Five Priority Compounds in Southeast Asia


1. Introduction

The study of emerging and persistent contaminants (EPCs) within the context of environmental sustainability is increasingly important due to their potential harm to aquatic ecosystems and human health [1,2,3]. EPCs originate from anthropogenic sources, and are released through activities such as industrial processes, manufacturing, medical/health services, and wastewater discharge. These contaminants frequently mix with other persistent pollutants, creating complex mixtures across various environmental matrices [4,5,6]. Insufficiently treated urban wastewater is a major source of EPCs in rivers globally, adding to the chemical loads, albeit typically modestly, that originate in headwater regions [7,8]. Even treated wastewater can introduce hard-to-remove compounds into rivers downstream of treatment plants [9]. Consequently, river systems often contain a complex blend of pharmaceuticals, personal care products, industrial and agricultural chemicals, microplastics, endocrine disruptors, heavy metals, household chemicals, and food additives [10,11,12].
Understanding how EPCs travel from different sources to river systems, including their connectivity with source activities, is essential for assessing environmental and social impacts, as well as the sustainability of the activities generating them [13,14]. This knowledge can inform health risk assessments, ecosystem threat evaluations, and the implementation of optimized environmental monitoring systems to detect and manage contamination events, including spills, to prevent or limit downstream contamination [15,16].
In a prior study investigating EPCs across different land uses in the Ping River Catchment in Northern Thailand, contamination was highest in urban waters, followed by rural areas with a mix of agricultural and domestic activities that are typical of peri-urban settings [17,18,19,20]. Notably, headwater streams were not necessarily immune to some degree of contamination by selected EPCs. Evidence also indicated that some EPCs may reach remote locations via atmospheric transport [18]. Nevertheless, urban areas had the highest concentrations of pharmaceuticals, daily-use compounds, and industrial chemicals, suggesting substantial wastewater inputs from manufacturing, medical activities, and residential discharges [17]. Daily use compounds like caffeine and acesulfame were widespread, indicating contamination from gray water sources.
On a larger spatial scale, minimal seasonal variation in EPC levels likely resulted from constant mixing or flushing within the northern Thai catchment [17]. The prior study also highlighted the complexity of EPC distribution, influenced by geographic factors and human activities [19]. It also underscored the need for further research on how hydrological connectivity—such as surface and subsurface pathways—affects EPC presence in complex environments [17]. Finally, the study indicated the importance of focusing on priority contaminants that pose the greatest risks to ecosystems and human health. A specific focus of this nature is sometimes needed, owing to the highs cost of sufficiently monitoring the concentration of any specific chemical in space and time.
Generally, the potential impacts on stream systems in locations not commonly identified as at risk are often overlooked in the EPC literature due to their remoteness from major urban centers or industrial activities where contamination sources are suspected [21,22,23]. Streams in minimally developed coastal regions, which frequently drain into ecologically sensitive coastal environments—such as estuaries, deltas, bays, wetlands, marshes, coastal lakes, and lagoons—are often overlooked despite their importance for biodiversity and other ecosystem services [24,25,26]. Investigating these areas is important for understanding the extent of EPC contamination in the total environment, as well as developing nuanced pollution management strategies when needed [27,28].
Our investigation focuses on EPC occurrence along a transect of the Kamphuan River in Ranong Province, Southern Thailand—a system flowing from mountain headwaters through developed areas before reaching mangrove and coastal ecosystems (Figure 1). This research provides insights for mitigating pollution from urban and agricultural sources in ecologically sensitive coastal surface waters, supporting local strategies and global sustainability goals to enhance water quality and ecosystem resilience [29]. We also review regional work performed on five priority EPCs to contextualize our findings.

2. Study Area

The study area, located in Suk Samran District, Ranong Province, along Thailand’s Andaman Coast, spans 89.16 km2 with diverse land cover (Figure 1). Evergreen forest is the dominant cover (50.30 km2), followed by 9.75 km2 of mangrove forest, highlighting the region’s natural biodiversity. Agricultural land includes 1.21 km2 of aquaculture, 1.63 km2 of fruit orchards, and 0.30 km2 of paddy fields. Perennial crops cover 17.41 km2, primarily taking the form of tree plantations like rubber and oil palm, with mixed perennial crops covering 0.61 km2. Miscellaneous land occupies 2.27 km2, and a small amount (0.56 km2) is used for pasture and farmhouses. Urban areas and water bodies account for 2.46 km2 and 2.67 km2, respectively, balancing ecosystem preservation, agriculture, and urban needs.

