Assessing the Potential Microplastic Sources and Retention Efficiency in Valencia’s Tancat de la Pipa Wetland

Abstract

Constructed wetlands are promising nature-based solutions for mitigating environmental pollution, able to retain suspended solids as microplastic (MP) particles. This study aims (i) to identify potential sources and transport pathways of MPs in the Tancat de la Pipa constructed wetland (Spain), using a conceptual source-attribution approach based on spatial distribution, particle characteristics, catchment characteristics, hydrological connections, and the available literature and official information, as well as (ii) to evaluate the effectiveness of the wetland in retaining MPs within the investigated size range. Sampling was conducted at six locations representing two inlet canals, two internal sites, and two outlet points of the wetland towards Albufera Lake. Higher MP abundances were recorded at the inlet, while lower concentrations at internal and outlet sites indicated partial retention, with apparent removal efficiencies ranging from 24% to 50%, depending on the sampling campaign. Blue and transparent MPs predominated, while polyethylene (47%) and polyester (18%) were the dominant polymers, followed by polymethyl methacrylate (10%), polystyrene (8%), and polypropylene (7%). The spatial distribution and particle characteristics, together with catchment land use and hydrological connections, indicated potential contributions from mixed sources, including packaging, agricultural activities, wastewater, traffic runoff, fisheries, and industrial activities. These findings provide an initial assessment of potential MP sources and transport pathways in a Mediterranean wetland and indicate partial retention of MPs within the investigated size range, supporting the need for longer-term and source-specific monitoring to improve source attribution and evaluate seasonal variability.

1. Introduction

Since the mid-20th century, global plastics production has grown exponentially. In 1950, approximately 1.5 million tonnes of plastics were manufactured worldwide, and by 2020 this figure had grown to 359 million tonnes annually [1]. Only 9% of discarded plastic products are recycled, and large amounts of plastic waste enter the natural environment, with the accumulation of plastics in the environment expected to reach 3.1 billion tons by 2050 [2,3,4].

Two categories of microplastics (MPs) are generally considered: primary and secondary. Primary MPs are intentionally manufactured at microscopic sizes for industrial and consumer applications, including cosmetic products, cleaning agents, resin pellets, and abrasive materials [5,6,7]. In contrast, secondary MPs are formed through the fragmentation of larger plastic items under the influence of physical, chemical, and biological weathering processes, such as mechanical abrasion, ultraviolet (UV) radiation, thermal degradation, and microbial activity [8]. These degradation processes break down plastic debris into microplastics (1 μm–5 mm) and, with further fragmentation, nanoplastics (<1 μm) [9]. Major sources of secondary MPs include the degradation of plastic packaging, agricultural mulch films, and vehicle tyres [10,11]. Unfortunately, both microplastic types (primary and secondary) exist in marine ecosystems at high concentrations. It has been estimated that about 245 tonnes of microplastics are produced each year, which end up in water bodies where they become ingested and incorporated into the bodies and tissues of marine organisms [12]. This dramatic increase reflects the global expansion of industrial, packaging, consumer, and construction applications of plastic materials. Global production is expected to continue rising due to growing demand and industrialization in emerging economies, with projections estimating continued growth into the mid-21st century [1,13]. Microplastic (MP) pollution has become an increasing environmental concern because small plastic particles are widely distributed in aquatic environments and can be transported through rivers, drainage networks, wastewater, runoff, and other hydrological pathways. Microplastics are widely detected in aquatic environments and are considered emerging pollutants due to their persistence, potential for bioaccumulation, and ecological impacts [14]. Wetlands, including constructed wetlands, play a crucial role in water purification, but may also function as sinks or conduits for MPs, depending on external inputs and internal processes [15]. The hydrological characteristics and dense vegetation of wetlands can retain microplastics from both aquatic transport and atmospheric deposition, thus acting as important accumulation areas [16]. Due to their unique location between terrestrial and aquatic ecosystems, covered by intermittent or permanent shallow water layers [17], wetlands are an important hub for the spread of MPs in global ecosystems [18]. Wetland microplastic contamination is primarily associated with surrounding anthropogenic activities [19]. Industrial sewage discharges are important sources of MPs in inland wetlands [20].

The physical processes responsible for the efficient removal of total suspended solids (TSS), including sedimentation, filtration, and vegetation-mediated retention, facilitate the capture and accumulation of MPs [21]. An important consideration in this regard is the subsequent management of the retained particles. Although their immobilization can reduce the transport of MP particles to receiving waters, this may also result in the progressive accumulation of MPs within the wetland system. Therefore, MP retention should not necessarily be interpreted as permanent removal from the environment. Retained particles may remain stored in sediments and organic matter and could potentially be remobilised under conditions of increased flow, flooding, sediment disturbance, or changes in hydraulic conditions. Recent studies have demonstrated that constructed wetlands (CWs) are effective nature-based solutions for reducing microplastic transport in aquatic environments, although their performance depends on wetland design, hydrology, and particle characteristics. In a horizontal subsurface-flow CW used as tertiary wastewater treatment, Wang et al. (2020) [22] reported a reduction in MP concentration from 6.45 MP L⁻¹ at the inlet to 0.77 MP L⁻¹ at the outlet, corresponding to an average 88% removal efficiency. The study identified fibres as the dominant particle shape throughout the treatment process and highlighted the role of CWs as efficient barriers preventing MPs from entering receiving water bodies. Cabañas-Mendoza et al. (2026) [23] have revealed the concentrations of MPs at the inlet (3.6 mg/L in water and 3.7 mg/g in sediments), higher root retention (24.5 mg/g), and a removal efficiency in the floating treatment wetlands of up to 80.2%. Ziajahromi et al. (2020) [24] investigated a stormwater floating treatment wetland and demonstrated that wetlands also function as sinks for road-derived microplastics, particularly tyre wear particles, which accumulated predominantly in wetland sediments, emphasizing the importance of stormwater wetlands in intercepting urban runoff before discharge into natural waters. Likewise, Lu et al. (2022) [25] evaluated five full-scale constructed wetlands and found inlet MP concentrations ranging from 0.4 to 3.8 MP L⁻¹ and outlet concentrations between 0.1 and 1.3 MP L⁻¹, representing removal efficiencies of approximately 40–75% depending on wetland type and hydraulic conditions. The authors further showed that MPs were preferentially retained in wetland sediments and vegetated areas, while finer particles (<300 μm) were more likely to remain suspended and be transported downstream. These concentrations are substantially higher than those measured in the present study, where only a few MP particles were detected per 100 L of water. The difference should, however, be interpreted cautiously because MP concentrations reported in the literature are influenced by sampling volume, size range analysed, sampling and extraction procedures, environmental conditions, and wetland characteristics. The substantial differences in reported MP concentrations among constructed wetlands highlight the influence of site-specific characteristics, hydrological conditions, wastewater inputs, catchment land use, and analytical methodology on observed MP abundance.

