Plastic sachet pollution on beaches of East Java, Indonesia: Spatial distribution, composition, and weathering characteristics

Flexible multi-layer sachet packaging is increasingly recognized as an important component of coastal plastic debris in many developing countries, particularly in Southeast Asia. This study presents a comprehensive assessment of sachet contamination across 14 beaches along the northern and southern coasts of East Java, Indonesia. A total of 1274 sachet items were quantified by numerical density (items m^-2) and mass density (g m^-2), classified by product category, measured for physical dimensions, and evaluated according to condition and weathering stage. Numerical and mass-based abundance were strongly correlated (r = 0.96, p < 0.001). Northern beaches exhibited significantly higher contamination levels (up to 1.15 items m^-2 and 1.60 g m^-2) than southern beaches (maximum 0.16 items m^-2 and 0.37 g m^-2) (Welch's t-test, p < 0.001), and showed more heterogeneous product composition, whereas southern beaches were largely dominated by food-related sachets. In contrast, weathering characteristics showed no significant regional variation, and the predominance of intact-fresh sachets indicated that contamination was largely driven by recent and recurrent inputs. Significant dimensional differences among product categories revealed packaging-specific variability that may influence environmental behavior. Complementary FTIR and SEM analyses demonstrated that weathered sachets underwent chemical and surface morphological alteration following environmental exposure. Overall, the findings highlight the importance of controlling land-based plastic leakage and provide new insights into the degradation behavior of sachet-derived plastic pollution in tropical coastal environments.