Heavy Metal Pollution in Wastewater-Irrigated Urban Soils of Ethiopia: A Systematic Review Using Pollution Indices and Multivariate Analyses

Freshwater scarcity driven by population growth, industrialization, and urbanization has increased the use of wastewater as a source of irrigation, leading to the accumulation of heavy metals (HMs) in soils. However, studies in Ethiopia remain focused on individual HM concentrations and lack an integrated evaluation of overall pollution levels. The data from 39 peer-reviewed studies published between 2012 and 2025, covering ten HMs (Cd, Pb, Cr, Ni, Zn, Cu, Co, Mn, As, and Hg), were synthesized to assess both concentrations and pollution levels of HMs in wastewater-irrigated urban soils in Ethiopia using pollution indices and multivariate statistical analyses, including principal component analysis (PCA) and cluster analysis (CA). The mean concentrations (mg kg−1) of Cd (7.46), Mn (979.78), and Hg (2.63) exceeded FAO/WHO limits, while those of other HMs remained within permissible limits. The mean contamination factor (CF) values for Cd (24.9) and Hg (6.57) showed very high contamination, while the mean geo-accumulation index (Igeo) values for Cd (2.01) and Hg (5.38) were classified as strong and extreme pollution, respectively. The mean values of ecological risk factor (Eri) for Cd (747.26) and Hg (262.87) revealed serious and high ecological risk, respectively. Similarly, the mean values for contamination degree (Cdeg) = 40.54 and modified contamination degree (mCdeg) = 4.05 showed high contamination. The mean values of potential ecological risk index (PERI), 1047.44, and pollution load index (PLI), 1.14, are classified as significantly high ecological risk and intermediate contamination, respectively. The results of PCA explained 69.53% of the total variance and identified three components, while CA grouped HMs into three clusters (As–Hg, Ni–Co and Mn–Zn–Cu–Cd–Pb–Cr). Strong correlations were observed for As–Hg (0.77), Cu–Zn (0.71), and Cr–Ni (0.59). Overall, Cd and Hg are the dominant contributors to both pollution levels and ecological risk, indicating environmental and health risks and the need for control measures involving policymakers, researchers, and farming communities.