Riverine microplastics derived from mulch film in Hainan Island: Occurrence, source and fate

Mulch film (MF) residues is an important source of microplastics (MPs) in farmland, but its transportation risk to the wider environment was still unknown. Some researches have pursued the sources of MPs found in exorheic rivers. Even so, a systematic study depicting the occurrence, source and fate of microplastics derived from mulch films (MPMF), the crucial component of MPs in farmlands, in exorheic rivers still lacking. Here, the combination of UV-Vis Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) was used to identify the full-size MPMF (1-5000 μm) in field sediment samples collected by single-diagonal systematic sampling. This study verified that MPMF, a polyethylene-matrix composite doped with additives, contributed a considerable part of MPs detected in upstream farmland soil and riverine sediments, and even had an abundance of 38 ± 11 items/kg to 82 ± 15 items/kg, accounting for 9.0%-13.7% of the total MPs in estuary sediments. Notably, upstream farmland was identified to the main source of the riverine MPMF by partial least square path modeling (PLS-PM), contributing to 94.7% of MPMF in riverside sediments and 85.0% of MPMF in estuary sediments. Our study first demonstrates that MPMF constitutes a non-negligible component of MPs in estuarine sediments and underlines the urgency of strengthening the management of MPs pollution in drainage areas with a high agricultural intensity.