The Valencian Community in eastern Spain has a diverse range of wetland ecosystems, from one of the largest coastal lagoons in Europe (Albufera) to a network of marshes, lagoon systems and constructed wetlands embedded in agricultural landscapes. The Tancat de la Pipa wetland is located in the Albufera Natural Park near Valencia, Spain, and is surrounded by urban, agricultural, industrial, and tourism activities. Understanding MP sources in this context is essential for environmental management and protection of the Albufera aquatic ecosystem.

This study aimed (i) to identify potential sources and transport pathways of MPs in the Tancat de la Pipa constructed wetland (Spain) using a conceptual source-attribution approach based on short-term spatial distribution, particle characteristics, catchment characteristics, hydrological connections, and available literature and official information, and (ii) to evaluate the effectiveness of the wetland in retaining MPs within the investigated size range.

2. Materials and Methods

2.1. Study Area

The present study was conducted in the Tancat de la Pipa wetland (39.368046, –0.345619 to 39.359275, –0.346540 area) (Figure 1), a 40-ha restored wetland located on the northern side of Albufera Natural Park (Valencia, Spain), a park declared as a Ramsar site, a Zone of Special Protection for Birds and a Natura2000 site. Tancat de la Pipa, an area previously used for rice cultivation, was restored between 2007 and 2009 as part of an ecological restoration project aimed at improving the water quality of Albufera Lake while increasing biodiversity. The design of Tancat de la Pipa follows nature-based solution principles, integrating shallow lagoons, sedimentation areas, and vegetated zones dominated by emergent macrophytes. These elements collectively promote hydrodynamic slowing down, particle settling, and biological processing of contaminants before water is discharged into the lagoon [26,27]. The system comprises approximately 9–10 ha of free water surface-flow wetlands, divided into four vegetated basins, together with two shallow lagoons (the Educative Lagoon, ~6 ha, and the Reserve Lagoon, ~8 ha) and an artificial pool of approximately 4.5 ha supplied by groundwater. Water enters from agricultural drainage channels and the Albufera lagoon, flows through the vegetated basins, passes into the lagoons, and is finally discharged back to the lake. The wetland is dominated by emergent macrophytes, including common reed (Phragmites australis), cattails (Typha spp.), bulrushes, rushes (Juncus spp.), sedges (Carex spp.), and yellow flag iris (Iris pseudacorus), while the lagoons support extensive communities of submerged macrophytes and charophytes [28]. This diverse vegetation promotes sedimentation, nutrient uptake, and habitat creation, making Tancat de la Pipa an important example of a nature-based solution for improving water quality and supporting aquatic biodiversity in the Albufera ecosystem. Together, these processes contribute to the removal of nutrients, suspended solids, organic contaminants, and emerging pollutants, including microplastics, from incoming waters [29,30].

Tancat de la Pipa receives water from two main tributaries: the Catarroja Canal/Acequia del Puerto de Catarroja (PC) and the 43.5 km long Poyo stream/Barranco del Poyo (BP) (Figure 2), which drains urban, industrial, and agricultural areas. Tancat de la Pipa discharges water into the Albufera Lake through a system of managed cells with inlets and outlets [31].

At the inlet points, where water enters from the Catarroja canal (PC) and from Poyo stream (BP), flow velocities are relatively high, favouring the transport of suspended particles, including microplastics. As water moves into the internal basins, flow velocities decrease due to basin widening, vegetation density, and increased hydraulic residence time. These conditions enhance sedimentation, filtration by macrophytes, and particle interception, which are recognized mechanisms for MP retention in constructed and natural wetlands [30,32].

Vegetated zones, in particular, act as physical barriers that trap particles through stem interception and biofilm adhesion, a process shown to be effective for both MPs and fine suspended matter [33]. The outlet points represent the final control points of the system, where only a fraction of the incoming microplastics is released back into Lake Albufera. Previous studies on constructed wetlands have demonstrated that such multi-cell designs significantly reduce microplastic abundance between inlet and outlet, with reported retention efficiencies often exceeding 50%, depending on particle size and hydrodynamic conditions [4,34].

Hydraulic travel time between the inlet and outlet zones was considered in the interpretation of the sampling design. Martin et al. (2014) [35] in previous hydraulic management documentation for Tancat de la Pipa reported a nominal hydraulic retention time (HRT) of approximately 3.3 days under the specified hydraulic loading conditions. This value represents the theoretical water residence time of the wetland and should not be interpreted as a direct measurement of the travel time of individual water volumes or microplastic particles from the inlet to the outlet. Martin et al. (2013) [36] stated that the hydraulic retention time of Tancat de la Pipa is dependent on the hydraulic loading and operational configuration of the wetland.

Smaller particles may remain in suspension and pass through the system, while larger fragments and fibres are preferentially retained in sediments and vegetation mats. This spatial organization makes Tancat de la Pipa particularly suitable for investigating not only the presence of microplastics, but also their potential sources, transport pathways, and retention efficiency within a wetland system.

The wetland is surrounded by a complex mosaic of land uses that exert diverse pressures on water quality, potentially contributing to MP transport and resuspension.

