Exploring plastic detectability on riverbanks using remote sensing.

Plastic pollution is an emerging environmental challenge, threatening terrestrial, freshwater and marine ecosystems. Rivers are major pathways and storage systems, and large-scale plastic monitoring is necessary to effectively reduce plastic pollution. This study investigates the detectability of plastics on riverbanks across spatial scales, ranging from in-situ hand-held spectrometers to large-scale satellites. We designed an experiment using two artificial plastic targets placed on the riverbanks of the Nederrijn, the Netherlands. The first target was a white polyester sheet of four different sizes (0.5x30 m, 1x30 m, 2x30 m, 3x30 m), and the second target consisted of transparent PET bottles with two different sizes and surface concentrations (3x30 m with 4 items/m, 15x30 m with 8 items/m). Data were collected with several sensors, covering a range of spatial, spectral, and temporal resolutions: the ASD Handheld 2 Spectroradiometer, the MAIA S2 multispectral camera, Sentinel-2, PlanetScope SuperDove, and EnMAP. We analyzed the reflectance spectra, developed a new index (SI-13), and applied a Naïve Bayes detection model to test the detectability of the plastic targets. Sentinel-2 images were successfully used to detect the three largest polyester targets. The PET targets were however not detected. In addition, we found high correlations (-0.93) between polyester target size and several spectral indices. Our results suggest that plastic detection satellite remote sensing is limited by both spatial resolution and plastic concentration. This paper serves as a proof of concept to show that plastic detection in riverbank environments using satellite and camera imagery is feasible and should be investigated further.