Land Use Leaves Its Fingerprint in Iran’s Metal-Polluted Soils, Major Review Finds

Soil has a long memory. Every industrial chimney, mine tailing, fertilizer spreader and congested highway leaves a chemical residue that can persist for decades, and a sweeping new analysis of Iranian soils shows just how deeply those residues have accumulated. A systematic review published in Environmental Geochemistry and Health, led by Mohammadmahdi Khalili of the University of Tehran together with Ahmad Nohegar and Touraj Nasrabadi, has synthesized nearly two decades of pollution research into the most comprehensive picture yet of toxic metal contamination across the country’s landscapes, and the verdict is sobering: human activity has left measurable fingerprints in every type of land the researchers examined.

The study, published on 27 September 2026 as volume 48, article 608 of the journal, set out to fill a striking gap. While individual studies of contaminated sites in Iran have proliferated, no one had systematically compared contamination levels across different land uses, leaving environmental planners without a national baseline. To build one, the team screened the scientific literature and selected 195 papers published by March 2025 in which anthropogenic activities demonstrably affected the heavy metal content of soil or sediment. That corpus became the raw material for a land-use-by-land-use audit of pollution intensity, mobility and ecological risk.

The analytical framework was deliberately broad. The researchers focused on ten heavy metals and metalloids: arsenic, cadmium, cobalt, chromium, copper, mercury, manganese, nickel, lead and zinc. These were tracked across nine distinct land-use categories, spanning industrial areas, mining areas, landfill sites, river sediments, roadside soils, agricultural lands, urban areas, coastal sediments and wetland sediments. For each combination of metal and land use, the team computed seven widely used soil pollution indices: the contamination factor, the enrichment factor, the pollution load index, the single-element ecological risk index, the potential ecological risk index, the risk assessment code and the geoaccumulation index. Each index probes a different facet of pollution, from simple enrichment relative to background geochemistry to the chemical forms in which metals are bound and how easily they can move into water, plants and food chains.

The headline finding is unambiguous: the impact of human activities was traceable in every land-use category. Yet the picture was not uniformly dire. For some metals, the signal was comparatively muted. Manganese and vanadium showed the smoothest conditions in mean contamination factor calculations, while cobalt, nickel, manganese and vanadium fared best in enrichment factor terms, and cobalt, chromium, manganese and vanadium were the least alarming under the geoaccumulation index. These elements, in other words, are less consistently elevated above natural background levels than the notorious quartet of lead, cadmium, arsenic and mercury that dominates much of the contamination record.

Two land uses stood out as relative bright spots. Wetland sediments were the only category in which the mean potential ecological risk index fell into the low ecological risk band, registering 136.89. The pollution load index, which integrates multiple metals into a single composite score, dropped below the critical threshold of 1 in both wetland and river sediments, with mean values of 0.837 and 0.856 respectively, indicating overall low pollution. That these aquatic sediment environments, which often act as sinks for contaminants washing off the land, remain in comparatively good condition offers a rare note of encouragement in an otherwise grim assessment.

The most technically consequential part of the analysis concerns mobility, measured through the risk assessment code, which classifies metals by the strength of their binding to sediment and soil phases. Metals locked into residual crystal lattices pose little immediate threat; metals held loosely on exchangeable sites or bound to carbonates and iron-manganese oxides can be released with a shift in pH, redox conditions or salinity. The review found that mean RAC values for cadmium and mercury each reached 46 percent, manganese 37 percent, lead 34 percent, and arsenic and vanadium 32 percent each. Values in this range are conventionally interpreted as high environmental risk, because a large fraction of the metal burden is chemically labile and available for uptake by organisms or transport through soil water.

When the researchers turned to the single-element ecological risk index, mercury and cadmium emerged as the country’s most dangerous soil contaminants. Mercury registered high ecological risk potential in agricultural areas, roadside soils, coastal sediments and river sediments, while cadmium achieved the same classification in roadside and industrial areas. This pairing is ecologically meaningful. Mercury is a potent neurotoxin that biomagnifies through aquatic food webs, and cadmium accumulates in kidneys and bones and is readily taken up by crop plants, so their prevalence in agricultural and roadside settings translates directly into exposure pathways for people and wildlife alike.

Across all nine land-use categories compared side by side, industrial areas emerged with the worst pollution status, a conclusion that will surprise no one who has studied the legacy of smelting, petrochemical processing and manufacturing, but which the review now documents at national scale. More striking still is the magnitude of some measurements. The authors report that certain published concentrations ran up to roughly two orders of magnitude higher than the corresponding ranges reported on a global scale, meaning some Iranian sites carry metal burdens a hundred times above international comparison values. Such extremes reflect a combination of rapid industrialization, aging infrastructure, and, in some regions, naturally metal-rich geology amplified by mining and ore processing.

Why does a national synthesis like this matter beyond Iran’s borders? The authors argue that their output can help environmental planners establish strategies for the optimal management of heavy metal pollution across different land uses, and that it paves the way for future research to build a more precise understanding of pollution levels in developing countries, specifically in the Middle East. The methodological lesson is equally transferable: contamination is not a property of a metal alone but of the interaction between a metal and the landscape it lands in. A roadside soil contaminated by brake wear and leaded-fuel legacy behaves differently from a paddy field dosed with cadmium-bearing phosphate fertilizer or a river delta receiving mine effluent, and risk indices must be interpreted within that land-use context.

There are also caveats worth bearing in mind. The seven indices employed here rest on comparisons with background or reference values, which vary between studies, and the risk assessment code, while informative about chemical mobility, does not by itself quantify doses to humans or ecosystems. The authors likewise note that no new datasets were generated for the study; its power lies entirely in the synthesis of prior field measurements. Even so, by assembling 195 studies into a single comparative framework, the review transforms a scattered literature into an actionable map of where toxic metals concentrate, which metals are most mobile, and which land uses demand the most urgent intervention. As soil pollution increasingly threatens agriculture and human health worldwide, the Iranian experience documented here offers both a warning and a template for other rapidly developing regions confronting the chemical debts of their own growth.

Subject of Research: Ecological risk assessment of heavy metal contamination in Iranian soils across different land uses

Article Title: Overview of ecological risk assessment of heavy metals with the approach of land uses affecting soil pollution

Article References: Khalili, M., Nohegar, A., & Nasrabadi, T. (2026). Overview of ecological risk assessment of heavy metals with the approach of land uses affecting soil pollution. Environmental Geochemistry and Health, 48(15), Article 608. https://doi.org/10.1007/s10653-026-03482-4

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03482-4

Keywords: heavy metals, soil pollution, ecological risk assessment, land use, Iran, systematic review, mercury, cadmium, contamination factor, pollution load index, risk assessment code, environmental geochemistry