Distribution and flux assessment of microplastic debris in the middle and lower Chao Phraya River, Thailand

Microplastics (MPs) now present a global debris problem with serious impacts on ecology and the environment. Plastics are made from non-renewable crude oil by several processes of fractionation and polymerization (Lithner et al. 2011). They are durable and accumulate in the environment causing serious concerns in aquatic ecosystems (Barnes et al. 2009). Macroplastics discharged onto the land break up into smaller pieces that are transported by surface runoff to rivers, and eventually to oceans, causing polluted habitat for benthic organisms.

Plastic debris breaks down into small pieces known as MPs, defined as small pieces of plastic with a size less than 5 mm in diameter (Barnes et al. 2009). MPs are categorized into two types depending on their origin. Primary MPs result from microbeads, while secondary MPs are broken down from large to small pieces of plastic by physical, chemical and biological interactions (Cole et al. 2011). MPs in the environment accumulate in aquatic animals by feeding. Previous studies have reported MP contamination in varied ecosystems such as beaches (Dowarah & Devipriya 2019), sediment (Firdaus et al. 2020), rivers (Kataoka et al. 2019), ocean (Kwon et al. 2020) and aquatic animals (Lefebvre et al. 2019) including indoor dust (Kashfi et al. 2022) and particulate matter (PM_2.5) from the atmospheric environment (Akhbarizadeh et al. 2021a).

MP contamination has also been reported in bottled drinking water (Akhbarizadeh et al. 2020a). The leaching of contaminants is an emerging concern in plastic bottled water. Phthalates or phthalic acid esters are added to plastics to make them more flexible. These plasticizers can contribute to the release of MPs into the environment. When plastic products that contain plasticizers are disposed of, they become brittle and breakdown into smaller particles that become MPs. These can associate with toxic substances and heavy metals in the environment, as found around the Persian Gulf Coastline in Iran (Dobaradaran et al. 2018; Hajiouni et al. 2022; Mohammadi et al. 2022). Akhbarizadeh et al. (2020b) found that canned tuna and mackerel were contaminated with MPs, thereby causing long-term impacts on human health.

Rivers are the main transportation routes of plastic and MP debris to the ocean from sewage discharge (Zhao et al. 2020). Plastic debris flow to the ocean in Thailand in 2015 was rated the sixth highest in the world (Jambeck et al. 2015). In Thailand, the Chao Phraya River draining the central part of the country has the highest water discharge flow to the estuary (HII 2012). Many anthropogenic activities in the Chao Phraya River basin discharge large amounts of plastic and MP debris into the riverine ecosystem. During the COVID-19 pandemic, studies showed that using personal protection equipment such as face masks and plastic gloves caused secondary MP pollution in the water environment (Akhbarizadeh et al. 2021b; De-la-Torre et al. 2022; Cabrejos-Cardeña et al. 2023).

The Chao Phraya River is a source of drinking water but the discharge of MP-polluted wastewater (i.e., brine) degrades water quality, making it unsuitable for drinking and industrial applications (Panagopoulos 2022; Panagopoulos & Giannika 2022). Wastewater management and desalination technologies are applied to produce drinking or agricultural water using renewable solar or wind farm energy to reduce the amount of plastic waste in the freshwater environment (Panagopoulos 2021).

Water treatment plants along the watercourse reduce plastic pollution from wastewater entering the aquatic environment. MP pollution is also removed during wastewater treatment processes (Takdastan et al. 2021). Proper methods to reduce MP pollution by wastewater treatment plants need to be developed to reduce the release of MPs and associated pollutants from anthropogenic activities into the environment in the future.

This study assessed the quantity of MPs in sedimentation and riverine flux in each province along the Chao Phraya River to clarify the MP pollution situation in Thailand. This research studied the distribution, abundance, composition and flux of MP debris in five provinces along the watercourse of the Chao Phraya River. MPs were collected in sediment using a sediment trap. Hydrological water quality data were also collected to estimate MP riverine flux and accumulation of MPs between provinces to clarify the extent of MP debris transported along the Chao Phraya River. The research focused on MP spatial distribution and seasonal variation, both quantitatively and qualitatively. MP riverine flux was estimated in each province along the Chao Phraya River using mathematical modeling modified from a box model.

