4.1. Methods of sampling and separating microplastics
The selection of sampling sites impacts the abundance and types of polymers found (Kim et al., 2015), because contamination of beach sand by microplastics occurs through rivers, sea currents, and anthropogenic activities along the coast, as well as transport by water movement or by air emissions. Transects were commonly used when surveying beaches and quadrants were used in many previous studies (Hanvey et al., 2017). The strandline is known for containing a higher abundance of marine debris. Therefore, many studies target this tidal line to survey microplastics (Faure et al., 2015; Nel and Froneman, 2015; Song et al., 2015). However, this may lead to bias in the results, because the studies are aimed at finding microplastics, rather than designing a systematic measurement of microplastics distribution on the beach. Therefore, we conducted a microplastics study at three different tidal lines for each sampling area.
The abundance and level of accumulation of microplastics were also affected by the thickness of the collected sand layer. Many studies only focus on the surface layer of the sand. However, understanding the vertical distribution of microplastics may reveal their fate and the potential pollution of the sampled areas. Some studies found a different distribution of microplastics with depth (Carson et al., 2011; Claessens et al., 2011; Turra et al., 2014). The distribution of microplastics decreased from the surface to the deeper layer; however, the proportion at each layer varied depending on the study. Therefore, in the present study, collecting sand samples vertically was done to evaluate the accumulation of microplastics in the sand.
After sampling and sieving, subsamples of beach sand were transferred into a beaker for density separation. Depending on the study design, 50 to 500 g of sand were selected (Korez et al., 2019; Li et al., 2018; Lots et al., 2017; Saliu et al., 2018). We conducted trial experiments to determine a suitable mass for subsample analysis. We started with 20 g of subsample and then increased the mass until microplastics were detected in approximately 200 g of sample. To extract microplastics from beach sand, sodium chloride (d = 1.2 g/mL) is commonly used, because many polymers float in this solution. However, to ensure the most common polymer types were extracted, a solution of 1.5 g/mL NaCl and ZnCl2 was used in the present study.
Microplastics were found in beach sand at four sampling areas as described in section 3.1. The occurrence of microplastics in beach sand may be explained by the geography of Can Gio compared with other areas. In addition, Can Gio is a suburban agricultural area of HCMC. Therefore, plastic waste and microplastics can be transported by rivers from inland regions and accumulate on the beach.
4.2.1. Comparison with other beaches
In the present study, sand samples were collected along three tidal lines at two different depths. The abundance of microplastics ranged from 0 to 6.58 pieces/kg d.w. and these results were compared with those of other published studies (Table 3). However, there were various sampling methods in other studies, so we recalculated the abundance of microplastics in different ways for comparison (Table 2).
Table 3
Different researches in beach sand in the world in comparison with this study
Reference | Research area | Sampling | Sampling area’s characteristics | Microplastics abundance |
(Eo et al., 2018) | South Korea | 3 tidal lines 100 m stretch 0.5 x 0.5 m quadrat Upper 2.5 cm of sand | beaches with different features: ● polluted beach. ● deserted peninsula. ● Port area, residential. The beaches have different geomorphology and tidal regime. | 1400–62800 pieces/m2 (small microplastics) 0–2088 pieces/m2 (large microplastics) |
(Wu et al., 2020) | China | 3 tidal lines 100 m stretch 0.5 x 0.5 m quadrat Upper 5 cm of sand | Recreational beaches with different levels of impact | 106.50 ± 34.41 items/kg |