The region has a tropical monsoon climate with over 2500 mm of annual rainfall, characterized by a wet season extending from May to October and a dry spell from December to April. Temperatures remain consistently warm year-round, averaging between 24 °C and 32 °C. Geological formations span multiple time periods and include Carboniferous–Permian rocks such as sandstone, mudstone, limestone, shale, and chert. Pleistocene deposits are abundant, consisting of gravel, sand, silt, and rock fragments resulting from weathering and erosion, particularly in hilly areas. Coastal lowlands have been shaped by tidal processes, with deposits of clay, silt, and fine sand. Soil development varies across the landscape, with poorly developed soils on steep slopes and richer mangrove and marine soils in the lower areas.

Figure 2.
Locations of sampling sites along the Kamphuan Stream in Suk Samran District, Ranong Province, Southern Thailand (Figure 1). Shown are the primary measurement locations: (a) headwater site and (e) downstream site. Panel f shows the downstream site following a rainstorm when the water was turbid (~100 NTU). Panels b, c, and d show reference (ref) locations, matching those identified in Figure 1. They are shown to provide a more detailed view of the stream system.

Figure 2.
Locations of sampling sites along the Kamphuan Stream in Suk Samran District, Ranong Province, Southern Thailand (Figure 1). Shown are the primary measurement locations: (a) headwater site and (e) downstream site. Panel f shows the downstream site following a rainstorm when the water was turbid (~100 NTU). Panels b, c, and d show reference (ref) locations, matching those identified in Figure 1. They are shown to provide a more detailed view of the stream system.
The Kamphuan Stream is about 15 km long, originating in the coastal mountain range east of Kamphuan town at about 1350 m elevation. Its dendritic system, characterized by perennial streams and moderate drainage, flows through forest and various mixed land uses before reaching the mangrove and other coastal ecosystems (Figure 1). Contaminant sources include agricultural runoff, road discharge, domestic wastewater, and commercial activities. Pollution is a concern due to the sensitive freshwater and aquaculture areas near the stream–mangrove interface along a section of the Andaman coast that is increasingly stressed by development activities.
Climate change-induced stressors, including extreme weather events and shifting tidal patterns, compound these pressures [30]. Developing communities face additional challenges from socio-economic changes, such as tourism and agriculture, which reshape their dependence on natural resources and often limit their adaptive capacity to respond effectively [31]. Beyond the visible threats, pollution adds a hidden layer of degradation to surface water resources, with contaminants like heavy metals, pesticides, and nutrients associated with surface runoff, some occurring as EPCs, stressing local ecosystems [32].
Emerging and persistent contaminants, rarely studied in non-urban systems in the region, pose uncertain risks as they accumulate in marine environments, where their toxicity weakens ecosystems already under pressure from expanding aquaculture and agriculture. This issue relates to the complexity of managing Andaman coastal ecosystems and stresses the need to address both visible and hidden threats to help communities and policymakers develop sustainable management strategies [33].

3. Methods

This exploratory study focuses on EPCs in water samples collected in two sections of the Kamphuan Stream: one near the headwaters (site “a” in Figure 1), the other below Kamphuan Town (site “e”) but above the mangrove, before the ocean tidal water influences the stream water quality (Figure 1 and Figure 2). The sampling design was guided by limited funding for analysis and the objective of exploring potential unknown sources of contamination. The focus was on areas above the coastal system but below the headwaters, where contamination levels were assumed to be low. Sampling was strategically conducted in both the early (July 2021) and late wet season (October 2021), during periods when surface runoff was expected. The third campaign was during the dry season (February 2022), when surface runoff was not active.

Grab samples (125 mL) were collected during the three campaigns in new high-density polypropylene (HDPE) amber bottles rinsed with ultrapure water. Prior to analysis, the samples were spiked with 1 g/l of sodium azide to inhibit microbial activity. The samples were stored at 4 °C for less than one week, then shipped cold to Singapore for analysis.

