The study, published in the journal Environmental Geochemistry and Health, is described by its authors as the first city-wide assessment in the United States to systematically measure contamination by potentially toxic elements in soils at community-reported illegal dumping sites. Led by Erica D. Walker of Brown University’s School of Public Health, the team measured eight elements: the metalloid arsenic along with cadmium, chromium, copper, manganese, nickel, lead and zinc. Jackson made an urgent and instructive setting. The city has struggled for years to maintain reliable waste collection, and a 2023 breakdown in negotiations halted residential trash pickup across the city, during which illegal dumping rose sharply.

The methodological heart of the study is portable X-ray fluorescence, or pXRF, an instrument technique that bombards soil samples with X-rays and reads the characteristic fluorescent signatures of the elements they contain. At each of the 45 sites, the researchers collected soil from 20 locations, at two depths, and combined the samples into composite surface and subsurface specimens. The soils were dried, sieved to below 250 micrometers, and analyzed in triplicate. On a subset of 13 sites, the team also ran laboratory analysis by inductively coupled plasma optical emission spectrometry, or ICP-OES, following standard acid digestion, to provide an independent comparison.

The results revealed a clear hierarchy of contamination. Lead, copper and zinc dominated the enrichment, co-occurring across sites and reaching striking extremes: lead peaked at 2,910 milligrams per kilogram at depth, zinc at 1,630, and copper at 1,220. The distributions were strongly right-skewed, so a handful of extreme sites drove the averages. Surface lead, for example, averaged 226 milligrams per kilogram against a median of just 84.7. Cadmium, by contrast, was barely detectable by the field instrument, and the few pXRF readings that did appear proved to be spectral artifacts, contradicted by laboratory values 30 to 570 times lower.

Perhaps the most elegant result is the separation of two distinct geochemical fingerprints using field-portable screening alone. Principal component analysis and hierarchical clustering, applied to the depth-specific measurements, independently grouped chromium, nickel and iron together, while placing lead, zinc, arsenic and copper in a separate cluster. The iron association points to a lithogenic origin, elements present because of the local geology, since Jackson’s soils sit on deep silt deposits of Peoria Loess. The lead-zinc-arsenic-copper group, by contrast, is consistent with waste inputs from tires, wiring, plumbing, electronics, painted debris and treated wood. Partial-extraction recoveries from the laboratory subset corroborated the split: chromium was recovered at roughly a tenth of its total concentration by nitric acid extraction, as expected for chromium locked in silicate phases, while lead and copper were recovered at about half, consistent with more readily extractable, waste-derived forms.

The depth data carry the study’s central practical warning. For every element measured, surface and 15-centimeter concentrations were significantly correlated, and for lead the correlation was nearly perfect, at r = 0.982. In the cluster analysis, every element paired with its own counterpart at the other depth before joining any other element, a structural signature of vertical coherence. The researchers are careful to note that they sampled sites as found, with waste present, so the depth data describe contamination already in the ground beneath active dumping sites. But the inference for cleanup policy is the same: clearing surface waste, or even removing the uppermost soil layer, would not eliminate the elemental burden at these locations.

To translate concentrations into health-relevant terms, the team ran a screening-level risk assessment following United States Environmental Protection Agency guidance, evaluating incidental soil ingestion, dermal contact and inhalation of resuspended particles for a hypothetical child and adult resident. For a child, the hazard index exceeded unity at 21 of the 45 sites, with a maximum of 2.98. For adults, it stayed below unity everywhere. But the drivers of that calculated hazard are largely natural: arsenic and manganese, both geogenic in this setting, dominated the index, and the EPA’s arsenic screening level of 0.68 milligrams per kilogram actually sits below the local background of 6.9. Remove arsenic from the calculation, and only one site remains above unity. The honest reading, the authors argue, is that the elements driving computed hazard are mostly natural, while the element driving regulatory exceedance is the one attributable to dumping.

That element is lead. Because the EPA assigns lead no reference dose, evaluating it instead through blood-lead modeling, the researchers compared depth-averaged lead concentrations against absolute regulatory thresholds. Lead exceeded the residential soil screening level of 200 milligrams per kilogram at 11 of the 45 sites, roughly a quarter, and exceeded the 600 milligram per kilogram removal management level at four sites. Exceedances appeared in five of the six wards containing sites, most often in Ward 5. Since lead is also the element that the component, cluster and extraction evidence identifies most clearly as waste-derived, the authors describe this combination of regulatory exceedance and unambiguous anthropogenic attribution as the study’s principal public health finding.

The study is equally candid about its own limits, and those limits carry lessons for how such screening should be used. The contamination factor and degree of contamination indices the team calculated are useful for ranking and prioritizing sites, but they depend entirely on the chosen background values, and the researchers show that classifications derived from total pXRF measurements and from partial-extraction laboratory measurements agreed for only one of 13 paired sites. Chromium hazard remains unresolved, because the field instrument measures total chromium and cannot distinguish the relatively benign Cr(III) form from the toxic Cr(VI); under a worst-case assumption the child hazard quotient for chromium would jump from 0.001 to 1.42. No conclusions about mobility or bioavailability were drawn, since leaching tests and organic carbon measurements were not performed.

