Trace metal distributions in the sediments of the Little Akaki River, Addis Ababa, Ethiopia

Abstract

The levels and distribution of trace metals (Cr, Mn, Co, Ni, Cu, Zn, Cd and Pb) at eleven water and sediment stations on the Little Akaki River (LAR) in Addis Ababa, Ethiopia, were determined. The binding forms of the metals in various geochemical fractions of the sediments were also quantified. The molar ratio of the sum of the simultaneously extractable metals (∑SEM) and acid-volatile sulphide (AVS)-as a measure for predicting metal-induced toxicity-was estimated. LAR trace levels in water for Cu, Zn, and, particularly Mn were, in most instances, higher than the recommended guidelines for healthy aquatic ecosystems. Total trace metal (TTM) contents in the LAR sediments at certain stations exceeded "threshold effect concentrations" and even "probable effect concentrations", especially in the cases of Zn, Cu, Ni, Pb, and at all stations for Mn. This became more apparent after applying "normalizations" to the relatively lower TTM adsorption capacities of coarse-grained, organic-poor sediments. Sequential extraction of the sediments showed that trace metals generally have a higher affinity for Fe-Mn oxide and organic matter/sulphidic fractions, followed by the residual fraction. Mn was relatively strongly bound to the exchangeable, carbonate bound fractions, whereas a large proportion of Cr was found in the residual fraction. The Σ[SEM]/[AVS] ratio pointed to potential metal-induced toxicity of sediments collected from seven out of the eleven stations. The results indicate that trace metal pollution pose risks to the health of ecosystems, and to human communities that use the river for a range of different purposes.

Soil Organic Carbon and Carbonates are Binding Phases for Simultaneously Extractable Metals in Calcareous Saltmarsh Soils

“…For Ni and Cu, the PCA results suggested a behavior not dissimilar to that of Zn. Nickel was, in fact shown to be weakly bound to AVS (Peng et al ), and its distribution among binding phases was reported to be similar to that of Zn (Akele et al ). The PCA analysis confirmed the strong affinity of Cu for calcium carbonates, on whose surface sorption processes occur rapidly (Compton and Pritchard ).…”

Estimating combined loads of diffuse and point-source pollutants into the Borkena River, Ethiopia

“…However, the trend of decreasing heavy metal concentration with increasing grain sizes was not straightforward (Figure 3 To account for the effect of OM and grain-size distributions in the content of heavy metals on sediments, it is important to normalize the variation based on these factors. Applying a normalization procedure as used in The Netherlands, comparing with a "standard sediment" with 25% fraction of < 63 µm and 10% OM content, yields results accounting for variation of heavy metals content owing to all existing ranges of grain-sizes and organic matter (Akele et al, 2016;Department of Soil Protection, 1994). This changes the estimated toxicity effect Preventing sustainable development: policy and capacity gaps for monitoring heavy metals in riverine water and sediments within an industrialising catchment in Ethiopia 57 (Table 3.4.…”

“…Several studies report that the heavy metals concentrations in rivers, pose high risks in many sub-Saharan countries (see Table 3.6.). As the dilution capacity of the rivers to pollutants is diminishing in the face of intensifying consumption of river water for irrigation, storage and global climate change (Abdel-Satar et al, 2017), the deterioration of the rivers and sediment quality is widespread (Zinabu et al, 2017a;Akele et al, 2016). A critical assessment of heavy metals and their impact on river water quality are thus pressing needs.…”

“…Industrial effluents containing heavy metals, and their accumulation in sediments and biota, present a persistent threat to ecosystems health (Xu et al, 2014;Kelderman, 2012;Jining and Yi, 2009;Gaur et al, 2005). This holds also for sub-Saharan African countries, where regular monitoring is limited (Ndimele et al, 2017;Akele et al, 2016). Thus, identifying effluent concentrations and discharge management are of increasing importance if environmental risks and hazards are to be addressed (Rudi et al, 2012).…”

“…Ethiopia provides a striking example of how high pollution risk from multiple point sources and farming remains unmeasured. Despite Government awareness of potential impacts from pollution, there is limited action to protect human or ecosystem health (Akele et al, 2016;Aschale et al, 2016;Beyene et al, 2009b).…”