Sulfur isotopes reveal hidden legacy of coal mine waste in England

More than three decades after the last coal was hauled from the Durham coalfield, the ground beneath a quiet corner of north-east England is still telling the story of its industrial past. A new study of the remediated South Hetton Colliery and Hawthorn Combine Mine and Cokeworks sites in East Durham shows that sulfur isotopes locked in soils, plants, coal fragments and water can map where legacy mining waste continues to shape the environment. The research, published in Discover Geoscience, demonstrates how a geochemical fingerprinting technique first developed in the 1940s can serve as a modern surveillance tool for the more than 1,500 post-mining landscapes scattered across England and Wales.

The team, led by Eve H. Rose, Caitlin S. Banbury and Darren R. Gröcke of Durham University’s Department of Earth Sciences, measured the ratio of the stable isotopes sulfur-34 to sulfur-32, expressed as δ34S values in per mil relative to the Vienna Canyon Diablo Troilite standard. Because different sulfur sources carry distinct isotopic signatures, the technique allows researchers to distinguish coal-derived sulfur from sulfate leached from Permian evaporite rocks, agricultural fertilisers, atmospheric deposition and groundwater inputs. In a landscape where remediation has churned natural soil horizons into a chaotic mosaic of imported fill and backstowed waste, that ability to attribute sources is invaluable.

Fieldwork in August 2024 covered a 1.4 square kilometre area divided into ninety quadrants of 200 by 100 metres. At the 56 accessible sites, the researchers extracted shallow soil cores with a T-handle auger, targeting depths of up to 40 centimetres, and collected 140 vegetation samples, 51 coal fragments and 18 water samples. Analysis at Durham’s Stable Isotope Biogeochemistry Laboratory used a Thermo Scientific Flash IRMS system coupled to a Delta V Plus mass spectrometer, calibrated against four international reference materials and achieving a precision of ±0.2 per mil. The resulting dataset was then interpolated in ArcGIS using Empirical Bayesian Kriging to build isoscapes, or maps of isotopic variation, for soil, plants and water.

The numbers reveal a strikingly heterogeneous environment. Topsoil δ34S values ranged from −8.7 to +14.9 per mil with a mean of +6.0 per mil. Plants spanned −4.1 to +18.4 per mil, coal fragments within the cores ranged from −0.2 to +13.6 per mil with a mean of +9.4 per mil, and water samples fell between +6.2 and +12.5 per mil. That spread reflects the multiple overlapping sulfur sources at play: pyrite-rich colliery spoil, sulfate-bearing minerals in the underlying Magnesian Limestone, NPK fertilisers applied to surrounding farmland, and residual atmospheric deposition from the era when coal smoke blanketed Britain.

The most compelling result is spatial. Across every soil depth analysed, from the surface 0–7 centimetre layer down through 7–14 centimetres and beyond 14 centimetres, δ34S values decline consistently from the southwest of the site, where South Hetton Colliery once stood, toward the northeast, where the Hawthorn Combine and Coke Works processed coal from 1959 until 1992. The researchers attribute this gradient to the oxidation of pyrite in unburnt colliery waste in the west. When pyrite weathers, the lighter isotope sulfur-32 is preferentially incorporated into the resulting sulfate, producing isotopically light sulfate that groundwater can then carry downslope, leaving the source material relatively enriched in sulfur-34.

Plants recorded the same story from a different vantage point. Vegetation δ34S tracked soil values closely, with an average offset of about 1.5 to 1.7 per mil, exactly the fractionation expected when roots preferentially take up the lighter sulfur-32 isotope during assimilation into the amino acids cysteine and methionine. This plant–soil offset matches values reported in previous studies despite the unusual, coal-influenced artificial soils of the site, suggesting that legacy mining inputs do not substantially distort the fundamental biogeochemistry of sulfur uptake. Notably, the highest plant values appeared on the margins of the artificial ground, in agricultural areas where fertiliser inputs and natural bedrock likely dominate, while the lowest values clustered around the former coke works.

The coal fragments themselves told a more complicated tale. Their mean δ34S of +9.4 per mil overlaps with regional pyrite data from the Northumberland Coalfield, which averages +5.4 per mil but with an enormous spread of ±15.9 per mil, and the difference between the datasets was not statistically significant. This wide range likely reflects both original depositional variability and post-depositional processes, including secondary pyrite mineralisation linked to the emplacement of the Weardale granites and Permian-era tectonic activity. Decades of remediation, which redistributed coal waste across the site, may have further homogenised the isotopic signal, blurring distinctions between the two former mining complexes.

