Heavy Metals and Arsenic Species Traced Across the Mae-Kok River System

The Mae-Kok River, which flows from the border highlands of northern Thailand through Chiang Rai Province before joining the Mekong system, has long been suspected of carrying an unseen burden of metal contamination from upstream mining and land-use pressures. Now, a comprehensive multi-matrix study published in the Archives of Environmental Contamination and Toxicology has delivered the most detailed picture yet of what is actually moving through this transboundary waterway, combining measurements of river water, soils, and sediments with sophisticated statistical source-attribution tools and cutting-edge synchrotron X-ray spectroscopy.

Led by Siwatt Pongpiachan of the National Institute of Development Administration in Bangkok, with corresponding author Patcharee Pripdeevech of Mae Fah Luang University, the thirteen-member research team found that concentrations of arsenic, lead, and nickel in the river water frequently exceeded international guideline values for safe water quality. This finding alone would be concerning, but the study goes considerably further than simply documenting exceedances. By analyzing multiple environmental compartments simultaneously, the researchers were able to distinguish between metals that are actively being flushed downstream in the water column and those that have accumulated over longer periods in soils and riverbed sediments, revealing two fundamentally different contamination stories unfolding along the same river.

A central technique in the study was enrichment factor analysis, a geochemical method that compares the concentration of a metal at a given site with its concentration relative to a conservative reference element, typically normalized against average crustal abundances. When the enrichment factor for a particular metal substantially exceeds one, it signals that the element has been augmented by sources beyond natural weathering of local bedrock. In the Mae-Kok system, the enrichment factor results pointed to substantial anthropogenic enrichment, with the most pronounced signals appearing in the upstream reach of the river. This spatial pattern is significant because it aligns with the geography of upstream extractive and industrial activities that have drawn public concern in recent years, though the authors are careful to note what their data can and cannot prove.

To probe the question of sources more rigorously, the team deployed a trio of multivariate statistical approaches: hierarchical cluster analysis, principal component analysis, and positive matrix factorization. Hierarchical cluster analysis groups elements according to the similarity of their concentration patterns across sampling sites, effectively revealing which metals travel together. Principal component analysis reduces the large dataset of correlated variables into a smaller number of independent factors that can be interpreted as underlying geochemical processes. Positive matrix factorization, originally developed for air quality research by Pentti Paatero and Unto Tapper in the 1990s, goes a step further by apportioning measured concentrations among a mathematically derived set of source profiles, using a weighted least-squares approach constrained to non-negative values. Together, these methods identified a coherent grouping of arsenic, uranium, cobalt, nickel, and copper, a suite of elements the authors describe as a polymetallic geochemical association.

The chemical logic behind this grouping is telling. Arsenic, cobalt, nickel, and copper are classically associated with sulfide ore deposits, where they occur together in minerals such as arsenopyrite and related sulfarsenide phases. The team’s results are compatible with upstream anthropogenic inputs involving sulfide-rich materials, possibly related to mining or ore processing that exposes these minerals to oxidation. When sulfide minerals weather in the presence of oxygen and water, they release their metal cargoes into solution, often generating acidity in the process that further mobilizes other metals. However, the authors emphasize an important caveat: the statistical patterns, while strongly suggestive, do not uniquely identify specific point sources. The fingerprint of a sulfide-associated element suite is consistent with mining-related inputs, but the same association could theoretically arise from other pathways, and receptor modeling of this kind attributes concentrations to source profiles rather than to named facilities.

In contrast to the polymetallic group, zinc and cadmium exhibited enrichment patterns and statistical behavior that set them apart, consistent instead with diffuse anthropogenic influences such as agricultural activities. Cadmium, in particular, is a well-known companion contaminant of phosphate fertilizers and certain agricultural practices, and zinc is widely used in agrochemicals and animal husbandry. Even here, the researchers exercise caution, noting that these associations are indirect. The distinction nonetheless matters for management: a point-source problem demands different interventions, such as treatment of discharges, than a diffuse one, which requires changes in land management across whole catchments.

