Sediment evaluation indices point to cadmium and selenium contamination: A simultaneous analysis of potentially toxic elements in the water and sediment along the upper and middle Awash River, Ethiopia

Abstract

This study presents a simultaneous analysis of potentially toxic elements (PTEs) in the water and sediment of the upper and middle Awash River for the first time. We used standard field and laboratory procedures, sediment assessment indices and guidelines, and multivariate statistical methods to analyze PTEs and assess their possible sources. Water and sediment samples were collected from nine sampling sites, and PTEs were analyzed using the Inductively Coupled Plasma Optical Emission Spectrometer. The PTEs concentrations arranged in descending order in water were Fe>Mn>B>Zn>Cr>Ni>Pb>Cu, ranging between 4.14±2.093 mg L 1 and 0.074±0.016 mg L 1 . The level of PTEs in the sediment varied between 24,493.6±16,640.3 mg kg -1 and 0.4±0.2 mg kg -1 and follows the order: Fe>Mn>Cr>Zn>Pb>Ni>B>Se>Co>Cu>Cd>Hg>As. The river-mouth of the upper Awash had the highest concentrations of Mn, Ni, Cr, and Cu in water and Fe, Mn, Cr, Ni, Pb, Co, Se, Cu, Cd, and AS in the sediment. Contamination factor, enrichment factor, and geo-accumulation index showed a very high level of contamination of Se and Cd. The ecological risk index, Nemerow pollution index, and pollution load index also indicated a high ecological risk and pollution by PTEs in the river. The levels of Fe, Pb, Ni, Mn, B, and Cr in the water surpassed Ethiopian/WHO's drinking water guidelines, while the levels of Cd, Hg, and Cr in the sediment exceeded Canadian/Ontario's probable/severe effect levels. This study revealed the presence of ecological degradation at the downstream sites of the upper and middle Awash. The most likely sources of contaminants were identified as small-scale agricultural pollution, residential and commercial waste from towns like Addis Ababa, Merti, and Metehara, and the inflow of Lake Beseka. We recommend an integrated management plan for the Awash River to ensure sustainable use of the water, guarantee the well-being of wildlife, and reduce public health risks.