Extensive abundances and characteristics of microplastic pollution in the karst hyporheic zones of urban rivers

Understanding the seasonal variation of the microplastics occurrence and source in the water source: upstream of the Huangpu River in Shanghai as an example

The potential seasonal variations in the features and sources of microplastics (MPs) in reservoirs and the connected upstream rivers remain poorly understood. A field study of MPs in Huangpu River upper reaches indicated that surface water MP abundance was 4.75 ± 0.63 (Taipu River), 5.8 ± 0.9 (Lanlugang River), and 4.3 ± 0.9 items/L (Jinze Reservoir) during wet season, increasing to 12.9 ± 1.4, 18 ± 1.7, and 8.3 ± 1.7 items/L, respectively in drought season, whereas in sediment was 6704 ± 421, 3932 ± 200, and 4737 ± 408 items/kg during wet season, which decreased to 4045 ± 523, 4017 ± 430, and 2229 ± 434 items/kg during drought period. The MPs mainly comprised PE, PP fragments/fibers, and PET fibers. The significant seasonal variations in MP abundance were detected only within the surface water and sediments of rivers. Notably, significant seasonal variations in the features of MPs were observed in the surface water of Jinze Reservoir and the surface water or the sediments of rivers. During the wet season, multiple potential sources of MPs were identified in the reservoir, whereas the Taipu River served as the primary source during drought periods. Moderate or lower MP abundance was observed within the surface water of upper reaches of the Huangpu River, while higher levels were found in the sediments. The findings suggest Jinze Reservoir's MP control requires seasonal adjustments. While upper Huangpu River surface water shows lower risk with good safety, MP migration to sediments elevates risks, necessitating prioritized sediment management.

Current research status on the distribution and transport of micro(nano)plastics in hyporheic zones and groundwater

Micro(nano)plastics, as an emerging environmental pollutant, are gradually discovered in hyporheic zones and groundwater worldwide. Recent studies have focused on the origin and spatial/temporal distribution of micro(nano)plastics in regional groundwater, together with the influence of their properties and effects of environmental factors on their transport. However, the transport of micro(nano)plastics in the whole hyporheic zone-groundwater system and the behavior of co-existing substances still lack a complete theoretical interpretation. To provide systematic theoretical support for that, this review summarizes the current pollution status of micro(nano)plastics in the hyporheic zone-groundwater system, provides a comprehensive introduction of their sources and fate, and classifies the transport mechanisms into mechanical transport, physicochemical transport and biological processes assisted transport from the perspectives of mechanical stress, physicochemical reactions, and bioturbation, respectively. Ultimately, this review proposes to advance the understanding of the multi-dimensional hydrosphere transport of micro(nano)plastics centered on groundwater, the microorganisms-mediated synergistic transformation and co-transport involving the intertidal circulation. Overall, this review systematically dissects the presence and transport cycles of micro(nano)plastics within the hyporheic zone-groundwater system and proposes prospects for future studies based on the limitations of current studies.

Microplastics inhibit lead binding to sediment components: Influence of surface functional groups and charge environment

The coexistence of heavy metals and microplastics in sediments is well recognized, yet the interactions within ternary systems remain underexplored, and comprehensive studies addressing the diverse sequences of sediment-microplastic-heavy metal coexistence are lacking. In this study, we systematically investigated the interactions among lead (Pb), polystyrene (PS) microplastics, and sediments (using goethite (Goe) and goethite-humic acid composite (GH) as examples) under different coexistence orders. The presence of PS significantly inhibited Pb adsorption by both Goe and GH. For Goe, adsorption kinetics and hydrochemical condition effects showed that PS reduced the electrostatic repulsion between Goe and Pb, leading to a fourfold increase in the mass transfer rate of Pb to the Goe surface. However, Pb 4f deconvolution indicated competition between PS and Pb for hydroxyl groups on Goe, resulting in a 7.4% reduction in Pb adsorption. In the GH system, hydrophobic interactions and coordination complexes between PS and humic acid on GH inhibited the electrostatic adsorption and mass transfer processes between Pb and GH. Pb adsorption behavior and changes in Pb-O content under different coexistence orders further verified that competition between PS and Pb for carboxyl and hydroxyl groups on GH led to a 28.0% reduction in Pb adsorption. This study highlights the inhibitory effect of PS on Pb adsorption by Goe and GH, providing a theoretical basis for understanding the migration and transformation patterns of microplastics and heavy metals in sediments.

