Microplastics in karst groundwater: a review of current findings and methodological biases

Karst aquifers are globally important yet highly vulnerable water resources. Their rapid recharge, limited natural filtration, and conduit-dominated flow make them susceptible to contamination, including by emerging pollutants such as microplastics (MPs). While MPs are well documented in marine, freshwater, and terrestrial systems, their occurrence and behaviour in karst groundwater remain poorly understood. This review synthesises 22 published studies on MPs in karst caves, springs, wells, and boreholes, assessing comparability using a widely applied quality assessment framework and a critical evaluation of methodological influences on reported MP concentrations and characteristics. Reported concentrations vary by several orders of magnitude (0.001–911 particles/L), which is likely to reflect both karst environmental variability and methodological differences in sampling design, detection limits, contamination control, and analytical specificity. Temporal datasets remain limited, but existing studies suggest high-flow events substantially increase MP loads. No single analytical technique is quantitative across the full diversity of MPs, each being inherently selective toward sizes, shapes, or polymers. Notably, stereomicroscopy appears to overrepresent fibres, while low-density separation, such as sodium chloride solutions, miss higher-density plastics. These findings highlight the need for clear reporting of the operational definition of MPs in each study to improve cross-study comparability. We propose a harmonised sampling and analytical workflow to support future karst MP research, alongside the continued need for robust quality assurance/quality control practices, broader geographic coverage, and higher temporal resolution sampling. More work beyond simple concentration reporting is needed to investigate mechanistic processes driving MP transport and fate.