
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12161-5
10.1016/j.heliyon.2024.e36130
e36130
Research Article
Microplastic accumulation in water from protected areas in Western Forest Complex of Thailand
Teampanpong Jiraporn a
Phanchaum Jiroj a
Rayaphak Aiina b
Duengkae Prateep prateep.du@ku.ac.th
c⁎
a Department of Conservation, Faculty of Forestry, Kasetsart University, Bangkok, 10900, Thailand
b Nam Tok Yong National Park, 5th Protected Area Administration Office, the Department of National Park, Wildlife, and Plant Conservation, Nakhon Si Thammarat Province, 80000, Thailand
c Department of Forest Biology, Faculty of Forestry, Kasetsart University, Bangkok, 10900, Thailand
⁎ Corresponding author. prateep.du@ku.ac.th
10 8 2024
15 9 2024
10 8 2024
10 17 e3613027 5 2024
9 8 2024
9 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Microplastics (MPs) are emerging as ubiquitous environmental pollutants worldwide. However, no research has been conducted in freshwater ecosystems within Thai protected areas (PAs), where biodiversity and natural resources are safeguarded for human well-being. The aim of this study was to explore the occurrence and abundance of potential MPs in the water of freshwater ecosystems in Thai PAs and to examine factors affecting their presence in the Western Forest Complex in Thailand (WEFCOM). Seventy water samples were collected from eight PAs in WEFCOM using a 20 μm plankton net. The water samples underwent digestion, density separation, and filtration. Potential MPs were visually identified under a stereomicroscope, and their chemical composition was further characterized using FTIR. The results revealed a 98.57 % potential MP prevalence in the water samples, with an average abundance of 0.30 ± 0.32 item·L−1. The quantities of potential MPs among PAs were significantly different (H = 17.88, p = 0.01). Fibers (68.93 %) were the most frequently identified potential MPs, with the dominant colors being blue (22.40 %) and black (20.03 %), mostly small-sized MPs (0.05–0.5 mm; 41.80 %). The major chemical plastic types included low-density polyethylene, polyethylene, polyethylene terephthalate, polypropylene, polyethylene/polypropylene copolymer, polyester, and natural fibers. The abundance of potential MPs varied significantly among PAs, shapes, colors, and sizes (p < 0.01). Furthermore, potential MP abundance correlated with the number of tourists (p < 0.05). Proximity to open waste dumping (p < 0.01) and lower elevation (p < 0.05) tended to accumulate more potential MPs. This research suggests potential sources of microplastics in Thai PAs from wastewater effluent, human activities, and recreational activities, highlighting the urgent need for research to develop appropriate waste management technologies in Thai PAs and to raise awareness among local people and tourists about microplastic pollution.

Highlights

• Microplastics were 98.57 % prevalent in water of eight Thai protected areas.

• Small (0.05–0.5 mm) microplastics with blue and black colors were very common.

• Six polymer types were identified with some natural fibers.

• Microplastic abundance was correlated to tourism and human activities.
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pmc1 Introduction

Microplastics (MPs) are harmful entities affecting Earth's processes and human society, requiring urgent attention [1]. Over 40 % of plastic is single-use, with 79 % accumulating in landfills and natural environments [2,3]. Thailand is responsible for 52 % of the world's mismanaged plastic waste [4], with 70.1 % of its waste contaminating freshwater and marine ecosystems [5]. Significant sources of microplastic pollution include improper disposal, tire abrasion, and textile degradation [4,5]. In 2023, key sources of microplastics in Thailand's waterways were improper plastic disposal, tire abrasion, and the breakdown of textiles and paint, releasing 26.46 tons of microplastics and 20,160 tons of chemical additives [4].

The release of MPs and their associated chemicals into water bodies from various sources has escalated environmental pollution issues. These minute plastic particles, along with associated chemical additives, pose a severe threat to ecosystems by entering the food chain and potentially harming a wide range of organisms across trophic levels, from plankton and fish larvae to worms, crabs, and marine mammals [6]. The study of environmental contamination by MPs and their toxicity to living organisms has gained increasing attention. Most research has been conducted in marine and coastal ecosystems and has gradually expanded to freshwater ecosystems [7].