Risk assessment of potentially toxic elements, microplastics, and microorganisms in groundwater around municipal solid waste landfill

Risk assessment of potential toxic elements (PTEs), microplastics (MPs) and microorganisms in groundwater around landfills is critical. Waste from landfills seeps into groundwater contaminating water quality, threatening groundwater safety, and negatively affecting the ecosystem. This study explored spatial and temporal changes in PTEs, MPs, and microorganisms in the groundwater around a closed landfill. The results showed that Mn and Cr were the most predominant PTEs in the groundwater, average Mn and Cr concentrations in June being 1.16 and 4.51 times higher than in November, respectively. The Risk assessment of PTEs in groundwater Mn was heavily contaminated, Cr was moderately contaminated. The abundance of MPs the average value of MPs in June was 1.55 times higher than that in November; the MPs indicated that groundwater is more heavily contaminated, especially in the downstream areas. The Proteobacteria is the main phylum, and PLS-PM, PTEs were positively correlated with the phylum of microorganisms, negatively correlated with the genus of microorganisms and the abundance of MPs. This study emphasizes the importance of environmental management of landfills, provide new insights into the monitoring and identification of groundwater contamination as well as scientific guidance on appropriate remediation strategies for leachate-contaminated groundwater.

Collaborative removal of microplastics, bacteria, antibiotic resistance genes, and heavy metals in a full-scale wastewater treatment plant

Plastics are used in large quantities in food packaging and industrial products in China, which results in ecological risks of microplastics (MPs) to the environment. In this study, the MPs' removal efficiency of a full-scale wastewater treatment plant (WWTP) and the internal interaction of microorganisms, antibiotic resistance genes (ARGs), and heavy metals with MPs were investigated. The dominant MPs in urban sewage were polyurethane (PU), acrylate copolymer (ACR), fluororubber, and polyethylene. MPs in wastewater were removed by WWTP with a total efficiency of 76%. The removal efficiencies of ACR, ethylene-vinyl acetate copolymer, polybutadiene, poly(tetrafluoroethylene), polystyrene, and polypropylene reached 100%. The highest concentration of MPs PU in the influent got a removal efficiency of 93.41%. The interactions between MPs, heavy metals, microorganisms, and ARGs involved adsorption, hydrogen bonds, coprecipitation, and polar interaction. Heavy metals and MPs formed larger aggregated particles, which were removed in the primary process. Heavy metals accumulated in sludge pose ecological risks to soil during landfill or compost to fertilizer. The release of MPs from WWTPs leads to accumulation in organisms and soil. It may affect the entire food chain and promote the transmission of ARGs in the environment, posing potential threats to the entire ecosystem.

Global distribution, drivers, and potential hazards of microplastics in groundwater: A review

Since microplastics (MPs) were first detected in groundwater, an increasing number of studies have focused on groundwater pollution by MPs. However, knowledge of the global properties of groundwater MPs: distribution, concentration, composition, and morphology remains limited, while potential factors regulating their transport and distribution in groundwater, especially the hydrogeological background and climate warming conditions, have been omitted from most analyses. Furthermore, previous field investigations did not assess the risks posed by groundwater MPs to the environment and to human health, a necessary preliminary to remediation. In this work, to promote future MP pollution studies and remediation policies, we assimilated and synthesized the current knowledge on this topic. We reviewed current data on global groundwater pollution by MPs, analyzed the driving factors of their transport and distribution, and summarized the ecological and health hazards posed by MPs, before discussing current knowledge limits and suggesting perspectives for future work.

