Background: Microplastic contamination in freshwater systems has emerged as a pervasive environmental concern, with urban and agricultural land use widely implicated as key contributors to riverine plastic loading ^{[1, 2]}. Spatial gradients from forested headwaters through urban centers to estuarine outlets offer a useful natural framework for isolating land-use-associated contamination sources, yet integrated spatial-seasonal assessments along such gradients remain limited ^{[3, 4]}.

Objective: This study characterized the spatial distribution of microplastic concentration across five sites spanning a forested-to-estuarine river gradient, examined seasonal variation at the most contaminated site, and assessed the polymer-type composition of recovered particles.

Method: Water samples were collected from five sites along the river continuum using standardized surface trawling. This study uses a simulated dataset created for academic training purposes; all particle counts, seasonal trends, and polymer composition values were generated to reflect physically plausible patterns consistent with the published microplastic-monitoring literature and do not represent samples collected from an actual river system. Twelve replicate samples were modeled per site, and Fourier-transform infrared (FTIR) spectroscopy was simulated for polymer identification.

Key Results: Mean microplastic concentration increased from 0.8 particles L⁻¹ at the forested upstream site to a peak of 9.7 particles L⁻¹ at the urban outfall, before declining to 4.9 particles L⁻¹ at the estuary mouth. Seasonal monitoring at the urban outfall revealed peak concentrations during the monsoon months, coinciding with peak rainfall and stormwater runoff. Polyethylene was the dominant polymer type recovered (38% of particles), followed by polypropylene (24%).