Microplastics (MPs), defined as plastic particles measuring less than 5 mm, are considered an emerging pollutant. Their presence in the water cycle and their interaction with ecological processes pose a significant environmental threat. As groundwater (GW) represents the primary source of drinking water, monitoring MPs in GW and investigating their potential sources and pathways is of urgent importance. This article offers a comprehensive overview of the primary contamination pathways of MPs from surface water, seawater, and soil into the GW. Moreover, it presents an examination of the occurrence of MPs in GW and identifies the challenges associated with their monitoring in GW. This study also discusses the difficulties associated with comparing research results related to MPs in GW, as well as indicating the need for implementing standardised techniques for their sampling and detection. On the basis of our experience and the literature review, we highlight the importance of understanding the specific hydrogeological and hydrogeographic conditions, collecting representative samples, using sampling devices with comparable specifications and comparable laboratory techniques for MP identification, and preventing contamination at all stages of the monitoring process. This review offers valuable insights and practical guidelines on how to improve the reliability and comparability of results between studies monitoring MPs in GW.

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.