2.1.1. Industry

The industrial belt surrounding the Albufera Natural Park, particularly within the municipality of Catarroja (l’Horta Sud, Valencia), represents a relevant potential source of microplastic pollution to the Tancat de la Pipa wetland. This area hosts a dense concentration of plastic manufacturing, plastic packaging, chemical processing, logistics, and material manipulation companies. The Catarroja Industrial Park hosts approximately 450 companies, with a notable share engaged in chemical, rubber, and plastics-related activities (≈6% of enterprises) alongside broader manufacturing and supply sectors [37]. Plastic pellets, fragments, films, and fibres can be unintentionally released during manufacturing, cutting, storage, transport, and waste handling phases. These losses are widely recognised as an important source of primary and secondary microplastics in aquatic environments, particularly in industrialised catchments [38]. The Catarroja Canal receives water from urban runoff and agricultural drainage, including rice fields and horticultural land, across municipalities such as Catarroja, Paiporta, and Torrent. Furthermore, runoff from the Port of Catarroja may contribute additional synthetic particles originating from urban runoff, combined sewer overflows (CSOs), and industrial activities. The Poyo stream (43.5 km) functions as an intermittent but highly effective drainage corridor that integrates runoff from urban, agricultural, and peri-urban industrial areas of the southern Valencia metropolitan region municipalities (e.g., Torrent, Picanya, Paiporta, Massanassa, and Catarroja). The Poyo stream is likely influenced by multiple microplastic input pathways, namely municipal wastewater, stormwater runoff, industrial and port-related activities with plastic manufacturing, packaging industries, construction material production, other polymer-processing facilities, and agricultural activities. Wastewater Treatment Plant (WWTP) effluents are commonly discharged into nearby irrigation/drainage channels, which then connect to the Albufera system, and these discharges are typically treated wastewater (secondary or tertiary treatment depending on the plant), reused for irrigation or released when excess flow occurs. Even when wastewater is treated, significant quantities of microplastics, especially fine fibres from synthetic textiles and small fragments, can pass through treatment processes and enter connected waterways [39,40,41]. Given this hydrological configuration, MPs originating from industrial activities may reach Tancat de la Pipa through indirect WWTP effluent discharges into the Catarroja canal and Poyo stream. In addition, CSOs during intense rainfall can introduce untreated domestic sewage, increasing MP loads rapidly during storm events. During rainfall events, this channel rapidly mobilises surface materials accumulated in its catchment, including sediments, nutrients, and anthropogenic debris such as plastics. Hydrological studies of the basin highlight its role as a conduit linking densely urbanised zones with downstream wetland environments, ultimately contributing inflows toward the Albufera system through connected drainage networks [42]. Navigable canals and boat traffic may contribute to microplastic inputs through paint abrasion, rope and net degradation, and resuspension of previously deposited plastic particles.

2.1.2. Agriculture

To the west and northwest, extensive rice cultivation areas dominate the Albufera Natural Park landscape, where plastic films, irrigation infrastructure, and agricultural inputs represent potential sources of microplastics transported through irrigation return flows and drainage canals. Albufera Natural Park consists of an interconnected landscape of shallow lagoons, marshes, and extensive rice-growing areas that occupy approximately 223 km² around the lagoon. In the context of Tancat de la Pipa, rice fields surrounding the wetland are frequently irrigated through a network of canals and drainage channels connected to the wetland system. Rice paddies create a distinct seasonal cycle of flooding and desiccation, as fields are intentionally flooded during the growing season and drained in spring and autumn, which influences water movement, sediment transport, and nutrient dynamics across the wetland landscape [43]. Given the shallow nature of Albufera lake and its connectivity with surrounding irrigation networks, cultivated rice lands represent a critical interface between terrestrial agriculture and aquatic ecosystems. Studies of organic contaminants have documented the presence of multiple agrochemicals, including herbicides, insecticides, and fungicides, throughout water and sediment matrices during the rice cultivation period, with herbicides frequently detected at high concentrations [44]. Approximately 35 tonnes of pesticides are yearly applied in the Albufera Natural Park, and on average, more than 2 ± 0.6 (mean ± standard deviation) tonnes of pesticides are discharged annually from rice paddies to ditches. Bentazone, a widely used herbicide, was identified as the single most applied compound in this regime [45]. The hydrological connectivity maintained through irrigation networks ensures that these agricultural signals propagate across the wetland, affecting ecological processes well beyond the cultivated fields [43]. Plastic mulch films, irrigation pipes, greenhouse materials, and fertilizer or pesticide packaging can degrade into microplastics and be transported through irrigation return flows and drainage canals [46,47]. Plastic films used for mulching and soil covering are particularly relevant because they are often made of polyethylene (PE) and are exposed to UV radiation, temperature changes, and physical abrasion, which accelerate fragmentation into micro-sized particles [48,49].

Extreme weather events, such as flash floods, likely play a major role in mobilizing stored plastics from soils, road margins, agricultural fields, and urban waste deposits, producing episodic pulses of MPs transported downstream. Overall, the Poyo stream represents a highly complex MP receptor system influenced by combined wastewater, urban runoff, industrial discharge, agricultural drainage, and flood-driven remobilization processes.

2.2. Sampling/Lab Processing

Sampling was conducted during three campaigns (20 February, 16 April, and 16 May 2024). These campaigns covered the late wet-season and spring period but did not encompass the complete annual hydrological cycle. Consequently, the results obtained in this study describe microplastic occurrence and distribution during the investigated sampling period and do not represent year-round microplastic dynamics in the Tancat de la Pipa wetland. The surface water samples were collected from 3 lines of Tancat de la Pipa: the inlet line from two inlet spots, with controlled flow of 30 L/s, one entrance spot from Poyo stream (urban/industrial/ agricultural inputs)—BP, with one entrance spot from Catarroja canal (urban/industrial inputs)—PC; the inner line of the wetland ELE, ELR spots; and from the outlet line from two exit spots to Albufera lake SLE, SLR (Figure 3). Sampling covers the entire hydraulic pathway of the wetland, including inlet, inner, and outlet zones, thus allowing an integrated assessment of microplastic transport, attenuation, and retention efficiency. Sampling depth varied according to the sampling location. At the two inlet points (BP and PC), water samples were collected directly from the respective water inlet pipes. At the internal sampling points (ELE and ELR) and outlet points (SLE and SLR), samples were collected from the surface water layer, at a maximum depth of 10 cm below the water surface. These sampling points were located within the shallow internal lagoons of Tancat de la Pipa, where Rodrigo et al. (2013) [50] in previous studies reported water depths of 0.30–0.60 m for the Educative Lagoon and 0.35–0.70 m for the Reserve Lagoon.

