Salgótarján is a city of roughly 35,000 people whose economic life was built on brown coal. Mining began in the mid-nineteenth century, and over at least 170 years the city developed a heavy industrial portfolio that included a coal-fired power plant, iron and steel smelting, glassworks, and mining machinery production. When the communist system collapsed in the early 1990s, the mines and heavy industries declined, but their waste did not disappear. Coal mine dumps, coal ash heaps, and smelter slag deposits remained scattered across the urban landscape. In several cases, these industrial leftovers were repurposed as filling material during post-industrial redevelopment, meaning that playgrounds, parks, and kindergartens may literally sit on top of the by-products of a vanished industrial era.

The research team focused on strontium because of an unusual geochemical property. Strontium has four naturally occurring stable isotopes, and the proportion of strontium-87 increases over geological time through the radioactive decay of rubidium-87, an element with a half-life of 48.8 billion years. As a result, ancient rocks and the soils derived from them tend to carry elevated ratios of strontium-87 to strontium-86, while industrial by-products such as coal ash typically carry lower ratios. Strontium also substitutes readily for calcium in minerals, which means it concentrates in the limestone additives and carbonate-rich materials used during coal combustion. When coal burns, strontium becomes enriched in the resulting ash: the Salgótarján brown coal contained 52.8 milligrams of strontium per kilogram, while a coal ash sample collected from a cone just half a kilometer from the former power plant contained 113.6 milligrams per kilogram.

To capture these signals, the researchers analyzed fifteen urban soil samples collected from kindergartens, playgrounds, parks, and roadsides in the northeastern part of the city, at depths of five to fifteen centimeters, following the Euro-Geo-Surveys sampling protocol. Samples were digested using microwave-assisted acid treatment, purified through extraction chromatography resin, and measured with thermal ionization mass spectrometry at a laboratory in Fukushima, Japan, using the multi-dynamic technique and the NIST-SRM-987 standard for quality control. The results revealed considerable variation: strontium concentrations ranged from 17.5 to 51.3 milligrams per kilogram, and the strontium-87 to strontium-86 ratios spanned 0.710303 to 0.719559, a spread wide enough to encode multiple distinct sources.

The spatial pattern was striking. More than half of the samples showed a strong positive correlation, with a coefficient of determination of 0.86, between the isotopic ratio and distance from the former coal-fired power plant. Soils closest to the plant, such as a roadside sample 0.6 kilometers away and a park sample 0.9 kilometers away, carried the lowest ratios, consistent with heavy coal ash input, since the local coal ash itself registered a ratio of 0.711652. At intermediate distances the ratios climbed, and beyond three kilometers they approached values typical of the loess deposits that blanket the Pannonian basin, indicating that coal ash influence had faded and local soil dominated. The local coal ash ratio is itself explained by the marine carbonates within the coal seams, which carry the globally uniform seawater signature of roughly 0.7080 to 0.7090, slightly modified by the limestone sorbents added during combustion.

Not every sample fit this tidy distance gradient, and the deviations proved just as informative. A playground sample located near the former iron and steel works recorded the lowest ratio of the entire dataset, 0.710303, even lower than the coal ash itself, despite not being the closest site to the power plant. Two other samples, from a kindergarten and a roadside location, showed similarly depressed values. The researchers attribute these anomalies to smelter slag, a known industrial source in the city that incorporates strontium from carbonate and phosphate minerals with inherently low isotopic ratios, and possibly to unknown technogenic soils transported by humans during land cover changes. Microscopic examination of these samples revealed reddish oxidized layers and slag grains, physical corroboration of the isotopic evidence.

At the opposite extreme, three samples exceeded even the local soil end-member, with the highest ratio reaching 0.719559 at a roadside site. These elevated values point to artificial infills, quartz-rich materials commonly introduced for road construction and soil stabilization during urbanization. Such materials derive from parent rocks rich in rubidium, like granites and metamorphic rocks, in which radiogenic strontium-87 has accumulated over geological timescales. In other words, even the apparently clean end of the isotopic spectrum in Salgótarján tells a story of human landscape engineering rather than untouched nature.

