A new methodological approach for the assessment of plastic pollution in estuarine waters: Guadalquivir River (SW Spain)

Rivers and estuaries are a key transport pathway for plastics from inland to the sea. These systems are subject to fluctuations depending on sources of plastics and local environmental factors, causing variation in the plastic concentration up to several orders of magnitude within limited time ranges. Due to this large variability, in freshwater systems, it is challenging to obtain representative monitoring data. This study presents a new methodological approach to determine the number of plastic particles needed to obtain representative data for plastic pollution characterization in estuarine waters. The method used monthly in situ observations during a period of two years in the Guadalquivir River estuary (SW Spain). The data allowed the characterization of plastic concentrations across all size categories (micro-, meso- and macroplastics). The items were categorized into different size classes (<5 mm, 5-25 mm, 2.5-5 cm, 5-10 cm and >10 cm) and a resampling simulation was applied to generate 95% confidence intervals for plastic concentration variability. Our results suggest that, when using a limited number of samples, there is an underestimation of all size classes, e.g., up to three quarters of the samples could underestimate the average number of particles. Differences up to 3 orders of magnitude can be established between the lower (<5 mm) and the higher (>10 cm) size class in terms of number of particles sampled. This approach will help to optimize time and define sample size for specific particle size classes to improve plastic monitoring in freshwater environments.