Each microplastics sample was extracted from 100 L of water, in situ filtered through stainless steel sieves with mesh sizes of 0.040 mm, 0.075 mm and 0.425 mm [51]. The sieve fractions were used as the operational basis for particle-size classification. Thus, particle size was not determined by measuring the maximum dimension of each individual particle; instead, particles were assigned to size fractions according to the sieve through which they passed or on which they were retained. The analysed microplastic size fractions were ≥40 µm, with particles retained in the 40–75 µm, 75–425 µm, and >425 µm fractions. Particles smaller than 40 µm were not systematically retained by the applied sieving procedure and were therefore outside the quantitative size range investigated in this study. The use of stainless-steel equipment was adopted to avoid plastic contamination during sampling, in accordance with established microplastic monitoring protocols [8,52]. The particles captured in the metal sieves were washed with distilled water in glass containers covered with aluminium foil and transported to the laboratory for further processing. Samples were then placed in the laboratory drying oven, maintained at 60 °C, and left to dry until complete evaporation of the aqueous phase was achieved. The organic matter was removed using 30% hydrogen peroxide (H₂O₂) [8]. H₂O₂ effectively removes organic matrices without significantly altering polymer composition, making it suitable for MP extraction from aquatic samples [53]. Following organic matter removal, MPs were isolated using density separation, employing two complementary high-density salt solutions: potassium iodide (KI) [54] and calcium chloride (CaCl₂).

These solutions allow the flotation of polymers with densities up to approximately 1.6–1.8 g·cm⁻³, enabling efficient separation from mineral particles and residual sediments. Density separation using KI and CaCl₂ is considered particularly suitable for freshwater and wetland samples with mixed particle compositions [55,56]. The supernatant obtained was filtered using a vacuum filtration system equipped with a glass microfiber filter, grade 692. A total of 18 surface water samples were collected from the Tancat wetland in Pipa, processed in the laboratory, resulting in 54 analysed filters.

2.3. Microplastic Identification

Microplastic particles were identified using a combination of visual inspection and spectroscopic analysis. Following filtration, suspected microplastic particles retained on metallic sieves were first examined under a stereomicroscope (Olympus SZX7 microscope, with high-resolution image, up to 600 lines per mm, and with high colour fidelity, Olympus, Tokyo, Japan) at magnifications ranging from 20× to 50×. Visual identification was based on commonly accepted criteria, including particle shape, colour, surface texture, and the absence of cellular or organic structures, in order to distinguish synthetic particles from natural debris [8,52]. Each particle was assessed according to its morphological characteristics, including shape, colour, surface appearance, and structural characteristics. Particles displaying characteristics inconsistent with synthetic polymers, such as recognisable biological structures or clearly mineral-like morphology, were excluded from the microplastic count. Particular attention was given to avoiding the classification of natural or inorganic particles as microplastics solely on the basis of their size or appearance.

To reduce the risk of misidentification associated with visual sorting alone, a subset of visually identified particles was further analysed using Raman spectroscopy for polymer confirmation. Selected individual microplastic particles were chemically characterised using a WITec alpha300 Raman-SNOM microscope equipped with an Ultra High Throughput Spectrometer (UHTS; WITec Wissenschaftliche Instrumente und Technologie GmbH, Ulm, Germany). Raman measurements were performed using 532 nm (green) or 633 nm (red) excitation wavelengths, with microscope objectives of 20×, 40×, and 50×, selected according to the characteristics and accessibility of individual particles. The confocal Raman microscope enabled individual particles to be optically located and targeted for spectral acquisition. This was particularly relevant for particles in the 40–75 µm fraction, which could be individually visualised and positioned under the microscope before Raman measurement. The dimensions of these particles were substantially larger than the sub-micrometre optical spatial resolution achievable with confocal Raman microscopy; therefore, particle size itself did not prevent optical localisation and targeting of individual particles. The excitation wavelength (532 or 633 nm) was selected according to the spectral response of the analysed particle and measurement conditions. The use of different excitation wavelengths provided flexibility when analysing environmental particles, particularly where fluorescence or other spectral interferences affected the quality of the Raman signal. For particles in the 40–75 µm fraction, the Raman measurement position was carefully placed on the visually identified particle to minimise the contribution of the underlying filter or surrounding material. Raman spectra were therefore acquired from individual particles rather than from the bulk material retained on the sieve. Polymer identification was based on comparison of the obtained Raman spectra with reference spectral information. Raman analysis enabled the identification of the chemical composition of selected particles by comparing obtained spectra with reference polymer libraries, allowing discrimination between synthetic polymers and non-plastic materials. Combining microscopic examination with Raman spectroscopic confirmation increases the reliability of microplastic identification by reducing false-positive classifications and providing chemical verification of visually selected particles [57,58]. The final microplastic dataset was based on particles that fulfilled the morphological criteria for suspected microplastics, with polymer identity confirmed by Raman spectroscopy for the analysed subset. The size of each particle was assigned according to the sieve fraction in which it was retained, rather than according to direct measurement of its individual dimensions.

Data analysis focused on descriptive spatial patterns across sampling points and campaigns. Colour distribution was analysed to support potential source hypotheses. Microsoft Excel was used for calculations, analysis and graphics. Raman spectroscopy was used for microplastic and polymer type detection. Google Earth/ Google maps were used for location images.

3. Results and Discussion

3.1. MPs by Location and Sieve Size

During three sampling campaigns, 18 surface water samples (100 L of water filtered through 3 different-sized sieves, from 6 spots, on 3 different days) were collected for analysis. Microplastics were detected in all Tancat de la Pipa sampling points and campaigns, confirming the permanent presence of MPs within the wetland system and its continuous exposure to anthropogenic pressures from both Albufera Lake and connected canals. A total of 152 microplastic particles (Figure 4) from 54 fiberglass filters (18 surface water samples × 3 different-sized sieves) were examined using the microscope and Raman equipment.

