Microplastic contamination in irrigation canals of a major agricultural region: a fluorescence-based monitoring study in Reynosa, Mexico

This study evaluates microplastic contamination within critical agricultural infrastructure by examining the Anzaldúas and Rodhe irrigation canals in Reynosa, Mexico. To ensure geographical representativeness while managing analytical throughput, we implemented an in situ composite sampling design, pooling field triplicates across four strategic locations. We processed the resulting matrices through vacuum filtration onto microstructured silicon nitride nanomembranes, followed by hydrophobic staining with Nile Red fluorophore and automated fluorescence imaging under a TRITC optical channel. Image processing utilized a customized digital watershed segmentation macro to isolate, count, and morphologically classify individual synthetic particles. Our findings reveal a pronounced spatial heterogeneity in pollutant loading, establishing a clear decreasing contamination gradient: Rodhe Internal (RI) > Rodhe External (RE) > Anzaldúas Internal (AI) > Anzaldúas External (AE). The Rodhe Canal emerged as the primary vector for microplastic transport; the internal site (RI) exhibited the highest contamination profile (55.73 particles/mL; 2.99 µg), followed by the external site RE (19.64 particles/mL; 1.52 µg). Conversely, the Anzaldúas Canal displayed substantially lower incidence, yielding 2.61 particles/mL (0.48 µg) at the internal site (AI) and a baseline minimum of 1.31 particles/mL (0.09 µg) at the external site (AE). Morphological analysis identified fragments and fibers as the predominant particle types. Crucially, all sites exceeded laboratory blanks by 13.05 to 557.3 times, demonstrating persistent anthropogenic alteration. This work establishes the first regional quantitative baseline within this vital agricultural corridor, validating an efficient, high-throughput diagnostic tool to guide targeted water management policies in northeastern Mexico.