Heavy Metal Levels in the Water and Sediment Samples of Dire and Aba-Samuel Reservoirs,Ethiopia
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Date
2019-05-05
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Addis Ababa University
Abstract
Dire is among the main sources of drinking water supply for Addis Ababa city while Aba-Samuel Reservoir is a multi-purpose reservoir for the nearby residents. However, the ever-intensifying anthropogenic activities in the catchment have increased the potential pollutants of these reservoirs. Of all the contaminants, heavy metals are non-degradable, can bio-magnify along the food chain and are probably toxic to humans and aquatic biota. Therefore, there is a need for continuous monitoring of the pollution levels in the reservoirs as the assessment provides evidence-based data to protect public health. The concentrations of selected heavy metals in water and sediment samples were collected and determined from three sites in each reservoir (Inlet; Site1-S1, Center; Site2-S2 and Outlet; Site3-S3) for three (3) consecutive months (April to June 2018). Physicochemical parameters (DO, temperature, EC, and pH) were measured at the established three sampling sites using portable Multimeter, while turbidometer was used to measure turbidity. The collected water and bottom sediment samples were analyzed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) for selected heavy metals (Cu, Zn, Mn, Cr, Cd, Hg, Pb, and As). The data generated from the present study were statistically analyzed using Analysis of Variance (ANOVA). Physicochemical parameters except for temperature in Aba-Samuel Reservoir varied spatially although the differences in their levels among sampling sites were not significant (p>0.05). Among the physicochemical parameters measured in the field, only turbidity surpassed the acceptable limit set for drinking water by different organizations. The mean concentrations of heavy metals in bottom sediments (mg Kg-1) were higher than those in subsurface water samples (mg L-1). The highest concentrations of all metals measured in water and sediment samples were recorded for Mn in both Dire (0.236±0.014, 1098.90 ± 13.25) and Aba-Samuel (0.504±0.023, 1198.39± 6.85) reservoirs, respectively. Mean concentrations of all metals except Mn in water samples of Dire Reservoir were not significantly different among sampling sites (P>0.05), while the reverse was true for those of Aba-Samuel Reservoir. Some heavy metals in sediment samples of both reservoirs (e.g. Mn, Cr, and Pb) showed significant variations (P<0.05) among sampling sites while Zn showed spatial variations, which were not significant (P>0.05). The mean concentrations of heavy metals in water samples were all below the guideline values set by WHO (2008) and USEPA (2011) with the exception of Mn in both Reservoirs and Cd in Aba-Samuel Reservoir. Concentrations of Zn, Mn, Cr, and Cd in sediment samples are, however, above the respective reference values (ISQG, 2002; USEPA, 2010), while those of Pb and Cu were below the respective reference values (ISQG, 2002; USEPA, 2010) in both reservoirs. The results of the present study signify the importance of anthropogenic loading of pollutants and serve as an early signal for the need to take a timely measure to prevent further degradation of the reservoirs. Because the reservoirs are shallow and polymictic, the high concentrations of heavy metals in the sediment can lead to their increased levels in the water column thereby affecting public health and aquatic biota. Therefore, strategies of controlling point and non-point sources located all over the catchment areas should be developed to ensure better protection of the reservoirs, public health, and aquatic and terrestrial life.
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Keywords
Physicochemical Parameters, Public Health, Tropical Reservoirs, Water Pollution