Perhaps the most conceptually interesting finding concerns the differences between the water matrix and the combined soil-sediment matrix. The clustering of elements differed between these compartments, highlighting the contrasting behavior of metals during short-term aqueous transport versus longer-term depositional accumulation. In the water column, metals are subject to rapid changes in pH, redox conditions, and complexation with dissolved organic matter, and they move quickly downstream. In soils and sediments, by contrast, metals accumulate over years to decades, binding to iron and manganese oxides, clay minerals, and organic matter. A metal that appears diluted in water today may be stored in the riverbed tomorrow and re-released under changed chemical conditions. This sediment archive is precisely why multi-matrix studies are considered essential for reconstructing contamination histories and anticipating future risks.

The most technically ambitious component of the study involved synchrotron-based X-ray absorption spectroscopy, performed in collaboration with the Synchrotron Light Research Institute in Nakhon Ratchasima, Thailand. Unlike conventional total-concentration measurements, X-ray absorption spectroscopy interrogates the local electronic environment of an atom, allowing researchers to determine not just how much of an element is present but in what chemical form. For arsenic, this distinction is critical to toxicity and mobility. The technique relies on tuning an intense, energy-scannable X-ray beam to the absorption edge of the element of interest and analyzing the near-edge region, known as XANES, alongside the extended oscillations that follow it. The position and shape of the absorption edge reveal the element’s oxidation state, while fits against reference compounds allow quantitative speciation of mixtures.

The results showed that arsenic in the Mae-Kok system occurs predominantly as arsenate, the pentavalent form As(V), which tends to bind strongly to iron oxide minerals under aerobic conditions and is generally less mobile and less toxic than its trivalent counterpart. However, the team also detected localized enrichment of arsenite, As(III), which indicates spatial variability in redox conditions within the river system. Where oxygen is depleted, in waterlogged sediments or stagnant zones, arsenate can be microbially reduced to arsenite, which adsorbs more weakly to mineral surfaces and can therefore be released into the water. This mechanism, famously implicated in the widespread arsenic contamination of groundwater in Bangladesh and West Bengal, serves as a warning that portions of the Mae-Kok system may hold arsenic in a form primed for mobilization if conditions shift. In practical terms, the finding means that total arsenic measurements alone would mask pockets of elevated risk.

Chromium told a more reassuring story. Spectroscopic analysis showed that chromium is present mainly as Cr(III), the reduced form of the element, which forms insoluble hydroxides and binds tightly to particles, suggesting limited mobility under the prevailing environmental conditions. This contrasts with hexavalent chromium, Cr(VI), a carcinogenic and highly soluble species, whose absence indicates that the river’s chromium burden, while possibly elevated, is currently locked in relatively inert chemical forms. The finding underscores why speciation, not merely concentration, should guide ecological and human health risk assessment.

Taken together, the study paints a picture of a multi-source contamination regime shaped by overlapping upstream and local anthropogenic influences, superimposed on whatever geochemical background the geology itself provides. The authors argue that this integrated approach, spanning three environmental matrices, three statistical tools, and synchrotron-level molecular scrutiny, substantially improves understanding of metal behavior in transboundary river systems and provides a scientific basis for future monitoring and management efforts. For a river whose waters serve agriculture, fisheries, and communities on both sides of international borders, that evidence base arrives at a critical moment. The research was funded by Thailand’s Program Management Unit for Human Resources and Institutional Development, Research and Innovation, and the team notes that while their methods can attribute contamination to coherent chemical fingerprints, pinpointing individual sources will require complementary investigations, including isotopic tracing and direct monitoring of candidate discharge points. What is already clear is that the Mae-Kok carries a chemically layered legacy, one whose most dangerous forms may be the ones currently at rest in the sediment, waiting for changing conditions to set them free.