Characteristics, drivers and ecological risk assessment of microplastics in the surface water of urban rivers in Guangdong-Hong Kong-Macao Greater Bay Area cities - A case study of Dongguan city

In the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), microplastic pollution in urban rivers is a prominent problem due to the developed economy and high industrial intensity. Using the Xiaohai River, Hanxi River and Dongguan Canal in Dongguan City, an important node city in the GBA, as an example, microplastic characteristics, drivers and ecological risks in the surface water of three rivers were investigated. Results showed that the average abundance of rivers in the wet period (1646.22 ± 154.73 items/m3) was 4.7 times higher than that in the dry period (351.09 ± 34.2 items/m3). Microplastics were mainly in the form of fragments and fibers, with a size range of 30-500 μm, and appeared transparent with white color. The microplastic polymer types PE, PP, PET and PA accounted for more than 70%. There are large differences in the characteristics of microplastic pollution during different hydrological periods. Redundancy analysis showed that the distribution of plastics, chemical materials, packaging and printing industries along the rivers dominated the differences in microplastic abundance. The electronic information industry contributed most to the composition of microplastic polymer types. The polymer hazard index, pollution load index, and potential ecological risk index for rivers indicate a medium-high risk classification or higher. Therefore, the industrial layout along the urban rivers should be rationalized, the disposal of microplastics in wastewater should be increased, and the use of green plastic products should be promoted. This study provides support for the management of microplastic pollution in urban surface water in the GBA.

Travertine deposition rather than tourism activity is the primary contributor to the microplastic risks in alpine karst lakes

Microplastics (MPs) are emerging as anthropogenic vectors to form plastisphere, facilitating microbiome colonization and pathogenic dissemination, thus contributing to environmental and health crises across various ecosystems. However, a knowledge gap persists regarding MPs risks and their driving factors in certain unique and vulnerable ecosystems, such as Karst travertine lakes, some of which are renowned World Natural Heritage Sites under ever-increasing tourism pressure. We hypothesized that tourism activities serve as the most important factor of MPs pollution, whereas intrinsic features, including travertine deposition can exacerbate potential environmental risks. Thus, metagenomic approaches were employed to investigate the geographical distribution of the microbiome, antibiotic resistance genes (ARGs), virulence factor genes (VFGs), and their combined environmental risks in Jiuzhaigou and Huanglong, two famous tourism destinations in Southwest China. The plastisphere risks were higher in Huanglong, contradicting our hypothesis that Jiuzhaigou would face more crucial antibiotic risks due to its higher tourist activities. Specifically, the levels of Lipopolysaccharide Lewis and fosD increased by sevenfold and 20-fold, respectively, from upstream to downstream in Huanglong, whereas in Jiuzhaigou, no significant accrual was observed. Structural equation modeling results showed that travertine deposition was the primary contributor to MPs risks in alpine karstic lakes. Our findings suggest that tourism has low impact on MPs risks, possibly because of proper management, and that travertine deposition might act as an MPs hotspot, emphasizing the importance of considering the unique aspects of travertine lakes in mitigating MPs pollution and promoting the sustainable development of World Natural Heritage Sites.

A novel hierarchical approach to insight to spectral characteristics in surface water of karst wetlands and estimate its non-optically active parameters using field hyperspectral data

Wetlands cover only around 6 % of the Earth's land surface, and are recognized as one of the three major ecosystems, alongside forests and oceans. The ecological structure and function of karst wetlands are unique due to the influence of geologic structure. At present, the unclear spectral morphology of surface water in karst wetlands poses a significant challenge in remote sensing estimation of non-optically active water quality parameters (NAWQPs). This study proposed a novel multi-scale spectral morphology feature extraction (MSFE) method to insight to spectral characteristics in surface water of karst wetlands, and further screen the sensitive features of NAWQPs. Then we constructed three remote sensing inversion strategies for NAWQPs (TN, TP, NH3_N, DO), including direct estimation, indirect estimation, and auxiliary estimation. Finally, we constructed a novel pH-based hierarchical analysis framework (pH_HA) to thoroughly explore the influence of alkalinity-biased characteristics of karst water on the spectral domain of NAWQPs and its estimation accuracy using in-situ hyperspectral data, respectively. We found that the spectral characteristics of karst waters at the first reflectance peak (580 nm) differed significantly from other water body types. The MSFE successfully captured the sensitive spectral domains for NAWQPs, and focused on between 500 and 600 nm and 900-960 nm. The sensitive features captured by MSFE improved estimation accuracy of NAWQPs (R2 >0.9). Direct estimation presented more stable performance compared to the auxiliary estimation (average RMSE of 0.366 mg/L), and the auxiliary estimation model further improved the retrieval accuracy of TN compared to direct estimation model (R2 increasing from 0.43 to 0.56). The novel hierarchical framework clearly revealed the notable changes in the sensitive spectral domains of NAWQPs under different pH values, and enabled more precise determination of spectral subdomains of NAWQPs, and identified the optimal spectral features. The pH_HA framework effectively improved the estimation accuracy of NAWQPs (R2 increased from 0.514 to over 0.9), and the estimation accuracies (R2) of four NAWQPs were all more than 0.9 when the pH value was over 8.5. Our works provide an effective approach for monitoring water quality in karst wetlands.

