Pollution and ecological risk assessment of heavy metals in anthropogenically-affected soils of Sudan: A systematic review and meta-analysis

Abstract

Soil is a sink for environmental hazardous materials including heavy metals (HMs). Developing countries face enormous environmental problems, including HMs in soils that reduce biodiversity and create serious human health risks. Thus, it is important to determine and track status of soil HM concentrations in different anthropogenic land uses (LUs) in such countries to support critical environmental management decisions. Due to the absence of soil HMs information for anthropogenically-affected soils across Sudan, this study conducted a systematic review and meta-analysis of 76 publications produced between 1996 and 2024 that reported on soil HM values in Sudan. Our work focused on 10 HMs (As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, and Zn) and 9 LUs (urban agricultural, industrial, roadside, tannery, waste dump, rural agricultural, coastal sediments, gold mining, and river sediments). The objective was to investigate HM levels and assess the potential ecological risks posed by soil HMs in the aforementioned LUs across the entire country. The averages for all HMs in all LUs combined exceeded their corresponding average common range (ACRs) in soils, with Cd, Cr, Cu, Hg, Ni, and Pb surpassing their WHO/FAO limits. The averages for Cd and Hg in urban agricultural were ≈158 and 340 times their ACRs and ≈3 and 10 times the WHO/FAO limits, respectively. The averages for Cd, Cu, Hg, and Pb (gold mining) were 122, 21, 407, and 24 times, for Pb (industrial) 89 times, and for Cr (tannery) 20 times higher than the ACRs. The average PI, CF, EF, and I_geo pollution indices indicated anthropogenic sources for all HMs except Mn in all LUs. The averages for single ecological risk (Er) for all HMs indicated low risks, whereas the potential ecological risk index (PERI) suggested considerable risk. The average PERI values were waste dump > industrial > gold mining > rural agricultural > river sediments > urban agricultural > roadside > tannery > coastal sediments. Industrial, gold mining and waste dump had extreme ecological risk values (PERI >900), with Cd and Hg contributing >70 % of the total PERI for waste dump and gold mining. Findings from this review and meta-analysis provide useful information on pollution levels and ecological risk posed by HMs on a national scale. We strongly recommend establishing national HM background concentrations for better understanding of HM pollution levels, taking into account the lithology and soil parent materials across the country. This can help guide future research priorities for soils in Sudan, specifically pollution control, management strategies and human health risk assessment.