While research on MPs has predominantly focused on marine environments, the impact on freshwater ecosystems—crucial habitats under increasing threat globally [8]—remains relatively understudied [7]. Freshwater environments may accumulate levels of MPs that are 4–23 times greater than those in marine environments [7]. These ecosystems facilitate the movement of other pollutants in the soil, impacting nutrient cycling and the reproductive processes of both direct and indirect terrestrial biota [9]. MPs transferring through food webs could lead to toxicity or environmental pressures [10]. Hence, rising MP accumulation in freshwater ecosystems, especially protected areas (PAs), is a concern that could impact ecological services [11] and cause biodiversity loss [10].

Western Forest Complex (WEFCOM), the largest network of contiguous PAs in mainland Southeast Asia, spans 19,000 km2 in the heart of the Dawna-Tanintharyi landscape. It is the high priority for wildlfie conservation [12] and possibly the only large viable population of Indochinese tigers (Panthera tigris corbetti) [13]. Western Thailand serves as a global priority for biodiversity conservation in Southeast Asia [14] and as the headwater for two [15] of the five high-priority catchments that release freshwater into the Upper Gulf of Thailand [5]. Despite MPs being considered a global emerging pollutant [9] that threatens biodiversity [[10], [16]] and causes unexpected ecological harm [17], research on MP accumulation and abundance is lacking in this area with high ecological significance. Existing studies only document the presence of microplastics in the carcasses and feces of terrestrial vertebrate species in some PAs of WEFCOM [18,19].

Therefore, this pioneering study aims to fill a crucial knowledge gap regarding MP occurrence and abundance in freshwater, a major resource for all living creatures in crucial Thai PAs. By investigating the quantities and characteristics of MPs in these areas and identifying the factors influencing their accumulation, this research seeks to shed light on a critical environmental issue and advocate for improved plastic waste management to safeguard biodiversity in Thai PAs. It is hypothesized that the PAs would exhibit varying levels of MP accumulation, categorized by their shapes, colors, and sizes, with human activities being the most significant contributing factor. As the inaugural study of its kind in Thailand, this research endeavors to augment the existing knowledge base regarding MP accumulation in freshwater ecosystems in Thai PAs. The findings are expected to heighten awareness and catalyze the implementation of enhanced plastic waste management strategies, ultimately safeguarding biodiversity within Thai PAs.

2 Materials and methods

In total, 70 sampling locations were randomly selected based on accessibility in the freshwater ecosystems of eight PAs in the WEFCOM in Kanchanaburi province (Fig. 1). The PAs consisted of five national parks (NP)—Khao Laem, Kheun Srinakarin, Thong Pha Phum, Sai Yok, Lam Khlong Ngu, and Erawan—and two wildlife sanctuaries (WS)—Salakphra and Thung Yai Naresuan West. The WEFCOM is home to over 2500 plant species, 153 mammal species, 490 bird species, 90 reptile species, 40 amphibian species, and 108 fish species [15], with at least 3133 households occupying land in these eight PAs [20].Fig. 1 Sampling locations of water samples in eight protected areas in Western Forest Complex lying only in Kanchanaburi Province, Thailand.

Fig. 1

2.1 Field data collection

Sampling took place at 70 stations in freshwater ecosystems in Western Thailand between July 2020 and June 2021. A water sample was collected 30 cm below the water surface from both lotic and lentic sites using a volume-reduced sampling method [21] with a a calibrated liter-aluminum bucket being filtered through the 15 μm net. The net was pre-cleaned with freshwater in the field before using it for filtering water sample and stored in glass bottles. A total of 30 L of surface water per station were collected from multiple representative sub-samples to ensure the representation of the entire body of water. Consequently, only one water sample was collected at each station and there were at least five stations at each PA.

The water flow rate (m/s), and the depth (m) and width (m) of the sampled waterways were measured. The coordinates of all sampling locations and the open dump site were recorded using a GPSMAP 60 CSX unit (Garmin). Elevation (m) and slope (%) were retrieved from the GPS for subsequent use.

