Plastic pellets in sandy beaches: mechanisms and deposition indicators

ABSTRACT In the coastal zone, the plastic found is the result of the mismanagement of this kind of waste, particularly in developing countries. The material comes directly from the dispose of offshore by vessels (fishing debris) and by marine currents and winds, as well as urban drainage systems, rivers and/or estuaries. Specifically, in the plastic pellets case, which are spheres with 2-5 mm that constitute the raw material for the manufacture of plastic products, the Santos Port and the plastic factories in Cubatão city, are considered the main local sources of this input at the (environment of the) São Paulo coast. Consequently, the beaches most affected for this pollutant are those near Santos estuary, like Enseada do Guarujá beach. However, some questions are still open, such as: what are the mechanisms which control the pellets deposition, and which locations are most favorable for deposition on the beach? Thus, to develop this work it took four steps: 1) Plastic pellets geodetic survey based on GNSS positioning; 2) Beach geomorphometric parameters (altitude, aspect, and slope) derived by Digital Elevation Model (DEM); 3) Strandline altitude estimated by WaveWatch III (WW3) wave model analysis and tide height; and, 4) Plastic pellets deposition Suitability Index (PSI). The joint analysis of the altimetric, geomorphometric and meteoceanographic aspects showed that the beach areas with altitudes higher than those calculated for the strandline (> 2.06 m), slope ~ 3º and facing the same direction of the higher energy waves (175 - 195°) were more susceptible to pellet deposition. This indicates that the accumulation of this pollutant on the beach is controlled not only by its physical characteristics but mainly by storm surge events. Besides, the work also guides future research surveys optimizing time and avoiding subsamples, as well as beach cleaning. Another advantage brought by the methodology is that: when working with a geodetic reference (fixed, univocal and relatively stable on time), the information collected can be compared with other beaches anywhere on the planet — thus contributing to a standardization of the survey methodology