Heavy metal contamination along the Duhok-Zakho highway soils: spatial analysis, risks, and ecological implications

This study investigates heavy metal contamination in soils of the Duhok-Zakho Highway of Iraq, where 20 samples were collected along a 50-km-long highway to assess Cu, Pb, Fe, Mg, Co, Ni, and Mn concentrations. Results revealed significant spatial variations in metal concentration, where the ranking of the metals followed: Fe > Mn > Mg > Ni > Co > Pb > Cu. The enrichment factor analysis indicated moderate to very high levels, while the geoaccumulation index exhibited a high level of metal enrichment, surpassing the standard background value. The ecological risk was assessed as moderate. Non-carcinogenic risk analysis revealed that Fe and Mn posed extreme non-cancer risk, with ingestion accounting for 99.95% of exposure. Children showed 10 times more vulnerability than adults to non-cancer risks. Carcinogenic risk derived from Ni and Pb exceeded the USEPA guideline threshold (1 × 10⁻⁶), with the children facing 25 times greater risks than the adults. The toxicity leaching test revealed substantial metals leaching (64–79%), suggesting a high potential for heavy metals to transport downstream the agricultural farmland and probable groundwater contamination. The findings highlight the impact of vehicle-induced metal pollution and industrial operations on heavy metal distribution in highway soils.