Baseline assessment of microplastic contamination in agricultural soils from the coastal stretches of Karnataka and Goa, Southwestern India

Microplastic (MP) contamination in terrestrial ecosystems-particularly in agricultural soils-remains an underexplored environmental concern in India, an agriculture-driven economy. This study assessed MP contamination in paddy fields across 15 sites in Udupi (Karnataka) and Goa, with soil samples collected from the surface and at three subsurface depths (10, 20, and 30 cm). MPs were extracted using density separation, identified via stereomicroscopy and ATR-FTIR, and further characterized using SEM-EDS to examine surface morphology and elemental composition. Microplastics were detected in all samples, with higher concentrations observed in Goa across most depths. In Goa, surface soils contained an average of 100.93 ± 64.19 pieces/kg, while subsurface layers at 10, 20, and 30 cm recorded 90.70 ± 40.37, 119.76 ± 169.44, and 48.34 ± 29.75 pieces/kg, respectively. In comparison, Udupi samples exhibited slightly lower concentrations, with 95.68 ± 30.69 pieces/kg at the surface, and 55.00 ± 29.33, 46.53 ± 9.04, and 16.10 ± 7.63 pieces/kg at corresponding depths. Shape analysis revealed regional differences: in Udupi, fibres dominated (86.8%), followed by films (10.3%) and fragments (3.0%), whereas in Goa, fibres were less dominant (50.8%), with a higher proportion of films (36.7%) and fragments (12.0%). Black-coloured MPs and polypropylene polymers were most prevalent overall. SEM analysis indicated signs of environmental weathering, and EDS confirmed the presence of toxic heavy metals (Fe, Cd, and As) on MP surfaces. The highest Polymer Hazard Index (PHI) value of 1859.9 was recorded in Goa, indicating Hazard Level V due to the presence of high-risk polymers such as polyamide and polystyrene. The Coefficient of Microplastic Impact (CMPI) analysis showed that fibre-shaped MPs in Udupi had values > 0.8, indicating strong ecological risk, while Goa exhibited a more balanced morphology impact (CMPI: 0.11-0.50). These findings underscore the widespread presence of MPs in agricultural soils and highlight the urgent need for routine monitoring and the development of mitigation strategies to protect soil health and food security.