Microplastics in freshwater ecosystemssources, pathways, and risks

Abstract

Plastics and microplastics (MPs, plastics below 5 mm) are now ubiquitously found in the natural environment. They are a versatile group of materials that are made of different polymers and can have different shapes (e.g., bead, fragment, fiber). MPs are either directly emitted into the environment in this size or form due to the breakdown of larger plastic particles. However, the contribution of different MP sources and their pathways into the environment and between compartments are not well understood. Wastewater and sewage sludge have been suggested as major MP entry pathways into aquatic and terrestrial environments. Wastewater and stormwater converge a wide range of potential MP sources. These MPs can be released directly into the environment if wastewater treatment infrastructures are not in place or due to combined sewer overflows. In wastewater treatment plants, most MPs are captured and concentrated in the sewage sludge, with a smaller proportion not retained and released with treated effluents to aquatic receptors. Moreover, if sludge is used as fertilizer on agricultural lands, these wastewater-derived MPs can re-enter terrestrial ecosystems. Subsequently, they may be driven to aquatic environments through surface water runoff. For instance, MP fibers released from textiles and MP particles from the abrasion of vehicle tires during driving are expected to be emitted in large quantities into the environment along these pathways. Once in the environment, MPs may negatively affect organisms through various physical and chemical mechanisms such as entanglement, external damage, internal abrasive damage or blockage of the digestive tract after ingestion, food dilution, or leaching of harmful additives. Furthermore, MPs can interact with other environmental pollutants through sorption processes, thus acting as transport vectors of these pollutants for aquatic organisms. This thesis aims: (1) To assess the current state of knowledge and identify data gaps regarding the sources, environmental pathways, loads and fate of plastic and MPs in aquatic and terrestrial ecosystems. (2) To determine MP pollution in a Mediterranean river catchment and elucidate the role of wastewater as a MP entry pathway. (3) To uncover the fate of MPs in agricultural soils following sewage sludge application and the importance of agricultural surface water runoff as a transport mechanism for MPs into aquatic ecosystems under semi-arid conditions. (4) To quantify the impacts of frequent MP types on freshwater invertebrates by considering environmentally relevant shapes, sizes, and concentrations and to perform a risk assessment for these organisms. (5) To assess the relevance of MPs as vectors of other chemical contaminants in the aquatic environment and their contribution to ecological risks. Following the general introduction in Chapter 1, Chapter 2 summarizes the current knowledge regarding the sources of plastics and MPs, their environmental pathways and load rates, and their occurrence and fate in different environmental compartments. It shows that knowledge of the sources of large-sized plastics, MPs, and nanoplastics to aquatic and terrestrial ecosystems is limited. Data on MPs occurrence in freshwaters is restricted to certain geographical regions, while data on soil ecosystems are generally limited. Furthermore, data limitations exist for pathways and fluxes into the environment and between different environmental compartments. This literature review also shows that the presence of MPs in wastewater is relatively well studied. However, the contribution of this pathway for MP contamination in aquatic ecosystems, compared to other pathways such as surface water runoff or the occurrence of large-sized plastics and their disintegration into MP and nanoplastic, are urgent research needs. Finally, this chapter highlights research areas that need to be advanced to improve the understanding of potential ecological risks of plastic pollution and provides recommendations to improve management and reduce plastic and MP inputs into the environment. In the next chapter (Chapter 3), MP pollution in a Mediterranean river catchment (Henares River catchment, central Spain) is assessed, and the contribution of wastewater to the MP catchment discharge is identified. This chapter shows that although wastewater treatment plants retain on average 93% of the MPs in the influent, the treated effluents and small amounts of untreated wastewater inlets still contribute up to 50% of the total MP catchment discharge. Thus, the wastewater system represents a major environmental pathway for MPs into Mediterranean rivers. Moreover, it shows that MP concentrations in river water and sediment strongly depend on land use and significantly increase with increasing anthropogenic pressure. Finally, MP pollution patterns in the catchment are found to be influenced by season due to high flow periods in Spring, with higher water concentrations but lower sediment concentrations. In Chapter 4, MP fate in agricultural soils receiving different sewage sludge treatments and the role of agricultural soil as contributors to MP pollution in aquatic ecosystems through surface water runoff is investigated. This chapter shows that the application of sewage sludge significantly increases MP concentrations in soils. These soil concentrations remain relatively constant over one year and little transport into deeper soil layers is observed. Additionally, the study indicates that MP concentrations in historically treated soils remain high five years after sludge application. Finally, this chapter highlights that surface water runoff has a negligible influence on the export of MPs from agricultural soils. Therefore, it concludes that agricultural lands can be considered long-term MP accumulators under the studied soil and weather conditions. In Chapter 5, the ingestion and effects of polyester fibers and car tire particles are assessed for freshwater invertebrates with different feeding strategies and habitat preferences. It shows that ingestion capability depends on particle shape and size, while ingestion quantity also depends on the exposure pathway and the feeding strategy of the test organism. No adverse effects on survival or reproductive output for benthic macroinvertebrates are observed in this study. However, it concludes that chronic exposure of Daphnia magna (pelagic species) to both MP types could affect reproduction and survival at concentrations, that are for the most part not environmentally realistic yet. Finally, this chapter shows that MPs affect organisms through different effect mechanisms. In Chapter 6, the relevance of MPs to act as vectors of organic pollutants to freshwater fish is investigated. This is done by assessing the bioconcentration and chemical stress caused by two hydrophobic organic chemicals in Danio rerio with and without MPs. This chapter shows that the presence of MPs does not enhance but rather decreases the bioconcentration of these chemicals. Moreover, the decrease in bioconcentration is greater for the more hydrophobic chemical. Chemical stress, determined through different biomarkers, is not enhanced in the presence of MPs. This confirms that MP contamination in freshwater ecosystems is not expected to aggravate the risks associated with the bioconcentration of organic contaminants in aquatic organisms. Other exposure pathways (i.e., uptake via respiration, skin permeability) may instead be of higher importance. Finally, the overall conclusions of this thesis are presented in Chapter 7, together with some recommendations for future research and MP pollution management.