Microplastic concentrations (particles/L) (

Table 1. Microplastic concentrations (particles/L) measured at different sampling locations and sieve dimensions during the monitoring campaigns conducted in February, April, and May 2024.

SAMPLING LINE	SPOT	Sieve Dimension	MP (Particles/L) 20 February 2024	MP (Particles/L) 16 April 2024	MP (Particles/L) 16 May 2024
INLET PC spot	PC 40	0.040 mm	0.05	0.00	0.02
INLET PC spot	PC 75	0.075 mm	0.03	0.02	0.01
INLET PC spot	PC 425	0.425 mm	0.00	0.03	0.02
INLET BP spot	BP 40	0.040 mm	0.02	0.09	0.03
INLET BP spot	BP 75	0.075 mm	0.11	0.05	0.04
INLET BP spot	BP 425	0.425 mm	0.00	0.04	0.02
INNER ELE spot	ELE 40	0.040 mm	0.06	0.01	0.00
INNER ELE spot	ELE 75	0.075 mm	0.03	0.06	0.04
INNER ELE spot	ELE 425	0.425 mm	0.02	0.03	0.01
INNER ELR spot	ELR 40	0.040 mm	0.06	0.04	0.02
INNER ELR spot	ELR 75	0.075 mm	0.06	0.04	0.02
INNER ELR spot	ELR 425	0.425 mm	0.02	0.04	0.01
OUTLET SLE spot	SLE 40	0.040 mm	0.02	0.04	0.00
OUTLET SLE spot	SLE 75	0.075 mm	0.00	0.05	0.01
OUTLET SLE spot	SLE 425	0.425 mm	0.05	0.00	0.03
OUTLET SLR spot	SLR 40	0.040 mm	0.02	0.02	0.00
OUTLET SLR spot	SLR 75	0.075 mm	0.05	0.02	0.02
OUTLET SLR spot	SLR 425	0.425 mm	0.02	0.01	0.01

) measured at different sampling spots and sieve dimensions during the monitoring campaigns conducted in February, April, and May 2024 indicate that the highest values are at inlet spot BP.

When the size of MPs is taken into consideration, the results show a clear trend regarding the categorization by MPs size (Figure 5). For the 0.040 mm sieve, the mean concentration decreased from 0.035 particles L⁻¹ at the inlet to 0.017 particles L⁻¹ at the outlet. For the 0.075 mm fraction, the inlet exhibited the highest concentration (0.043 particles L⁻¹), followed closely by the inner sampling line (0.042 particles L⁻¹), with concentrations decreasing to 0.025 particles L⁻¹ at the outlet. This corresponds to a retention efficiency of 42%, which may reflect the combined effects of sedimentation, interception by wetland vegetation, attachment to suspended solids and organic matter, aggregation, and temporary storage within low-flow zones and sediments. For the 0.425 mm fraction, the inlet-to-outlet difference was negligible (approximately 9%), indicating limited net reduction across the sampling pathway during the investigated period. The stronger decrease observed for the 0.040 mm fraction should not necessarily be interpreted as permanent removal. Particles in this size range may remain suspended and can be redistributed within the wetland depending on hydraulic conditions, sediment interactions, other particle transformation processes, and local water movements. Similarly, the relatively small difference between the inlet and outlet concentrations for the 0.425 mm fraction suggests that larger particles were not consistently associated with greater net retention under the conditions investigated. The higher concentration observed at the inner sampling line for the 0.425 mm fraction compared with the inlet may indicate spatial heterogeneity in particle distribution, although the limited number of detected particles prevents a robust assessment of this pattern. Overall, the results indicate size-dependent spatial variability in MP occurrence, but they do not provide sufficient evidence to attribute the observed differences to a specific retention mechanism.

Microscopic examination revealed microplastic particles with diverse morphologies, sizes, and colours (Figure 6). The retention efficiency relative to particle size suggests that the 40 µm fraction is more readily removed by sedimentation, filtration by emergent vegetation, and physical trapping within the wetland, although the occurrence of other particle transformation processes cannot be excluded. However, the relatively low particle numbers limit the strength of mechanistic interpretations.

Microplastic concentration (particles/L) across the three sampling campaigns (February, April, and May 2024) along the sampling transect (inlet–inner–outlet) in the studied wetland system reveals an average of 0.021 MP particles/L (Figure 7) was detected at the outlet points from Tancat de la Pipa toward Albufera Lake. The results show a slight decrease in microplastic abundance from the inlet and inner sections (0.032 particles/L) towards the outlet (0.021 particles/L), indicating a potential retention effect within the system. Given the limited number of detected particles and the short sampling period, this difference should be interpreted as a site- and period-specific observation rather than as evidence of a constant wetland removal efficiency.

The spatial distribution of microplastics observed in Tancat de la Pipa wetland reveals a complex mixture of potential sources, reflecting the combination of urban, industrial, agricultural, and recreational pressures in the Albufera Natural Park. The presence of two distinct inlet pathways, the Catarroja canal (PC) and Poyo stream (BP), allows an interpretation of the origin of microplastic particles, supported by the observed size distribution, temporal variability, polymer type and colour composition. Internal redistribution of microplastic particles appears to be controlled by hydrodynamics, sedimentation, vegetation trapping, biofilm formation, aggregation with suspended particles and organic matter, adsorption, and retention within wetland sediments. The predominance of small MP particles suggests that potential contributors from diffuse sources, particularly water from wastewater treatment plant (WWTP) discharges and urban runoff, are likely to represent contributors to the observed microplastic load [59,60,61]. The analysed size range was limited to particles ≥40 µm, and particles smaller than 40 µm were not quantified. Consequently, the present results should be interpreted as an assessment of the spatial distribution of MPs within the analysed size range and sampling period rather than as a complete assessment of MP transport or removal in the wetland.

Urban stormwater runoff, including drainage managed through sustainable drainage systems, may contribute to the MP transport into wetlands and surface waters, with nature-based drainage features potentially modulating these inputs [62].