2.2 Microplastic separation

The water samples were treated with 30 % H2O2 for a minimum of 24 h at room temperature in the dark to break down organic matter without affecting any plastic contents [22]. Saturated NaCl was added, mixed thoroughly, and left for settlement for 24 h, lidded Petri dishes. Then, each sample was filtered through a 1.2 μm glass fiber filter (GF/C, 47 mm Ø, Whatman) and its container was rinsed with distilled water using a vacuum pump, and kept in clean Petri dishes, followed by drying at 50 °C for at least 4 h.

2.3 Microplastic identification

Visual identification under a stereomicroscope (ZEISS Axioskop40) at 40× magnification was used to classify and count potential MPs into categories of fiber, fragment, film, foam, and pellet based on shape, color, and size. The MP sizes were classified into eight classes: very small (<0.05 mm), small (0.05–0.5 mm), slightly small (0.5–1 mm), moderate (1–2 mm), slightly large (2–3 mm), large (3–4 mm), very large (4–5 mm), and debris (>5 mm), as adapted from Wang et al. [23]. After visual identification under the stereomicroscope, each sample was kept in aluminum foil for further analysis.

2.4 Chemical classification of microplastics

Potential MP samples from each type and color were chosen for analysis using FTIR (Type II; PerkinElmer) in the wavelength range 400–4000 cm−1. Only readings with at least 60 % confidence were considered reliable [24].

2.5 Quality assurance and control

Precautions were taken to minimize MP contamination. Sampling and sample processing utilized glass apparatus and containers. During sample processing, individuals wore white cotton laboratory coats, and all lab surfaces were cleansed with distilled water before use. The isolated MPs were preserved in pre-cleaned, lidded Petri dishes before being transferred to aluminum foil after visual detection under the stereomicroscope.

To check for potential MP contamination during sampling and analysis, field blank tests and laboratory blank tests were conducted. The field blank was applied to only distilled water using to rinse the net after filtering the 30-L of water sample to make sure no residual MPs remaining on the plankton net at each sampling for 10 replicates. Meanwhile, the laboratory blank tests were applied to distilled water, H2O2, saturated NaCl solution, and the laboratory air for 10 replicates. The laboratory air was tested by vacuum filtering it for 1 h through a 1.2 μm glass microfiber filter paper [23]. The blanks contained relatively low levels of identical particles in distilled water, H2O2, saturated NaCl solution, plankton net, and the air—30 %, 20 %, 20 %, 30 %, and 30 %, respectively, with a maximum of two items per filter. These findings suggest some risk of contamination within the laboratory settings, especially from atmospheric microplastics.

2.6 Data analysis

All statistical analysis were conducted using the R statistical software (version 4.2.3) [25], with the level of statistical significance set at α ≤ 0.05. The frequency of occurrence (FO) was reported as a percentage. Potential MP abundance was calculated by dividing the total number of MPs by the 30 L water sample volume (item·L−1). Values were presented as mean ± SD. Overall MP abundances were differentiated among PAs using the Kruskal-Wallis test, with a Dunn-Bonferroni post-hoc test if a non-normal distribution occurred. The aligned rank transformation (ART), non-parametric test [26] was used to examine patterns among PAs and the shapes, colors, or sizes of MPs using the ART package [27].

To determine factors influencing potential MP contamination, a generalized linear model (GLM) with a negative binomial distribution was developed using the package MASS 7.3 in R [28] due to overdispersion in MP counts. The factors investigated that affected MP abundance were: slope, elevation, land use, proximity to landfills, tourist sites, local landmarks, and villages. Additionally, Kendall's tau rank correlation was employed to examine the correlation between the number of tourists (2020–2021) in each PA and potential MP abundance due to the small sample size. Finally, the Spearman rank coefficient was utilized to examine correlations in MP abundance with flow rate (m/s) and the width and depth of the watercourses (m).