The Chao Phraya River is a principal waterway located in the central part of Thailand draining an area of 20,523 km² (HII 2012). The Chao Phraya River originates in Nakhon Sawan Province at the confluence of the Ping, Wang, Yom and Nan Rivers, 379 km inland from the estuary (HII 2012). This major river flows through the central plains of Thailand where intensive agricultural, aquacultural, industrial and residential areas are located.

Water velocities were recorded at all sampling stations at a depth of 50 cm from the surface by an Alec electromagnetic current meter (JFE Advantech, Japan). Data for calculating the cross-sectional areas of the river were collected from a speedboat positioned perpendicular to the riverbanks using a handheld global positioning system (Garmin, Switzerland) and portable depth meter (Hondex, Japan), with results calculated by Surfer 22.0. Water quality data including salinity, temperature, pH and dissolved oxygen were collected by a multiparameter water quality meter (YSI, USA) with transparency data recorded using a Secchi disc.

The water and sediment sampling processes were modified by Chinfak et al. (2021). One water sample was collected in the middle of the river channel at each sampling station. About 200 L of water were collected at each sampling station in pre-cleaned 10-L stainless steel containers. The samples were then filtrated through three layers of 330-, 150- and 15-μm mesh size plankton nets stacked from top to bottom. The water samples were stored in 250 mL bottles, and 10% formalin (QReC, New Zealand) was added before analysis to inhibit decomposition and degradation from microorganisms.

One sediment sample was collected at the side of the river channel at each sampling station by a 15 cm × 15 cm × 10 cm Ekman grab as a 2-cm surface layer using a stainless steel spoon. All sediment samples were stored at −20 °C before analysis in a laboratory at Kasetsart University.

MP separation protocol was modified from the method of the National Oceanic and Atmospheric Administration (NOAA). Water samples were measured for volume and the wet peroxide oxidation (WPO) method was followed by adding 30% hydrogen peroxide (QReC, New Zealand) and 1% potassium hydroxide (QReC, New Zealand) at 9:1 and increasing the temperature to 75 °C while stirring until the organic gas bubbles disappeared. The organic matter was removed after 24 h. MPs were recovered using the density separation procedure by adding 1.3 g/cm³ NaCl solution and stirring with a glass rod. The supernatant was collected after standing for 24 h and filtered through a glass microfiber filter (Whatman GF/C pore size: 1.2 μm; diameter: 47 mm). The filter papers were dried in an oven at 60 °C and stored in glass Petri dishes with glass lids for later visualization and identification.

The sediment samples were homogenized, dried and weighed before analysis in the laboratory. The MPs were separated from the sediment by adding 1.3 g/cm³ NaCl solution, stirring with a stirrer and then waiting for 24 h before collecting the supernatant. The same protocol was followed for the water samples using the WPO method, density separation procedure, filtration and drying in an oven. After separation, the samples were stored for visualization and identification. Sediments from the sediment trap were analyzed following the same procedure used for the sediment samples.

The number, size, shape and color of the MP particles on the filter papers were observed and the number of pieces was counted using a stereo microscope (Olympus, Germany). The hot needle test was used to separate the MPs (De Witte et al. 2014). MP sizes were measured by the ImageJ program and categorized into three size ranges: 15–150 μm, 150–330 μm and 330–5,000 μm. All MPs were classified as fibers, fragments, films, pellets and foams (Ding et al. 2019). Six colors of MPs were recorded as black, blue, red, green, yellow and white with clear items also counted. The MPs were grouped by shape and color and verified at about 10% of the total MP particles in the water and sediment samples, while sedimentation traps were analyzed at 100%. The MP particles were separated as one item per Petri dish and verified by comparing reflection wavelengths and reference data in the database using a Fourier Transform Infrared Spectrophotometer (FTIR Type II; Perkin Elmer, UK).

