Microplastic (MP) is of concern as an emerging pollutant in our plastic age because its ubiquitous presence in the environment poses a potential risk to human and animal health. A better understanding of MP pathways in the environment would benefit the development of mitigation measures for plastic pollution. For this reason, this study investigated potential point sources of land-based MPs into the aquatic environment, focusing on wastewater treatment plant (WWTP) effluent and stormwater from the drainage sewer systems. With the River Weser as the study focus, MPs in the effluent of the WWTP Bremen-Seehausen was monitored for one year to (1) determine the extent to which treated wastewater (TWW) contributes MPs to the recipient and (2) capture the annual variations of MPs. Also, water samples were taken from a runoff storage basin to (3) investigate MP occurrence in stormwater and have it compared to that of TWW. In this study, MPs were isolated from environmental samples through enzymatic-oxidative purification and subsequent density separation with ZnCl2. For characterization, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and focal plane array (FPA)-based micro-FTIR imaging were applied, allowing the detection down to a size of 11.6 μm. Overall, MPs were present in all analyzed samples. MP concentration in TWW varied throughout the studied year without a clear pattern. Acrylates/polyurethanes/varnish/lacquer, polypropylene, polyethylene, ethylene vinyl acetate and rubber were the most abundant polymers detected. MPs were dominant in subclasses with a size range of 10-100 μm. There was no significant difference found between MP occurrence in TWW and stormwater. In conclusion, this thesis provided a comprehensive data set of MP occurrence in TWW with annual variations observed. Also, the contribution of stormwater to MP pollution in receiving waters was elucidated, in turn emphasizing the need of studying MPs in stormwater.