Sixteen surface (5) and groundwater (11) samples were collected from the south-eastern part of Attica, Greece, and analysed for physico-chemical parameters and microplastics (MPs) by optical microscopy and Raman microspectroscopy (RS). A total of 3399 particles were optically identified in all sixteen samples, ranging from only 16 particles/L in a sample from a deeper borehole to 513 particles/L in a sample from a shallow water well. They were then visually classified into eight categories based on their color, texture, size, reflectivity, shape, and general morphological properties. Raman microspectroscopy was performed on the particles on the filters and revealed four different types of MPs, namely polyethylene (PE, 35%), polypropylene (PP, 30%), polystyrene (PS, 10%), and polyethylene terephthalate (PET, 25%). The samples from the shallow phreatic aquifer contained more MPs than the samples from the deeper borehole, which contained fewer MPs and categories. This is to be expected, since the phreatic aquifer (a) is generally more contaminated, as it is close to human activities that generate MPs and its infiltration depth is only a few metres, which means that many microplastics can infiltrate at such shallow depths, and (b) it is exposed to the atmosphere, so they can also be suspended in the air. On the other hand, it is interesting to note that MPs, especially PET and PE, were detected in the borehole sample, suggesting that MPs can migrate to greater depths through water infiltration. Chemical analyses of the groundwater samples revealed high values of E.C., Ca²⁺, Mg²⁺, Cl⁻, and Na⁺, which indicate that seawater intrusion is taking place in the coastal aquifer system of the Erasinos basin. The increased concentrations of NO₃⁻ and PO₄³⁻ also indicate the impact of agricultural activities.

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.