(Bosker et al., 2018) | Netherland | Strandline 0.5 x 0.5 m quadrat Upper 5 cm of sand | highly exposed area to seasonal extreme events (tropical hurricanes) | 68 ± 19–620 ± 96 pieces/kg d.w. (261 ± 6 pieces/kg d.w.) |
(Wessel et al., 2016) | USA | Wrack line 0.25 x 0.25 quadrat Upper 3–6 cm of sand | • Gulf of Mexico Fourth largest estuary in the USA | 5–117 pieces/m2 |
(Saliu et al., 2018) | Maldives | Drift line 1 x 1 m grid Upper 1 cm of sand | 2 beaches have different characteristics: ● An area has coral, the banks are gentle, shallow, and polluted. • An area has strong, deep waves, no anthropogenic activities. | 22.8 ± 10.5 pieces/m2 |
(Korez et al., 2019) | Slovenia | Strandline Upper 4 cm of sand | Slovenian beaches | 0.5 ± 0.5 pieces/kg d.w. to 1.0 ± 0.8 pieces/kg d.w. |
(Kunz et al., 2016) | Northern Taiwan | Middle of tidal zone 0.5 x 0.5 m quadrat 2 layers of sand (0–5 cm, 5–10 cm) | ● rocky shore, narrow sandy beach. Sampling beaches proximity to industrial zones, residential areas, estuaries, aquaculture zones, and seaports. | 0.23–30.4 pieces/kg d.w. |
(Tran Nguyen et al., 2020) | Danang, Vietnam | Transect from water-edge to vegetation zone 2 layers of sand (0–5 cm, 5–10 cm) | ● one of biggest and major coastal cities of Vietnam ● sandy beaches | 9238 ± 2097 items/kg d.w. synthetic fibers |
(Garcés-Ordóñez et al., 2019) | Colombia Caribbean | Random sites 1 m2 | Ciénaga Grande de Santa Marta mangrove | 31–2863 pieces/kg d.w. |
(Nor and Obbard, 2014) | Singapore | Low tide Upper 3–4 cm of sand 1.5 x 1.5 m quadrat | Mangrove Ecosystems | 60.7 ± 27.2 pieces/kg d.w. |
(Naji et al., 2019) | Iran | Random sites Upper 5 cm of sand | The Iranian mangrove forest is located between the Persian Gulf and the Oman Sea | 19.5 ± 6.36 to 34.5 ± 0.71 pieces/kg d.w. |
(Zhou et al., 2020) | China | Random sites 0.3 x 0.3 m quadrat Upper 2 cm of sand | China's Southeast mangroves | 8.3–5738.3 pieces/kg d.w. |
This study, 2019 | Vietnam | 3 tidal lines 100 m stretch 0.5 x 0.5 m quadrat 2 layers of 2 cm (surface and 5 cm depth) | • Frequently affected by erosion • Receive the flow from mangrove • Beaches with different anthropogenic activities | 0 to 6.58 pieces/kg d.w. (0–26.32 pieces/m2) |
Some studies selected sampling sites along three 100 m stretches of the tidal lines (upper shoreline, middle line, and water-edge line) and used 0.5 × 0.5 m quadrants and one layer of sand (upper 2.5–5 cm). Therefore, the abundance of microplastics in the present study, which was consistent with this sampling method, was 0–9.55 pieces/kg d.w. (0–38.2 pieces/m2). This abundance was much lower than that of studies in South Korea (1400–62,800 pieces/m2 small microplastics; 0–2088 pieces/m2 large microplastics) and China (106.50 ± 34.41 items/kg) (Eo et al., 2018; Wu et al., 2020). Other studies sampled the strandline (or high tide line) and also collected only one layer of sand (upper 1–6 cm of sand). The abundance in the present study was 0–26.94 pieces/kg d.w. (0–107.76 pieces/m2) following that sampling method. Compared with the results in the Netherlands (261 ± 6 pieces/kg d.w.) (Bosker et al., 2018), the abundance in Can Gio, Vietnam was lower and was similar to the abundance of microplastics observed in the USA (5–117 pieces/m2) (Wessel et al., 2016) and higher than that of the Maldives (22.8 ± 10.5 pieces/m2) (Saliu et al., 2018) and Slovenia (0.5 ± 0.5 pieces/kg d.w. to 1.0 ± 0.8 pieces/kg d.w.) (Korez et al., 2019). Several studies collected sand at two depths (0–5 cm and 5–10 cm), but with different collection sites on the beach (Table 3) (Kunz et al., 2016; Tran Nguyen et al., 2020). The abundance of microplastics in these studies (0.23–30.4 pieces/ kg d.w., Northern Taiwan; 9238 ± 2097 items/kg d.w. synthetic fibers, Danang, Vietnam) was higher than that of this study (0 to 6.58 pieces/kg d.w.). The results indicate that microplastics in Can Gio are less abundant than those in the Mangrove Ecosystems of Singapore (Nor and Obbard, 2014), the Ciénaga Grande de Santa Marta mangroves in the Caribbean of Colombia (Garcés-Ordóñez et al., 2019), and China's Southeast mangroves (Zhou et al., 2020).
