Spatial and vertical distribution of microplastic within the soil of a sustainable urban drainage system
Résumé
As urban roads represent a significant aspect of human activity, evaluating their impact as a source of macro-litter and microplastics is necessary to better understand plastic transfer in continental environments. In particular, the microplastic input into soils of sustainable urban drainage systems via urban runoff and their infiltration through soils remain to be assessed. One way to estimate the microplastic input related to road activity and determine the role played by urban runoff in their transport, is to focus on an infiltration ditch on the side of a road.
In this study, the soil of a seepage ditch on the side of a high traffic suburban road in the Parisian region was studied for microplastic content, along with its incoming urban runoff water and outlet water. Soil samples were collected by manually coring at a depth of 35 cm. To assess a potential spatial variation along the seepage ditch, a total of 10 cores were collected. Each core was slighted by depth into 4 samples at [0-5[, [5–15[, [15–25[, [25–35[ cm. After collection, microplastics were isolated from the samples using a chemical treatment of organic particles (H2O2) followed by a density-based removal of minerals (NaI). Finally, samples were deposited on alumine filters and their microplastic content was determined using an automated Fourier Transform Infrared Micro-Spectroscopy mapping analysis (for particles > 25 µm). Preliminary results showed an overall median concentration of 13.4 MP/g of soil, with no significant difference between samples of different depth. Preliminary results on water samples remain to be assessed, though seem to point toward significantly lower microplastic concentrations at the outlet of the ditch than in the urban runoff (inlet), suggesting that the soil retains efficiently microplastics larger than 25 µm.