To move from qualitative fingerprints to quantitative estimates, the team applied a two-end-member mixing model, plotting the isotopic ratio against the inverse of strontium concentration. Around eighty percent of the samples fell along a strong linear trend, with a coefficient of determination of 0.89, confirming that local soil and coal ash are the two dominant contributors. The model estimated that coal ash accounts for roughly 77 percent of the strontium in a roadside sample, 71 percent in a park sample, and 67 percent in another park located 4.4 kilometers from the plant, where ash was likely introduced as fill rather than by atmospheric deposition. Conversely, samples from kindergartens showed local soil contributions of 71 to 97 percent, suggesting minimal industrial contamination at those particular sites. The authors emphasize that these figures are semi-quantitative, intended to indicate spatial trends rather than precise apportionment, since the urban environment likely contains additional sources that a two-component model cannot fully resolve.

The physicochemical context reinforced the isotopic story. The soils were predominantly weakly alkaline, with a mean pH of 7.28, and samples rich in coal ash showed elevated pH reflecting the calcium and magnesium content of combustion residues, while samples near the smelter slag hill were more acidic, consistent with the secondary minerals like gypsum and iron oxy-hydroxides that form as slag weathers over decades. Phosphorus and sulfur contents peaked at sites influenced by industrial by-products, and total organic carbon and total nitrogen were highest in vegetated parks, where organic matter appears to help stabilize strontium in the soil matrix. Notably, a previous study by part of the same team had already documented reduced microbial activity at several of these sites, linked to arsenic, lead, mercury, and cadmium from coal ash and slag, underscoring that the strontium signal tracks co-contaminants of direct health concern.

The broader significance of the study lies in its demonstration that strontium isotope geochemistry can serve as a robust, relatively inexpensive tracer for legacy industrial contamination in post-industrial cities worldwide. Comparable isotopic signatures have been reported in urban soils from Kyrgyzstan and in contaminated sediments from Chinese rivers, suggesting the approach travels well across different geological and industrial settings. For city planners and environmental agencies, the message is practical: before repurposing industrial waste as fill, and before siting playgrounds and kindergartens on former industrial land, the isotopic composition of the soil can reveal exactly what lies beneath. As former industrial regions across Central and Eastern Europe continue to be redeveloped, the humble strontium atom may become one of the most valuable witnesses to what the twentieth century left in the ground.

Subject of Research: Strontium isotope tracing of industrial contamination sources in urban soils of a former Hungarian coal and steel city

Article Title: Strontium isotopic signatures and source apportionment in urban soils of Salgótarján: unraveling the impact of industrial emissions

Article References: Maghsoudlou, M., Tserendorj, D., Abbaszade, G., Kavasi, N., Völgyesi, P., Tóth-Bodrogi, E., Sahoo, S. K., Štrok, M., Inoue, K., Kovács, T., & Szabó, C. (2026). Strontium isotopic signatures and source apportionment in urban soils of Salgótarján: unraveling the impact of industrial emissions. Environmental Science and Pollution Research, 33(30), 15829-15848. https://doi.org/10.1007/s11356-026-38225-6

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38225-6

Keywords: strontium isotopes, urban soils, coal ash, coal-fired power plant, smelter slag, soil contamination, source apportionment, Salgótarján, heavy metals, geochemistry, post-industrial cities, environmental forensics

The research, published in Environmental Geochemistry and Health by a team from Fire University, the Central Laboratory for Radiological Protection and Warsaw University of Technology in Poland, addressed a long-standing blind spot in the assessment of naturally occurring radioactive materials, known as NORM. Most previous studies have treated coal and its combustion residues as independent sample sets, comparing average ash concentrations from power plants with average coal concentrations from mines. That approach obscures the crucial question of whether combustion itself acts as a predictable amplifier of radiological risk. The Polish team instead burned nine hard coals and measured the radioactivity of each ash against its own parent coal, establishing a direct quantitative link between the two.

The nine coals were deliberately chosen for geological diversity. Samples came from the Bowen Basin in Queensland, Australia; the Central Appalachian coalfield in Virginia, United States; the Karaganda and Ekibastuz basins in Kazakhstan; the Moatize Basin in Mozambique; the Cerrejón Formation in Colombia; and two Polish sources, including the Carboniferous Upper Silesian Coal Basin. Together they span depositional ages from the Carboniferous to the Paleogene and represent markedly different tectonic and sedimentary settings. All were collected from shipments transhipped at the Port of Gdansk using a random representative sampling procedure compliant with the ISO 18283:2008 standard, and each was treated as an independent observation of a distinct geological origin.