The lower abundance of MPs in Can Gio compared with other areas may be explained by the following reasons. First, Can Gio is a suburban agricultural area of HCMC. Despite its large area, it has a low population density and is primarily used for growing rice, fruit trees, salt making, and aquaculture. Tourism activities, such as swimming, do not occur much, so this may be an explanation for the low abundance of microplastics (0.5–5 mm) in Can Gio compared with other studies. Second, beach sand in Can Gio is small and fine because the beach is accreted by alluvium from the Sac mangrove. Furthermore, erosion frequently occurs so the accumulation of microplastics may be low.
4.2.2. Distribution of MPs at different sampling areas and sand depths
In this study, DHM is at the intersection between the Dong Tranh River and the Can Gio Sea, located near the Dong Hoa market and receiving water flow from inland regions. These are probably the sources of plastic and microplastic emissions, resulting in a high concentration of microplastics (4.49 pieces/kg d.w. or 18.66 mg/kg d.w.). PNPR is one of Can Gio’s most famous tourist resorts and the plastic wastes found at the sampling sites were primarily Styrofoam boxes, plastic instant noodle cups, spoons, forks, and plastic straws. 30AB is the most famous beach in Can Gio, which is in the middle of the Can Gio coastline. Compared with other sites in which stone embankments were built at a 100 m distance along the coast to prevent erosion, 30AB has no such construction. As a result, waves usually climb up the coast and erode the coastal dunes. In recent years, volunteer beach cleaning campaigns are regularly held in this area, so this may contribute to a lower abundance of microplastics. AA is located near Ghenh Rai Bay and the microplastics found there were primarily in the form of fibers, such as fishing gear. The sources of plastic and microplastic emissions may be from coastal aquaculture and sea waves. CGP is an area receiving waste flowing out of the Ghenh Rai Bay and from swimming activity. The plastic waste found has similarities with PNPR, such as sausage packaging and instant noodle containers. This beach hosts cleaning activities, which results in a lower concentration of microplastics (0.68 pieces/kg d.w.). In conclusion, the source of microplastics at the sampling sites in Can Gio may be from livelihood activities, aquaculture, coastal tourism, and waste percolating from the Dong Tranh River and Ghenh Rai Bay, as well as waves washing ashore.
