Abstract
Plastics originating from land are mainly transported to the oceans by rivers. The total plastic transport from land to seas remains uncertain because of difficulties in measuring and the lack of standard observation techniques. A large focus in observations is on plastics floating on the water surface. However, an increasing number of observations suggest that large quantities of plastics are transported in suspension, below the water surface. Available underwater plastic monitoring methods use nets or fish traps that need to be deployed below the surface and are labor-intensive. In this research, we explore the use of echo sounding as an innovative low-cost method to quantify and identify suspended macroplastics. Experiments under controlled and natural conditions using a low-cost off-the-shelf echo sounding device show that plastic items can be detected and identified up to 7 m below the river surface. Eight different debris items (metal can, cup, bottles, food wrappers, food container) were characterized based on their reflection signature. Reflectance from plastic items diverged significantly from organic material and non-plastic anthropogenic debris. During a multi-day trial field expedition in the Guadalete river, Spain, we found that between 0.8 and 6.3 m depth considerable quantities of plastics are transported. As most plastic monitoring and removal strategies focus on the upper layer below the surface (up to approximately 1.5 m depth), a substantial share of the total plastic transport may be neglected. With this paper we 1) demonstrate that echo sounding is a promising tool for underwater plastic monitoring, and 2) emphasize the importance of an improved understanding of the existing plastic loads below the surface.
Highlights
Plastic pollution in aquatic ecosystems is of increased global concern due to its negative impact on ecosystem health and human livelihood (Cózar et al, 2014; Lau et al, 2020; van Emmerik and Schwarz, 2020)
The main goal of this research is to explore the potential of echo sounding for riverine macroplastic (>0.5 cm) monitoring below the water surface using an off-the-self low-cost sensor
We systematically investigated the use of sonar for plastic monitoring through 1) indoor controlled tests, 2) semi-controlled outdoor tests, and 3) uncontrolled application under natural conditions
Summary
Plastic pollution in aquatic ecosystems is of increased global concern due to its negative impact on ecosystem health and human livelihood (Cózar et al, 2014; Lau et al, 2020; van Emmerik and Schwarz, 2020). Much of the plastic daily discarded on land is leaked into rivers, and transported into the world’s oceans (van Emmerik and Schwarz, 2020; Meijer et al, 2021). Estimates of plastic transport from rivers into the oceans are associated with great uncertainties due to methodological difficulties to accurately quantify land-based plastic fluxes into the aquatic. Plastics are abundant in all components of river systems: floating at the surface, accumulated on riverbanks and floodplains, deposited in the sediment, and suspended in the water column (Schwarz et al, 2019; van Emmerik et al, 2020). As rivers are assumed to be one of the main plastic input sources into the oceans, there is a need to monitor the suspended plastics in rivers
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