Heavy metal contamination in thermal power plant ashes and leachate-impacted soils at Reppe dumpsite in Addis Ababa, Ethiopia

Abstract

Heavy metal contamination created from the waste-to-energy (WtE) ash and leachate-impacted soils was examined at the Reppe open dumpsite and near the WtE plant in Addis Ababa, Ethiopia. Bottom ash (n = 3), fly ash (n = 3), and leachate-impacted soil (n = 6, 0–15 cm depth) samples collected during the dry season (February–March 2024). Samples were processed (United States Environmental Protection Agency (USEPA) Method 3050B) and scrutinized for Cadmium (Cd), Chromium (Cr), Copper (Cu), Lead (Pb), Zinc (Zn), and Mercury (Hg) using atomic absorption spectrophotometry (AAS). Quality assurance included certified reference materials (NIST 2711a, NIST 1633c), technical blanks, triplicate analyses (RSD < 5%), and permissible limits of 0.01–0.08 mg L⁻ 1 . Contamination Factor (CF), pollution load index (PLI), toxicity risk index (TRI), and ecological risk index (RI) were used, and source apportionment was made using principal component analysis (PCA) and Pearson correlation. Ash samples presented as Hg 2630 μg L⁻ 1 (13,150-fold above the United States Environmental Protection Agency (USEPA) soil limit of 0.2 μg L⁻ 1 ) and Cd 1530 mg L⁻ 1 (510-fold above the World Health Organization (WHO) guideline of 3 mg L⁻ 1 ), the highest records. All ash samples revealed “very high” ecological risk (RI = 1086–3000; threshold RI ≥ 600). Leachate-impacted soils indicated considerable to very high risk (RI = 255–1065). PCA elucidated 94.5% of total alteration, recognizing an anthropogenic component (PC1: 70.5% Pb-Cd-Cu) and a mixed geogenic/combustion component (PC2: 24% Cr-Hg). Pearson correlation indicated strong Pb-Cd associations in ash (r = 0.91), demonstrating shared combustion backgrounds, while soil correlations revealed Cr-Cu-Pb associations (r = 0.69–0.71) from waste disposal. CF recognized Hg as the main contaminant (ash CF = 12.9–65.8; soil CF = 5.1–25.4). Source apportionment computed anthropogenic contributions at 58% in ash versus 38% in soil. Action is vital to solve the severe contamination challenges from fly ash and leachate soils in Ethiopia and across the globe. Regular monitoring and justifiable land-use practices are recommended to mitigate ecological and human health risks and protect the Reppe ecosystem.