Water samples, though limited by exceptionally dry field conditions, added a hydrological dimension. The lowest water δ34S values, around +7.1 to +7.4 per mil, occurred in the northwest, where they closely resembled plant values and may point to agricultural fertiliser-derived sulfate. Higher values above +10.6 per mil in the central and southeastern regions could reflect microbial sulfate reduction in groundwater or simply longer water residence times combined with preferential biological uptake of the lighter isotope. Crucially, the isotopic patterns in soil are consistent with earlier investigations of groundwater flow at the site, reinforcing the idea that isotope mapping can independently corroborate hydrogeological models in landscapes where direct monitoring is sparse.

The practical implications extend well beyond one Durham hillside. The study site, now a patchwork of arable farmland, woodland, footpaths and children’s play areas, sits beside two Sites of Special Scientific Interest, and Dalton Beck, which drains the area to the North Sea, has previously failed ecological assessments for priority hazardous substances including mercury compounds. Leachate from deep coal spoil heaps is recognised as one of the greatest long-term risks to groundwater in former coalfields, and communities in mining regions continue to experience elevated health burdens. The authors argue that spatially resolved δ34S mapping offers a cost-effective complement to conventional concentration-based monitoring, capable of distinguishing overlapping sulfur sources that bulk chemistry alone cannot separate.

There is a certain poetry in the site’s latest chapter: the £2 billion Eastern Green Link 1 project is now constructing a 400 kV converter station there to pipe offshore wind power into the national grid, transforming a node of the coal economy into an artery of the renewable one. Yet as this study makes clear, the transition does not erase the past. Sulfur isotopes act as a memory of industrial activity, persisting in soils and vegetation long after the headstocks have gone. For the hundreds of reclaimed coalfield sites still awaiting careful assessment, the message is that the ground remembers, and with the right geochemical tools, scientists can read it.

The sulfur isotope method used at South Hetton carries a lineage stretching back to pioneering work in the Canadian Rockies, where researchers first showed that coal-derived emissions could be distinguished from natural sulfate by their isotopic composition. Since then, the approach has been deployed in settings ranging from Swedish mine tailings to groundwater systems in mining districts worldwide, and it has repeatedly proved its value in identifying microbial sulfate reduction within acid-mine drainage. What distinguishes the East Durham study is its deliberate combination of isotope chemistry with systematic geospatial sampling, treating an entire reclaimed landscape as a single analytical unit rather than a scatter of discrete sampling points.

The regional geology helps explain why the isotopic signatures are so variable. UK Carboniferous coals, such as those mined in the Durham coalfield, contain sulfur predominantly in organic and pyritic forms, with the pyrite produced by bacterial sulfate reduction during and after deposition. Coal sulfur isotope values globally span nearly twenty per mil, and County Durham pyrites are famously scattered, from values below −5 per mil to above +32 per mil. Against this backdrop, individual measurements can rarely be attributed to a single source on their own, which is precisely why the spatial coherence of the isoscape matters more than any one data point.

Atmospheric context has also shifted in ways that make such studies timely. Before the mid-twentieth century, coal combustion deposits masked the natural isotopic baseline across UK ecosystems. The Clean Air Act and the subsequent collapse of coal use have progressively cleared that signal, allowing background patterns to re-emerge while the so-called memory effect of legacy pollution lingers in soils and vegetation. Establishing modern baselines is therefore essential, because deviations from them can reveal continuing inputs from industrial activity, fertiliser application or long-range atmospheric transport.

Plant physiology adds another interpretive layer. Roots take up sulfate produced by the oxidation of pyrite, microbial activity and the decomposition of organic matter, and plant tissues typically register values around one to two per mil lower than their substrate, with roots and stems generally lighter than leaves. Rooting strategy matters too: shallow-rooted species interacting with dynamic surface horizons show greater variability than deep-rooted taxa drawing on more stable sulfur pools, and associations with mycorrhizal fungi can further modulate uptake. These general patterns held at South Hetton despite the artificial ground, which strengthens confidence that vegetation can serve as a reliable sampling medium even in heavily disturbed terrain.

For land managers, the study suggests that isotope mapping could guide targeted remediation, indicating where colliery spoil still drives sulfate mobilisation and where agricultural or bedrock sources dominate. Repeated surveys could also track whether the isotopic gradients persist, migrate or fade as the landscape continues its post-industrial evolution.

Subject of Research: Using sulfur stable isotope ratios to trace legacy coal mining waste in a remediated post-industrial landscape

Article Title: Tracing legacy coal mine waste using sulfur isotopes

Article References: Rose, E. H., Banbury, C. S., & Gröcke, D. R. (2026). Tracing legacy coal mine waste using sulfur isotopes. Discover Geoscience, 4(1), Article 341. https://doi.org/10.1007/s44288-026-00679-z

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00679-z

Keywords: sulfur isotopes, coal mining, legacy contamination, soil geochemistry, isoscapes, acid mine drainage, pyrite oxidation, groundwater, remediation, Durham coalfield, environmental monitoring, biogeochemistry