3 Results

3.1 Microplastic accumulation

In total, 634 pieces of potential MPs were identified in 98.57 % of the 70 water samples. The number of potential MPs per sample ranged from 0 to 60, with an average abundance of 0.30 ± 0.32 item·L⁻1. Various shapes of potential MPs, including fibers, fragments, foams, films, and pellets, were found contaminating the water. Fibers were the most frequent potential MPs (68.93 %), with an average value of 0.21 ± 0.17 item·L⁻1. The second most frequent shape was fragments (25.87 %), with an average value of 0.08 ± 0.19 item·L⁻1, followed by films, foams, and pellets at 3.47 %, 0.95 %, and 0.79 %, respectively.

Thirteen colors of potential MPs were found, with blue having the highest frequency of occurrence (FO) (22.40 %) and an average value of 0.07 ± 0.09 item·L⁻1, followed by black (20.03 %), red (17.64 %), and light blue (11.36 %), with average values of 0.06 ± 0.07, 0.04 ± 0.07, and 0.03 ± 0.12 item·L⁻1, respectively. The remaining colors each had an FO of less than 7 %. The highest FO was observed for small-sized MPs (0.05–0.5 mm) at 41.80 %, with an average of 0.13 ± 0.20 MP L⁻1. The second and third highest FO sizes were slightly small (20.03 %) and medium (19.56 %) MPs, with averages of 0.06 ± 0.07 item·L⁻1 and 0.03 ± 0.04 item·L⁻1, respectively as shown in Fig. 2. The ART revealed significant differences in potential MP abundance among shapes, colors, sizes, and PAs and their interactions (Table 1).Fig. 2 The total count (upper) and abundances of microplastics (MP.L−1: lower) in water comparing by shapes, colors, and sizes found in all protected areas.

Fig. 2

Table 1 Statistical results using the aligned rank transformation (ART) non-parametric test to decimalize microplastic abundance among protected areas and their characteristics as classified by shapes, colors and sizes.

Table 1Factor	F	df1, df2	p	
 Shape	113.06	7, 310	<2.22 × 10−16	
 Protected area (PA)	14.62	7, 310	<2.22 × 10−16	
 Interaction (shape × PA)	6.22	28, 310	<2.22 × 10−16	
 Color	12.07	7, 744	1.22 × 10−14	
 PA	27.16	7, 744	<2.22 × 10−16	
 Interaction (color × PA)	2.31	77, 744	1.26 × 10−8	
 Size class	50.36	7, 496	2.22 × 10−16	
 PA	16.45	7, 496	2.22 × 10−16	
 Interaction (size × PA)	3.07	49, 496	2.46 × 10−10	

3.2 Microplastic detection and accumulation in different protected areas

The highest number of water samples (14) was obtained from Thong Pha Phum NP, and the lowest from Erawan and Lam Khlong Ngu NPs (6 samples each). All water samples from most PAs (100 %) except from Khao Laem NP contained MPs. MP abundance was highest in Erawan NP, with decreasing levels in Sai Yok NP, Salakphra WS, Thong Pha Phum NP, Khao Laem NP, Thung Yai Naresuan West WS, Kheun Srinakarin NP, and Lam Khlong Ngu NP, respectively. There were significant differences in MP abundances among PAs (H = 17.95, df = 7, p = 0.012), particularly between Erawan NP and Khao Laem NP (Z = 2.79, p = 0.049, Fig. 3). The total number of water samples, the number of samples with MP presence, and the abundance of MPs by PAs are shown in Table 2.Fig. 3 Comparisons of microplastic abundance among eight protected areas with the statistical results using Kruskal Wallis Test.

Fig. 3

Table 2 Descriptive statistics of microplastic detection and accumulation of microplastics in eight protected areas in Western Forest Complex, located in Kanchanaburi Province of Thailand.

Table 2Protected area	No. of samples (%)	No. of samples with MP detection (%)	x‾ ±sd	median	se	
Erawan NP.	6 (8.57 %)	6 (100 %)	0.84 ± 0.73	0.65	0.30	
Khao Laem NP.	12 (17.14 %)	11 (91.67 %)	0.22 ± 0.29	0.13	0.08	
Thong Pha Phum NP.	14 (20.00 %)	14 (100 %)	0.26 ± 0.10	0.23	0.03	
Kheun Srinakarin NP.	5 (7.14 %)	5 (100 %)	0.19 ± 0.03	0.20	0.01	
Lam Khlong Ngu NP.	6 (8.57 %)	6 (100 %)	0.12 ± 0.08	0.12	0.03	
Sai Yok NP.	12 (17.14 %)	12 (100 %)	0.36 ± 0.24	0.28	0.07	
Salak Phra WS.	7 (10.00 %)	7 (100 %)	0.29 ± 0.33	0.13	0.12	
Thung Yai Naresuan West WS.	8 (11.43 %)	8 (100 %)	0.22 ± 0.07	0.20	0.02	
Total	70 (100 %)	69 (98.57 %)	0.30 ± 0.32	0.20	0.04	