3.1.1. Inlet Line

The analysis focused primarily on the two inlet spots. A difference in the abundance of MPs can also be observed between the sampling spots in the inlet line, with PC with an average of 0.030 MP particles/L and BP with an average of 0.044 MP particles/L, respectively. Thus, at the collection BP spot, a much higher abundance of MP particles is observed compared to the PC spot. The distribution of MPs between inlet sources was markedly uneven (Figure 8). The BP inlet contributed 69% of the MPs, whereas the PC inlet contributed 31% of the MPs. This result clearly indicates that the Poyo stream represents the dominant source of microplastics entering the wetland, while the Catarroja canal represents a potential pathway for MP inputs to the wetland, with lower MP abundances than those observed at the Poyo inlet during the investigated sampling campaigns.

Inlet Line—PC Inlet Spot

The PC inlet, located at the connection between the Catarroja canal and the Tancat de la Pipa wetland, exhibited notable microplastic particle inputs across all sampling dates. At the PC inlet, the total observed MP abundance decreased from 0.08 particles/L in February to 0.05 particles/L in April and May. This reduction was accompanied by a shift in particle-size distribution. In February, the 40–75 µm fraction accounted for 62.5% of the observed MPs, whereas no particles in this fraction were detected in April. Conversely, particles >425 µm, which were not detected in February, accounted for 60% of the observed MP abundance in April. In May, the distribution was more balanced among the investigated size classes. These observations indicate temporal variability in the size distribution of MPs entering the wetland; given the limited number of sampling campaigns and the exclusion of particles <40 µm, the observed changes cannot be interpreted as a definitive temporal trend in the total MP load.

The spatial pattern and size distribution at PC suggest contributions from urban and industrial sources upstream. Urban runoff, stormwater discharges, combined sewer overflows (CSOs), and WWTP effluents have been identified in the literature as potential pathways for MPs into riverside and canal systems [60]. The predominance of small MPs at the PC inlet (0.040–0.075 mm) is consistent with the expected output from wastewater treatment plants in indirect discharge canals, which commonly release microfibers and small fragments despite high removal efficiencies [59,61]. Industrial activities in the villages to the west of the wetland, including plastic manufacturing and processing, may also contribute to MPs in the canal through direct discharges or poor waste management [63]. Plastic pellets, fragments, and synthetic fibres released during industrial activities are known to enter adjacent waterways and are frequently observed in surface waters and sediments near industrial and port zones, and represent additional potential pathways for microplastic inputs to the canal [7,64]. Furthermore, traffic-related sources such as tyre and brake wear may contribute additional MPs to the canal via road runoff, particularly during rainfall events. Tyre wear particles are typically black and sub-millimetre in size, aligning with the presence of small MPs in the PC samples [65,66].

Inlet Line—BP Inlet Spot

The BP inlet showed the highest total MP abundance among all sampled sites, with an average of 0.044 MP particles/L detected overall. This pattern suggests a background input of fine MPs likely associated with continuous sources such as wastewater-derived microfibers, urban runoff, and diffuse contamination transported through the canal. In April 2024, the counts increased markedly, indicating an episodic or seasonal input of both small and larger fragments. This increase may be explained by intensified spring agricultural activities (soil preparation, irrigation return flows, and mobilization of degraded plastic films), enhanced stormwater runoff transporting urban litter and tyre wear particles, and/or increased hydrodynamic disturbance and resuspension within connected canal-lake systems. Additionally, spring rainfall events or higher WWTP discharge volumes could increase the transport of fine MPs into the Poyo stream, resulting in a temporary spike in MP abundance. May 2024 again showed moderate values. This indicates a return to moderate inflow conditions, likely reflecting stabilization after the April peak. The high abundance and persistence of small MPs (0.040–0.075 mm) at BP suggest continuous inputs from activities associated with the Poyo stream route. Overall, the temporal variability at BP inlet point suggests that microplastic contamination is not only controlled by continuous background sources (urban wastewater and runoff), but also by seasonal pulses related to agricultural management and episodic hydrological events.

The two inlet points exhibited distinct patterns of MP abundance and particle-size distribution. The mean observed concentration was approximately 2.2 times higher at BP than at PC. MPs detected at inlet line spots, BP and PC, across all three sieve size fractions (0.040 mm, 0.075 mm, and 0.425 mm), had a clear dominance of smaller particles. Of the total MPs identified: 36% were retained by the 0.040 mm sieve, 45% by the 0.075 mm sieve, and 19% by the 0.425 mm sieve. Overall, approximately 80% of all MPs in the inlet were smaller than 0.075 mm, highlighting the predominance of fine microplastics at the wetland inlet. This dominance of small-sized MPs is characteristic of secondary microplastics, resulting from the fragmentation and degradation of larger plastic debris, rather than primary industrial pellets. The predominance of fine particles is consistent with secondary microplastics produced through the environmental weathering of larger plastic items, rather than with direct emissions of primary industrial pellets [7,67]. Microplastics: Smaller MP particles are preferentially transported over longer distances and are less efficiently removed during wastewater treatment processes, explaining their high abundance at wetland inlets [59]. The size distribution strongly supports fragmentation-driven sources rather than primary microplastic inputs. The observed decrease applies only to the detected ≥40 µm fraction, while smaller fractions were not investigated.

3.1.2. Inner Line

The internal wetland points (ELE and ELR) show a moderate concentration of MPs, indicating partial retention. Within the vegetated basins, microplastics are retained through complementary physical and biological processes, including sedimentation, particle settling, interception by emergent vegetation, and attachment to biofilm-covered surfaces. However, the continued presence of MPs indicates that not all particles are retained and some may be transported downstream. These internal points likely reflect the combined effects of hydrological circulation, sedimentation, and macrophyte trapping, which can temporarily retain MPs before redistribution within the system. The constructed wetland vegetation and substrate can significantly influence microplastic retention, as observed in full-scale treatment wetlands where macrophyte roots and sediment complexes enhanced MP capture [51]. The internal wetland may also act as a secondary source if MPs stored in sediments are resuspended during water flow or wind events. This could explain the persistence of MPs in the internal area despite the wetland’s retention capacity.

