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Surface Water Monitoring with Sedimentation Boxes: Assessing the Sampling Performance and Its Effect on Microplastic Concentration
Summary
This study tested whether passive "sedimentation box" samplers — devices that slow water flow to settle out suspended particles — can effectively capture microplastics from surface waters over time, rather than relying on infrequent spot sampling with nets or pumps. Results showed the boxes reliably retained a range of particle sizes and types, and their performance aligned with the physical behavior of other suspended particles in water. Passive samplers like these could help create more consistent, time-integrated microplastic monitoring that better captures the true variability of plastic pollution in rivers and lakes.
Currently, there are still no harmonized and thus reproducible methods for microplastics (MP) sampling. Infrequent spot sampling with, e.g., nets, pumps, or containers, does not reflect the large spatial and temporal variety of MP abundance, and there is little experience with time-integrated, passive sampling methods. However, passive samplers have been applied thoroughly to recover suspended particulate matter (SPM) from water bodies. The physical and chemical characteristics of MP are in range with those of other materials belonging to SPM, and we state that MP are an integral component of SPM. In general, passive samplers like the sedimentation box decrease the flow velocity, enhancing the sedimentation of SPM within the device. The retention rates of particles in sedimentation boxes depend on various factors such as the flow velocity, the SPM size and density, but precise information remains scarce. Therefore, we performed laboratory tests to assess the retention rates of the polymers polystyrene and polyethylene and analyzed the dependency of sedimentation on the flow velocity and particle sizes. The quantification of MP in samples collected by sedimentation boxes underestimates the concentration of smaller-sized particles due to their lower retention rate, and MP concentrations should be reported accordingly. Subsequently, we carried out a series of field experiments with sedimentation boxes and showed that MP can be retained from different water bodies with diverse characteristics. Due to their robust sampling mechanism, sedimentation boxes are promising devices for time-integrated, long-term sampling of MP.