Floating GPS tracker measurements reveal intermittent and shoreline-driven transport of floating plastic in urban waterways

Riverine plastic pollution is commonly quantified using point-based flux measurements, yet the transport pathways and residence times of floating litter between monitoring locations remain poorly understood. This limits both the interpretation of flux data and the effective placement of interception and cleanup strategies. In this study, we investigate whether GPS tracers can be used to characterize the movement patterns of floating plastic in (semi)urban waterways.

Multiple field experiments (±60 sensors) were conducted across Dutch waterways (Alkmaar, Zaandam, Leeuwarden) in different seasons, during which floating GPS trackers were released and tracked over periods ranging from several days to eight weeks. The resulting trajectories were analysed in terms of cumulative distance travelled, lateral distribution within the channel, and temporal movement patterns.

Across all release locations (six in total), transport was found to be highly intermittent: long periods of little to no movement were frequently interrupted by short bursts of rapid displacement, resulting in stepwise cumulative distance curves. Spatially, trajectories showed a strong preference for near-shore transport rather than mid-channel flow. Floating proxies were repeatedly observed to accumulate along banks, where movement was often halted by obstructions such as vegetation, moored vessels, and infrastructure. These stagnation periods strongly influenced overall travel distance and residence time.

The results demonstrate that floating plastic transport in urban water systems cannot be approximated as continuous downstream movement. Instead, it is governed by intermittent mobilisation and frequent temporary retention along channel margins. GPS-based measurements therefore provide critical complementary information to flux monitoring, helping to explain variability in observed litter counts and supporting more effective design and placement of monitoring and interception systems. This approach offers a scalable pathway to bridge the gap between local flux measurements and system-wide transport dynamics of urban plastic pollution.