Point-Source Contribution of Food Packaging Manufacturing to Microplastic Concentration in Urban Stormwater: A Distance-Decay Study

Microplastics (MPs) are common pollutants in the environment, but there is limited data linking specific industrial sources to urban drainage systems. This study investigates the contribution of food-packaging manufacturing (FPM) facilities to MP presence in urban stormwater infrastructure. It is hypothesized that MP concentrations would display a significant inverse relationship with distance from the FPM facility, indicating these sites as primary sources of pollution rather than just contributors to the overall urban pollution levels of stormwater drains. To test this, stormwater samples were collected from two distant FPM sites (Site A and Site B) during hydrological events in late 2025 and early 2026. A ‘Close’ vs. ‘Far’ sampling design was utilized, with samples processed via wet-transfer sieving, density separation, and optical microscopy. Plastics were classified by morphology—fibers and fragments. The results confirmed a consistent distance-decay relationship at both locations. At Site A, MP concentration decreased from 705.07 particles/L to 264.80 particles/L as distance increased, yielding a network distance slope of -1.301 particles/Lm and a geodesic distance slope of -0.426 particles/Lm. Site B mirrored this trend, decreasing from 325.05 particles/L to 224.10 particles/L with a steeper network slope of -2.584 particles/Lm and a geodesic distance slope of -1.297 particles/Lm. These findings validate FPM facilities as significant point-source pollutants. This research provides the evidence needed to update stormwater management policies and emphasizes the urgent need for effective control of industrial runoff to reduce the spread of microplastics in the environment.