Contamination of the environment with plastic debris : “Development, improvement, and evaluation of monitoring methods”

Improper disposal of plastics, coupled with their durability and low weight, has led to the widespread environmental pollution of plastic debris. For larger plastic debris negative ecological, cultural, economic, safety, and health impacts are reported and well known. For microplastics (particle size ≤5 mm), harmful effects are still a matter of debate. Nevertheless, microplastics are the distinct subject of national and international marine monitoring directives (i.e. MSFD, NOAA), due to their bioavailability to a wide range of organisms, their omnipresence in the marine environment, and the lack of removal techniques once introduced. Microplastic contamination levels have been intensively examined within marine habitats. And even though the relationship of human activities and plastic debris inputs are known, significant knowledge gaps exist on the sources, transport, and accumulation areas in terrestrial environments. Thus, the first objective of this thesis was the identification of potential sources, pathways, and accumulation areas of plastic debris in terrestrial environments. Three case studies on overlooked, yet potentially plastic debris containing sources and accumulation areas, were carried out. As plastics frequently enter biowastes through misthrows, we exemplarily investigated organic fertilizer from biowaste fermentation and composting as input source of microplastic debris to farmlands. Our results indicate that, depending on receiving wastes, pretreatment of the substrate, and the technical state of the plant, organic fertilizers can contain high concentrations of microplastics. When applied to farmlands, a potential input of 35 billion to 2.2 trillion microplastic particles per year was calculated for German arable land. As around 50% of land use in Germany is agricultural, we further investigated plastic debris contamination of a farmland neither subjected to known plastic-containing fertilizer or to plastic applications. We detected 206 large plastic pieces, and 158,100 to 292,400 microplastic pieces per hectare. Additionally, we were the first to investigate the hyporheic zone of streambed sediments, a transition zone between fresh- and groundwater. Our exemplary study at the Rote Main river indicated that especially small microplastics (<50 μm) are infiltrated into sediments of the hyporheic zone of streambeds. Even though, results from this study are based on one sample, it points towards another temporal sink and relevant transportation pathway for microplastics. The lack of sufficient sample replication is a common issue in microplastic studies, mainly due to the high costs of sampling, sample processing, and analytics. Consequently, the second objective was to improve existing sampling and sample processing methods for microplastics. Concerning sample processing, environmental samples often contain a high number of natural particles that impair spectroscopic identification of microplastics if not removed. Thus, I contributed to the development of a gentle sample purification protocol that is adaptable to a broad range of environmental samples. With the application of a series of specific enzymes, we achieved high removal efficiencies of organic matter from surface water samples (>95%) and high recovery rates of microplastics (>80%). Yet, sample replication is still a compromise between representativeness and feasibility within a project. To assess sufficient sample replication for beaches, we studied the spatial distribution of microplastics in beach sediments of the Po River Delta, in northern Italy. Our analysis of microplastics >1 mm for three different accumulation areas suggests that for the high tide line, the recommendation by the “Technical Subgroup on Marine Litter” of five replicates is sufficient. If accumulation areas farther from the waterline are sampled, a minimum of 10 replicates should be taken. The highly variable polymer type distribution among the accumulation areas further indicated that for a comprehensive assessment of microplastic contamination, different accumulation areas need to be sampled. However, concerning water surface samples from coasts and the open ocean, a representative sampling will be limited simply because of their mere dimensions. Hence, the third objective was the development of alternative monitoring methods that could provide additional information on sources, sinks, and transport pathways of buoyant plastic debris. A three-dimensional hydrodynamical model, coupled with a Lagrange particle tracking module, was utilized to forecast the transport of microplastics emitted by the Po River branches and subsequent off-washing onto adjacent beaches. A correlation with in-situ measured microplastic abundances on the beaches was not present. In another approach, we assessed if water constituents depictable from satellite images (e.g., chlorophyll-a, suspended particulate matter, and colored dissolved organic matter) could be used as proxy to indirectly map microplastic distribution. Under the assumption that microplastic transport is driven by similar processes, such as wind and currents, we tested if a correlation between microplastics and those water constituents exists. The results of three field data acquisitions on three different river systems showed no clear relationship, with only one data set showing a spatial correlation between microplastics and the proxy water constituents. Nevertheless, model simulations and remote sensing techniques are able to provide information on larger spatial and temporal scales, which is why the development of this methods should be followed in future.