The FO of MPs, classified by shape, color, and size, was similar across each PA (Fig. 4, Fig. 5, Fig. 6). Additionally, there was a significant correlation between the accumulated number of tourists (2020–2021) and MP abundance in water in each PA (Kendall's tau = 0.59, z = 2.01, p = 0.044).Fig. 4 The abundance of potential microplastics in water classified by shapes, comparing among eight protected areas.

Fig. 4

Fig. 5 The abundance of potential microplastics in water classified by colors, comparing among eight protected areas.

Fig. 5

Fig. 6 The abundance of potential microplastics in water classified by sizes, comparing among eight protected areas.

Fig. 6

3.3 Chemical properties of microplastics

Of the 28 pieces of potential MPs (4.49 % of 634 pieces), 50 % were plastic, consisting of five thermoplastics—low-density polyethylene (LDPE), polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), and poly(ethylene-propylene) copolymer (EPC)—and one thermosetting plastic called polyester (PES). Among the 14 pieces of MPs identified as plastics, they could be classified as 21.43 % each of PP and PET, and 14.29 % each of EPC, PE, LDPE, and PES.

Additionally, 32.14 % of the samples were made of natural fibers, with the remaining 17.86 % being inorganic materials such as diatomaceous earth and silica gel. Fig. 7 illustrates examples of FTIR spectra of representative microplastic polymers from the study.Fig. 7 Some examples of FTIR spectra of representative microplastic polymers from this study. The upper spectrum is the FT-IR measurement of our MP samples. The lower spectrum is the reference spectrums from the FT-IR library.

Fig. 7

3.4 Factors influencing microplastic accumulation

Based on the GLM results, factors affecting potential MP accumulation in water were proximity to open waste dumping (p = 0.01) and elevation (p = 0.02) (Table 3). In other words, the closer to open waste dumping and the lower the elevation, the higher the potential MP accumulation in water.Table 3 Results from the generalized linear model with negative binomial using data expressed as number of MPs in water as an offset term.

Table 3Coefficient	Estimate	se	Z	p	
Intercept	2.82	0.21	13.77	<2 × 10−16	
Proximity to open waste dumping	−4.26 × 10−5	1.66 × 10−5	−2.56	0.011	
Elevation	−8.63 × 10−4	3.82 × 10−4	−2.26	0.024	
Null deviance = 86.77 on 69 df; Residual deviance = 72.55 on 67 df, AIC: 442.78 Theta = 2.16, SE = 0.44,
2 × log-likelihood: 434.78	

Kendall's tau was applied to check the relationship between potential MP abundance and the characteristics of watercourses, with no significant association with water flow (m/s; Kendall = −0.02, z = 0.025, p = 0.81), width of the watercourse (m; Kendall = −0.15, z = −1.52, p = 0.13), and depth of the watercourse (m; Kendall = −0.02, z = −0.25, p = 0.81).

4 Discussion

4.1 Abundance of microplastic accumulation

This study is the first to report potential MP contamination in water samples from eight Thai PAs in WEFCOM, with an FO of 98.57 % and an average concentration of 0.30 ± 0.32 item·L⁻1. Comparing these results with other studies is challenging due to discrepancies in collection, separation, and identification methods. However, it is evident that park managers need to be made aware of this issue. This research provides the first baseline for MP monitoring in Thai PAs.

