Assessing Depth-Dependent Heavy Metal Concentrations in Former Industrial Soils: An Environmental Sustainability Perspective

Former industrial sites can retain metal residues long after industrial activities have ceased, yet their contamination patterns may remain poorly resolved when assessments rely only on surface soils. This study provides a reconnaissance-scale, depth-resolved characterization of heavy metals in a former industrial zone in Tabuk, northwestern Saudi Arabia, an arid environment where limited precipitation, alkaline conditions, and sandy soils may influence contaminant retention and redistribution. Twenty-seven soil samples were collected from three closely spaced investigation points within an approximately 100 m2 area, representing three sampling depths: 0–2 cm (surface interval), 5 cm, and 10 cm (nominal depths), with triplicate sampling at each depth. Eight potentially toxic elements (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) were quantified by inductively coupled plasma mass spectrometry (ICP-MS), complemented by soil physicochemical characterization, depth-based statistical analysis, Pearson correlation analysis, and two reference-normalized contamination indices: the Contamination Factor (CF) and Geo-accumulation Index (Igeo). The results revealed pronounced metal-specific and depth-dependent patterns. As ranged from 70 to 115 µg/kg, Cu from 95 to 335 µg/kg, Ni from 165 to 555 µg/kg, and Zn from 440 to 1650 µg/kg, whereas Cd was quantified only at selected locations (20–25 µg/kg). Cr, Hg, and Pb remained below their respective limits of quantification. Significant depth-related variability was observed for As (p = 0.006) and Ni (p = 0.031). Strong positive correlations among Zn, Cu, and Ni (r ≈ 0.84–0.93) further suggest shared sources or comparable transport and retention processes. Based on mean concentrations, CF values were below 1 for all quantified metals, while Igeo values ranged from −8.23 to −3.74, indicating low reference-normalized contamination and practically uncontaminated conditions. All quantified concentrations were also below applicable Saudi regulatory limits. The study demonstrates that low overall contamination does not preclude localized subsurface variability. The combined use of multi-depth sampling, statistical analysis, and contamination indices therefore provides a useful baseline for monitoring legacy metal behavior in arid post-industrial soils. The findings support environmental sustainability by providing evidence to guide soil-quality monitoring, sustainable land management, and the responsible use of land in arid environments.