Soil Covers Tame Toxic Copper Mine Tailings, But Not for Every Metal

In the mining towns of Zambia’s Copperbelt Province, the hills on the horizon are not natural. They are tailings storage facilities, vast engineered dumps of the finely ground waste rock left behind after copper ore is processed. These structures hold millions of tonnes of crushed material laced with potentially toxic elements, and where they sit uncovered, wind and rain work on them year after year. A new study published in Environmental Monitoring and Assessment has now put hard numbers on what a simple intervention, a soil cover, actually does to the chemistry of these dumps, and the answer is more nuanced than a straightforward success story.

The research team, led by Misozi Makangila of the Copperbelt University’s School of Mines and Mineral Sciences together with colleagues at the British Geological Survey, compared two tailings facilities in the Copperbelt: one that had been rehabilitated with a soil cover and one left open to the elements. By running both materials through an identical battery of physical, chemical and leaching tests, the researchers could isolate the effect of the cover itself rather than guessing from regional averages. The work was funded through the Copperbelt University’s Africa Centre of Excellence for Sustainable Mining, the British Geological Survey University Funding Initiative, NERC national capability programmes and a Royal Society International Exchange grant, with sampling permission granted by Mopani Copper Mines.

The physical characterisation began with the basics that determine how a tailings deposit behaves in the field. The team measured organic matter content, specific gravity and particle size distribution, and used automated mineralogy to identify exactly which minerals make up the waste. This mineralogical detail turned out to be the crux of the whole study. The covered tailings were rich in dolomite and calcite, two carbonate minerals that act as natural antacids. The uncovered tailings, by contrast, were dominated by quartz, which made up around 60 percent of the material and is essentially chemically inert, offering no buffering capacity against the acids that form when sulphide minerals oxidise.

That difference in mineralogy translated directly into a difference in chemical behaviour. The uncovered tailings carried markedly higher concentrations of labile elements, meaning elements held in forms that are easily released into water. Copper reached 4,470 milligrams per kilogram in the uncovered material, alongside zinc at 12 milligrams per kilogram, chromium at 12, lead at 7 and arsenic at 2. These numbers matter because the Copperbelt’s tailings dams sit close to settlements, and previous work in the region has documented metals accumulating in soils, fruits and trees around the dumps. The total concentration, however, is only half the story; what determines environmental risk is how much of that metal can actually move.

To measure mobility, the researchers applied three complementary leaching tests, each simulating a different environmental scenario. The field leaching test captures conditions closer to what the material experiences in place. The toxicity characteristic leaching procedure, a standard regulatory test developed by the US Environmental Protection Agency, uses an acidic extraction to approximate worst-case disposal conditions. The synthetic precipitation leaching procedure simulates the direct action of rainwater percolating through waste. Running all three on the same samples allowed the team to bracket the range of release rates the tailings might exhibit from mild to aggressive conditions, a methodological point that matters because single-test assessments routinely understate or overstate real-world leaching.

The results for copper were striking. Under the mild conditions of the synthetic precipitation leaching procedure, the uncovered tailings were estimated to release around one tonne of copper per year in leachate. The covered tailings released just 0.004 tonnes per year, a reduction of roughly two orders of magnitude. The mechanism is well understood in acid rock drainage science: a soil cover limits the ingress of oxygen and water, slowing the oxidation of sulphide minerals that generates acidity. With less acid produced, the abundant carbonates in the covered material can neutralise what little forms, keeping pH in a range where copper stays locked in the solid phase rather than dissolving into percolating water.

But the cover did not suppress every metal, and this is where the study earns its relevance for rehabilitation practice. Zinc behaved in the opposite direction: the covered tailings showed increased zinc mobility compared with the uncovered material. The researchers attribute this to ion exchange and complexation reactions facilitated by the higher carbonate content. Carbonate-rich environments can keep zinc in solution through the formation of soluble complexes, and ion exchange on mineral surfaces can displace zinc ions into pore water even as copper is immobilised. In other words, the same geochemical conditions that stabilise one potentially toxic element can mobilise another, a reminder that cover system performance cannot be judged on a single-element basis.

The element-specific nature of this outcome has direct implications for how mine waste is closed out, both in Zambia and in other sulphide-rich mining districts worldwide. Cover systems are a cornerstone of tailings rehabilitation internationally, used from Scandinavia to Indonesia to cut off the oxygen and water supply that drives acid generation. What this study adds is paired, quantitative evidence from the Zambian Copperbelt showing that a cover delivers dramatic copper stabilisation while potentially increasing zinc flux. Risk assessments and closure designs that assume a cover uniformly reduces all metal mobility would miss that trade-off entirely. The findings also feed into a broader debate about the long-term stewardship of tailings facilities, which globally number in the thousands and include many legacy structures never rehabilitated at all.

The timing is significant for Zambia itself. The country has set an ambitious target of raising copper production to three million tonnes within a decade, which means more tailings will be generated, and the legacy dumps of a mining district worked for nearly a century already pose ongoing exposure questions for nearby communities. Studies like this one provide the baseline data needed to prioritise which facilities to rehabilitate first and to design covers that account for the full suite of metals present, not just the headline contaminant. The authors position their results explicitly as input for future mine waste rehabilitation and risk mitigation studies in Zambia and comparable mining-impacted regions.

There is also a methodological lesson embedded in the work. The agreement and divergence among the field leaching test, the TCLP and the SPLP illustrate why regulators and researchers increasingly insist on multi-test characterisation before certifying a waste as stable. A single aggressive extraction might flag the uncovered tailings as hazardous while saying little about the slow, chronic copper release that actually dominates environmental loading under rainfall. Conversely, a mild test alone would have missed the zinc mobilisation that emerges under the carbonate-rich chemistry of the covered facility. As automated quantitative mineralogy becomes faster and cheaper, pairing it with tiered leaching protocols offers a template for characterising not just Copperbelt tailings but the enormous global inventory of sulphidic mine waste whose stability depends on mineralogy as much as on chemistry.

Subject of Research: Leaching behaviour and environmental stability of copper mine tailings with and without soil covers in Zambia's Copperbelt Province

Article Title: Characterisation of copper mine tailings for environmental stability from the Copperbelt Province, Zambia

Article References: Makangila, M., Maseka, K. K., Hara, Y. R. S., Watts, M., & Harimana, J. (2026). Characterisation of copper mine tailings for environmental stability from the Copperbelt Province, Zambia. Environmental Monitoring and Assessment, 198(10), Article 1128. https://doi.org/10.1007/s10661-026-15932-8

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15932-8

Keywords: mine tailings, copper, zinc, soil cover, leaching tests, TCLP, SPLP, acid rock drainage, Zambia, Copperbelt, potentially toxic elements, mine rehabilitation