Abstract:
Assessing the sediment contamination with heavy metals; which indicates the contamination
of fresh water ecosystem is one of the challenges strongly influence the sustainable
development and ecosystem health in Ethiopia in general and in the central part of Ethiopia
in particular. The main objective of this study was to investigate sediment contamination with
heavy metals around Holeta and Sebeta demolished floriculture farms of central Ethiopia.
The sediments from 30 sampling points were acid digested and the level of selected heavy
metals: Cd, Pb, Cu, Zn, Ni, Cr, Co and Mn were determined by Flame Atomic Absorption
Spectroscopy (Agilent technologies 200 series AA). Heavy metal pollution and their
ecological risks in the sediment were assessed by Contamination factor, Pollution load index,
geo-accumulation index and potential ecological risk index. The average metal
concentrations (mg/kg) in the sediments were increased as follows: Cu<Co<Ni<Cr<Zn<Mn
in both dry and rainy season. The spatial concentration (mg/kg) ranges of the trace metals
were 3.1-34.8 for Cu, 1.9-71.4 for Co, 26.2-146.4 for Ni, 12.9-148.2 for Cr, 57.56-121.32 for
Zn and 663.4-3579.4 for Mn. Except for Mn, the average concentrations of all selected heavy
metals in the rainy season were higher than the dry season. In both seasons, almost the
calculated geo-accumulation values indicated that the sediments from the 30 sampling points
were found to be in the class 0, thus which observed in the uncontaminated standard level.
The mean potential ecological risk index value for the study area was 11.27 for the dry
season and 18.53 for the rainy season, indicating low ecological risk of the sediment with
heavy metals. The contamination factor values of all selected heavy metals in the two seasons
were observed between low contaminations to considerable contamination. In the rainy
season the values of pollution load index were observed greater than 1(one) at many
sampling points which indicates sediment was observed n the polluted standard level with
heavy metals. The comparison of the mean concentration of examined heavy metals with
sediment quality guidelines were reflects that, the sediments were moderately polluted to
heavily polluted with Ni and Cr in both dry and rainy seasons. These pollution status in study
area would not stay static, as floriculture activity continuous and waste control mechanisms
have less attention, more pollution was emerging; even the activity is finished, residues of
wastes could be contaminate the stream ecosystem as a whole. To establish sustainable
conditions for the society around the demolished floriculture industries and reduce ecological
risks, an immediate plan for mitigation and remediation and the periodic monitoring are must
be carried out.