The compounds analyzed for targeted analysis (TA) included five priority compounds (diclofenac, gemfibrozil, metformin, naproxen, and fluoxetine), determined based on a prior regional synthesis [34]. Additionally, nine indicator compounds were analyzed, including two pesticides (atrazine and fenobucarb), one industrial compound (TBEP), three daily-use compounds (acesulfame, caffeine, and sucralose), and three pharmaceuticals (fexofenadine, gabapentin, and ibuprofen). These EPCs were chosen for their relevance in identifying pollution from various anthropogenic activities in Thailand [17].
The determination of the EPC concentrations was based upon methods described in full elsewhere [17]. Briefly here, we used online Solid-Phase Extraction (SPE) coupled with high-resolution liquid chromatography triple quadrupole mass spectrometry (LC-MS/MS). Raw water samples were filtered through 0.2 μm PES syringe filters. Then, 100 μL of a 10 ppb internal standard (ISTD) stock solution was spiked into each 5 mL filtered sample. Chromatographic separation was performed using InfinityLab Poroshell 120 superficially porous columns (EC-C18; 3.0100 mm; 2.7 μm) and a compatible guard column (EC-C18; 2.15 mm; 1.9 μm). Mass spectrometry was conducted using an Agilent 6495 triple quadrupole tandem mass spectrometer with a dual AJS-ESI source.
The limit of detection (LOD) for each targeted EPC and four quality control samples at a concentration of 100 ng/L distributed across the samples were determined for each sampling campaign to ensure method and instrumental consistency (for QA-QC data, see the supplementary files of prior work [17]). The LOD was taken as a signal-to-noise ratio of at least 10 above laboratory blanks and field blanks. LOD is sometimes referred to as below detection limit (bdl), particularly in tables. Samples with low ISTD counts were retested in the potential event of inaccurate additions of the standard during sample preparation instead of presuming that it was due to matrix effects. All data were preprocessed using Agilent MassHunter Quantitative Analysis software (version 10.0 for QQQ). Post-processing was performed in R (version 4.2.2).

Other basic water variables were collected monthly from May 2021 to June 2022 (n = 1 at each location per month), including pH, conductivity, salinity, and total dissolved solids. As water temperature was not measured in situ, specific conductivity (SEC) was converted from conductivity using ambient water temperature in the laboratory. Additionally, values were determined from near-surface grab samples rather than depth-integrated samples. However, this is unlikely to significantly impact the results, as stream water depths at both sampling locations were consistently less than 50 cm.

5. Episodic Stream Contamination: Emerging Patterns and Pathways

Identifying pollution sources in mixed-use catchments can be challenging due to the existence of multiple pathways, including runoff events, groundwater transport, atmospheric deposition, and untreated gray water discharge. Urban drains during storms channel contaminants into streams, resulting in fluctuating pollution levels due to variable storm characteristics and runoff pathways [18]. The “episodic” pattern observed in the Kamphuan Stream system contrasts with more constant signals that could be associated with groundwater or constant direct discharges, which are often indicative of non-point sources.

Across the three sampling campaigns, significant EPC levels were detected only in the late rainy season (October), demonstrating that stormwater runoff is likely the primary mechanism for EPC entry to the stream system. The other rainy season sampling period was not affected by a runoff event. The absence of consistent EPC presence across all sampling campaigns further suggests that storm events, rather than diffuse or steady sources like groundwater, represent the main contamination pathway affecting the samples. Low EPC concentrations upstream also support this finding, in part, as the site lies above the anthropogenically affected catchment areas.

