Distribution and migration characteristics of microplastics in farmland soils, surface water and sediments in Caohai Lake, southwestern plateau of China

Microplastics (MPs) have become a new environmental pollution problem of global concern, and there are relatively few studies on the migration pattern of MPs in farmland soils, water and sediments in lake ecosystems. To address the abovementioned issues, the present study investigated the relative abundances and morphological distributions of MPs in water, sediments and farmland soils in the Caohai Lake region. The order of the relative MP abundance distribution in the Caohai Lake region was farmland soils (3000–8640 n/kg), freshwater estuarine sediments (3300–4260 n/kg), lake sediments (1320–4260 n/kg), freshwater estuarine water (3.4–12 n/L) and lake water (4.6–10.1 n/L). It was indicated that the estuary in study area, as a key point in the whole transportation process of MPs, and was considered as a "potential sink" of MPs. The MPs that were detected in this study were divided into four types, inclusive of fibers, pellets, fragments and films. Transparent and black MPs accounted for large proportions of the MPs in the five environments, and their possible sources were mulching films, greenhouse films, shopping bags and woven bags. The proportions of MPs with particle sizes of 0–0.5 mm for the studied locations was farmland soils (85.74% ± 6.10%) > lake sediments (84.70% ± 7.04%) > freshwater estuarine sediments (79.12% ± 2.99%) > freshwater estuarine water (73.52% ± 14.41%) > lake water (73.46% ± 11.00%), and it was exhibited that most of MPs cracking occurs in the farmland. In the present study, the relative abundance of MPs was a weak correlation with the physical and chemical indices in farmland soils and water bodies in the Caohai Lake region. The relative abundance of MPs was positively correlated with total organic carbon, total nitrogen, total phosphorus and alkali hydrolysed nitrogen in the sediments. Our investigation suggested that the MPs in farmland soils were mainly derived from plastic film and domestic sewage and that estuarine water is the first temporary carrier of MPs after they enter the water environment. These findings provide a theoretical reference for the treatment of MPs in freshwater environments.