In this study, all microplastics were found at the upper shorelines. If the abundance of microplastics was only assessed at the upper shoreline, the highest abundance of microplastics would be 19.74 pieces/kg d.w. at PNPR. In total, there were 30 sand samples collected at the upper shoreline, and microplastics were found in six sand samples (p < 0.05). The method of determining the sampling location was different for each study. Samples can be collected along the upper shoreline where a lot of plastic wastes accumulate (de Jesus Piñon-Colin et al., 2018; Young and Elliott, 2016), in the middle of the intertidal zone (Kunz et al., 2016), or along the water-edge line and the upper shoreline (Eo et al., 2018). The number of sand samples collected in each study was different but ranged from 1 to 12 sand samples per site (de Jesus Piñon-Colin et al., 2018; Eo et al., 2018; Young and Elliott, 2016). The number of sand samples and sampling sites can affect the reliability of the study (Besley et al., 2017). According to Besley, among 22 studies of beach sand, the sampling process was optimal when the number of sand samples collected per 100 m of each tidal line was from 3 to 5 sites, depending on the desired reliability. In the present study, Fig. 2c shows that most of the detected microplastics distributed at the surface layer of sand (0–2 cm) accounted for 83.3% of the total microplastics found. Only one sample of PNPR showed the presence of microplastics at a 5 cm depth. There is no consensus on sand sampling depth in the literature. Some studies collected one surface sand layer with a 2 to 5 cm thickness to identify microplastics (de Jesus Piñon-Colin et al., 2018; Eo et al., 2018; Li et al., 2018). Other studies examined the distribution of microplastics by depth (Besley et al., 2017; Young and Elliott, 2016). The results indicated that the distribution of microplastics in beach sand at different depths is heterogeneous, but microplastics tend to accumulate closer to the surface. The results of this study on the distribution of microplastics at two depths in beach sand from Can Gio are similar to those of other published studies.
4.3. Physical characteristics of microplastics
Compared with other studies, the shape distribution of microplastics in the present study was similar to that published in Singapore (Nor and Obbard, 2014) and the Maldives (Saliu et al., 2018). The fragments accounted for a significant proportion, whereas fibrous microplastics accounted for the smallest percentage (21.4%) (Fig. 3b). The shape of microplastics may relate to their emission sources (primary or secondary microplastics). For example, according to Zhou et al. (Zhou et al., 2020), a high percentage of the fiber will be found at the sampling sites receiving effluent discharge containing a lot of microfibers. More than 93% of the microplastics in beach sand and sea sediments of mangroves are fragmented, which indicates that the origins of these microplastics are fragments of larger plastics. Besides, the impacts of other pollutants and environmental processes also affect the appearance of microplastics. They may be found in coastal areas and can have a basic shape or deformation resulting from erosion, solar radiation, or biodegradation (Zhou et al., 2020). The basic shapes are discharged directly as primary microplastics, such as pellets, granules, and spheres. Others are caused primarily by the decomposition of plastic items (secondary source) consisting of fibers and unspecified shapes. During cracking, the surface morphologies of plastic can change significantly, because of erosion or adherence of organisms (Imhof et al., 2013). Differences in surface morphologies relate to the longevity in the environment, the physical and chemical properties of the plastic, and other environmental factors. Therefore, to obtain more insights into the origin of microplastics, the chemical composition of macroplastics were also determined in the same samples including foam boxes, disintegrated polystyrene, or small pieces of plastic broken down from ropes.
The predominant colors of microplastics identified in the present study were white, blue, and green. White and blue were the two predominant colors (Fig. 3c) with a percentage of 40.5%. Blue microplastics were detected at all sampling sites. CGP only contained blue microplastics, which may be fragments of the plastic ropes used to tie sandbags to prevent corrosion along the coastline in this area. White granular microplastics found in DHM had intact and smooth surfaces that looked like resin pellets (possibly primary microplastics). However, the white microplastics found in PNPR were fragments and shards from Styrofoam (Fig. 5). Microplastics with similar colors were detected in other studies, such as in the Taiwan study (Walther et al., 2018), in which 60% of the microplastics were white and translucent. White is also a common color for Styrofoam fragments found in mangroves in China (Zhou et al., 2020), whereas green (23.1%) and blue (19.2%) were two common colors associated with fibrous microplastics. Blue fibrous microplastics accounted for 34.7% of the total. Other studies also showed a diversity of microplastics. For example, in the mangroves of Iran, black, blue, and white were the most common colors, with black representing the highest percentage at 41% (Naji et al., 2019).
