhibited no significant seasonal variations (p ≥0.05) in water for surface sediments. The River Riana was contaminated by all the determined trace metals and this indicated potential toxicity risk, thus the need for regular monitoring. We recommended determination of other toxic metals in water, sediments and biota such as fish form this river