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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Date
2007-07
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Addis Ababa Universty
Abstract
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.
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Keywords
Spatial and temporal water quality