3.1. Abundance of microplastics
Analysis results show that microplastics were detected at 4 of 5 investigated sites with 42 pieces and the concentrations of microplastics were in the range of 0–6.58 pieces/kg d.w. The average mass of microplastics at sampling sites was also recorded, showing the number from 0 mg/ kg d.w. to 18.66 mg/kg d.w. Table 1 and Fig. 3 indicate the distribution of microplastics in different sites. Site 2 had the highest average abundance of microplastics with 6.58 pieces/ kg d.w, followed by site 1 with 4.49 pieces/ kg.d.w, and site 3 did not show any appearance of microplastics. In terms of microplastics’ load, site 1 found the most contaminated with the weight of microplastics found was about 18.66 mg/kg d.w, followed by site 2 with 13.33 mg/ kg.d.w. This study's results were compared to other previous research on microplastics contamination in beach sand (Table 2). The microplastics average concentration of Can Gio is similar to that of Maldives (22.8 ± 10.5 pieces/m2) (Saliu et al. 2018) and Taiwan (0.23–30.4 pieces/kg sand) (Walther et al. 2018), especially the concentration of microplastics in the beach sand of the Korean study (1400–62800 pieces/m2) (Eo et al. 2018) is much higher than this study. Besides, comparing the results to the concentration of microplastics in the Slovenian beach (0.5 ± 0.5 pieces/kg d.w to 1.0 ± 0.8 pieces/kg d.w) (Korez et al. 2019), the data in Can Gio is higher. These studies’ results showed that areas with higher microplastic concentration are in areas with residential activities, commercial location, seaports or shallow seas, large tidal ranges, complex shoreline and farming activities, aquaculture (Eo et al. 2018; Saliu et al. 2018; Walther et al. 2018; Zhao et al. 2018).
Table 1 Concentration of microplastics and plastics in beach sand in Can Gio, Ho Chi Minh city, Vietnam in November 2019
Site symbol
|
Location
|
Coordinate
|
Microplastic concentration
|
Plastic concentration
|
pieces/ kg d.w
|
mg/ kg d.w
|
pieces/ kg d.w
|
mg/ kg d.w
|
Site1_DHM
|
Dong Hoa Market
|
10.376830, 106.879996
|
4.49
|
18.66
|
3.72
|
2482.3
|
Site2_PNPR
|
Phuong Nam Pearl Resort
|
10.377025, 106.894204
|
6.58
|
13.33
|
5.71
|
7019.2
|
Site3_30AB
|
30th April Beach
|
10.389663, 106.928722
|
--
|
--
|
1.37
|
23.6
|
Site4_AA
|
Aquaculture area
|
10.400064, 106.954130
|
2.37
|
6.09
|
3.39
|
49.5
|
Site5_CGP
|
Can Gio Park
|
10.413471, 106.974338
|
0.68
|
0.20
|
2.76
|
1408.5
|
Table 2
Different researches in beach sand in the world in comparison with this study
Reference
|
Research area
|
Sampling area’s characteristics
|
Microplastics concentration
|
(Walther et al. 2018)
|
Northern Taiwan
|
● 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
|
(Eo et al. 2018)
|
South Korea
|
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)
|
(Saliu et al. 2018)
|
Maldives
|
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
|
Slovenian beaches
|
0.5 ± 0.5 pieces/kg d.w to 1.0 ± 0.8 pieces/kg d.w
|
(Garcés-Ordóñez et al. 2019)
|
Colombia Caribbean
|
Ciénaga Grande de Santa Marta mangrove
|
31–2863 pieces/kg d.w
|
(Nor and Obbard 2014)
|
Singapore
|
Mangrove Ecosystems
|
60.7 ± 27.2 pieces/kg d.w
|
(Naji et al. 2019)
|
Iran
|
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
|
China's Southeast mangroves
|
8.3–5738.3 pieces/kg d.w
|
This study, 2019
|
Vietnam
|
Sac Mangrove
|
0 to 6.58 pieces/kg d.w (0–26.32 pieces/ m2)
|
Can Gio beach is affected by the sedimentation because Dong Tranh river flowing from Sac mangrove carries alluvium, dead plants, and garbages. The average concentration of microplastics in this study was compared to some similar studies. The results show that microplastics in Can Gio have lower concentration(Garcés-Ordóñez et al. 2019; Zhou et al. 2020). The distribution of microplastics in beach sand in this study can be explained as Can Gio location in the funnel-shaped area on the estuary of Dong Nai river, which is also the intersection area between rivers and the sea. Therefore, plastic wastes and microplastics can be transported from the Sac mangrove as well as from rivers. There are many wharves and local markets along the rivers, so this is also a potential microplastics' source (Critchell and Lambrechts 2016; de Jesus Piñon-Colin et al. 2018). Besides that, erosion frequently happens in Can Gio, leading to microplastics hardly accumulate along the coastline. In addition, the beach sand here is fine, combining with alluvial and clay with small grain size (1 µm − 62.5 µm) (2017) and compact. Therefore, larger pieces of microplastics are not easy to accumulate according to depth. Can Gio is a suburban agricultural district, despite its large area but low population density, mainly living by growing rice, fruit trees, salt making, and aquaculture, tourism activities such as swimming is not much, so this can be an explanation for the low concentration of microplastics in Can Gio compared to other studies. The reasons for this distribution of microplastics are also discussed in the studies by O. Garcés-Ordóñez in Colombia (Garcés-Ordóñez et al. 2019) or in the Singapore mangrove forest (Nor and Obbard 2014), distribution and concentration of microplastics are influenced by population density and sea currents.