SR refers to the sedimentation rate (g/m²/day); m refers to the mass of sediment (g); MSR refers to the microplastic sedimentation rate (items/m²/day); MP_flux refers to the microplastic riverine flux (items/day); MP_n refers to the quantity of microplastics in sediment trap (items); MP_conc refers to the contamination of microplastics in water (items/m³); A_s refers to the sediment trap opening cover area (m²); A refers to the cross-sectional area (m²); v refers to the current velocity (m/s); t refers to the soaking time (day).

Three blank controls obtained by distilling deionized water were analyzed in our laboratory following the same procedures used for the water and sediment samples. Results from the three blank controls showed no MP contamination.

Normal distribution was tested by the Shapiro-Wilk test (P-value = 0.05). Nonparametric statistics were also performed. The Kruskal–Wallis H test and pairwise comparison were used to test for MP abundance between provinces and seasons (P-value = 0.05).

Hydrological data were collected during the neap tides of September 2021 (wet season) and March 2022 (dry season). Current velocities and cross-sectional areas are shown in Table 1. Current velocities ranged from 0.08 to 0.27 m/s and 0.1 to 1.13 m/s in September 2021 and March 2022, respectively. In the wet season, current velocities during the sampling periods were lower than in the dry season caused by tidal elevation in the daily cycle. In March 2022, all data were collected during the ebb tide (11.00–18.00), while in September 2021 all data were collected during the flood tide (9.00–17.00). Tidal cycles caused different current velocities and water elevation in the Chao Phraya River. The highest cross-sectional area was recorded at C1 in Samut Prakan Province at the estuary of the Chao Phraya River for both sampling times. Water quality data collected at all sampling stations are shown in Table 2. The composition of ions in the Chao Phraya River water varies depending on several factors such as agricultural runoff, industrial discharge and urban wastewater discharge. Water contains various ions including bicarbonate, calcium, magnesium, sodium, chloride, sulfate and nitrate as well as other trace elements such as iron, manganese and zinc (Horiuchi et al. 2020). Water quality in the Chao Phraya River has been a concern for many years; it is impacted by various pollutants including heavy metals, organic compounds, nutrients and mismanagement debris. Dissolved oxygen was generally below 4 mg/L, indicating poor water quality in the Chao Phraya River Estuary that can affect aquatic animals. However, water quality data such as temperature, salinity, pH, transparency and dissolved oxygen were not significantly correlated with the abundance of MPs.

Comparison of our results with other studies should consider diverse sampling protocols, extraction, analysis and reporting units (Aliabad et al. 2019). Our results used similar protocol and reporting units as previous studies. Different water velocities during sampling also impact MP abundance. Talbot & Chang (2021) suggested that increased current velocity reduced MP contamination in the surface water of the river. MP abundances concurred with studies on the Tapi-Phumduang River, Thailand (0.68–2.81 items/L) (Chinfak et al. 2021) and Hudson River, USA (0.98 items/L) (Miller et al. 2017), while our concentrations of MPs were lower than in the Yangtze River Basin (0.3–3.1 items/L) (Su et al. 2018) and the Wei River (3.67–10.7 items/L) (Ding et al. 2019).

MP contamination in water values was similar but river size, water flow, turbulence and community density along the banks are also crucial factors that must be considered when comparing and analyzing contamination in water and sediment. Differences in these factors can significantly affect the amount of MPs released into the environment. Large rivers with high water runoff have a greater impact on the amount of MPs discharged into the environment than small rivers with low water runoff. Waste management potential in each province and region is another factor that impacts the amounts of MP pollution in watercourses and accumulation in sediment throughout Thailand.

Highest abundances of MPs in the water were found near the estuary in Bangkok and Samut Prakan Provinces, indicating that MPs have a long residence time before flowing out of the estuary because tidal fluctuations near the estuary increase MP accumulation in surface water (Oo et al. 2021). Higher contamination of MPs in the surface water of riverine ecosystems also resulted from higher anthropogenic utilization near the sampling stations.