Measurement relied on gamma-ray spectrometry using a MAZAR analyser coupled to a sodium iodide scintillation probe housed in lead shielding. The activity concentrations of radium-226 and thorium-232 were determined indirectly through their secular equilibrium daughter products, bismuth-214 at 1764 kiloelectronvolts and thallium-208 at 2610 kiloelectronvolts, while potassium-40 was measured directly from its 1460 kiloelectronvolt photopeak. Each crushed and sieved sample was sealed in a 1.7-litre Marinelli vessel and stored for four weeks to allow radioactive equilibrium to be reached before being measured nine times over 18,000-second counting intervals. Combined relative uncertainties typically fell within 10 to 15 percent for radium and thorium and 10 to 12 percent for potassium, with minimum detectable activities of a few becquerels per kilogram.

The combustion stage took place in a Kolton UNIX 20 solid-fuel boiler, a domestic bottom-feed unit operating under natural draught in a fixed-bed grate configuration, with active-zone temperatures typically between 700 and 1050 degrees Celsius. Each coal was burned to completion, the boiler cooled, and the bottom ash carefully collected. The authors acknowledge that domestic grate firing differs from industrial pulverised-coal or fluidised-bed systems in temperature profile, residence time and ash fractionation, but the fundamental mechanism is the same: the organic fraction of coal is oxidised to gas, while mineral-bound radionuclides are retained and concentrated in the solid residue. Notably, fly ash separated in industrial plants often carries even higher radionuclide concentrations than bottom ash, meaning the enrichment factors reported here may be conservative.

In their raw state, all nine coals proved radiologically unremarkable. Activity concentrations of radium-226, thorium-232 and potassium-40 sat below global average values for hard coal and for the Earth’s crust, with the lowest readings in the Colombian sample and the highest radium and thorium values in the Mozambican coal. The Polish Upper Silesian sample contained the most potassium-40. None of the coals exceeded any of the standard radiological screening indices: radium equivalent activity ranged from 19.3 to 57.7 becquerels per kilogram, far below the 370 becquerel per kilogram safety threshold, external hazard indices stayed well under unity, and gamma dose rates of 1.74 to 25.90 nanogray per hour remained below both the global average of 54 nanogray per hour and the Polish average of 47.4. As fuels, these coals posed negligible radiological concern.

Combustion changed the picture dramatically. Enrichment factors relating ash activity to coal activity ranged from approximately 3.8 to 9.9, clustering around a median of about six for every radionuclide and every derived hazard index. A paired Wilcoxon signed-rank test confirmed the increase was statistically robust, with a very large effect size of r = 0.889 and complete directional consistency: in all nine coal-ash pairs, every measured parameter rose. The mechanism is essentially a mass-balance effect. When the organic matter burns away, the ash yield drops to an estimated 10 to 27 percent of the original coal mass, so the same absolute quantity of radionuclides is packed into a much smaller mass. Earlier studies had reported enrichment factors of two to five; the tighter, higher and more uniform amplification observed here likely reflects the paired experimental design, which excludes the confounding effects of fuel blending and heterogeneous ash streams in large power stations.

The radiological consequences of this amplification were visible in every index calculated for the ash. Radium equivalent activity in the ashes ranged from 149 to 418 becquerels per kilogram, external hazard indices from 0.46 to 1.13, gamma dose rates from 68 to 185 nanogray per hour, annual effective doses from 0.018 to 0.226 millisieverts per year, and the radioactivity concentration index from 0.52 to 1.46. The most radioactive ash, derived from the Mozambican coal, exceeded regulatory thresholds across multiple parameters. Critically, the external gamma dose rate surpassed the recommended reference level of 54 nanogray per hour in all nine ash samples, even though most other indices remained within limits. The authors stress that the study assessed the potential for environmental impact based on the elevated activity of the ash itself, not on direct observation of radionuclide migration into soil, water or air.

Why does this matter beyond the laboratory? Coal ash is one of the most heavily reused industrial by-products on Earth, finding its way into concrete, road base layers, land reclamation schemes and even, occasionally, agricultural applications mixed with sewage sludge. Each of these uses creates a potential pathway for concentrated NORM to disperse into soil, groundwater and the built environment. The difference between a two- or three-fold enrichment and a six-fold one is far from trivial when millions of tonnes of residue are involved, and the authors argue that risk assessments based on average literature ash values may systematically underestimate exposure if coal-specific transformation factors are ignored. Under the Euratom Basic Safety Standards, which set activity concentration reference levels for building materials, some combustion residues may fall within regulatory scope when used in construction.

