Effects of long-term wastewater irrigation on microplastics pollution in agricultural soil

Microplastic (MP) pollution in agroecosystems is a growing concern with unknown consequences for sustainable agricultural activities. Few studies have revealed MPs in soil as a result of wastewater irrigation, despite the increasing application of wastewater irrigation and inadequacy of conventional wastewater treatment plants in removing microplastics (MPs). In this study, the effect of treated wastewater (TWW) irrigation on MPs accumulation in agricultural soils of Konya City (in Türkiye) and the potential risks on agricultural ecosystem were investigated. For this purpose, 202 soil samples taken from 90 TWW irrigated lands and 11 non-agricultural control lands, at depths of 0-10 cm and 10-20 cm, were analyzed for color, shape, and polymer type. The risk level of MPs pollution was determined by the pollution factor (CF), pollution load index (PLI), and polymer risk index (H). The relationship between some physico-chemical properties of the soil and MPs pollution level was also analyzed. The findings of this study revealed a significant difference (P < 0.0001) in the MP count in the TWW irrigated soils, and control soils. While the average numbers of MPs for control soils were 169 ± 46.8 MPs/kg (100-220 MPs/kg) and 140 ± 44.7 MPs/kg (80-240 MPs/kg) for the 0-10 cm and 10-20 cm soil depths, respectively; 329 ± 139.5 MPs/kg (100-840 MPs/kg) and 295 ± 115.4 MPs/kg (80-660 MPs/kg) were identified for TWW irrigated soil samples taken from the same soil depths. Fiber, film, and fragment type MPs were found to be dominant polymer types in TWW irrigated soil, with 56%, 23%, and 16%, respectively. Transparent colored MPs were predominant. MP decreased from 0-10 cm to 10-20 cm depths. Most of the samples were significantly contaminated with MPs (3 ≤ CF < 6), categorized in hazard category class I (PLI < 10). The findings of this study indicate that TWW irrigation increases the accumulation of MPs in agricultural soils, which poses a higher risk to more fertile soils with higher organic matter, total nitrogen, and available phosphorus content. Hazard index assessments reveal that the soils of Konya, often referred to as the "granary of Türkiye," are at risk of MPs contamination. The findings showed that MPs, a neglected type of pollution for soil, will become an even more important problem with increasing wastewater irrigation.