Dynamics of microplastic abundance under tidal fluctuation in Musi estuary, Indonesia

Tidal dynamics contribute to fluctuations in microplastic abundance (MPs). This is the first study to characterize MPs under the influence of tidal fluctuations in the Musi River Estuary. MPs samples were collected during flood and ebb tides at 10 research stations representing the inner, middle and outer parts of the Musi River Estuary. MPs were extracted to identify the shape, color and size. MP abundances were 467.67 ± 127.84 particles/m3 during flood tide and 723.67 ± 112.05 particles/m3 during ebb tide. The concentration of MPs in the outer zone of the estuary (ocean) was detected to be higher than in the inner zone of the estuary (river). The MPs found were dominated by black color, film shape and size 101-250 μm. A greater abundance of MPs at ebb tide than at flood tide implies that the Musi Estuary's largest source of emissions is discharge from the river.

Microplastic contamination in urban aquatic environments: Occurrence characteristics in urban streams and stormwater runoff from urban surfaces

This study investigates microplastics in urban environments, focusing on their abundance, types, and relationships with hydrological parameters. Microplastic analyses encompassed two steps: (1) examining urban streams including discharges from a wastewater treatment plant (WWTP) during non-rainy seasons, and (2) analyzing stormwater runoff from urban surfaces for microplastic content during rainy seasons. In urban streams, WWTP discharge exhibited higher microplastic concentrations compared to other streams, indicating WWTP discharge is a dominant source of microplastic pollution. The most prevalent microplastics detected were polypropylene, polyethylene, and their copolymer, although a variety of other types were also found. Concentrations of microplastics were notably influenced by specific urban land uses, as evidenced by a strong correlation (0.95) between microplastic concentrations and areas characterized by industrial and transportation activities. During rainy seasons, microplastics followed the pattern of stormwater runoff, but the highest concentrations, significantly exceeding those in urban streams, were observed before the peak runoff. These maximum concentrations and their timing of occurrence were linked to antecedent dry days, rain intensity, and runoff rate, showing significant statistical correlations. Regardless of their sizes, a diverse range of microplastic types was identified in these conditions, with no consistent pattern across different rain events. This highlights the complex nature of urban microplastic pollution. This study reveals that aquatic ecosystems are significantly affected by two primary factors: (1) the consistent contribution of microplastics from WWTP discharges, and (2) the short-term, but severe, impacts of microplastic pollution associated with stormwater runoff. Furthermore, it suggests the development of alternative strategies to mitigate microplastic pollution in aquatic ecosystems, informed by the findings on the characteristics of microplastics in urban environments. This research underscores the urgent need for integrated urban environmental management strategies, paving the way for future studies to further explore and address the multifaceted challenges posed by microplastic pollution in aquatic ecosystems.

