Interactions Between Heavy Metals and Microplastics in Surface Marine Sediments, Chanthaburi River Mouth, Eastern Gulf of Thailand

Sediment quality in global estuaries was reported by assessing the degree of anthropogenic input and the corresponding ecological risks. This research intended to categorize the quantities of marine pollution at the mouth of the Chanthaburi River, on the Eastern Gulf of Thailand, by examining the interactions amongst the heavy metals (Pb, Cd, Cu, Zn) and microplastics (MPs) in surface marine sediments. Marine pollution severity was classified using the Geo-accumulation Index (Igeo), Sediment Enrichment Factor (SEF), and Pollution Load Index (PLI). Spatial distribution of pollutants and geostatistical covariance were examined via Geographic Information System (GIS) and Principal Component Analysis (PCA). The average concentrations determined in sediment samples were as follows: Pb, 0.369 ± 0.022 ppm; Cd, 0.0042 ± 0.0004 ppm; Cu, 5.424 ± 0.007 ppm; Zn, 33.756 ± 0.182 ppm; and microplastics, 1.36 ± 0.06 particles/g. All metal levels were below the WASV, CCV, and TRV reference thresholds. Igeo and SEF indicated that Zn was moderately accumulated with minor enrichment, while other metals were unpolluted. PCA explained 90.85% of the variance, mainly reflecting Zn accumulation in downstream sites. We also found an inconspicuous correlation between heavy metals and MPs, which may be caused by distinct sources, physicochemical properties, and potential biological synergistic effects that remain unclear. A key originality of this study lies in the integration of GIS-based spatial interpolation with the PLI data to visualize and distinguish site-specific accumulation zones. The study did not assess biological uptake or biomarkers, limiting insight into actual bioavailability and toxicity to marine species. These findings provide spatially explicit evidence for targeted estuarine management and highlight the need for future studies on bioavailability and ecological risks.