Multi-matrix Investigation of Heavy Metals and Arsenic Speciation in the Mae-Kok River System: Chemical...

The Mae-Kok River in northern Thailand is a transboundary river system influenced by multiple upstream and downstream anthropogenic pressures. This study presents a multi-matrix assessment of metal contamination by integrating river water, soils, and sediments with enrichment factor (EF) analysis, multivariate statistical approaches, and arsenic speciation. Concentrations of arsenic, lead, and nickel in river water frequently exceeded international guideline values, while EF results indicate substantial anthropogenic enrichment, particularly in the upstream reach of the river. Hierarchical cluster analysis, principal component analysis, and positive matrix factorization identify coherent groupings of elements (As, U, Co, Ni, and Cu) characterized by polymetallic geochemical associations. These patterns are compatible with upstream anthropogenic inputs involving sulfide-rich materials but do not uniquely identify specific point sources. In contrast, zinc and cadmium exhibit enrichment patterns and statistical character distinct from the polymetallic element group, consistent with diffuse anthropogenic influences such as agricultural activities, although these associations are indirect. Differences in clustering between river water and soil-sediment matrices highlight contrasting metal behavior between short-term aqueous transport and longer-term depositional accumulation. Synchrotron-based X-ray absorption spectroscopy shows that arsenic occurs predominantly as arsenate [As(V)], with localized enrichment of arsenite [As(III)] indicating spatial variability in redox conditions. Chromium is present mainly as Cr(III), suggesting limited mobility under prevailing environmental conditions. Overall, the results indicate a multi-source contamination regime shaped by overlapping upstream and local anthropogenic influences. This integrated approach improves understanding of metal behavior in transboundary river systems and provides a scientific basis for future monitoring and management efforts.