Each sample was washed with tap water using a stacked box mesh consisting of 5 mm and then 300 μm size opening respectively. Samples were washed until the wastewater ran clear indicating fine silt and mud was removed. Samples were placed in pre-weighted stainless-steel trays and oven-dried at 80℃ until completely dry. Sample dry weight was then recorded at room temperature. An initial density separation step was done to recover microplastics in the sediment and reduce sample volume by adding super-saturated NaCl (1.2 g mL -1 ) solution prepared by adding 358.5 g of NaCl (FUJIFILM Wako Pure Chemical Co., Ltd.) to 1 L of ultrapure water . Dried sediment was added to a 3000 mL glass beaker and the NaCl solution was added until 8-10 cm above the sediment. This mixture was agitated by stirring with a stainless-steel spatula for 2-3 minutes and left to rest for 1 hour allowing the sediment to settle and, microplastic and other buoyant materials to float to the top. All floating matter was poured into a custom-made filtration apparatus consisting of a PVC tube, with a 315 μm nylon mesh attached to one end via a hose clip. Ultrapure water was used to wash the floating material to remove excess NaCl after each recovery. The agitation and filtration steps were repeated three times for each sample using a new NaCl solution each time to ensure maximum recovery of plastic fragments. Filtered material containing microplastic was allowed to dry in a covered glass petri dish at 80°C for 24-48 hours to reduce moisture in the sample. After these additional steps were completed, digestion and final extraction were carried out using the same methods for surface water in preparation for microscopic analysis.