Spatiotemporal variation in microplastics derived from polymer-coated fertilizer in an agricultural small river in Ishikawa Prefecture, Japan

Marine plastic pollution has highlighted the need to address the disposal of plastic materials used in agricultural fields and prevent their runoff. To assess the status of microplastics derived from polymer-coated fertilizers (microcapsules), we investigated their seasonal and daily variations in a small agricultural river in Ishikawa Prefecture, Japan, throughout the irrigation period of April to October 2021 and 2022. We also investigated the relationship between microcapsule concentration and water quality. The mean microcapsule concentration over the study period ranged from 0.0 to 783.2 mg/m3 (median 18.8 mg/m3) and was positively correlated with total litter weight, but it was not correlated with common water quality parameters such as total nitrogen or suspended solids. Concentrations of microcapsules in river water showed distinct seasonal variations, being particularly high in late April and late May (median 55.5 mg/m3 in 2021, 62.6 mg/m3 in 2022) and almost undetectable thereafter. The timing of the increase in concentration coincided with the timing of the outflow from paddy fields, suggesting that microcapsules that flowed out of the paddy fields would reach the sea relatively quickly. The results of a tracer experiment supported this conclusion. Intensive observations revealed that microcapsule concentrations varied widely over time, with differences reaching a maximum of 110-fold (range 7.3-783.2 mg/m3) over a 3-day period. Daytime concentrations were higher than those at night, reflecting the fact that microcapsules are discharged from paddies by daytime operations such as puddling and surface drainage. Microcapsule concentrations in the river were not correlated with river discharge, making estimating their loading a future research challenge.