3.1.3. Outlet Line

The outlet value is lower than the average MPs (0.032 MP particles/L) recorded at the inlet points, indicating that the wetland functions as a net retention system for microplastics, as revealed in

Table 2. Microplastic (MP) retention efficiency (%) of the Tancat de la Pipa constructed wetland during the sampling campaigns conducted on 20 February, 16 April, and 16 May 2024, and the average retention value. MP removal efficiency (%) was calculated as (C_in − C_out)/C_in × 100, where C_in and C_out are the microplastic concentrations measured at the inlet and outlet sampling lines, respectively.

TANCAT DE LA PIPA Sampling Lines	MP Removal Efficiency (%) 20 February 2024	MP Removal Efficiency (%) 16 April 2024	MP Removal Efficiency (%) 16 May 2024	AVERAGE MP Removal Efficiency (%)
INLET-OUTLET	24%	39%	50%	36%

.

Similar reductions in MPs abundance between inflow and outflow have been reported in both constructed and natural wetlands, confirming their role as effective interception systems for particulate pollutants [10,68]. Wetlands reduce microplastic transport primarily through decreased flow velocities, enhanced sedimentation, and interactions with vegetation and biofilms, which favour particle trapping, although the occurrence of other particle transformation processes cannot be excluded. Despite these retention processes, the persistence of fine MP particles at the outlet indicates that complete removal is not achieved under the hydraulic conditions observed in this wetland [18]. The lower MPs abundance at the outlet supports the interpretation of Tancat de la Pipa as a net sink rather than a net source of microplastics to Albufera Lake.

The calculated microplastic removal efficiencies should be interpreted as reductions in particle abundance within the analysed size range rather than as definitive evidence of permanent microplastic removal from the wetland system. The observed decrease in particle counts between inlet and outlet may result from physical retention processes, such as sedimentation and filtration by the wetland matrix and vegetation, but other particle transformations cannot be excluded. In particular, fragmentation of larger microplastic particles could generate smaller particles that were not quantified in the present study because particles below 40 µm were outside the analysed size range. Consequently, a reduction in particles ≥40 µm at the outlet could partly reflect a shift towards smaller size fractions rather than complete removal from the system. Chemical or biological degradation of polymers may also contribute to changes in particle abundance, although the present study was not designed to quantify polymer degradation or transformation. Therefore, the reported removal efficiencies represent an apparent reduction in the concentration of microplastic particles within the investigated size.

The outlet points (SLE and SLR) show lower concentrations of fine MPs, but a higher proportion of larger MPs (0.425 mm) in February. The presence of larger MPs at the outlet indicates that the wetland is not fully effective in trapping all plastic sizes, and that it may act as a conduit for MP transport to the Albufera lake. From a system-scale perspective, the export of MPs from Tancat de la Pipa represents a reduced and filtered contribution relative to upstream inputs. The wetland does not constitute a primary MP source to Albufera Lake, but rather moderates the MP fluxes and limits the transfer of larger particles. The observed reduction in downstream microplastic flux supports the role of constructed wetlands as nature-based systems that mitigate, rather than eliminate, the transport of particulate pollutants. The key potential sources transported to the outlet are resuspended sediments from the wetland, transport of MPs originating from inlet sources, and navigation-induced resuspension and transport.

Study Limitations

The total number of microplastic particles identified in the Tancat de la Pipa samples was relatively low, with 152 particles detected from a total processed water volume of 1800 L. This corresponds to an overall observed abundance of approximately 0.084 particles L⁻¹ within the size fractions investigated (≥40 µm). Therefore, the study area should not be characterised as heavily contaminated based on the concentrations measured during the sampling campaigns. The relatively low particle abundance also limits the strength of conclusions that can be drawn from individual sampling points or individual size fractions, particularly where only a small number of particles were detected. The observed differences among inlet, internal, and outlet zones should consequently be interpreted as patterns within the investigated dataset rather than as definitive evidence of differences in microplastic contamination across the wetland. Similarly, the distribution among sieve fractions provides information on the size distribution of the detected particles but should not be interpreted as representing the complete size distribution of microplastics in the wetland. The findings are therefore best considered as an assessment of microplastic occurrence and spatial distribution under the specific sampling and analytical conditions employed in this study. Larger datasets involving more sampling campaigns, additional sampling points, and repeated sampling would be required to characterise spatial and temporal variability with greater statistical confidence.

A methodological limitation of this study is the exclusion of microplastic particles smaller than 40 µm. The smallest sieve used for the Tancat de la Pipa samples had a nominal mesh size of 40 µm; consequently, particles below this threshold were not systematically collected or quantified. The reported microplastic abundances therefore represent the particles retained within the analysed size fractions and should not be considered an estimate of the total microplastic abundance across all particle sizes. This limitation is particularly relevant because smaller microplastic particles can exhibit greater mobility and may have an increased potential for biological uptake and interactions with aquatic organisms.

A limitation of the present study is also the restricted temporal coverage of the sampling campaigns. Samples were collected on three dates between February and May 2024 and therefore represent only a portion of the annual hydrological and meteorological variability of the Tancat de la Pipa wetland. In particular, the study did not include the autumn period, when rainfall and runoff events can be important in the Valencia region. Consequently, the measured microplastic concentrations and the observed differences between inlet, internal, and outlet sampling zones should be interpreted as representative of the conditions prevailing during the investigated sampling period rather than as annual averages or year-round patterns. Rainfall, runoff, water discharge, and associated hydrological conditions may influence the transport and redistribution of particulate material within wetland systems. Therefore, microplastic inputs and retention may vary between seasons and between individual rainfall events. The present dataset does not allow the magnitude or direction of such seasonal variability to be quantified.

The identification of microplastic particles at the sampling locations provides evidence of their presence and spatial distribution within the Tancat de la Pipa wetland, but does not by itself allow their specific sources to be determined. In the present study, source attribution was therefore based on the spatial position of the sampling points, polymer composition, morphology, colour, documented characteristics of the surrounding catchment and hydrological connections, and information available from the literature and official sources, rather than on direct chemical fingerprinting or source-tracking measurements. Definitive source apportionment would require additional evidence, such as long-term monitoring, source-specific sampling, chemical fingerprinting, or quantitative source-apportionment approaches.