The FO of potential MPs in this study was slightly higher than in the Snake River, which is part of the Grand Teton National Park and Lower Columbia River complex in the USA (92.8 %) [29]. The average abundance in this analysis was higher than that reported for the Bueng Boraphet Ramsar site in Thailand (0.0014 ± 0.0034 item·L⁻1) [30], Antarctic Specially Protected Area (Byers Peninsula, Livingston Island (0.00047–0.00143 item·L⁻1) [31], and for the Snake River in the USA (no MPs) [29]. The higher MP abundance observed in this study could be attributed to the sampling locations being limited to sites with vehicle accessibility via unpaved or paved roads or short walks. Accessibility by road could introduce MPs from vehicle tires [32], road runoff [33], and human clothing and footwear [34]; consequently, the chances of MP detection would be higher than at sites rarely used by humans and not accessible by road.

However, the MP abundance in this investigation was lower than that reported for PAs on the American continents (16–36 item·L⁻1) [35] and on Mount Everest (1 item·L⁻1) [36]. The lower number of tourists in eight Thai PAs within WEFCOM, compared to PAs in the American continents and Mt. Everest, could be one of the reasons for the lower microplastic levels in freshwater in these Thai PAs. This is supported by evidence that the rise in tourism is anticipated to increase the deposition of microplastics near Mt. Everest [36].

4.2 Microplastic accumulation by shape, color, and size

The main types of MPs found were fibers (68.93 %) and fragments (25.87 %), which aligns with other studies in Thailand. For example, fibers and fragments were found in 93.8 % and 6.2 % of samples, respectively, at the Bueng Boraphet Ramsar site [30]; in 63 % and 81.9 % of samples, respectively, at the Chi and Pong Rivers [37]; and in 88 % and 12 % of samples, respectively, in the U-Taphao and Tapi-Phumduang Rivers [38].

In this research, blue (22.40 %) and black (20.03 %) were the most prevalent colors. Blue was also the most widespread MP color reported in many studies in Thailand at over 60 % [38] and in the Yangtze River Basin at 40–60 % [39]. However, MP colors may vary across regions and human-related activities. Black and red colors (37.5 % each) were dominant in adjacent community areas, whereas blue dominated agricultural areas at the Bueng Boraphet Ramsar site [30]. White translucent (24.3 %) was the second most prevalent color in the U-Taphao and Tapi-Phumduang Rivers [38].

MPs smaller than 2 mm were the most ubiquitous (81.39 %) in our study, with the 0.05–0.5 mm size class having the largest proportion (41.80 %). These findings align with other research in Thailand, which reported that MPs smaller than 1 mm were the most prevalent [[36], [37], [38]]. The small MPs might have originated from the photodegradation of plastic waste into smaller polymers in both lotic and lentic environments, resulting in MPs in sizes ranging from 0.07 to 0.24 mm [40]. Plastic bags used in check dam construction might have become a main source of synthetic microfibers contaminating Thai PAs (Fig. 8A).Fig. 8 Examples of local plastic waste disposal near streams and sources of microplastics in and adjacent to PAs. The use of reusable woven plastic bags for creating checkdam along local streams (A), uses of plastic mulch, sunshade net, netting for agriculture (B), remaining fishnet in Vachiralongkorn Dam as part of Khao Lam National Park, recovered and disposed by Baimai Environmental Enterprise (C), human settlement near watercourses (D), open dump as a common plastic waste disposal in rural areas and remote ranger stations (E), and social norm of ignorance on plastic waste the environment (F), were the main sources of microplastics in and around protected areas in WEFCOM.

Fig. 8

MPs in water from this site may have originated from washing clothes, especially official uniforms, as both student and university uniforms release blue fibers [37]. Small-sized MPs (0.05–0.5 mm) from laundry activities could be released into the environment because they are too small to be retained by the filter bags in laundry machines [41]. The abundant number of black MPs may have derived from vehicle tires [32], plastic mulch, sunshade netting [42] and trash bags used in housing, agricultural areas, plant nurseries in PAs (Fig. 8B). Diverse-colored MPs, especially red ones, might have come from PP ropes used for various purposes in Thailand. Other potential MP sources include atmospheric deposition [34,43], road runoff [33], and ropes and fishing gear [38] (Fig. 8C). Fragmented MPs may have been degraded from pipes or containers used on farms and in dwellings (Fig. 8D).

