Finding Riverine Plastics in Floating Plant Patches using Worldview-3 Satellite Imagery

Water hyacinths appear to play an important role in gathering and transporting macroplastic litter in riverine ecosystems. These fast-growing and free-floating invasive freshwater plants tend to form large patches at the water surface, which makes it possible to detect and map them in freely available imagery collected by the European Space Agency (ESA) satellites. In polluted rivers, hyacinth may thus serve as a viable proxy for macroplastics. However, at the ~10m spatial resolution offered by the Sentinel-1 and Sentinel-2 satellites, it’s not possible to discriminate smaller items of plastic caught up within large plant patches.

We present a case study from the Saigon River around Ho Chi Minh City, Vietnam. Here, we were able to successfully discriminate plastic debris within hyacinth patches using MAXAR Worldview-3 multispectral optical data (1.24m) and panchromatic imagery (0.31m). For the optical data, we selected the ACOLITE atmospheric correction and applied a novel detection index that leveraged the panchromatic band and the red band (band 5) to highlight differences between vegetation and debris. This approach allowed for the detection of riverine plastics within hyacinth patches floating downstream from Ho Chi Minh City towards the coast. Initial results from the Han River and coastal waters of Da Nang in Vietnam suggest that our plastic litter discrimination method is transferable to other aquatic environments. This research is preparatory for further remote sensing monitoring of 'plastic plants' in riverine ecosystems, and will be supporting clean-up operations being trialled in 2022.