What emerges is a practical blueprint for resource-limited cities. Portable XRF screening, embedded in a public repository where residents and field teams document dumping locations, waste composition and exposure observations, makes city-wide assessment affordable and repeatable, while costlier laboratory analysis is reserved for high-priority sites. The spatial pattern in Jackson argues for site-by-site triage rather than uniform cleanup standards, because contamination is decentralized and scattered across individual locations that differ in size and contents. The authors’ closing message is blunt: a former dumping site converted into a community garden or playground may look fully restored while still carrying lead, arsenic and other elements that a visible cleanup did nothing to remove. A site is not recovered when it merely looks clean; it is recovered when a city can show that the land is safer for the people who live, work, garden or play on it.

Subject of Research: Soil contamination by potentially toxic elements at illegal dumping sites in Jackson, Mississippi

Article Title: Hidden toxicities: assessing city-wide contamination by potentially toxic elements from illegal dumping in Jackson, Mississippi

Article References: Walker, E. D., Klevan, C., Mathews, K., Mandalapu, S. V., Nica, C., & Ibrahim, N. (2026). Hidden toxicities: assessing city-wide contamination by potentially toxic elements from illegal dumping in Jackson, Mississippi. Environmental Geochemistry and Health, 48(15), Article 603. https://doi.org/10.1007/s10653-026-03450-y

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03450-y

Keywords: illegal dumping, soil contamination, potentially toxic elements, lead, portable X-ray fluorescence, Jackson Mississippi, environmental geochemistry, public health, risk assessment, urban soil, ICP-OES, environmental justice

The study, published in the journal Discover Soil, focuses on two soil profiles located near the city of Miercurea Ciuc along the Olt River valley. The first, designated Profile P1, sits on the roughly 20-meter terrace of the Olt River under long-term arable cultivation and was classified as a Cambic Hortic Chernic Phaeozem. The second, Profile P2, lies in the floodplain itself beneath permanent grassland dominated by moisture-loving grasses and sedges, and was classified as a Sapric Histosol of the Eutric, Fluvic variety. The sites are approximately three kilometers apart, sharing a similar climate but occupying fundamentally different positions in the landscape.

The Phaeozem profile proved remarkable in its own right. It exhibits a humus-rich sequence extending to a depth of 80 centimeters, with soil organic carbon concentrations exceeding two percent throughout the humic horizons and remaining at 1.51 percent even in the transitional AB horizon below the ploughed layer. The surface Ap horizon is very dark greyish brown, riddled with earthworm channels and granular structure shaped by decades of ploughing and bioturbation. Beneath it, two dark humic horizons collectively meet the strict diagnostic criteria for a chernic horizon, the signature feature of chernozemic soils. Deeper still, the profile transitions into cambic horizons showing incipient clay-humus coatings on ped faces and secondary carbonate accumulation at depth, with numerous filled animal burrows, known as krotovinas, threading down to about one meter.

One of the most intriguing findings concerns the boundary between the humic sequence and the underlying cambic material at approximately 80 centimeters. Particle-size analysis revealed a sharp textural change at this depth: clay content drops from 27.1 percent in the AB horizon to 14.8 percent in the underlying Bw horizon, while the silt-to-clay ratio jumps from 1.09 to 2.91 and remains elevated throughout the deeper horizons. On its own, this might suggest a major geological discontinuity. However, portable X-ray fluorescence measurements told a different story. The major oxide composition of the profile, including silicon, aluminum, calcium and iron oxides, remains relatively stable from top to bottom, and calculated titanium-to-zirconium ratios show no abrupt shift at the humic-cambic boundary. The authors interpret this combination of evidence as indicating that the soil developed in compositionally related but granulometrically stratified terrace sediments, subsequently integrated into a single profile by pedogenic processes and intense biological mixing.

This interpretation points toward a polygenetic model of soil development, in which successive phases of sediment accumulation and soil formation overlapped during the Holocene. The darker subsurface humic horizon may represent a relict feature inherited from earlier phases of soil formation, or a buried and welded horizon related to episodic sedimentation. Earthworm activity, long recognized as a major driver of chernozem formation and homogenization, appears to have played a central role in vertically redistributing organic matter and progressively welding the sedimentary units together. The researchers also considered whether the profile might preserve evidence of an earlier hydromorphic, waterlogged stage, a model previously proposed for chernozemic soils in the Ciuc Depression, but no diagnostic relict gleyic features were identified in this particular profile, leaving that broader regional hypothesis unconfirmed by the present data.

