Validating monitoring methods for riverine macroplastic pollution

Riverine macroplastic pollution (>0.5 cm) is omnipresent and can negatively impact ecosystems and livelihoods. Monitoring data are crucial for understanding the nature and extent of pollution as well as aiding the design of effective intervention strategies. Recent years have marked the development of methods to collect surveillance data, primarily focusing on the monitoring of floating plastics and plastics deposited on riverbanks. Today, these methods need validation. Criteria that are essential in robust monitoring are the accuracy and precision of collected data, and the minimum observable particle size. Addressing these challenges, we have conducted field experiments aimed to review the most widely employed protocols: the RIMMEL protocol for floating macroplastics and the river-OSPAR protocol for macroplastics deposited on riverbanks. We find that the recovery of larger pieces ranges between 80-90% for both methods, with the accuracy decreasing significantly when considering smaller items sizes, item colour, number of observers, and factoring in external variables such as bridge height or riverbank surface type. The precision, however, varied greatly between the different experiments. These results indicate that the limits & usage of data from different protocols are highly context dependent. It further highlights the urgent need to include these uncertainties in their communication and utilization. Our result show the urgency of standardizing the operating protocol to optimize the accuracy and precision for measuring riverine macroplastics, and of the necessity to quantify uncertainty in studies estimating plastic fluxes using the two protocols.