Spatial heterogeneity distributions and seasonal variations of microplastic contaminations in surface waters and sediments and microplastic pollution level assessments of the inner Gulf of Thailand

Abstract

Microplastics or the plastic particles less than 5 mm are the emerging pollutant that may be threat in marine ecosystems. Due to uncertainty in behavior of microplastics in the environment, this research objectives to analyze the characteristics of microplastic contaminations in surface waters and sediments of the inner Gulf of Thailand. The surface waters and sediments in dry season and wet season of 2018 were sampled from the coastal sea and the river estuaries of Mae Klong, Ta Chin, Chao Phraya, and Bang Pakong rivers. The analysis found that the influence of wet season increased contamination of microplastics from dry season. The average abundance of microplastics in water samples was increased from 8.70±15.34 pieces/L in dry season to 34.59±46.02 pieces/L in wet season, while that in sediment was increased from 7297±5960 pieces/kg of dry weight sediment in dry season to 12216±7079 pieces/kg of dry weight sediment in wet season. The distributions of microplastics illustrated the freshwater runoff and seasonal surface current controlled the transportation of microplastics in surface water; however, the microplastics tend to be aggregated with suspended solids and sedimented in the river estuaries more than that in coastal sea. The identification of plastic types indicated that polyethylene and polypropylene were the common plastic types in the study area as they were the most observed plastic types of microplastics in the water samples, while polyamide and poly (acrylonitrile: butadiene) were the major plastic types of microplastics in the sediment samples. In addition, the analysis found the contamination of heavy metals in the microplastic samples, especially in fouling accumulated on microplastics. The pollution level assessment revealed the contamination of microplastics in surface sediments of the inner Gulf of Thailand was in the level of very high contamination, while the ecological risk evaluation found the risk of microplastic contamination was in the moderate to high levels. The ecological risk index represents that the risk of microplastics was mainly regulated by plastic monomer chemicals, while the contaminated of heavy metals in microplastics resulted the small proportion in the total risk of microplastics. The revised risk assessment for the contamination of microplastics in this study could be included to the risk assessment of the contaminations of the other toxic substances for evaluating the potential ecological risk of the contaminations in sediment of the considered area.