A comparison of acesulfame and sucralose concentrations in the stream also points to urban runoff as a likely source of these sweetener compounds. The trends of the other pharmaceuticals support this assertion as well (Table 2). Both acesulfame and sucralose were detected at the downstream site (e) in October, with sucralose concentrations significantly higher (9068 ng/L) than acesulfame (2197 ng/L). This pattern aligns with findings from the Ping River in Northern Thailand, where sucralose, which is commonly associated with food industry sources, showed elevated levels in urban areas [17]. However, acesulfame and sucralose appeared less frequently in the Kamphuan Stream than in the Ping River. Upstream (site a), levels of EPCs were low, except for caffeine, which was consistently present, likely due to its widespread use and potential atmospheric transport, as documented globally [36,37], including in Northern Thailand [17] and other regional locations in Asia [38,39,40].
The local hospital was initially considered a potential EPC source, given the reliance on drain-field wastewater systems in the area [41]. However, our data do not support groundwater as an important pathway for EPCs entering the stream system, minimizing the hospital’s role as a source of contamination of pharmaceuticals. The pesticides atrazine (37 ng/L) and fenobucarb (215 ng/L) were detected downstream only in October, indicating that peri-urban runoff may be an active source in addition to agricultural lands. This pattern is consistent with findings from the Ping River Catchment, where selected herbicide pesticides were commonly present in urban waters [17,19].
These results underscore the importance of scrutinizing urban, peri-urban, and agricultural sources when assessing stream contamination [42,43,44,45,46,47,48,49]. Given the episodic nature of EPC concentration occurrence in the Kamphuan Stream, future work will focus on high-frequency, longitudinal sampling during storm events to accurately capture the dynamic nature of contamination and better assess all land-use contributions to pollution [50,51,52]. Expanding the analysis to include additional EPCs, their metabolites, and transformation products will further clarify risks to sensitive downstream coastal ecosystems. Complementary research on transport mechanisms, surficial and subsurface, will be attempted to fully understand the EPC dynamics at the site.

7. Toward Sustainable Stream Management of Emerging Pollutants

Considering the scarcity of studies in Thailand and surrounding Southeast Asian countries, there is a pressing need to assess the risks posed by EPCs in sensitive ecosystems, identify their sources, and develop cost-effective mitigation strategies where needed [1,103,104,105]. While ‘snapshots’ in time and space from limited sampling are informative, they must be supplemented with dedicated monitoring efforts to truly identify environmental issues. Both traditional and innovative approaches, such as probabilistic sensor deployment strategies, have been proposed to optimize monitoring networks and improve the cost-effectiveness of contamination detection, even when resources are limited [16]. However, such monitoring programs are resource-intensive and often justifiable only when contamination is known and reaches severe levels.
Our study focused on five regionally prioritized compounds, yet they likely represent only a fraction of the contaminants posing risks, as many unexamined chemicals may be even more hazardous to ecosystems and human health. A critical issue is that many EPCs are only partially removed by standard wastewater treatment, forming complexes, toxic derivatives, and byproducts that contribute to dynamic partitioning [106,107].
The compounds selected were guided by existing laboratory capabilities and previously established methods [17], ensuring continuity in our analytical approach. However, prioritization remains challenging, given the limited understanding of many emerging contaminants and their transformation products in terms of toxicity, persistence, and environmental pathways [34,53,108]. Addressing these gaps will require further research into metabolites, bioaccumulation potential, transport mechanisms, and transformations—challenges that mirror global needs [17,109,110]. Finally, local toxicity data are scarce, complicating risk assessments, since values derived elsewhere may not capture the unique vulnerabilities of local ecosystems.
There remains a significant gap in establishing harmonized guideline thresholds for managing contaminants, particularly those accounting for local species sensitivities, unique environmental conditions, and pollutant interactions. While various PNEC values exist [35], they often fail to address these region-specific factors, potentially underestimating ecological risks [111,112]. To improve accuracy and robustness, there is a pressing need to develop unified toxicology thresholds based on both experimental and predicted data, ideally complemented by bioanalytical toxicity information. In the absence of such standardized benchmarks, contaminant prioritization in local monitoring programs often depends on speculative or ad hoc approaches, a limitation also highlighted by earlier findings that PEC-based methods alone may be inadequate without the integration of ecotoxicological data [113].
Nontargeted analysis is potentially valuable for the environmental screening of EPCs, but its application in risk assessment is limited due to challenges in deriving the quantitative data needed for precise hazard and exposure evaluations [20,114,115,116]. Addressing these issues could help improve the understanding of EPC occurrence in the Kamphuan Stream and other water resources in the region moving forward.
The high costs of environmental monitoring significantly constrain EPC assessment, as many detection methods, such as mass spectrometry, require specialized equipment, skilled personnel, and substantial financial resources. For instance, previous studies on EPCs in Thai rivers have often relied on laboratories in Singapore and Korea due to limited local capabilities [17,18,19,64]. These high costs frequently force researchers to narrow their study scope to a few selected compounds and limit sampling efforts. This was evident in our study of the Kamphuan system, which focused on regionally prioritized compounds using a limited number of sampling campaigns, with the analyses performed in Singapore. Nonetheless, small datasets still offer valuable information on peak concentrations and seasonal trends, laying the groundwork for future research in a region where such studies remain sparse. Notably, many of the studies reviewed in Section 6 were based on limited sampling, yet are necessary for showing the range of concentrations that have been reported to date, contextualizing our findings.
Further, the spatial and temporal variability in the EPC concentrations reviewed in Section 6 highlight the need for dedicated research across additional rivers and streams to fully understand the impacts of EPCs in surface water systems in the region. Generalizing from existing data is challenging, even between urban and non-urban contexts. In densely populated areas, EPCs enter rivers from various point and non-point sources, including industry, transportation, hospitals/clinics, wastewater treatment, and household activities [17]. In remote areas, however, the lack of distinct pollution indicators complicates source identification beyond agricultural lands where erosion features are noticeable [19]. Furthermore, the role of the atmospheric transport of EPCs to remote systems remains unclear, underscoring the need for comprehensive research across diverse environmental settings [18,117].
Sustainable stream contaminant management generally requires coordinated efforts among diverse stakeholders [118]. Polluters, if identifiable, may resist regulatory pressures, particularly where there are limited incentives for improvement. Affected communities often lack the political leverage needed for stronger protection, while policymakers face competing priorities and constrained resources, which can delay action. Effective management, therefore, relies on a robust scientific foundation combined with locally adapted solutions.