3.2. Microplastic Polymer Type

Nine different types of polymers were the most commonly found in Tancat de la Pipa wetland, dominated by polyethylene (PE, 47%), followed by polyester (PES, 18%), acrylic/PMMA (10%), polystyrene (PS, 8%), polypropylene (PP, 7%), Polyvinyl Chloride (PVC), Tyre rubber (TWP), Polyurethane (PU), %), and polycarbonate (PC), as shown in Figure 9.

The polymer composition further supports the multi-source interpretation. The dominance of polyethylene (PE, 47%), together with polypropylene (PP, 7%) and polystyrene (PS, 8%), suggests that packaging waste, degraded consumer products, and agricultural plastic films represent major contributors, consistent with runoff and canal transport from surrounding rice cultivation areas and nearby urban settlements [8,67]. The relatively high contribution of polyester (PES, 18%) strongly indicates wastewater-derived contamination, as synthetic textile fibres are widely reported as dominant MP types in WWTP influents and effluents [61,64]. In addition, the detection of acrylic/PMMA (10%), along with smaller proportions of PVC (3%), polyurethane (PU, 3%), and polycarbonate (PC, 2%), suggests contributions from industrial activities and infrastructure-related materials, including plastic processing, coatings, construction products, and port-associated sources within the catchment [69].

The occurrence of tyre wear particles (TWPs, 2%) further highlights the influence of road traffic and stormwater runoff as a relevant pathway of synthetic particle input into the wetland system [61]. Overall, the polymer fingerprint reflects a combined impact of agricultural drainage, urban runoff, wastewater discharge, and industrial activities surrounding the Albufera wetland system.

3.3. MPs Colour Distribution

In terms of colour, the majority of the MPs were blue (42%) and colourless (41%), followed by red (10%), black (3%), green (2%), and white (2%) (Figure 10), indicating a strong prevalence of visually low-contrast and blue-coloured microplastics in the sampled environment.

The distribution of microplastic colours in Tancat de la Pipa provides additional clues on potential pollution sources. In this study, blue and colourless microplastics dominated the samples (127 items in total), followed by red, black, green, and white particles. Microplastic colour can provide useful information on potential sources and environmental weathering processes [70]. Blue coloured MPs (42%) have likely sources among synthetic textile fibres (polyester, polyamide, acrylic), fishing gear (nets, ropes, lines), or packaging materials and tarpaulins. Blue fibres are widely reported as the most abundant MP colour in aquatic environments, particularly in wetlands and lagoons connected to human activity [70,71,72]. Dris et al. (2015) [73] and Browne et al. 2011 [64] indicated that blue and black fibres dominate atmospheric and aquatic microplastic fallout, highlighting textiles as a primary source. The high proportion of colourless MPs (41%) is consistent with weathered polyethylene- and polypropylene-based materials, including agricultural plastics, although packaging materials and urban litter may also contribute. Prolonged exposure to sunlight, temperature fluctuations, and mechanical abrasion promotes pigment loss and fragmentation, resulting in aged colourless particles [67,74,75].

The high proportion of colourless MPs supports the hypothesis that agriculture is a major source of MPs in Tancat de la Pipa, although additional sources such as packaging materials and urban litter cannot be excluded [15,49,76,77]. Plastic mulch films, widely used in rice cultivation to control weeds, conserve soil moisture, and enhance yields, are primarily composed of polyethylene (PE) and polypropylene (PP). Red coloured MPs are commonly associated with consumer plastics and industrial materials, indicators of direct anthropogenic inputs in freshwater systems [78]. Their moderate abundance suggests episodic and relatively recent inputs. Red pigments are less prone to rapid fading, indicating shorter environmental residence times compared to colourless MPs. Black MPs are frequently linked to tyre wear particles, rubber and industrial plastics. Tyre wear is a major MP particle source, often underrepresented in surface water surveys [65]. The relatively low proportion observed may reflect preferential sedimentation or analytical underestimation due to visual detection limits. Green and white microplastics (~4%) are typically minor components in freshwater MP assemblages [79]. Additionally, white microplastic particles are commonly associated with packaging materials and polystyrene products, which are widely used for fertilizers, pesticides, and agricultural tools. These materials can enter fields as waste or debris and later be transported by runoff into the wetland [76,80].

4. Conclusions

This study provides a conceptual assessment of potential microplastic (MP) sources affecting the Tancat de la Pipa wetland, supported by short-term, spatially distributed sampling across six sites during three campaigns (February, April, and May 2024). The results indicate that the Catarroja (PC) and Poyo (BP) inlets are the primary pathways through which microplastics enter the wetland. The observed abundance, particle size distribution, colour, and polymer composition suggest that these inputs arise from a combination of urban and industrial activities, wastewater treatment plant (WWTP) effluents, stormwater runoff, navigation, fishing activities, and agricultural practices in the surrounding rice-growing areas, but the available data do not allow individual particles to be assigned unequivocally to specific sources. The predominance of blue and colourless polyethylene microplastics is consistent with contributions from both aquatic and agricultural sources. Blue particles are likely associated with fishing gear, ropes, and boating materials, whereas colourless particles may originate from weathered agricultural plastics, including mulch films, irrigation infrastructure, and packaging materials, although additional contributions from urban plastic waste cannot be excluded. The predominance of polyethylene and polyester, together with the occurrence of polypropylene, polystyrene, acrylic, polyvinyl chloride, polyurethane, polycarbonate, and tyre wear particles, further supports the influence of multiple diffuse and point sources within the catchment.

Comparison of inlet and outlet concentrations demonstrates that the wetland retains a proportion of incoming MPs, achieving an estimated numerical reduction of approximately 36%. Vegetation, other particle transformations and sedimentation processes appear to play an important role in particle retention; however, the continued detection of MPs at the outlet confirms that the wetland cannot completely prevent downstream transport, particularly for smaller particles.

Although based on a limited sampling period, this study demonstrates that conceptual source assessment combined with targeted field observations can effectively identify the dominant pathways of MP contamination and provide an initial evaluation of wetland retention performance. These findings contribute to a better understanding of the role of constructed wetlands as nature-based solutions for mitigating microplastic pollution and highlight the need for long-term monitoring, expanded polymer characterization, and sediment and biota analyses to improve source attribution and quantify the ecological benefits of wetland systems.