Microplastics pollution in groundwater remains significantly underreported within scientific literature. This paper presents a comprehensive protocol outlining the methodology for the sampling of groundwater from boreholes, as well as the steps of microplastics separation and analysis. It provides an extensive description of a filtration sampling system designed specifically for this purpose, along with the detailed sampling procedure. In addition, it presents the laboratory analysis of microplastic particles, including their characterization based on size, shape, color, transparency, and chemical structure using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and micro-FTIR spectroscopy. Factors that can influence results are discussed, and special attention is paid to preventing contamination of samples. The methodology described also considers the requirements of the Annex of Commission Delegated Decision (EU) 2024/1441 of 11 March 2024, supplementing Directive (EU) 2020/2184 of the European Parliament and of the Council. This comprehensive written protocol, accompanied by video guidance, is intended to support the development of a synchronized methodology for monitoring microplastics in groundwater or drinking water. This resource will be of interest to researchers in the field of microplastics worldwide.

Understanding the transport of microplastics (MPs) in the unsaturated zone (UZ) is essential for assessing their environmental impact, particularly regarding groundwater contamination. MPs are increasingly detected in groundwater systems; however, their transport mechanisms through the UZ remain poorly understood, and field-scale experimental evidence is scarce.This study presents a field-based methodological approach using lysimeter experiments to investigate MP migration under realistic environmental conditions. Four lysimeters, each designed as a vertical column with a diameter of 0.6 metres, were installed outdoors to closely simulate natural UZ environments. The columns were packed with 16 cm of sand and gravel of known granulometric fractions, incorporating variable grain sizes to represent diverse porous media and hydraulic properties. Comprehensive granulometric analyses and infiltration tests were conducted to characterise the physical properties of the columns. Commercial Polypropylene (PP) MPs of different shapes (fibres, fragments, and spheres) were applied as tracers, alongside deuterium oxide (D₂O) as a conservative tracer for hydraulic characterisation.This experimental approach provides valuable data on MP transport under controlled yet realistic conditions, reducing uncertainties associated with laboratory-only studies. The experimental design allows for the quantification of retention and breakthrough behaviour of MPs under variable hydraulic regimes. Furthermore, the integration of multiple tracer shape-types facilitates the differentiation between physical transport processes, providing a robust framework for future modelling efforts. A comprehensive description of the experimental setup, together with the initial results derived from the lysimeter studies, will be presented. This research is part of the project “Improved methods for determination of transport processes and origin of microplastics in groundwater resources—(GWMicroPlast)” (J1-50030), funded by the Slovenian Research and Innovation Agency (ARIS).

The occurrence of microplastics in groundwater remains poorly understood. This study investigates the occurrence and behaviour of microplastics in urban alluvial aquifers and karst aquifer systems and highlights both the complexity of microplastic transport across contrasting hydrogeological settings and the influence of environmental and anthropogenic factors. It also provides the first baseline assessment of microplastic contamination in Slovenian groundwater, which plays a critical role in drinking water supply and overall water security. Groundwater was sampled at 19 sites (8 alluvial and 11 karst) using an in situ filtration system that enabled the collection of three parallel samples of one cubic metre per site. Microplastics were identified using stereomicroscopy, and polymer composition was confirmed using ATR-FTIR spectroscopy.,Microplastics were detected at nearly all sampling sites, with concentrations ranging from 0 to 13.33 particles/m³ (mean: 4.65 particles/m³). While no statistically significant differences in abundance were observed between aquifer types, slightly higher average concentrations were found in alluvial systems. Particle morphology varied by hydrogeological setting, with fragments predominant in alluvial aquifers, and fibres proportionally more abundant in karst springs. In the latter, larger particles were more frequently detected, reflecting differences in filtration capacity and groundwater flow pathways. In both aquifer types, polyethylene and polyethylene terephthalate were the dominant polymers. A comparison of microplastic concentrations and water physicochemical parameters indicated a link between microplastic concentration and anthropogenic influences.,These findings underscore the significance of hydrogeological context for groundwater microplastic assessments and provide essential baseline data for future monitoring and management strategies.

The occurrence of microplastics in groundwater remains insufficiently documented, particularly across contrasting hydrogeological settings. This study provides the first baseline assessment of microplastic contamination in Slovenian groundwater by comparing its occurrence in urban alluvial and karst aquifers, which represent important drinking-water resources in Slovenia. Groundwater was sampled at 19 sites (8 alluvial, 11 karst) using a large-volume in situ filtration system, with three 1 m³ replicates collected at each site. Polymer composition was confirmed using attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). Microplastics were detected at nearly all sampling sites, with concentrations ranging from 0 to 13.33 particles/m³ (mean: 4.65 particles/m³). Although no statistically significant differences in microplastic occurrence were observed between aquifer types at the available sample size, average concentrations were slightly higher in alluvial aquifers, whereas karst systems showed greater site-level variability. Differences in particle characteristics were observed between aquifer types, with fragments predominating in alluvial aquifers, and fibres and larger particles proportionally more abundant in karst springs. These patterns are consistent with differences in groundwater flow and filtration processes between karst and alluvial aquifers. Polyethylene and polyethylene terephthalate were the most frequently identified polymers across all samples. Correlations between microplastic concentrations and selected water-quality parameters, particularly temperature and nitrate, were identified. Overall, the results emphasise the relevance of hydrogeological context for interpreting microplastic occurrence in groundwater and provide baseline information to support future monitoring efforts.