Quantifying microplastics in environmental waters: Mass concentrations are superior to abundance

Abstract Microplastics are contaminants of global concern that are primarily studied in marine and urban environments. Understanding of microplastics in drained agricultural watersheds is lacking. We aimed to evaluate microplastics in ditch and tile drainage water through periodic sampling. Water samples were filtered to capture particulates that were digested to remove organics, then stained and evaluated using fluorescence microscopy and image analysis. Further, we compared and contrasted microplastic abundance, the current reporting standard, with microplastic mass concentration, often unreported, to determine the most accurate assessment. Open‐ditch drainage had greater microplastic contamination than drainpipe outlets. Agricultural drainage contained 2–6 orders of magnitude less mass concentrations of microplastics than sampled urban surface waters and laundry graywater. However, when evaluated by abundance, the difference was not apparent. These findings improve our understanding of microplastics in agricultural watersheds and demonstrate the importance of evaluating microplastic contamination based on mass concentrations for accurate assessments. Core Ideas Mass concentration (ng/L) is a better predictor of microplastic contamination than abundance (counts/L). Agricultural drainage water had lower microplastic mass concentrations than surface water or laundry graywater. Open‐ditch drainage had greater microplastic contamination than drainpipe outlets. Mass of microplastic pieces from smallest to largest: drainage water < river water < lake water = laundry water.