Scientists clarify how much metal in soil is “too much” for people and the environment​

Heavy metals in soil are often measured in bulk, but those totals can badly overestimate the real risk to crops, ecosystems and human health, according to a new scientific review. The article, published in Environmental and Biogeochemical Processes, explains that only a fraction of the metal present in soil is actually accessible to plants, microbes and people – a concept known as bioavailability.

“Metal pollution in soil is a genuine global concern, but decisions about food safety, clean‑up and land reuse need to be based on the portion of metals that organisms can really take up, not just on headline numbers,” said lead author Willie J G M Peijnenburg of Leiden University and the National Institute for Public Health and the Environment in the Netherlands. “Our goal was to translate decades of research into a practical toolbox that scientists, consultants and regulators can actually apply in the field.”

Why bioavailability matters

The review begins by outlining how soils become contaminated by metals such as cadmium, lead, zinc, copper and arsenic through industry, agriculture, waste disposal and urban development. Unlike many organic pollutants, these metals do not break down over time, yet their risk depends strongly on soil pH, mineral composition, organic matter, redox conditions and microbial activity.​

In many sites, high total metal concentrations coexist with relatively low biological exposure, because metals are tightly bound to soil particles or locked in mineral forms that organisms cannot easily access. In other places, modest total levels can still pose a problem if soil conditions make metals mobile and readily taken up by crops or soil life.​

Tools to measure the “accessible” fraction

To turn this complex chemistry into actionable data, the review synthesizes three main groups of methods: chemical extractants, biological assays and in situ passive samplers. Common chemical tests using solutions such as calcium chloride, DTPA or EDTA are widely used to estimate labile or potentially mobile metal pools, while more elaborate sequential extraction schemes divide metals into exchangeable, reducible, oxidizable and residual fractions.​

Plant growth tests, earthworm and springtail assays, and microbial and enzyme activity measurements then reveal which soil metals are actually taken up or cause toxicity under realistic exposure conditions. Passive tools such as diffusive gradients in thin films and ion‑exchange resins bridge the gap by measuring freely available or weakly bound metals directly in the soil environment over time.​

Models, machine learning and regulation

Beyond measurements, the paper reviews a suite of models that predict metal bioavailability from basic soil properties and site conditions, ranging from simple regression equations to mechanistic speciation codes and biotic ligand models. More recently, machine learning approaches such as random forests, support vector machines and neural networks have been used to map bioavailable cadmium or zinc in agricultural regions and to forecast metal levels in crops.​

Case studies from Europe, North America and China show how bioavailability‑based approaches can change real‑world decisions. Examples include using bioaccessibility tests to avoid unnecessary excavation at urban redevelopment projects, and combining extractants, plant uptake data and models to manage cadmium in rice‑growing areas more precisely.​

Toward harmonized, practical frameworks

A key contribution of the review is a decision tree that helps practitioners select appropriate tools based on soil type, contaminants of concern, data needs and regulatory context. The framework encourages combining chemical indicators with biological and modeling information in a “weight of evidence” approach, rather than relying on any single test.​

“Many countries are beginning to incorporate bioavailability into soil quality criteria, but methods and thresholds still differ widely between regions,” Peijnenburg said. “If we want risk assessments to be both protective and affordable, the next step is international harmonization, better field validation and more open data on bioavailable metal fractions.”​

Future directions

The review highlights emerging molecular and microbial tools, such as metagenomics, transcriptomics and whole‑cell biosensors, that can detect early biological responses to bioavailable metal exposure. It also points to hybrid modeling approaches that combine physics‑based speciation models with data‑driven machine learning to improve predictions under changing climate and land‑use conditions.​

Ultimately, the author argues that focusing on bioavailability can lead to smarter remediation, more realistic soil standards and better protection of food security and ecosystem health worldwide. By summarizing the strengths, limits and regulatory relevance of each tool, the article offers a roadmap for scientists and policymakers who need to decide how much metal in soil is “too much” in practice, not just on paper.​

=== 

Journal reference: Peijnenburg WJGM. 2025. Bioavailability of heavy metals in soil: a review of tools, models, and regulatory applications. Environmental and Biogeochemical Processes 1: e011  

https://www.maxapress.com/article/doi/10.48130/ebp-0025-0011  

=== 

About the Journal:

Environmental and Biogeochemical Processes is a multidisciplinary platform for communicating advances in fundamental and applied research on the interactions and processes involving the cycling of elements and compounds between the biological, geological, and chemical components of the environment. 

Follow us on Facebook, X, and Bluesky.