The floodplain Histosol tells a completely different carbon story. Profile P2 contains a 55-centimeter-thick sequence of strongly decomposed sapric peat overlying gleyic, water-affected loamy sand alluvium, with the groundwater table visible near the base of the excavation pit. Soil organic carbon in the sapric horizons exceeds 17 percent, roughly eight times the concentration found in the Phaeozem’s humic horizons. Here, preservation is achieved not through organo-mineral associations in a well-drained mineral matrix, but through persistent water saturation, which limits oxygen availability and suppresses microbial decomposition. The near-neutral to slightly alkaline reaction, high base saturation and high cation exchange capacity of the peat all point to a base-rich, groundwater-fed environment known as a minerotrophic fen.

The geochemistry of the Histosol yielded one of the study’s most thought-provoking results. Portable X-ray fluorescence revealed unusually high calcium concentrations in the peat horizons, with calcium oxide values ranging from 7.15 to 13.24 percent. Yet the carbonate contents measured by the classical Scheibler gas-volumetric method were surprisingly low, at only 1.42 to 2.38 percent calcium carbonate equivalent. When both measurements were converted to elemental calcium equivalents, the comparison indicated that an estimated 86 to 94 percent of the calcium in the sapric horizons exists in non-carbonate forms. In the mineral substratum below, by contrast, carbonate-bound calcium becomes relatively more important, accounting for more than half of the estimated total calcium in the deepest gleyic horizon. The specific forms of this non-carbonate calcium, whether exchangeable on cation exchange sites, organically complexed, or associated with fine mineral particles, cannot be resolved from the available dataset, but the pattern is consistent with the calcium-mediated organo-mineral interactions thought to reinforce organic matter preservation in groundwater-fed fen systems.

The significance of calcium in soil carbon dynamics extends well beyond this single profile. While research on soil organic carbon stabilization has traditionally emphasized acidic soils and the role of aluminum and iron minerals, calcium has received comparatively less attention, particularly in alkaline environments. Exchangeable calcium is widely recognized to correlate positively with soil organic carbon concentration and its resistance to oxidation, yet the underlying mechanisms remain incompletely understood. Proposed pathways include cation bridging between organic molecules and mineral surfaces, calcium-facilitated aggregation, and direct interactions with organic functional groups. The Ciuc Histosol, with its coexistence of very high organic carbon, elevated calcium, near-neutral pH and high base saturation, is consistent with such calcium-mediated stabilization, though the authors are careful to note that this remains a plausible interpretation rather than a directly demonstrated process. Direct chemical fractionation, extraction of exchangeable cations or spectroscopic analyses would be needed to confirm the specific calcium pools involved.

The broader context of the study is equally compelling. Peatlands hold approximately one third of the global terrestrial organic carbon pool, yet most research has concentrated on the ombrotrophic bogs of northern latitudes, leaving mid-latitude minerotrophic fens comparatively understudied. Intramontane depressions of the Eastern Carpathians, with their cold-air pooling, persistent thermal inversions, shallow groundwater tables and thick lacustrine-alluvial sedimentary fills, create conditions favorable for organic matter preservation over millennial timescales. In the Ciuc Depression specifically, minerotrophic fens and chernozemic soils occur in close spatial association, covering roughly 10,000 hectares, about 15 percent of the basin, along the Olt River terraces and alluvial fans. This co-occurrence under a single climate regime makes the depression a natural laboratory for comparing contrasting pedogenic pathways of carbon enrichment.

The authors emphasize that the two profiles should be viewed as complementary case studies rather than a statistically replicated soil-landscape sequence, since geomorphic position, drainage, sedimentary history, parent material, land use and vegetation all vary simultaneously between the sites. Even so, the comparison demonstrates how integrating field profile description, standard physico-chemical analyses and portable XRF geochemical screening can identify sedimentary, pedogenic and geochemical patterns at the profile scale in intramontane basins. The team notes that future work across the full Giurgeu-Ciuc-Brasov depression chain, combined with bulk density measurements and full-depth carbon determinations, would allow landscape-scale carbon stocks to be quantified. As climate and hydrological regimes shift, understanding where and how mountain basins store their carbon becomes an increasingly urgent question, and these Romanian profiles provide a detailed snapshot of the mechanisms at work beneath some of Europe’s least-studied carbon-rich landscapes.

Subject of Research: Pedogenic controls on soil organic carbon accumulation in Phaeozem and Histosol profiles of the Ciuc intramontane depression, Eastern Carpathians, Romania

Article Title: Pedogenic controls on organic carbon accumulation in Phaeozem and Histosol profiles of the Ciuc intramontane depression, Eastern Carpathians, Romania

Article References: Pîrnău, R. G., Roșca, B., Vasiliniuc, I., Secu, C. V., Patriche, C. V., Keresztesi, Á., & Szép, R. (2026). Pedogenic controls on organic carbon accumulation in Phaeozem and Histosol profiles of the Ciuc intramontane depression, Eastern Carpathians, Romania. Discover Soil, 3(1), Article 163. https://doi.org/10.1007/s44378-026-00322-w

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00322-w

Keywords: soil organic carbon, Phaeozem, Histosol, pedogenesis, Eastern Carpathians, intramontane depression, minerotrophic fen, portable X-ray fluorescence, calcium stabilization, chernic horizon, peat preservation, Ciuc Depression