4.3 Polymer types of microplastics and their origins

This research identified the polymer types of MPs including LDPE, PE, PP, PET, EPC, and PES. These polymers are frequently found in freshwater ecosystems in Thailand, as reported for PP, PE, and PET in the Khwae Noi Watershed [44], the Chi River Basin [37], and the U-Taphao River [38]. Additionally, PES has been reported in both the Khwae Noi Watershed [44] and the Chi River [37]. LDPE, PE, PET, PP [45], and PES are widely used in various commodities, and EPC is a base material for products used in automotive engines, electrical cables, and construction [46], suggesting that the MPs found in this study could be secondary MPs.

It is possible that more polymer types were present than reported for two main reasons. First, NaCl is less effective at separating dense plastic polymers denser than 1.2 g/cm³, such as PET (1.40–1.50 g/cm³) and polyvinyl chloride (PVC: 1.30–1.70 g/cm³) [47]. Although PET was detected in this study, PVC might have been lost. NaCl was selected due to its cost-effectiveness, availability, and lower environmental impact compared to other salts [47]. Second, the low number of potential MPs (4.49 %) tested for polymer types using FT-IR, due to limited funding, might have caused the loss of some polymer types such as polystyrene (PS), which is prevalent in aquatic environments and can be effectively separated using NaCl [48].

4.4 Factors associating with microplastics accumulation

The context of this research revealed that waterways closer to open dump sites and at lower elevations tended to accumulate more MPs. MPs are released to leachate, leachate sediment, groundwater, and surface water from landfills [49], which are the main MP sources in the Mae Klong watershed, Thailand [5] (Fig. 8E).

The greater number of MP samples at lower elevations could be associated with a denser human population [44]. Additionally, MPs are commonly transported from upstream to downstream [37], so MP numbers tend to decline at higher elevations.

This result supports the hypothesis that MPs may have come from tourists, as MP abundance was correlated with the number of tourists in each 10.13039/100006131 PA . Other research has confirmed that tourists introduce MPs into PAs through sources such as vehicle tires [30], road runoff [31], and clothing and footwear [34].

4.5 Implication for reducing impacts of microplastics in protected areas

This research provides the first evidence of MP accumulation in water in Thai PAs, which are areas preserved for special uses such as biodiversity conservation and ecosystem resilience. This finding is not surprising, as MPs in freshwater have been pervasive in PAs in the USA [50] and countries in other continents [[33], [34], [35]].

Even with small MP abundance, monitoring is required [51]. The current findings support the importance of the Department of National Parks (DNP) implementing a standard protocol for MP monitoring to avoid potential biodiversity loss [9]. When MP abundance exceeds three item·L⁻1, a mitigation response regarding MP sources is necessary [50].

Mitigating microplastic pollution in Thai PAs is challenging and essential. Two main actions are needed initially: reforming existing open garbage dump sites with incorrect plastic separation [52], and standardizing plastic waste management to achieve the Zero Landfill Initiative (ZLI) [53] in both PAs and remote areas of Thailand [54] using the circular economy [55].

The second action is to raise awareness through education, outreach programs, and campaigns to encourage behavioral changes in plastic waste disposal by local communities and tourists regarding the ZLI and circular economy. This action is more challenging, as Thai local people have become accustomed to discarding waste in the environment [56] (Fig. 8F). This research findings also found mismanagement of plastic waste by park rangers at remote stations. Addressing these moral norms is crucial for fostering desirable behavior towards the ZLI [53] for the long-term success of reducing microplastic pollution in Thai PAs. Further social science research is required to tackle the issue of moral norms in plastic waste management in the Thai context.

4.6 Study limitations

There are various reasons why this research has limitations, which can be categorized into two primary types: challenges related to standardized methodologies and constraints due to insufficient research funding. Methodological limitations encompass issues on the absence of recovery study and the collection of only one single sample at all sampling locations.

