Microplastics in Urban Stormwater: Sampling and Separation Method

Microplastics (MPs) are considered emerging contaminants of concern in surface waters. To date, several monitoring studies have reported the presence of MPs in surface water systems, including beaches, waters, and sediments. The harmful impact of MPs on aquatic and marine life has been well documented. Several studies suggested that inland litter transported via stormwater runoff is one of the primary sources of plastics in surface water. Recent studies to quantify the number of MPs in urban stormwater have applied traditional stormwater sampling methods and NOAA methods for analysis. In this paper, we present a critical review of applying existing sampling and extraction methods for MPs in stormwater and present our experience with adapting and revising methods intended to accurately quantify MPs and optimize the separation process. Three designs of metal strainers were built and tested, and the best design was placed in storm drains to collect stormwater samples. Existing methods for soil and sediment analysis suggest wet oxidation prior to density separation; this results in using large volumes of hydrogen peroxide. In addition, stormwater samples could be rich in grass and other organics that are not readily decomposable. A step was added at the beginning to separate floatables and reduce the sample amount for oxidation. This paper tested the efficacy of using a semi-automated mixing to fasten the extraction process and reduce the possibility of losing bio-fouled plastic particles. The paper will also discuss the challenges of laboratory analysis and separation.