Sediment Organic Fingerprints Expose Hidden Metal Risks in Coal-Mining Lakes

Beneath the calm surfaces of coal-mining subsidence lakes in eastern China, an invisible negotiation is constantly underway between water and mud. Metals such as lead, copper, and cadmium shuttle between the overlying water column and the sediment layer below, and whether they stay locked away or slip back into the water depends to a surprising degree on the chemistry of dissolved organic matter, the vast and chemically diverse soup of carbon compounds that pervades both phases. A new study of the Linhuan mining area in Anhui Province argues that the organic matter sitting on the sediment side of that boundary has been chronically overlooked, and that its optical fingerprints carry signals that conventional water-only monitoring simply cannot see.

The research, published in Environmental Monitoring and Assessment by Yachen Ren of Anhui University and colleagues, took an unusually symmetrical approach. Instead of characterizing dissolved organic matter only in the water, as most monitoring programs do, the team collected 24 matched pairs of water and sediment samples from subsidence lakes formed where longwall coal mining has caused the land to sink below the water table. From each pair they profiled the dissolved organic matter in the overlying water and, in parallel, the water-extractable organic matter, or WEOM, that can be leached from the surface sediment. This paired design allowed them to ask a deceptively simple question: do the two organic-matter pools tell the same story, and which one better explains how potentially toxic elements partition between water and sediment?

The answer, in short, is that they tell very different stories. Using ultraviolet-visible absorbance spectroscopy, fluorescence indices, and excitation-emission matrix spectroscopy coupled with parallel factor analysis, a statistical decomposition technique known as EEM-PARAFAC, the researchers found that the overlying-water DOM looked comparatively fresh and autochthonous, meaning it bore the hallmarks of organic matter produced within the lake itself by algae and microbial activity. The sediment WEOM, by contrast, was markedly more aromatic, more humified, and apparently larger in molecular size, consistent with organic matter that has undergone extended microbial processing and diagenetic alteration within the sediment column.

Those differences matter because dissolved organic matter is not a passive bystander in metal cycling. Its carboxyl, phenolic, and other functional groups bind metal ions with varying strength, and the composition of the organic pool influences whether a metal remains dissolved and mobile, complexes into the water column, or adsorbs onto sediment particles. When the team computed apparent distribution coefficients, expressed as log Kd values, for seven potentially toxic elements, arsenic, cadmium, chromium, copper, nickel, lead, and zinc, they found that all seven showed positive values, indicating a net preference for the solid phase. But the strength of that preference varied systematically: lead was partitioned most strongly into sediment, followed by copper and nickel at roughly similar levels, then zinc and chromium, with arsenic and especially cadmium showing the weakest apparent retention.

This ranking is chemically coherent. Lead and copper are well known to form strong inner-sphere complexes with humic and fulvic substances, whereas cadmium tends to form weaker outer-sphere associations and remains comparatively labile in aquatic systems. Arsenic, which in its common oxyanion forms behaves quite differently from the cationic trace metals, occupied its own middle ground. The fact that the measured ordering matches expectations from coordination chemistry lends credibility to the apparent partitioning values, even though the authors are careful to describe them as apparent, since a single sampling snapshot cannot fully capture the dynamic exchange occurring across the sediment-water interface.

The study’s most consequential finding emerged when the team related the optical descriptors to those partitioning values. Sediment WEOM properties showed clearer, more element-specific statistical associations with apparent metal partitioning than the instantaneous overlying-water DOM descriptors did. Copper provided the clearest worked example: its apparent partitioning was associated with sediment WEOM characteristics related to molecular size and humification degree. In other words, the more processed, larger, more humified the extractable organic matter in the sediment, the more the associated metals appeared to be held on the sediment side. The overlying-water measurements, by contrast, captured only a momentary chemical state of a water column that can shift with rainfall, pumping, algal blooms, and mining-related groundwater discharge.

The researchers also introduced cross-phase optical fingerprint indicators, metrics that compare the fluorescence character of paired water and sediment samples directly. Rather than predicting partitioning themselves, these indicators primarily characterized how the two organic-matter pools are organized differently at each site, revealing component-level resemblance between type-aligned PARAFAC components alongside systematic phase-specific deviations. That combination of shared components and divergent processing histories is exactly what one would expect at an active sediment-water interface, where organic matter produced in the water settles, is transformed in the sediment, and can be partially released back upward. The paired fingerprints effectively record that exchange in optical form.

For environmental managers, the practical implication is significant. Coal-mining subsidence lakes are a growing feature of the landscapes of eastern China and other intensively mined regions, and they are frequently repurposed for aquaculture, irrigation, and recreation even as they continue to receive mining-affected groundwater. Monitoring programs built around water chemistry alone can miss the sediment-side signals that foreshadow secondary mobilization, the scenario in which metals that appear safely sequestered today are released tomorrow as conditions change. Changes in pH, redox state, or organic matter composition, whether driven by seasonal stratification, sediment resuspension, or remediation interventions, can all destabilize metal-organic associations. The Linhuan results suggest that a modest optical screening of sediment WEOM, which requires only a water extraction and standard spectroscopic instrumentation, could flag which lake basins warrant deeper investigation before problems become visible in the water column.

The approach also carries broader scientific weight. Dissolved organic matter is increasingly recognized as a master variable in aquatic biogeochemistry, shaping not only metal fate but also carbon cycling, nutrient availability, and even the treatability of drinking water. Yet most field studies still treat it as a water-column property. By demonstrating that paired water-sediment optical fingerprints are feasible at field scale and that the sediment-side descriptors carry independent, element-specific information, the study offers a template that could be adapted to other mining-impacted and industrially stressed aquatic systems, from acid mine drainage streams to estuarine sediments contaminated by industrial outfalls.

The authors are appropriately measured about the limits of their work. Apparent log Kd values derived from single-episode sampling are snapshots, not kinetic descriptions, and statistical associations between optical descriptors and partitioning do not by themselves establish mechanistic causation. Spectroscopic indices are operational proxies for molecular properties that ultimately require techniques such as high-resolution mass spectrometry to resolve fully. Still, the core message stands: in lakes born from coal mining, the mud remembers things the water forgets. Reading the optical signatures of sediment organic matter alongside the water column gives scientists and regulators a far richer, and potentially earlier, warning of where toxic elements may be poised to move. As subsidence lakes continue to multiply across former coalfields, that paired perspective may prove essential to keeping their hidden metal inventories where they belong.

Subject of Research: Dissolved organic matter optical characterization and potentially toxic element partitioning at sediment-water interfaces in coal-mining subsidence lakes

Article Title: Paired water–sediment DOM optical fingerprints reveal sediment-side signals of apparent potentially toxic element partitioning in coal-mining subsidence lakes

Article References: Ren, Y., Zhu, Y., Zhao, Y., Wang, L., Wang, K., & Zheng, L. (2026). Paired water–sediment DOM optical fingerprints reveal sediment-side signals of apparent potentially toxic element partitioning in coal-mining subsidence lakes. Environmental Monitoring and Assessment, 198(10), Article 1076. https://doi.org/10.1007/s10661-026-15845-6

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15845-6

Keywords: dissolved organic matter, sediment-water interface, coal-mining subsidence lakes, potentially toxic elements, EEM-PARAFAC, fluorescence spectroscopy, water-extractable organic matter, partition coefficient, heavy metal mobility, Linhuan mining area, environmental monitoring, lake geochemistry