The main reason that the methodology in this research did not include a recovery test is that, at the time this research was designed and conducted in 2020–2021, none of the accessible full research articles addressed the standardized protocols or procedures for a recovery test of MPs in freshwater, nor did they report the recovery rate of MPs either during the digestion methods or separation methods [57,58]. It is important to note that recent published research articles on MPs in surface water of freshwater ecosystems in Thailand and other countries did not address the recovery test [[59], [60], [61], [62]]. Therefore, the recovery test is required for future work on MPs in Thailand.

Finally, limited research funding presented two challenges in this study: collecting only a single water sample at each station and testing only 4.49 % of the MP samples for polymer verification using FTIR. Collecting only one water sample per station may introduce sample variability due to sampling error. By collecting a 30-L water sample from multiple substations and pooling them at each station can address this deficiency to ensure the inclusion of heterogeneous suspended particles in freshwater. It is noted that collecting one water sample per station has been applicable in recent research on MPs in freshwater ecosystems in Thailand [30,44,59]. This method was also used to determine the spatial distribution of MPs in surface water [30], which aligns well with the research objective of reporting the spatial distribution and occurrence of MPs in freshwater ecosystems across the vast landscape of PAs in western Thailand and adjacent areas.

The last main challenge in subsampling only 4.49 % of the MP samples for polymer verification with FTIR, considering types and colors might be too minimal to define the predominant polymer types of MPs necessary for identifying sources of plastics for management in this study area. This limitation may have resulted in some suspected items not being identified as MPs. While it cannot be verified whether every piece is plastic, other research indicates the main composition of the MPs found in freshwater in Thailand consists of a significant proportion of plastic polymers [30,37,38].

5 Conclusions

This study offers valuable insights into the potential of freshwater ecosystems in eight PAs of WEFCOM to become significant reservoirs of microplastics. It is the first to address microplastic pollution in these PAs in Thailand. Microplastic abundance in Thai PAs was comparable to or slightly higher than that in other freshwater ecosystems within Thailand and worldwide PAs, yet lower than in some freshwater ecosystems in other countries. The contamination primarily consisted of blue and black fibers and fragments, predominantly smaller than 2 mm in size, comprising various types of polymers, synthetic fibers, and natural fibers. This suggests a mixture of materials originating from poorly managed large plastics from tourism, household activities, agriculture, and runoff. A standardized monitoring protocol for microplastics in Thai PAs is required to better understand and manage plastic pollution. The findings from this study can serve as indicators for future monitoring of microplastic pollution. Future research should focus on understanding the fate of microplastics and associated risks to develop strategies to promote better plastic waste management practices among local communities and tourists in remote Thai PAs. This aims to foster more sustainable behaviors and protect the ecological integrity of these vital ecosystems.

Funding

Jiraporn (JT) Teampanpong was supported under Research and Innovation Grant 2020 by 10.13039/501100004704 National Research Council of Thailand and used her personal funds.

Data availability statement

The data and code availability will be made available on request.

CRediT authorship contribution statement

Jiraporn Teampanpong: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Jiroj Phanchaum: Investigation, Formal analysis, Data curation. Aiina Rayaphak: Investigation, Formal analysis, Data curation. Prateep Duengkae: Writing – review & editing, Validation, Resources.

Declaration of competing interest

The authors declare no competing interests.

Acknowledgements

We acknowledge Mr. Kusol Tankjaipitak (deceased), the laboratory technician at the Department of Forest Biology and Mr. Tatchakorn Hensawang at the Department of Conservation, Faculty of Forestry, Kasetsart University, Bangkok, Thailand and Miss Roochira Sukhsangchan, a researcher at the Department of Marine Science, Faculty of Fisheries, Kasetsart University for providing equipment and laboratories and assisting in laboratory operations. Miss Waranya Yimprasert, Miss Darinee Samranruen, Miss Kwanjira Sawatwong, Mr. Ramil Kohkaew, and Mr. Kaiwad Phanthanoo assisted with field data collection. We thank Dr. Sampan Tongnunui and Dr. Amnuay Wattanakornsiri during the field work and comments on this research. The 10.13039/501100017234 Department of National Parks, Wildlife and Plant Conservation of Thailand gave permission to collect samples in the protected areas, while the park superintendents at the eight protected areas where the samples were collected provided support logistics and accommodation, as well as suggesting sampling locations.
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