MP average abundance in sediment samples in March 2021, September 2021 and March 2022 ranged from 316.74 ± 83.49 to 546.18 ± 86.84 items/kg dw, 198.02 ± 63.90 to 313.13 ± 153.05 items/kg dw and 183.84 ± 38.76 to 361.83 ± 62.38 items/kg dw, respectively. Average seasonal abundances of MPs in each province are shown in Figure 3. The highest average MP concentration was found in Nonthaburi Province in March 2021 (Kruskal–Wallis, P < 0.05) at 546.18 ± 86.84 items/kg dw. The lowest average MP concentration was found in Phra Nakhon Si Ayutthaya Province during March 2022 at 183.84 ± 38.76 items/kg dw.

Concentrations of MPs in sediment were lower than those found by Ta et al. (2020) at 2,290 items/kg. MPs are primarily generated from the breakdown of larger plastic materials. Reducing plastic utilization following the roadmap on plastic waste management (2018–2030) in Thailand will decrease the amount of MPs released into the environment. Improved waste management practices, including recycling and proper disposal, will also reduce the amount of plastic waste that enters the environment, thereby decreasing MP contamination in rivers. Implementation of regulations to limit the use of single-use plastics or the release of MPs into the environment can also lead to a reduction in MP contamination, while changes in hydrological patterns, such as increasing river flow could also result in decreasing MP contamination in sediment. Anthropogenic utilization around sampling sites should also be considered. Our results showed higher values than in the Tapi-Phumduang River system in Thailand (55–160 items/kg) (Chinfak et al. 2021), while waste accumulation in the Thames River in the UK was similar to our study (185–660 items/kg dw) (Horton et al. 2016). Interestingly, accumulations of MPs in river system sediment around the world were generally higher than in our study, with Pearl River (80–959 items/kg) (Lin et al. 2018), Wei River (360–1,320 items/kg) (Ding et al. 2019) and the Rhine River (228–3,763 items/kg) (Klein et al. 2015).

Spatial distributions of MPs between provinces were not significantly different but low MP accumulations were recorded in the middle part of the Chao Phraya River in Pathum Thani and Phra Nakhon Si Ayutthaya Provinces. MP accumulation increased in the lower part of the Chao Phraya River in Samut Prakan, Bangkok and Nonthaburi Provinces. The number of MP particles in the sediment at the river mouth increased, indicating that tidal fluctuations increased the probability of MP sedimentation. This phenomenon occurred because of the longer residence time of MPs near the estuary. Various mechanisms such as biofouling also increased MP density leading to increased MP settling to bed sediments near estuary sites (Kumar et al. 2021).

The abundance of MPs in the sediment increased from anthropogenic utilization near sampling stations, while hydrological factors also caused diverse accumulation of MPs in riverine ecosystem sediments. In the dry season (March 2021), highest accumulation of MPs was recorded, indicating that low water velocity and low water discharge levels affect the accumulation of MPs in sediment by increasing settling probability (Jiwarungrueangkul et al. 2021; Kundu et al. 2022).

In sediment samples, the 330–5,000 μm MPs were dominant in all seasons (Kruskal–Wallis, P < 0.05) as 75.6, 76.8 and 76.9% in March 2021, September 2021 and March 2022, respectively (Figure 4(b)). MPs in sediment were generally found with high proportions of large size. This result concurred with the Yong Feng River in China, where more than 60% of MPs were larger than 330 μm (Rao et al. 2019) and also the Skudai and Tebrau Rivers in Malaysia (Sarijan et al. 2018). MPs can remain suspended in water for a long time before settling into the sediment. Degradation processes in water from UV light are faster than mechanical degradation processes such as waves, turbulence or water currents. If MPs in rivers sink into the sediment, degradation processes will slow down. Large-sized MPs were mostly accumulated in sediment, with a higher probability of sinking into sediments than small-sized particles (Nizzetto et al. 2016). All sizes of MPs can disrupt the marine environment, especially benthic ecosystems. MPs can absorb and desorb hazardous pollutants which cause serious problems in the aquatic environment (Cox et al. 2019). Small-sized MPs can easily be ingested by organisms, leading to human health impacts in the food chain.