The effect of Can Gio characteristics on microplastics concentration at the sampling sites can be explained as follows: site 1 is the intersection between Dong Tranh River and Can Gio sea, located near Dong Hoa market, this is the place receiving the ruins from the Sac mangrove as well as on this river plus the market is populated, these are probably the sources of plastic and microplastics emissions that lead to the highest concentration of microplastics here accounting for 48.73% of the total number of microplastics found (4.49 pieces/kg d.w or 18.66 mg/kg d.w). Site 2 is located right next to Phuong Nam Pearl Resort, one of Can Gio’s most famous tourist resorts. The plastic waste found here is mainly the styrofoam boxes, plastic instant noodle cups, spoons, forks, plastic straws also partly reflect the origins of plastics and microplastics. Site 3 is April 30th beach, the most famous beach in Can Gio, located in the middle of Can Gio coastline. Compared to other sites where stone embarkments are built with a 100 m distance along the coast to prevent erosion since April 30th beach is a tourist attraction. Therefore, this area has no such construction, leading to waves usually climb up the coast, destroying the coastal dunes, causing erosion. In addition, in recent years, volunteer beach cleaning campaigns are regularly held in this area, so this may be the reason why the concentration of microplastics here is the lowest. Site 4 is a clam aquaculture area, which located near Ghenh Rai Bay. Microplastics found there mostly were in the form of fibers like the fishing gears. The source of plastic and microplastic emissions can be from coastal aquaculture and received from sea waves. Site 5 is Can Gio Park - the area receiving wastes flowing out of the Ghenh Rai Bay and swimming activities. Plastic wastes found have similarities with site 2, such as sausage packaging, instant noodles container. This site was offered many beach cleaning activities, leading to the concentration of microplastics was low (0.68 pieces/kg d.w). In conclusion, the source of microplastics of the sampling sites in Can Gio could be from livelihood activities, aquaculture, coastal tourism, and waste receiving from Dong Tranh river and Ghenh Rai bay, as well as the waves washed ashore.
All microplastics were found at the upper-shore line. If only assess the distribution of microplastics at the upper-shore line, the highest concentration of microplastics can be 19.74 pieces /kg d.w at site 2. In total, there were 30 sand samples collected at the upper-shore line, 6 sand samples showing microplastics detected. The t-test showed a significant difference in the concentration of microplastics at this tidal line (p-value < 0.05). The method of determining the sampling location was different in each research. Samples can be collected along the upper-shore line where a lot of plastic wastes accumulated (Young and Elliott 2016; de Jesus Piñon-Colin et al. 2018) or in the middle of the intertidal zone (Walther et al. 2018) or along the water-edge line and the upper-shore line (Eo et al. 2018). The number of sand samples collected in each study was different but ranged from 1 to 12 sand samples/site (Young and Elliott 2016; de Jesus Piñon-Colin et al. 2018; Eo et al. 2018; Walther et al. 2018). The number of sand samples and sampling position can affect the reliability of the study (Besley et al. 2017). According to Besley, among 22 studies on beach sand were compared, the results show that the sampling process is optimal when the number of sand samples collected per 100 m of each tidal line is from 3 to 5 positions, depending on the desired reliability. In this study, Fig. 4 shows that most of the detected microplastics distributed in the surface layer of sand (0–2 cm) accounted for 83.3% of the total microplastics. Only 1 sample of site 2 reported the presence of microplastics at 5 cm depth. There is no consensus on sand sampling depth in published research. A number of studies collected one surface sand layer from 2 to 5 cm thickness (de Jesus Piñon-Colin et al. 2018; Eo et al. 2018; Li et al. 2018). Other studies have examined the distribution of microplastics by depth (Young and Elliott 2016; Besley et al. 2017; Walther et al. 2018), the results preliminarily interpreted that the distribution of microplastics in beach sand at different depths is heterogeneous and featured for each study area, but microplastics tend to accumulate in the sand layers closer to the surface. Thus, the distribution of microplastics in beach sand in Can Gio was pretty similar to other published researches.
