Spatio-heterogeneity risk assessment from microplastic contaminations in water and sediment of the inner gulf of Thailand

The inner Gulf of Thailand, a shallow semi-enclosed marine basin, plays a vital economic and ecological role for Thailand, yet faces increasing threats from microplastic (MP) pollution driven by urbanization, industrial and agriculture developments, and riverine discharges. This study aimed to assess the spatial and vertical distributions, polymer characteristics, and ecological risks associated with MP contamination in water columns and surface sediments throughout the inner Gulf. Sampling was conducted at 13 stations, with water collected from surface, mid-depth, and bottom layers, and sediment samples taken from corresponding surface sediments. MPs were analyzed for abundance, size, shape, color, and polymer composition using stereomicroscopy and Fourier-transform infrared microscopy (micro-FTIR). Results revealed revealed moderate to high MP contamination in surface waters, with concentrations ranging from 0.72 to 12.21 pieces/L, and an average of 4.98 pieces/L. Surface sediments exhibited MP abundances between 73.1 and 351.2 pieces/kg, with higher levels near estuarine zones influenced by riverine inputs. Polymer identification showed that PTFE (52.17%), PP (45.17%), and PE (7.31%) were the dominant types in both water and sediment. While polystyrene (PS), although less abundant, was associated with higher ecological risks. Fibers and small particles (16–100 µm) were the most prevalent morphologies and size fractions. Vertical profiles in the water column demonstrated MP presence at all depths, with surface dominance in nearshore stations and mid-to-bottom accumulation offshore, likely driven by biofouling, flocculation, and sediment resuspension. Contamination Factor (CF) values indicated moderate to extremely high contamination in surface waters (average CF = 49 ± 28) and moderate to considerable contamination in sediments (average CF = 2.72 ± 1.19), particularly near the Chao Phraya and Bang Pakong River mouths. Ecological risk assessments using Polymer Risk Index (PRI), and Ecological Risk Factor (Er) indicated low overall ecological risk (Er= 0.87 ± 0.83), but moderate polymer-specific risk (PRI = 95.35 ± 133.45), especially in areas with elevated PS concentrations. Correlation analyses revealed that polymer type, rather than MP abundance, was the primary driver of ecological risk. This study provides the first integrated assessment of both water column and sediment-associated MPs in the inner Gulf of Thailand, highlighting their spatial and vertical heterogeneity and ecological implications. The findings establish a scientific basis for continuous monitoring and serve as essential input for policymakers and coastal resource managers. Targeted mitigation strategies focusing on source reduction and polymer-specific pollution control are recommended to reduce future environmental impacts and safeguard the region’s marine ecosystem health.