Experimental method for quantifying macroplastic fragmentation in rivers

Direct field measurements of macroplastic fragmentation during its transport in rivers are unavailable, and there is no method to perform such measurements. Recent theoretical works have hypothesised that river channels may be hotspots of macroplastic fragmentation. Here, we propose a methodology for quantifying riverine macroplastic fragmentation by conducting repeated measurements of tagged macroplastic items' mass before and after their transport in the river. A 52-65-day experimental test of the proposed methodology allowed us to provide the first quantification of fragmentation of 1-liter PET bottles during their transport in a mountain river channel. We calculated the mass loss of tracked bottles (n=43), ranging from 0.025 grams/year (0.07%/year) to 1.0 gram/year (3%/year), with a median of 0.26±0.04 grams/year (0.78%/year), and the rate of bottle surface degradation, ranging from -0.29 μm/year to -11.88 μm/year (median = 3.77±0.43 μm/year). These results suggest that the total fragmentation time for a PET bottle under conditions represented by our experiment (low to medium flow) ranges from 33.63 years to 332.81 years (median = 128.92±31.07 years). Our methodology can be flexibly adapted to quantify macroplastic fragmentation in various types of rivers and other environments where macroplastic is transported