3.2. Physical Characteristics
The study focused on microplastics with a size from 0.5–5 mm. The size of microplastics was classified into three ranges from 0.5 to 1 mm, from 1 mm to 2.8 mm, and 2.8 mm to 5 mm. Figure 5 shows that microplastics with the size from 2.8 to 5 mm accounted for the highest percentage, with 71.46% on average. Microplastics with the size of 1 to 2.8 mm show the smallest percent, especially in site 5, with no appearance of microplastics with this size range. Microplastics smaller than 1 mm were often found as secondary microplastics fragmented from larger plastic such as filament or fragmented pieces found in sand samples.
Three shapes of microplastics found are fragment, fiber, and pellet (Fig. 5). Among the four sites detecting microplastics, sites 1 and 4 showed the highest concentration of fragmented microplastics, with 2.86 pieces/kg d.w (63.64% of the total microplastics of site 1) and 1.35 pieces/kg d.w (57.14% of the total microplastics of site 4). In site 2, pellet was dominant with 59.76% equivalent to 3.93 pieces/kg d.w. In total, pellets and fragments accounted for the highest percent, 41.31%, and 38.5%, respectively. Compared with other studies, the distribution of microplastics in this study is similar to those published in Singapore (Nor and Obbard 2014) or the Maldives (Saliu et al. 2018), fragmented microplastics showed a significant proportion while fibrous microplastics accounted for the smallest percentage. The shape of microplastics related to their sources which are primary or secondary. For example, a high percentage of the fiber found in the sampling sites was associated with the effluent discharge because the fibrous microplastics were correlated with the effluent output when the wash-holding process was identified as the primary source of microfiber. The distribution of thin-film microplastics could result from the transport of nearby salt fields or the fragmentation of plastic mulch used in horticulture (de Jesus Piñon-Colin et al. 2018). More than 93% of microplastics in the beach sands and sea sediments of mangroves are fragmented, predicting the origin of microplastics as secondary sources from large plastic fragments (Zhou et al. 2020). These conclusions are relatively appropriate with the proportion of microplastic shapes due to different anthropogenic activities of the sampling sites. Besides, the impacts of other pollutants and environmental processes also affect the appearance of microplastics. Microplastics found in coastal areas can have a basic shape or deformation due to 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 mainly 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 and cover by organisms (Imhof et al. 2013). Differences in surface morphologies relate to the longevity in the environment, physical and chemical properties of the plastic, and other environmental characteristics. Therefore, in order to get more insights into the origin of microplastics, in this research, besides monitoring microplastics in beach sand, macroplastics in the same sample, such as foam boxes, disintegrating polystyrene, or small pieces of plastic broken down from the rope, were also studied.
Microplastics with colors white, blue, green were mainly found, and the results show that white and blue were two dominant colors in this study (Fig. 5) with the percentage of 39.09% and 39.94%, respectively. Blue microplastics were detected at most among sampling sites. Its concentration ranged from 0.68 pieces/kg d.w to 2.65 pieces/kg d.w. Only sites 1 and 4 observe green microplastics with almost the same percentage (27% and 29 %). Site 5 only observes blue microplastics’ appearance, which can come from the fragmentation of the plastic ropes used to tie the sandbags to prevent corrosion along the coastline. The shape of white microplastics found in site 1 was pellets that if they were observed closer, their surfaces were intact and smooth, possibly primary microplastics. However, the white microplastics found in site 2 are fragments, shards from styrofoam (Expanded polystyrene). Microplastics with similar colors were detected in other studies, such as in Taiwan research (Walther et al. 2018); 60% of the microplastics were white and translucent. White is also a common color for styrofoam fragments found in a mangrove in China (Zhou et al. 2020), while green (23.1%) and blue (19.2%) were two common colors with fibrous microplastics, blue fibrous microplastics accounting for 34.7%. Some other studies also showed the diversity of microplastics. For instance, in Iran's mangroves, black, blue, and white were the most common colors, with the highest percentage is black at 41% (Naji et al. 2019).
3.3. Chemical Composition
Microplastics were confirmed the identity by using FTIR-ATR. The microplastics selected for chemical composition confirmation must satisfy the condition that they represent a group of microplastics from the same sand sample with the same color, size, shape, and similar surface morphology. In this study, polypropylene (PP), polyethylene (PE), and polystyrene (PS) were found, PE accounted for the lowest percentage with 25.69% compared to PP (32%) and PS (42.31%) (Fig. 6). PP microplastics appeared in all sampling sites, accounted for 4.55–100% of each site (from 0.34 to 2.64 pieces/kg d.w). PS was dominant in site 2, which is the tourist area with 59.09% equivalent to 4.40 pieces/kg d.w. There was only the presence of PP in site 5, other sampling sites showed all three types of plastic, with different distribution of each site. In site 1, PP was the plastic type that accounted for the majority of microplastics with 64%, and PE was the one with the lowest proportion with 9% in site 1. Site 2 showed the dominance of PS with 59%, which can be explained since most of the microplastic pieces obtained there were fragmentation from styrofoam packaging. Site 4 reported that PP was the most popular plastic type with 41%, whereas PS and PE shared the same proportion. In general, PS was detected near the tourist attraction area without regular cleaning activities. PP and PE were popular in the aquaculture location or near the market; the origin was usually plastic ropes or packaging. The data showed that the microplastics’ shape and their composition are related. From the results of this study, most of pellet microplastics were PS and they were in white color. Meanwhile, the fragmented and fibrous shapes were either PP or PE (Fig. 7).
