Microplastic Occurrence in Water Matrices: Metadata Analysis and Occurrence Trends

Microplastics are polymer fragments < 5mm either manufactured in small sizes or resulting from material degradation and have been detected in water matrices from around the world. There is concern that ingested microplastics may pose a hazard to humans or wildlife. To inform mitigation strategies, research has been conducted to understand the prevalence of microplastics in aquatic environments. Standard methods for extraction of microplastics proposed by NOAA in 2015 were improved upon by the recent publication of ASTM methods, while standards for polymer identification are still in development. This has resulted in a wide variety of methodologies being reported with varying degrees of reliability and reproducibility. The aim of this study was to analyze current literature to understand (1) microplastic occurrence in various water matrices, (2) the effectiveness of different water treatment processes, and (3) the potential impact of study design on results. Metadata and results were collected from a total of 80 published studies on microplastic observations in drinking water, stormwater, surface water, and wastewater and incorporated into a database with concentration data for >2300 samples. Criteria for inclusion in the database required the use of chemical analysis (e.g., FTIR, PyrGC-MS, Raman) to confirm microplastic polymer type. Currently, the database includes 50 studies of microplastics in wastewater that were found to meet the criteria for inclusion in the database, compared to 19 for drinking water, and 11 for stormwater. A variety of study quality assurance and control practices were reported: 75% of studies reported using blanks (63.5% of which had some contamination) and 20.2% used matrix spikes to confirm particle recovery. With respect to extraction techniques, most studies applied wet peroxide oxidation (52.2%) to remove surface contaminants and reduce interferences from easily oxidizable particles, 39.2% applied some form of density separation, while few applied the ASTM recommended proteinase and lipase enzymatic degradation for complex matrices. Researchers applied analytical techniques to confirm a set number or percent of visually identified particles, particles stained with lipophilic dyes, or all particles. The potential impact of these extraction and analysis choices on results will be explored. Secondary wastewater treatment plants achieved an average of 88.8% removal of microplastics as opposed to tertiary wastewater treatment plants which produced an average removal of 92.2%. This 3.4% increase in the extent of treatment indicates that current treatment processes after secondary treatment may have a negligible effect on the removal of microplastics.