Spatiotemporal assessment of microplastic incidence in the Atoyac basin — a key watershed in Mexico

This spatiotemporal study quantified and analyzed the types, shapes, and sizes of microplastics (MPs) in freshwater and sediments from the Atoyac sub-basin in Puebla, Mexico, a region impacted by highly populated areas, agriculture, and extensive industrial activity. Microplastics were quantified via fluorescence microscopy and analyzed using complementary techniques: attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and pyrolysis-gas chromatography/mass spectrometry (pyro-GC/MS). Sediment concentrations ranged from 1.0 to 23.8 mg MPs kg, while item counts varied from 7.15 to 135 particles g. In water, MP concentrations reached 238 ± 42 particles L. Key trends revealed higher MP concentrations in urban and industrial areas, directly linking human activity to pollution. We observed seasonal variation in MP distribution: for both water and sediments, sites 1, 2, and 3 showed higher concentrations in the dry season, while sites 5 and 6 exhibited the reverse trend in the wet season, challenging typical MP transport assumptions due to drought-influenced flow rates. In sediments, we found complementary results for polymer composition, with rayon prevalent via ATR-FTIR, and PE, PP, and PS dominating pyro-GC/MS analyses. This study highlights the importance of combining complementary techniques to quantify and identify MPs, revealing the complex environmental factors-including spatial variations, sources, sinks, flow, seasonality, water depth, and biological factors (microphyte consortia)-governing their fate and distribution. Ultimately, this work advances our understanding of MP sources, fate, and transport, which is critical for developing effective mitigation strategies in similar environments.