Can groundwater geochemistry and contaminants of emerging concern help elucidating microplastic sources and possible transport pathways?

The evidence of microplastics (MPs) in groundwater is still restricted to a few sites worldwide, mostly concentrated in China, South Korea, India, a few European countries, and the USA. Even when field data are presented, they are rarely combined with auxiliary data such as hydrogeological characterisation of the area, description of the well profiles and construction  materials, screen location, water geochemistry, isotopic data, and other contaminants of emerging concern. Because MP pollution is virtually ubiquitous, analysing MPs data by itself can reveal scant information on its possible sources and pathways in the subsurface. This study focuses on the Massaciuccoli Lake Basin (Central Italy), a Ramsar-designated marshy coastal wetland characterised by long-term anthropogenic influence. Over the past century, extensive land reclamation, agricultural intensification, and canalization have altered the natural hydrological regime. Documented land subsidence of approximately 2–3 m over the last 70 years has further modified groundwater gradients and surface–subsurface interactions. The initial step includes building a robust hydrogeological conceptual model of the area, combining landuse, geological background information, stratigraphic profiles, groundwater analysis of major ions and trace elements, water stable isotopes, and contaminants of emerging concern. Secondly, we combine the conceptual model with MP data from five sampling points over two sampling campaigns to help identify MPs sources and pathways in the subsurface environment.  Geologically, the area is characterised as a heterogeneous sequence of marine, transitional, and continental sediments. Stratigraphic logs available for four of the five piezometers indicate an alternation of sand, silt, clay, and peat layers. This stratification promotes both vertical segregation, where low-permeability clay and peat layers restrict downward transport, and lateral compartmentalization, where permeable sandy units act as preferential flow paths. Hydrogeologically, the system is characterized as a shallow unconfined alluvial aquifer with a strong meteoric contribution and direct hydraulic connection to surface waters. Electrical conductivity, major ion chemistry, and the contaminants of emerging concern occurrence patterns indicate that groundwater composition is distinct for each piezometer, suggesting limited lateral mixing and emphasizing localized flow systems. Despite this spatial variability, hydrochemical parameters show only minor temporal variation between sampling campaigns, indicating hydrological stability at the seasonal scale. The combination of stratigraphic heterogeneity and stable flow conditions suggests that MPs detected at individual piezometers are more likely to reflect local sources and short-range transport, rather than basin-scale homogenization. Consequently, MPs are expected to exhibit site-specific distributions, with transport dominated by near-surface flow paths and attenuation occurring through physical retention at lithological boundaries.