4.4. Composition of microplastics
In general, PS was detected near tourist attraction areas without regular cleaning activity (PNPR) (Fig. 3d). PNPR contained predominantly PS at 59.1% (most of the microplastic pieces obtained were fragmented Styrofoam packaging). PP and PE were common in the aquaculture location (AA) or near the market (DHM), possibly originating from plastic ropes or food packaging. There were differences in microplastics distribution among the beaches; however, the results indicated that the shape, color, and chemical composition may be related. Fig. 4 shows the distribution of the shape, color, and chemical composition of microplastics, as well as the relationship between these characteristics. The majority of PS microplastics were white and had a granular shape. Moreover, PE and PP microplastics were observed in green or blue and had a fragmented or fiber shape. From these characteristics, the possible origins could be predicted as discussed below.
4.5. Prediction of microplastic origins
Besides determining the presence of microplastics in sand samples, identifying macroplastic pieces (>5 mm) was also performed. Macroplastics (>5 mm) found in all sampling sites had an average abundance ranging from 1.37 pieces/kg d.w. (30AB) to 6.10 pieces/kg d.w. (PNPR) (Fig. 2b and Table 1). The mass of macroplastic waste was quite high, up to 7,403.80 mg/kg d.w. (PNPR area). The abundance of macroplastics and microplastics at the same sampling site was compared and the results indicated that macroplastics were mostly present in a 2 cm surface layer. In addition, the distribution of macroplastics at the three tidal lines was similar to that of microplastics, in which 73.7% of the macroplastics were found at the upper shoreline. Macroplastics were also identified by FTIR-ATR analysis and the results showed that PP accounted for the highest percentage at 45.8%, followed by 31.3% for PE, whereas PS accounted for 22.9%. PP, PE, and PS are the three popular plastic types used worldwide. Macroplastics in the sand samples were primarily plastic bags, confectionery packaging, spoons, and forks. These are common items found on the beach in which anthropogenic activities occur. They are primarily made from PP and PE, which explains their abundance. The PS pieces found were mostly Styrofoam packaging, which may be the source of the corresponding microplastics in the same sand samples.
The preliminary origin of the microplastics and macroplastics was predicted based on the shape, color, and composition found in the same sand sample. In some samples, microplastics were found as fragments or fibrous shapes (Fig. 5a and b). They had the same color, composition, and surface morphologies as some of the macroplastics. These microplastics are created secondarily from the environmental degradation of larger pieces. Therefore, microplastics are possibly degraded from macroplastics in the same sand sample with similar characteristics and composition. Meanwhile, in some sand samples of DHM, microplastics had a cylindrical shape with a smooth surface (Fig. 5c). These pieces were of the original shape and identified as resin pellets (classified as granules in this study), which are the raw material for manufacturing plastic products. These microplastics were likely brought ashore by ocean waves after falling off ships during transportation.
DHM, PNPR, and AA had a higher abundance of microplastics and macroplastics. This may have occurred because DHM received water flow from the Dong Tranh River, which carried plant residues of the Sac mangrove, whereas PNPR was located next to the Ngoc Phuong Nam tourist area and the plastic wastes were primarily single-use products, such as foam packaging and plastic cups. AA is an aquaculture area that has agricultural tools. On the other hand, 30AB and CGP exhibited much lower concentrations of microplastics compared with the other areas. Common characteristics of these places were tourism and frequent visits from local residents. However, these places often have beach cleaning campaigns, which resulted in a smaller number of plastic waste. In addition, DHM, PNPR, and AA were constructed with stone embankments to prevent erosion, leading to more microplastic accumulation compared with other areas. The results showed some similarities to the research in the same region by Tien Giang and Vung Tau (Hien et al., 2020a). Microplastics in both studies shared some common characteristics in terms of distribution and chemical composition of the plastic. Tien Giang, Vung Tau, and Can Gio had similar characteristics of windiness, tidal regime, and water flow from the Dong Nai and Saigon rivers. Therefore, studies on the effect of marine dynamics on microplastics migration should be carried out to provide more insight into the fate of microplastics in the environment.