The study’s conclusions come with honest caveats. Nine coal-ash pairs, while sufficient for the non-parametric statistical framework applied, cannot capture the full global variability of coal composition, and a single prepared sample per pair means the reported uncertainties reflect counting statistics rather than within-material heterogeneity. The domestic boiler used cannot reproduce industrial combustion conditions exactly. Yet the central message stands with unusual statistical clarity: combustion acts as a proportional and predictable amplifier of natural radioactivity, so the radiological properties of an ash can be inferred from its parent coal. As Poland and other nations continue importing coals from geologically diverse basins while promoting circular-economy reuse of combustion residues, the authors call for continuous monitoring of both domestic and imported coal, and for transformation processes to be built into environmental risk assessment frameworks. A material that enters the furnace radiologically harmless may not leave it that way.

Subject of Research: Radionuclide enrichment and radiological hazard transformation during coal combustion

Article Title: Radiological characteristics of coal-to-ash transformation: a paired coal-ash assessment of natural radionuclides

Article References: Łukaszek-Chmielewska, A., Rachwał, M., Rakowska, J., Piotrowska, B., Isajenko, K., & Szyłak-Szydłowski, M. (2026). Radiological characteristics of coal-to-ash transformation: a paired coal-ash assessment of natural radionuclides. Environmental Geochemistry and Health, 48(15), Article 612. https://doi.org/10.1007/s10653-026-03509-w

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03509-w

Keywords: coal ash, NORM, radionuclides, gamma-ray spectrometry, radiological hazard, TENORM, bottom ash, coal combustion, environmental radioactivity, radium-226, thorium-232, potassium-40

The research team, led by Max Kolton of Ben-Gurion University and Florida A&M University together with colleagues at the Savannah River Ecology Laboratory, compared floodplain soils with a long history of coal-ash contamination to nearby reference soils that remained relatively pristine. Coal ash, the residue left after coal is burned for power, carries a cocktail of metals including arsenic, chromium, and other elements that are toxic to living cells even at moderate concentrations. Because floodplains sit at the intersection of rivers and uplands, they are natural collection points for such industrial residues, and they are also among the most biologically active soils on the landscape, making them an ideal natural laboratory for asking what chronic metal stress does to the invisible life below ground.

To capture the full picture, the researchers sampled across five seasonal periods, an unusually thorough design for this kind of work. Microbial communities in soil are not static; they shift with temperature, moisture, and plant activity through the year. By sampling repeatedly across seasons, the team could distinguish a consistent contamination signal from the background noise of natural seasonal variation. The strongest divergence between contaminated and reference soils appeared during late-summer peak conditions, when heat and drought likely amplified the physiological stress that metals impose on microbial cells, pushing the two soil types furthest apart in composition and function.

The core of the study rested on three complementary molecular techniques. First, the team sequenced the 16S rRNA gene, a standard marker used to identify which bacteria and archaea are present in a sample and how diverse the community is. Second, they used a tool called PICRUSt2, which predicts the functional capabilities of a community from its taxonomic profile, offering a computational glimpse into what the microbes might be doing. Third, and most decisively, they turned to digital PCR, a highly sensitive technique that counts individual DNA molecules, to precisely quantify the abundance of 16S rRNA genes and a suite of nitrogen-cycling marker genes, including the ammonia monooxygenase genes carried by both bacteria and archaea, nitrite reductase genes, and the nrfA gene associated with dissimilatory nitrate reduction to ammonium.

The diversity results told a clear story. Metal contamination reduced both taxonomic and phylogenetic diversity, meaning that contaminated soils hosted fewer kinds of microbes drawn from a narrower slice of the evolutionary tree. Yet, crucially, the total abundance of prokaryotic DNA, measured by counting 16S rRNA gene copies, did not consistently decline. This is a subtle but important distinction. The contaminated soils were not sterile wastelands; rather, they had undergone a restructuring, with the microbial biomass largely maintained but redistributed among a smaller cast of survivors. Pollution, in other words, pruned the tree of soil life rather than cutting it down.