Increased numbers of MP fragments in sediment samples were found in March 2022 (49.5%), while fibers were also dominant in March 2021 (59.6%) (Kruskal–Wallis, P < 0.05). Fibers were the dominant group in sediment, while fragments increased in September 2021. This result concurred with Ding et al. (2019) in the Wei River. They found that fibers showed potential for suspension in the water column before settling to sediment more than fragments over time (Kumar et al. 2021). The large surface area of fragments leads to biofouling, with increasing density compared to fibers of the same size. High proportions of fibers, fragments and films in the riverine ecosystem indicated that the most common types of MPs in our study area were secondary MPs. MP films were the third highest type at 10.9, 8.9 and 16.3% in March 2021, September 2021 and March 2022, respectively. Films were found at all sampling stations and were also prevalent in previous studies of the Chao Phraya River (Sukhsangchan et al. 2020; Ta et al. 2020). Film shapes emanated from rotten plastic bags used for various purposes. Plastic bags degrade by several breakdown mechanisms including photodegradation (Kumar et al. 2021). Small pieces of plastic are easily transported by surface runoff to increase accumulation in aquatic environments.

Plastic pellets were recorded at 0.5–1.8%, with only a small proportion of foams (Figure 5(b)) found in surface water and sediment. Low levels of pellets and foams possibly resulted from the roadmap on plastic waste management (2018–2030) in Thailand which banned the use of microbeads in personal care products. However, single-use foams as food carriers are still in use.

For fibers, 45.0 and 45.2% found in water and sediment were blue, while 64.5 and 39.4% of fragments were also blue. Green was dominant for fragments in March 2022 (Kruskal–Wallis, P < 0.05), while black was found in fibers at all sampling stations. Red fibers were found at all sampling stations, with lower numbers of yellow fibers. Clear MPs were only found as fibers and films, while white MPs were mainly fragments and foams. Blue MPs were dominant in both water and sediment samples. These results concurred with Sarijan et al. (2018) who suggested that blue color was dominant in MPs and played an important role in preventing plastic photodegradation. Different wavelengths of ultraviolet irradiation impact the photodegradation of plastics. Light energy is transmitted by blue rather than red, and plastic degradation rates increase when the color does not effectively absorb UV transmittance (Zhao et al. 2022).

Results demonstrated that fiber polymers in water and sediment were PET at 81.9 and 70.2%, with PP at 16.8 and 28.8%. Fragmented MPs in water and sediment comprised PP at 55.8 and 43.3% with PE at 22.8 and 34.7%, respectively. Poly (ethylene:propylene:diene), PBE and PEI were found with fragment shapes in green, blue and white, respectively. Pellets and foams in water and sediment samples were PP, whereas films in water and sediment were mostly PP (68.6%) with small proportions of PE (21.0%) and 2,2,4,4-tetramethyl-7-oxa-3-20, diazadispiro (10.3%). Oo et al. (2021) found that MPs in the Chao Phraya River Estuary were mostly PP and PE, while our results showed PET as the dominant polymer group, as also reported in the Yangtze and Hanjiang Rivers in China (Wang et al. 2017) and Jiangzhou Bay (Zheng et al. 2019).

