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

The research team examined six affected watersheds, all located within national parks and preserves in northwest Alaska’s Brooks Range, an area protected from development precisely because of its untouched character. Between the summers of 2022 and 2024, the researchers collected water samples from the main channels of these rivers, from their tributaries, and from hillside seeps where groundwater emerges from thawing soil. Their goal was to quantify how acid rock drainage — the same class of chemical process that makes mine runoff toxic — behaves when it is triggered not by excavation but by a warming climate.

The underlying mechanism begins underground. Arctic soils sit atop permafrost, ground that has remained frozen for centuries or millennia. Within that frozen matrix are sulfide minerals, including pyrite, the iron sulfide often called fool’s gold. As air temperatures rise, permafrost thaws and water that once ran off the surface instead trickles down into newly unfrozen soil. There it meets minerals that have never been exposed to oxygenated water. The resulting oxidation reactions release iron, sulfuric acid, and a suite of other metals into the groundwater, which then discharges into streams. The iron precipitates as rusty hydroxides, staining the water orange, while the acid and dissolved metals travel with the flow.

What the new study reveals is how far that contamination travels and how concentrated it becomes. Metal concentrations in the affected rivers measured roughly seventy times higher than baseline levels as far as sixty miles downstream from the points where the metal-rich groundwater entered the channels. Senior author Brett Poulin, a professor in UC Davis’s Environmental Toxicology department, emphasized that this is genuinely acidic, metallic water — comparable to or worse than what drains from mines — despite originating in wilderness. The chemistry of these plumes, he noted, can become more toxic as the water moves downstream, meaning the impact zone extends far beyond the hillside seeps where the process begins.

There is, however, a measure of resilience built into the river systems themselves. The study found that the main channels did not become more acidic overall, even as acidic tributary inputs accumulated. The rivers appear to buffer the incoming low-pH water, a capacity the researchers interpret as a sign that the system can partially absorb the shock. Poulin described this buffering as good news, but he paired it with a caution: while the pH of the mainstem holds, dissolved metals increase downstream and can be transported over very long distances, carrying the contamination into habitats well removed from the thaw zones where it originates.

To understand when and why this phenomenon began, the team turned to long-term water quality records for the region. The analysis traced the onset of the acid rock drainage event to 2019, a year that coincided with the hottest summer on record in Alaska, followed by an unusually snowy winter. That combination, the researchers argue, set the stage for a region-wide chemical trigger. Deep snow insulates the ground beneath it, preventing the soil from refreezing during winter. When the next summer arrived, water was able to penetrate deeper into the soil column than it had in previous years, reaching mineral deposits that had remained sealed behind the frozen layer.

The records showed abrupt, large spikes in sulfate and zinc concentrations in 2019, followed by a steady decline as the watersheds entered what the authors describe as a recovery phase. First author Taylor Evinger, a Ph.D. candidate in Poulin’s lab, explained the team’s interpretation: water moved deeper into the soil because of thaw, interacted with the sulfide minerals trapped there, and triggered all of these watersheds at roughly the same time. The synchrony across multiple independent drainages is itself telling. It suggests that a single climatic anomaly — one extreme summer and one insulating winter — was sufficient to switch on acid-generating chemistry across an entire region of the Arctic.

The scale of the phenomenon continues to grow. Previous studies had already documented more than 200 rivers and streams affected by rusting water, and since Poulin’s group published its initial 2024 paper attributing the color change to thawing permafrost, similar occurrences have been reported in boreal and Arctic Canada and in other permafrost-bearing parts of the world. Poulin stressed that this process was never forecast, predicted, or included in any assessment of how the Arctic will change under a warming climate. It represents an entirely unanticipated pathway by which climate change alters water chemistry — not through melting ice or shifting precipitation alone, but by unlocking geochemical reactions that had been locked away in frozen ground for thousands of years.

For now, the direct risks appear contained. There are no known adverse impacts to people or wildlife from the acid rock drainage entering these remote streams, and researchers have not observed massive fish die-offs or drinking water quality problems harming villages. But the authors are careful to frame that reassurance as provisional. Additional research is underway to examine the risks more deeply, particularly because metal toxicity in aquatic food webs can accumulate gradually and because the affected watersheds feed into larger river systems. The team also notes that thawing permafrost and the ground subsidence that accompanies it carry implications for local infrastructure, an added dimension of concern for communities across the Arctic.

Evinger summarized the study’s dual message as concerns and hopes. The hope lies in the rivers’ demonstrated buffering capacity and in the apparent recovery phase following the 2019 spike, which suggests the systems are not in runaway decline. The concern lies in the sheer number of affected watersheds — hundreds of them — and in what the phenomenon reveals about the reach of climate change. These rivers are essentially untouched by human activity, and yet they are undergoing chemical transformation as severe as anything produced by industrial mining. That a warming atmosphere can generate mine-strength acid drainage in protected wilderness, the researchers argue, is a new reality of climate change, one that water quality assessments across the circumpolar North will now have to confront.

Subject of Research: Climate-driven thawing of permafrost triggering acid rock drainage and metal contamination in Arctic rivers

Article Title: Alaska’s orange rivers more metallic than acid mine drainage

Article References: Alaska’s orange rivers more metallic than acid mine drainage. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: permafrost thaw, acid rock drainage, rusting rivers, Alaska, Brooks Range, water quality, heavy metals, Arctic climate change, sulfide minerals, river chemistry, UC Davis, AGU Advances