【Objective】Microplastics (MPs) represent a pervasive class of emerging contaminants that have been widely documented across global environmental compartments. Despite this recognition, significant knowledge gaps remain regarding their fate and behavior within fragile karst groundwater ecosystems, particularly those that serve as critical drinking water resources. The unique hydrogeological features of karst systems, characterized by rapid infiltration through fractured limestone, raise serious concerns about MP transport and human exposure pathways. These systems, however, remain severely understudied. This study aims to address these uncertainties through a comprehensive investigation of the occurrence, distribution, underlying mechanisms, and potential ecological and health risks associated with MP contamination in a representative karst groundwater system used for drinking water.【Method】To account for temporal variability in these dynamic systems, we systematically collected water samples every quarter across four seasons (spring, summer, autumn, winter) from both pristine karst springs (Raw Water, RW) and downstream household tap water endpoints (End Water, EW) in a typical karst terrain in Southwest China. Laboratory analyses employed high-resolution microscopic Raman spectroscopy for definitive polymer identification and characterization of isolated MP particles. Subsequent statistical analyses used the Mantel test to rigorously examine correlations between MP concentrations and a suite of water quality parameters. Additionally, the Pollution Load Index (PLI) was calculated to provide an integrated assessment of MP pollution levels and to quantify potential environmental and human health risks. 【Result】The findings revealed significant MP contamination throughout the karst aquifer system. A total of 2,273 MP particles were identified across all 72 water samples. The detected MPs exhibited distinct physical characteristics: fragments were dominant (50.37%), followed by fibers (40.61%). Particle size analysis showed a predominance within the 100–300 µm range (59.08%), and most particles appeared transparent (78.79%) under microscopy. Polymer composition analysis indicated that polyethylene (PE; 44.08%) was the most abundant polymer, followed by polyethylene terephthalate (PET; 35.02%) and polypropylene (PP; 16.89%). The mean MP abundance was 1.58±1.43 items·L⁻¹ across all samples. Notably, significantly higher concentrations were observed in raw water sources (RW: 1.89±1.70 items·L⁻¹) compared to finished tap water (EW: 1.26±1.05 items·L⁻¹), suggesting partial removal during transport or treatment. Pronounced seasonal variations were observed, in the following order: autumn (2.67±2.08 items·L⁻¹) > spring (1.58±1.30 items·L⁻¹) > summer (1.13±0.63 items·L⁻¹) > winter (0.94±0.53 items·L⁻¹) (P < 0.05). The calculated Pollution Load Index (PLI = 1.10) indicated that the current MP pollution level in this karst groundwater system poses a relatively low risk. 【Conclusion】Key correlation analyses provided insight into MP transport mechanisms. A significant positive correlation was found between MP abundance and dissolved organic carbon (DOC) concentrations (P < 0.05), suggesting that organic matter facilitated MP mobilization. A negative correlation was observed between calcium ion (Ca²⁺) concentration and MP abundance (R² = 0.40), possibly reflecting interactions influenced by aquifer mineralogy. In contrast, no significant correlation was found between MP levels and antecedent monthly precipitation (P > 0.05), indicating that direct flushing during storm events may not be a major influence. These results collectively underscore that intrinsic water chemistry plays a more decisive role in MP transport and accumulation within karst aquifers than transient hydrological events. Based on these findings, we recommend urgent strengthening of protective measures for karst spring water sources, establishing long-term MP monitoring programs in vulnerable karst regions, and developing mitigation strategies along water supply chains to prevent future escalation of MP pollution risks.