Microplastics (MPs) are increasingly recognised as contaminants of groundwater, although their occurrence and behaviour remain less well understood than in surface water and soils. This systematic review evaluates 82 field studies from 28 countries published between 2019 and 2026, covering eight hydrogeological settings. MPs were detected in all investigated settings, with reported mean concentrations ranging from 0.001 to 2,103 particles/L. This wide range demonstrates both the widespread occurrence of MPs in groundwater and the considerable variability between individual studies. 

The occurrence and transport of MPs are controlled by interactions between contamination sources, particle characteristics and hydrogeological conditions. Surface recharge and sources such as wastewater, landfills, agricultural activities and urban environments can introduce MPs into aquifers. Their subsequent migration depends on aquifer properties including porosity, permeability, fractures and groundwater–surface water connectivity, as well as particle size, shape and density. Polyethylene (PE) and polypropylene (PP) are among the most frequently detected polymers, while fibres are the dominant particle morphology reported across different regions. 

A major conclusion of the review is that the enormous variability in reported MP concentrations cannot be explained by environmental differences alone. Sampling and analytical methodologies strongly influence measured concentrations. Groundwater sample volumes vary from only 0.1 L to 15,000 L, resulting in substantial differences in detection capability. Moreover, only 8.5% of the reviewed studies reported well purging before sampling. Differences in filtration, minimum detectable particle size and polymer identification techniques further limit direct comparison between datasets. 

Quality assurance and contamination control represent additional challenges. Microplastic samples are particularly susceptible to contamination by airborne fibres, sampling equipment and laboratory materials. Inconsistent use of blanks and correction procedures therefore introduces uncertainty into reported concentrations. The authors emphasise the need for harmonised groundwater-specific protocols covering well preparation, sampling volume, filtration, particle-size reporting, blank correction and spectroscopic polymer identification. 

The review concludes that MPs are widespread in groundwater, but current data are not yet sufficiently standardised to establish robust groundwater-specific threshold values. Methodological limitations are therefore directly linked to existing regulatory gaps. Developing harmonised and cost-effective monitoring approaches will be essential to generate comparable datasets, distinguish genuine hydrogeological patterns from methodological artefacts, and provide the scientific basis for groundwater quality criteria and future regulatory frameworks. 

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