Microplastics in groundwater: An updated systematic review of occurrence, hydrogeological controls, methodological...

Microplastics (MPs) have been detected in groundwater worldwide, yet a robust scientific basis for defining groundwater-specific threshold values is still lacking. This systematic review synthesizes evidence from 82 field studies conducted in 28 countries between 2019 and 2026, providing the most comprehensive assessment to date of MP occurrence in groundwater. The findings are organized across eight hydrogeological settings, all of which contained MPs, demonstrating that none of the groundwater environments investigated to date is free from contamination. Reported average concentrations span six orders of magnitude (0.001-2103 particles/L). This variability reflects differences in hydrogeological and environmental conditions but, more importantly, substantial methodological inconsistencies among studies. Sample volumes ranged from 0.1 to 15,000 L, well purging was reported in only 8.5% of investigations, and blank-correction procedures frequently failed to meet recommended quality-assurance criteria. Polyethylene (PE) and polypropylene (PP) were among the dominant polymers across regions, while fibers were the most frequently reported particle morphology. The review also identifies ten key regulatory gaps in relation to EU Council Document ST-14144/2025, which requires the inclusion of MPs in the groundwater watch list once reliable and cost-effective sampling and analytical methods become available. Addressing these methodological and regulatory gaps before the 2029 Drinking Water Directive reporting deadline is essential to move groundwater MP research from occurrence-based assessments toward standardized monitoring and enforceable regulatory frameworks.