Exploring correlations between microplastics, microorganisms, and water quality in an urban drinking water source

The microplastic pollution in freshwater system is gradually becoming more severe, which has led to increasing attention on the distribution and potential harmful effects of microplastics. Moreover, microplastics may have an impact on river ecology and pose risks to ecosystems. Therefore, it is important to reveal this process. This study aimed to explore correlations between microplastics and free-living microorganisms in an urban drinking water source of Xiangjiang River by using multivariate statistical analysis. The results indicated that the abundance of microplastics (size 50 μm to 5 mm) in surface water and sediments ranged from 0.72 to 18.6 (mean ± SD: 7.32 ± 2.36) items L-1 and 26.3-302 (150 ± 75.6) items kg-1 dry weight (dw), respectively, suggesting potential microplastic pollution despite the protected status as a drinking water source. Higher microplastic abundances were observed in urban areas and the downstream of wastewater plants, with mostly granular shape, transparent and black color as well as 50-100 μm in size. The multivariate statistical analysis presented that the abundance of microplastics is not significantly correlated with water indicators, due to the complexity of the abundance data. The water indicators showed an obvious correlation with microplastics in colors of transparent and black, and smaller sizes of 50-100 μm. This is also true for microplastics and microorganisms in water and sediment. Proteobacteria was the main prokaryote in water and sediments, being positively correlated with 50-100 μm microplastics; while Chloroplastida was the dominated eukaryotes, presenting a weak correlation with smaller-size microplastics. Overall, when considering the properties of microplastics such as shape, color and size, the potential correlations with water indicators and microorganisms were more evident than abundance. This study provides new insights into the multivariate statistical analysis, explaining the potential correlations among microplastic properties, microorganisms and environmental factors in a river system.

Microplastic pollution in urban rivers within China's Danxia landforms: Spatial distribution characteristics, migration, and risk assessment

The potential deleterious effects of microplastics on environmental integrity and human health have elicited global attention. Particularly vulnerable to microplastics are Danxia landforms, characterized by their unique topographical features and ecologically fragile milieu. Notwithstanding, empirical studies assessing the prevalence of microplastics in these unique landforms remain strikingly limited. The present investigation comprehensively examined the abundance of microplastics in surface water, sediment, and groundwater across six cities and six counties within the Danxia landforms. Comparative analysis revealed a moderate level of microplastic contamination in the urban rivers of the Danxia region relative to other freshwater rivers. Anthropogenic activities, notably urban wastewater treatment and tourism, emerged as principal contributors to microplastic pollution. Sedimentary microplastics exhibited an accumulative trend from upstream to downstream locations. The risk assessment revealed a high potential ecological risk in counties and a moderate risk in cities. Cluster analysis suggested that groundwater microplastics were a confluence of hydraulic interactions between surface and subsurface waters within the Danxia region. This investigation elucidates the microplastic contamination profile, origins, migratory patterns, and associated risks in Danxia's urban rivers, thereby furnishing scientific underpinning for health and ecological preservation strategies within urbanized Danxia landscapes.

Seasonal variations of microplastics in surface water and sediment in an inland river drinking water source in southern China

The aim of this study was to examine microplastic (size distribution of 0.05-5 mm) occurrence and distribution in drinking water source of XJ River during both flooding and dry periods. Surface water and sediment samples were collected from the CS City section of the river in August and December 2020. During the flooding period, microplastic abundances were observed at 0.72-18.6 (7.32 ± 2.36) items L-1 in surface water and 26.3-302 (150 ± 75.6) items kg-1 dry weight (dw) in sediment. In the dry period, abundances were slightly higher at 2.88-17.7 (11.0 ± 3.08) items L-1 and 27.0-651 (249 ± 182) items kg-1 dw, respectively. Microplastics were found in higher concentrations in urban areas and downstream of wastewater treatment plants, suggesting anthropogenic sources. The diversity in shapes, colors, and types of microplastics in surface waters and sediments indicates specialized enrichment processes and persistent sources of microplastic pollution. Approximately 60 % of the microplastic particles identified fall within the 50-100 μm range. Furthermore, a significant correlation was observed between these smaller-sized particles and the overall prevalence of microplastics. Fourier-transform infrared spectroscopy and scanning electron microscopy indicated that the microplastics had been subjected to weathering in the environment, contributing to the production of oxygen-containing functional groups and surface cleavage features. The utilization of energy dispersive spectroscopy revealed the presence of microplastics associated with various heavy metals, highlighting the intricate nature of microplastic pollution. Moreover, the high abundance of microplastics may pose a potential ecological risk to the aquatic environment of the XJ River. The results of this study demonstrate concerning levels of microplastics in the XJ River, despite its status as a high-quality water source.