The pruning followed a predictable ecological logic. Contaminated soils were enriched in generalist taxa, microbes with broad environmental tolerances that can cope with a wide range of conditions, and depleted in specialists, organisms finely adapted to particular niches but vulnerable to disturbance. Ecologists call this pattern biotic homogenization: as environmental filters like metal toxicity eliminate the sensitive and the specialized, the remaining communities across contaminated sites come to resemble one another, dominated by the same resilient cosmopolitan players. The loss of specialists matters beyond simple headcounts, because specialist microbes often perform narrow but vital functions, such as breaking down specific organic compounds or mediating particular steps in nutrient transformations, that generalists may not fully replace.

The most surprising findings emerged from the nitrogen-cycling analysis. PICRUSt2’s functional predictions suggested that nitrification, the process by which microbes convert ammonia into nitrate, should be reduced in contaminated soils. But when the researchers actually counted the relevant genes with digital PCR, they found the opposite: the abundance of ammonia-oxidation genes was increased, driven primarily by archaeal ammonia oxidizers rather than their bacterial counterparts. This mismatch between prediction and measurement is itself a lesson in method. Gene-based functional prediction tools are calibrated largely on well-studied bacteria and can miss the idiosyncrasies of archaea, which are known to dominate ammonia oxidation in many soil environments. Direct gene quantification revealed a reality that the predictive model had inverted.

Why would archaeal ammonia oxidizers thrive under metal stress? Archaea are ancient, often extremophile lineages, and their ammonia-oxidizing members are famously tolerant of harsh conditions, including low pH and, apparently, elevated metal concentrations. As metal-sensitive competitors and grazers were filtered out, the archaeal oxidizers may have faced reduced competition for ammonia, allowing their populations to expand. Whatever the precise mechanism, the consequence is a shift in the architecture of the nitrogen cycle itself, with ammonia oxidation gaining ground in contaminated floodplains relative to uncontaminated ones.

The gene ratio analyses added a second layer of reorganization. Relative to the nrfA gene, which marks the dissimilatory nitrate and nitrite reduction to ammonium pathway, or DNRA, the contaminated soils showed an increased proportion of genes for ammonia oxidation and for denitrifying nitrite reduction. This matters because the two pathways have opposite consequences for the ecosystem. DNRA conserves nitrogen within the soil by converting nitrate back into ammonium, a form that plants and microbes can retain. Denitrification, by contrast, converts nitrate into gaseous forms, including nitrous oxide, a potent greenhouse gas, which escape to the atmosphere. A community shifted away from DNRA and toward denitrification is, in effect, a community that leaks nitrogen rather than holding it, with potential downstream consequences for fertility and emissions.

The authors caution that these are measurements of genetic potential rather than direct observations of nitrogen fluxes, and that linking gene abundances to actual process rates will require further work. Even so, the study carries a sober implication for the many floodplains worldwide that bear the legacy of industrial metal pollution, from former mining districts to power plant ash basins. Remediation efforts typically focus on the chemistry of the contamination itself, measuring metal concentrations and immobilizing them in place. This research shows that the biological legacy runs deeper and differently: even where microbial life persists in abundance, the identity of the organisms and the genetic toolkit they carry have been permanently reorganized. Restoring a contaminated floodplain, the findings suggest, may mean more than detoxifying the soil. It may mean waiting for, or actively assisting, the slow return of the specialist microbes that keep nitrogen locked in the landscape, a recovery measured not in years but potentially in decades, and one that begins with recognizing that a soil can look alive while functioning in an entirely altered way.

Subject of Research: Effects of legacy coal ash metal contamination on floodplain soil microbial communities and nitrogen-cycling gene abundance

Article Title: Legacy Metal Contamination Alters Floodplain Soil Microbiomes and Reorganizes Nitrogen-Cycling Genetic Potential

Article References: Kolton, M., Chukwujindu, C., Oo, W. Y. M., Pathak, A., Fincher, K., Xu, X., & Chauhan, A. (2026). Legacy Metal Contamination Alters Floodplain Soil Microbiomes and Reorganizes Nitrogen-Cycling Genetic Potential. Microbial Ecology. https://doi.org/10.1007/s00248-026-02865-5

Image Credits: AI Generated

DOI: 10.1007/s00248-026-02865-5

Keywords: coal ash, metal contamination, floodplain soils, soil microbiome, nitrogen cycle, archaeal ammonia oxidation, denitrification, DNRA, digital PCR, PICRUSt2, biotic homogenization, Savannah River Site