Municipal landfills receive a high amount of plastic waste and due to the occurring physical and biochemical degradation processes, could be significant sources of microplastics (MP). Evaluating the threat to groundwater through the transfer of MP via landfill leachates require more research. The former “Prairie de Mauves” landfill, operated from 1963 to 1987 by the municipality of Nantes (France), and located above the alluvial groundwater of the Loire River, represents a good candidate for such investigations. Leachates and groundwater were sampled along a transect line from upstream to downstream of the landfill, in March and June 2022. MP (>25 µm) were quantified and characterized using µFTIR imaging in transmission mode. MP were observed in every sample with concentrations ranging from 0.71 to 106.7 MP/L. Concentrations in the leachates and the alluvial groundwater illustrate a migration of MP. Twelve polymers were identified and polyethylene (PE) and polypropylene (PP) were predominant. After a conventional rainfall event (14.3 mm), higher concentrations, diversity, and size ranges of MP were observed. Water infiltration through the heterogeneous geological substratum therefore enhanced the migration of larger MP towards the alluvial groundwater of the Loire River.