Microplastic retention within infiltration basins used for managed aquifer recharge: Effectiveness and key controlling factors

Increasing water demands and climate change pressures have led to the development of managed aquifer recharge systems like infiltration basins. The environmental performance (i.e., protection of groundwater bodies) of aquifer recharge using infiltration basins has been largely evaluated for several contaminants but it remains poorly investigated for emerging pollutants like microplastics (MPs). This study aimed to quantify the effectiveness of the infiltration media of infiltration basins in retaining microplastics and to identify which factors (sediment characteristics or basin management practices) could influence the MP retention. With this aim, undisturbed vertical layers of sediments were collected for each of the 11 studied infiltration basins used for groundwater recharge with surface water: a phototrophic biofilm surface layer (0-1 cm), an intermediate fine sediment layer (4-6 cm) and a bottom sand layer (8-10 cm). For each basin, the vertical distribution of MPs (abundance, size and polymer type) was determined. In addition, sediments were analyzed for total organic carbon and granulometry. Data regarding the management of each infiltration basin (water filling, water levels in the basins, age of the basin) were provided by the stakeholders. Results showed that MPs concentration decreased by 70% from the uppermost surface layer to the deepest sediment layer, indicating the effectiveness of the infiltration media to retain MPs. Microplastics concentration was mainly controlled by sediment depth but also by hydrological conditions with a positive influence of the number of inundation days on MP abundance. Polymer type composition among basins differed significantly in the uppermost surface layer indicating the occurrence of local sources of microplastic contamination. This study highlights the critical importance of the water-sediment interface in protecting groundwater from particulate pollution like MPs.