Retention efficiency for microplastic in a landscape estimated from empirically validated dynamic model predictions
Magnus Dahler Norling, Rachel Hurley, Theresa Schell, Martyn N. Futter, Andreu Rico, Marco Vighi, Alberto Blanco, José L. J. Ledesma, Luca Nizzetto
Abstract
Soils are recipients of microplastic that can be subsequently transferred to the sea. Land sources dominate inputs to the ocean, but knowledge gaps about microplastic retention by land hinder assessments of input rates. Here we present the first empirical evaluation of an upgraded microplastic fate model operating at landscape level. The mechanistic dynamic model accounts for hydrology, soil and sediment erosion, particle characteristics and behavior. We predict microplastic concentrations in water and sediments of the Henares river (Spain) within the measurement uncertainty boundaries (error factors below 2 and 10, respectively). Microplastic export from land and discharge by river fluctuates in a non-linear manner with precipitation and runoff variability. This indicates the need of accurate dynamic descriptions of soil and stream hydrology even when modeling microplastic fate and transport in generic scenarios, including at low temporal resolution. A time-averaged landscape retention efficiency was calculated showing 20-50% of the microplastics added to the catchment over a multiannual period were retained. While the analysis reveals persistent uncertainties and knowledge gaps on microplastic sources to the catchment, these results contribute to the quantitative understanding of the role of terrestrial environment in accumulating microplastics, delaying their transport to the sea. Soils are main recipients of microplastic pollution that is subsequently transferred to aquatic ecosystems. Land-based sources dominate microplastics inputs to the ocean but knowledge gaps about their accumulation in soil, run-off and in-stream transport has hindered assessments of regional/global distributions and marine ecosystem exposure. These gaps must be filled to prioritize environmental protection actions. Verified mechanistic models capable of describing this process are the main avenue to advance knowledge in the field. This paper fills this gap by presenting the first empirical validation of a mechanistic microplastic fate model operating at the river catchment scale.