3.4. Interpretations Of Microplastics Origin In Sand Samples
Besides investigating the presence of microplastics in sand samples, the distribution of macroplastic pieces was also assessed to interpret the relationship between them. Macroplastic wastes found in all sampling sites with an average concentration range from 1.37 pieces/kg d.w (site 3) to 5.71 pieces/kg d.w (site 2) (Fig. 3). The mass of macroplastic waste was quite large, up to 7019.2 mg/kg d.w (site 2). The concentration of macroplastic was compared to the corresponding concentration of microplastics, showing a similarity in all sampling sites. Macroplastic were mostly detected in a 2 cm surface layer of sand with 87.29%. In addition, the distribution of macroplastics at three sampling tide lines was similar to microplastics’ distribution, 76.34% of macroplastics found at the upper-shore line. White macroplastics accounted for the highest percentage with 33.79%, followed by green and transparent with 22.71% and 18.17%, respectively. Transparent macroplastics, possibly due to the fragmentation of plastic bags, were not found as microplastics. On the other hand, blue was still the color found in all sites with 15.51%. Macroplastics were also identified by FTIR-ATR and the result showed that PP was the plastic type showing the highest percentage with 46.99%, followed by 35.33% of PE, PS accounted for only 17.67%. PP, PE, and PS are also three popular plastic types used in everyday life. Macroplastics in the sand samples were mainly plastic bags, confectionery packaging, spoons, and forks. These are common items found on the beach that has anthropogenic activities. They are mostly made from PP and PE, explaining for the higher proportion of them. The PS pieces found were mostly styrofoam packaging, which may be the source of the corresponding microplastics in the same sand sample. PS was observed with the highest percentage at site 2 with 39.59% - this is also the sampling area with the highest PS microplastic concentration.
The preliminary origin of the microplastics was predicted based on the shape, color, and composition of the microplastics and macroplastics found in the same sand sample. As in some sand samples of site 1, microplastics had a cylinder shape with a smooth surface. These pieces were created in such an original shape and identified as resin pellets, which were the raw material for manufacturing plastic products. These microplastics were diagnosed to be brought ashore by ocean waves due to leakage during transportation. Meanwhile, in some sand samples, microplastics were found in fragmented or fibrous shapes. They had the same color, composition, and surface morphologies as some of the macroplastics. These microplastics can be created secondarily due to environmental degradation from larger pieces (Fig. 8). Pearson correlation coefficient was calculated to conclude the relationship between microplastics and macroplastic wastes in color and chemical composition. The results show a positive correlation of average concentration of white macroplastic and white microplastic at the corresponding sites. Concentrations of microplastic and microplastic with PP or PS composition also showed correlation. Therefore, microplastics were possibly degraded from macroplastics of the same sand sample with similar characteristics or composition (Fig. 9).
Out of the five sampling sites, sites 1, 2, and 4 had a higher concentration of microplastics and macroplastics. It can be explained that site 1 located behind Dong Hoa market, where received flows from the Dong Tranh river, carried the remnants of the Sac mangrove while site 2 was the location next to Ngoc Phuong Nam tourist, the plastic wastes here were mainly single-use products such as foam packaging, plastic cups. Site 4 is the aquaculture area that observed the presence of agricultural tools. Whereas sites 3 and 5 had a much lower microplastic concentration compared to other areas. Common characteristics of these places were tourist destinations, frequently visited by tourists and residents. However, these places often have beach cleaning campaigns, resulting in a small number of plastic wastes. In addition, sites 1, 2, and 4 were constructed with stone embankments to prevent erosion, leading to more microplastic accumulation than other sites. The results showed some similarities to the research in the same region: Tien Giang and Vung Tau (Hien et al. 2020). Microplastics in both investigations shared some common characteristics in terms of the distribution, the chemical composition of the plastic. These study areas had similar characteristics of wind regime, tidal regime, and received the currents of Dong Nai River, Saigon River. Therefore, researches on the effect of marine dynamics on microplastics migration should be investigated further for more insights into the fate of microplastics in the environment.