Spatial and temporal water quality trend analysis using sediment cores and water samples from Aba Samuel Lake, south west of Addis Ababa, central Ethiopia

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Addis Ababa Universty


The first objective of this work is to assess the temporal variation of Cu, Zn, Pb, Cd, Cr and Ni in sediments of the Aba Samuel Reservoir Lake, south west of Addis Ababa. Two sediment cores with 44 and 49 cm length were recovered at the proximity of the dam in the south (Core 1) and at the western shore (Core 2). Sediment slices have been taken from top, middle and bottom parts of the cores for analysis. Sedimentation rate was estimated using constant rate and therefore by linear extrapolation using the age of the dam as a base line (68 yrs). Considering 4m average depth of sediment over the span of time the dam construction took, an average sedimentation rate of 6cm/yr has been calculated. The core bottom sediments contain therefore the history over the last 7 years. The result of the analyzed heavy metals shows a general increasing trend from the bottom to the top samples in the cores. The amounts currently obtained, except for Nickel and copper, are still below the standard value. However, the value of all analyzed metals will be higher than the standard if the current concentration is to be projected towards the next 7 years. Nickel concentration is higher than the standard value for both sediment cores. Comparing the concentrations in the respective sediment cores, sediment core 2 has an average value of 51.4 mg/kg and sediment core 1 contains 42.5 mg/kg on average. This shows the slight spatial variation within the lake. The variation could be due to differential trapping along the path of the water and the closeness of sediment core 2 to the inlet point of the lake than sediment core 1. Nutrient analyses have been conducted for the lake water. Nitrate and phosphate show decreasing trends from the inlet towards the outlet of the lake; this could be due to the uptake of these nutrients by water hyacinth and the settlement of nutrients along with sediments. While, ammonia increases from the inlet to the outlet, possibly owing to the prevalence of a reducing environment which results in the formation of ammonia. From this study it was possible to observe that, the pollution load in the Aba Samuel Lake is increasing. This confirms the importance of anthropogenic load becoming important in the background of possible constant natural supply. Therefore, proper monitoring of pollutants in the lake environment is crucial.



Spatial and temporal water quality