PET was mostly found in fiber shape. This polymer is widely used in food packaging as a fabric material with various applications (Dhaka et al. 2022). Polyethylene is also widely used in various products such as film packaging, caps and storage containers including bottles (Andrady & Neal 2009). PP is commonly used in various base materials for household applications (Alsabri et al. 2022) and was found in all shapes. Poly (ethylene:propylene:diene) is widely used to make automotive and electronic appliance parts and also in the construction industry (Athawale & Joshi 2016) and was mostly found as green fragments. Polyethyleneimines-PEI is used in thermosetting plastic material (Acebo et al. 2017) and was mostly found as white fragments, while polyester cotton (65:35) was found as green fiber. This mixture of polymers is generally used in the textile industry. Poly (bisphenol A-co-epichlorohydrin)-PBE is a thermosetting plastic used in epoxy resin (Jones et al. 2015) and was mostly found as blue fragments. 2,2,4,4-Tetramethyl-7-oxa-3,20-d is an additive in polymeric material with antiultraviolet properties (Lahimer et al. 2017) and was only found as blue films and fragments, while poly (acrylonitrile ethylene propylene styrene) or AES is generally used in automotive parts and was only found as yellow fragments.

Our MSRs were higher than those recorded in Huruslahti Bay in Finland at only 88.8 items/m²/day (Saarni et al. 2017). However, polymer composition and shape were similar to PP, PE and PET with fiber and fragment shapes as the dominant groups.

Our main study limitation was the identification of MP polymer types by FTIR. In this study, 10% of the total MPs in water and sediment samples were analyzed, while all the MPs in sedimentation traps were analyzed. Categorizing MPs by shape and color using the hot needle test will help to identify MPs and resolve the weakness of the investigation on MP type verification.

MPs were present at all sampling stations. Average abundance in all provinces in water samples ranged from 0.54 ± 0.05 to 1.07 ± 0.28 items/L, while in sediment samples ranged from 183.84 ± 38.76 to 546.18 ± 86.84 items/kg dry weight throughout all seasons. Significant seasonal variations were found in water and sediment samples. Spatial distribution was not significantly different for MP abundance. MP size ranges in water samples were different from sediment samples. The size range of MPs in sediment was mostly between 330 and 5,000 μm, while sizes of MPs in water varied and shifted from small to large throughout the study period. Fiber MPs were abundant in both water and sediment samples, with fragment MPs significantly higher in sediment samples than in water samples. Blue MPs were mostly fibers and fragments. Polymer compositions were mostly PET, PP and PE whereas PET was only found in fiber shape. Some polymers were additives in MPs. 2,2,4,4-Tetramethyl-7-oxa-3,20-d is an additive in polymeric material for antiultraviolet properties. Spatial and temporal MP abundances along the Chao Phraya River were associated with hydrological factors such as current velocity and tidal fluctuation. SRs of MPs were not related to SRs of sediment samples. Significant differences between seasonal variations of MSRs were found. River current velocity is an important factor that determines the SR of MPs in riverine ecology. High water flow prevents MPs from settling and the SR will decrease. MP input from the terrestrial load into the riverine environment during the wet season and dry season was highest in Samut Prakan Province at 1.60 × 10⁵ million items/day and Nonthaburi at 1.21 × 10⁵ million items/day, respectively. Highest MP riverine flux discharged into the sea during the wet season and dry season was recorded in Samut Prakan Province at 1.83 × 10⁵ million items/day and 1.60 × 10⁵ million items/day, respectively. The composition of MPs from each province showed that synthetic MP fiber, especially the PET polymer type, was mostly discharged to the middle and lower sections of the Chao Phraya River. Our results indicated that fiber MPs emanated from the shredded cloth after washing in households from residential areas along the river. Large amounts of these pollutants enter the riverine ecosystem from sewerage waste into the Chao Phraya River and the Gulf of Thailand. Our findings will help to elucidate the types and abundance of MPs that flow into the Gulf of Thailand and can be used to set guidelines and measures for better MP and plastic debris management in Thailand in the future.

In future studies, SRs should be collected in different areas, with water volume data collected from various floodgates that transfer from each province into the Chao Phraya River. This will improve the assessment of the transport of MPs, accumulation, distribution and sedimentation processes in the environment.

This research and innovation activity was funded by the National Research Council of Thailand (NRCT). The authors would like to thank all members in the laboratory of the marine environment, Department of Marine Science, Faculty of Fisheries, Kasetsart University for their support during this research.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.