Assessment of water quality and time accumulation of heavy metals in the sediments of tropical urban rivers: Case of Bumbu River and Kokolo Canal, Kinshasa City, Democratic Republic of the Congo

The aim of this study is to assess the status of tropical urban rivers contamination by heavy (toxic) metals and to evaluate the potential human and environmental risks. In this study, water and surface sediment samples from Bumbu River (BR) and Kokolo Canal (KC, named Jerusalem River) draining Kinshasa, the capital city of the Democratic Republic of the Congo were collected during the period of 2015-2017 from the same sites to assess the status of rivers contamination by heavy metals. Water samples were characterized for temperature (T), pH, electrical conductivity (EC) and dissolved oxygen (O2). The concentrations of heavy metals including Cu, Cr, Zn, Cd, and Pb in sediment samples were determined using Inductive Coupled Plasma-Mass Spectroscopy (ICP-MS). The Hg analysis was carried out using Advanced Mercury Analyzer (AMA). The evaluation of the pollution degree was based on Sediment Quality Guidelines (SQGs) as well as probable effect levels (PELs), Enrichment Factor (EF) and Geoaccumulation Index (Igeo). Spearman's Rank-order correlation and principal component analysis (PCA) were used in order to investigate the potential source of contaminants. The results show the high seasonal variability of the water physicochemical composition; for example, pH values varied significantly during the three years and ranged from 4.49-8.24 and 7.05-7.81 for BR and KC, respectively. The EC values ranged between 197-396 and 198-614 µS cm-1 for BR and KC respectively. The highest concentrations of heavy metals in sediments reached the values of (in mg.kg-1) 190.2 (Cu), 58.6 (Cr), 1117.9 (Zn), 1.2(Cd), 149.2 (Pb), and 11.8 (Hg). The results indicate high variability in metal concentrations in surface sediments according to sampling period and sites. In general, all metal concentrations showed a gradual increase with time, e.g. Cd and Pb consistently increased during the three sampling years. All analysed metals were above SQGs during the year 2017 and 2 to 4 times higher than PELs. EF and Igeo showed severe enrichment to extremely severe enrichment, and moderate to extremely polluted by Zn and Pb. Correlation analysis suggested that the metal contamination in the studied sites is caused by the anthropogenic activities as well as the influence of urban effluents discharge into the river receiving systems. The results of the present study will be helpful to assess and intervening effective management strategies at the local input source pollution level and similar environments