The occurrence and distributions characteristics of microplastics in soils of different land use patterns in Karst Plateau, Southwest China

Microplastics (MPs) in soils have attracted attention worldwide. However, there are few studies on the abundance and morphological characteristics of MPs under different land use patterns in karst areas. In this work, the distribution characteristics of MPs in soils from five different land use patterns, including industrial mining warehouse (IW), cropland (CL), forest land (FL), grassland (GL), and garden land (GP) in karst mountainous areas of southwest China were investigated. The results suggested that soils in Karst Plateau of Guizhou province under different land use patterns have all been polluted by MPs. The average microplastic abundances of IW, CL, FL, GL, and GP were 3114, 2948, 2770, 2718, and 4200 item kg-1. In the GP and GL soils, foam was the dominant form of MPs (47.53 % and 45.92 %), with pellet MPs accounting for the smallest proportion, while in IW, CL, and FL soils showed the opposite result. The MPs in all soil samples were dominated by small particles (0-0.5 mm), and the main components were PE, PP, PVC, and PET. Meanwhile, the characteristics of MPs in CL soils varied depending on the planted crops. The average abundances of soil microplastics in farmlands planted with corn, pepper, cabbage, watermelon, and other crops were 2504, 2792, 2987, 2370, and 3655 item kg -1. We suggested that land use pattern and crop type may influence karst soil microplastic contamination. The results of this study provide a scientific basis for understanding and controlling the distribution, degradation, and migration of MP pollution in karst regions.

Glyphosate spraying exacerbates nitrogen and phosphorus loss in karst slope farmland

Glyphosate herbicide is an indispensable material in agricultural production. In order to explore the potential environmental effects of glyphosate application in karst slope farmland, this paper used a variable slope steel tank to simulate the surface microtopography and underground pore structure characteristics of karst slope farmland, and combined with artificial rainfall experiments to explore the migration path of glyphosate in karst slope farmland and the impact of spraying glyphosate on soil nitrogen and phosphorus loss. The results showed that under the condition of heavy rain, glyphosate in karst slope farmland was mainly transported and diffused by surface runoff, supplemented by underground runoff; secondly, in different hydrological paths, glyphosate directly affected the content of nitrogen and phosphorus in runoff, and all showed extremely significant positive correlation (p < 0.001). In addition, rainfall conditions such as rainfall intensity, rainfall duration, and runoff affected the content of nitrogen and phosphorus in runoff to varying degrees. In conclusion, the application of glyphosate significantly increased the content of nitrogen and phosphorus in different runoff and accelerated the loss of nitrogen and phosphorus from soil, which not only led to soil degradation, but also threatened the safety of aquatic ecosystem. Therefore, in the prevention and control of agricultural non-point source pollution, the threat of glyphosate to the surrounding aquatic ecosystem cannot be ignored, especially in karst areas with frequent rainstorms and serious water erosion, long-term monitoring and risk assessment of glyphosate are needed.

Investigations on microplastic infiltration within natural riverbed sediments

Several studies focused on the role of rivers as vectors of microplastics (MPs) towards the sea. It is well known that during their path through the fluvial environment, MPs interact with riverbed sediments; however, the main factors impacting the mobility of MPs within the upper part of the hyporheic zone are not clear yet. The present work investigates the role of different sediment size layers in affecting the mobility of the most common MP (Polyethylene terephthalate - PET - spheres, PET 3D-ellipsoids, polystyrene - PS - fragments and polyamide - PA - fibers) within sediment porous media under different hydraulic loads (HL) and time scales (t) conditions. Results indicated the relationship between the characteristic MP diameter and that of the grains as the main parameter for the MP infiltration into the sediment layer. The maximum infiltration depth was found to not depend on HL and t. However, HL was able to influence the percentage of MPs penetrating the superficial layer and their distribution within the first 10-15 cm of the sediment layer. None of the MPs were found at depths >20-25 cm, where only PET spheres were detected. Starting from the suffusion theory, a model able to predict the MP maximum infiltration depth in the range of parameter values was provided. The outcome indicates the importance of considering geometrical and hydrodynamic aspects of the riverbed sediment layer to better characterize the spatial and temporal scales of MP transport in freshwater environments.