Investigation of microplastic pollution in the lower Chao Phraya River, Thailand

In recent years, microplastics (MPs) have received special attention in the public and scientific community due to their potential risks to the environment. MPs are plastic particles smaller than 5 mm in diameter. MPs are categorized into primary that is manufactured to microscopic sizes and secondary that is generated from the fragmentation of large plastics. Although freshwater environments are reported as major origins and pathways of MPs to the oceans, knowledge of MP pollution in the environment is insufficient. This study is conducted to bridge the knowledge gap for MPs pollution in the freshwater environment of Thailand. For this purpose, MPs were analyzed from sediment and surface water of the lower Chao Phraya River. Samples were collected at three parts of the river with different land-use. These three locations are Pa Mok (Ang Thong) which is an agricultural zone, Tha Pra Chan (Bangkok) is an urban zone, and Pak Nam (Samut Prakan) is the mouth of the river to the ocean with aquaculture and industrial zones. The main objective of this study is to obtain baseline data of MPs in the Chao Phraya River. The concentration and number of MPs in collected sediment and surface water samples were observed. Polymer types, morphologies, and colors of MPs were also investigated for tracking their origin. The impacts of land-use at sampling sites and seasons (rainy and dry) on the abundance of MPs were also reported in this study. Moreover, concentrations of heavy metals adsorbed on MPs were also reported. In this study, surface water samples were collected by a manta trawl with an aperture size of 0.3 mm, and sediment samples were collected by a Van Veen grab sampler. Particles in water samples were separated into four size ranges (i.e., 0.05 – 0.3, 0.3 – 0.5, 0.5 – 1.0, and 1.0 – 5.0 mm). Particles larger than 0.5 mm were examined by an optical microscope and characterized using Fourier transform infrared spectroscopy. Particles with a size smaller than 0.5 mm are digested with hydrogen peroxide. Density separation was also conducted with sodium iodide solution (1.5 g/cm3). After drying, the size fraction of 0.3 – 0.5 mm was examined in the same way as the particles larger than 0.5 mm. For the size fraction of 0.05 – 0.3 mm, particles were stained by Nile Red solution and were inspected by a fluorescence microscope. Polymer analysis of the particles was done by a micro-Fourier transform infrared spectroscopy. For sediment samples, duplicate 500 g wet sediment at each sampling point was used for analysis. The samples were dried and determined for total solids. MPs were extracted from the sediment by density separation using the same sodium iodide solution in the water samples. The supernatants from the extraction were treated similarly to the water samples. Moreover, the content of heavy metals (Cr, Cu, Ni, and Pb) on MPs was extracted by aqua regia solution and analyzed by ICP-OES.MPs were found in all surface water and sediment samples collected from the Chao Phraya River. At the agricultural zone, the mean numbers of MPs in surface water were 11 ± 2 and 6 ± 2 items/m³ for the sampling in the dry and rainy season, respectively. For sediment samples, the mean numbers of MPs were 3 ± 1 and 8 ± 1 items/kg for the dry and rainy seasons, respectively. At the urban zone, the mean numbers of MPs surface water were 80 ± 60 and 45 ± 13 items/m³ for the sampling in the dry and rainy season, respectively. For sediment samples, the mean numbers of MPs were 62 ± 11 and 41 ± 5 items/kg for the dry and rainy season, respectively. At the estuary zone, the mean numbers of MPs surface water of the river were 11 ± 4 and 48 ± 8 items/m³ for the sampling in the dry and rainy season, respectively. For sediment samples, the mean numbers of MPs were 11 ± 4 and 37 ± 14 items/kg for the dry and rainy seasons, respectively. These results demonstrated that the abundance of MPs increases with the increase of the urbanization rate. The abundance of MPs also affects by the season changes. The main morphologies of MPs were fragments and fibers at all sampling sites. Polypropylene and polyethylene particles were the most abundant in all surface water and sediment samples. Toxic metals (Cr, Cu, Ni, Pb, Cd, and Zn) were also found in MPs from surface water samples at three sampling sites. The metals are found higher at the agricultural and estuary zones in the rainy season. The Chao Phraya River is the major raw water supply and is an important aquaculture zone of Thailand. Therefore, the presence of MPs in the river may severely impact human health and aquaculture activities. As far as we know, this study is the first to report MP pollution in the Chao Phraya River. Therefore, this study provides important background data to bridge the knowledge gap for MP pollution in the freshwater environment of Thailand and the ASEAN region.