For the Kamphuan River site specifically, longitudinal data collection is needed to accurately identify the sources and pathways of EPCs entering the stream system, as well as to identify sensitive organisms in the downstream mangrove and adjacent ecosystems. Additionally, a comprehensive assessment of water management practices in nearby (peri)urban and agricultural areas is necessary to support sustainable management efforts moving forward.

In closing, while this study provides valuable insights, its methodological limitations include a focus on a limited number of compounds and restricted sampling campaigns. Addressing these issues through expanded, high-frequency monitoring and comprehensive analytical approaches is essential for improving our understanding of EPC dynamics and their impacts on sensitive ecosystems. However, even daily sampling may fail to capture nuanced changes in flashy stream systems, where rapid shifts in pathway connectivity and water chemistry during storm events can greatly influence contaminant transport and transformation. Additionally, fine spatial resolution will be needed to identify contamination sources in mixed-use catchments, where multiple pathways and land uses contribute to complex pollution dynamics.

8. Conclusions

Data from our exploratory study indicate that urban stormwater runoff is likely the primary source of EPCs in the Kamphuan Stream, with contamination occurring episodically during rainfall events rather than persistently. Upstream contamination was minimal, but downstream concentrations of several compounds rose during the rainy season, highlighting how storm runoff events create temporary hydrological connections that transport EPCs into the stream system, then draining into coastal ecosystems.

A comparative analysis with regional studies suggests that most detected EPCs pose a low risk; however, the maximum concentrations of diclofenac, gemfibrozil, ibuprofen, sucralose, caffeine, and fenobucarb exceeded the predicted no-effect concentrations (PNECs) for marine or freshwater environments. Notably, ibuprofen surpassed both thresholds, signaling that it may be a priority concern. Nonetheless, the concentrations of all EPCs were not persistent, and the limited scope and frequency of sampling constrain our full understanding of contaminant dynamics and associated risks.

These results emphasize the need for expanded research on EPCs in Southeast Asia, particularly in sensitive river systems that are often overlooked due to their remoteness or lack of significant pollution sources. Sustainable contaminant management will require an integrated approach that prioritizes local capacity building for monitoring, improves analytical methods, and fosters stakeholder engagement. Addressing episodic contamination and developing cost-effective mitigation strategies tailored to local environmental and socio-economic contexts are essential to protecting sensitive ecosystems and supporting long-term water quality goals in the face of increasing development pressures.



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Theodora H. Y. Lee www.mdpi.com