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Microplastic Pollution Intrudes Carbonate Critical Zones During Flood Events
Summary
Continuous monitoring of a Missouri cave stream over one year found microplastics in all water samples at concentrations up to 82.5 particles per liter, with fibers comprising 93% of particles and flood events dramatically increasing transport. This study demonstrates that karst aquifer systems — which supply drinking water to millions of people — are vulnerable to microplastic contamination that surges during rainfall events.
Carbonate critical zones (CZs) are often characterized by the presence of dissolution features in bedrock that lead to high connectivity between the surface and subsurface. Carbonate aquifers can therefore be highly susceptible to anthropogenic contamination compared to aquifers in silicate CZs. Microplastics (plastic < 5 mm) are emerging contaminants that are ubiquitous in the environment. Because they degrade slowly and are highly mobile, microplastics can travel long distances and be easily ingested by wildlife. Microplastic research mainly focuses on marine and surface freshwater environments, with groundwater systems remaining understudied. Thus, our study identifies microplastic sources and transport mechanisms through an aquifer in a carbonate CZ. We continuously monitored in situ water quality (e.g., temperature, specific conductivity, pH) and level for a stream issuing from a cave hosted in St. Louis Limestone (Cliff Cave; St. Louis, Missouri) from February 2020 to February 2021. We also collected water samples under a range of flow conditions, employing both weekly and high frequency flood sampling (four flood events total). Samples were analyzed for microplastic content and characteristics as well as other analytes (e.g., total suspended solids (TSS), ion chemistry, O and H isotopes). Microplastics were found in all samples, with concentrations of 2.1 – 82.5 counts/L. For all microplastics, the dominant morphology was fiber (93.0%) and the most common color was clear (59.7%). Total microplastic concentrations had significant, positive correlations with water level and TSS (R 2 > 0.18; p < 0.05) but significant, negative correlations with specific conductivity and pH (R 2 > 0.17 p < 0.05). Our findings indicate that microplastic transport is enhanced during floods in karst systems, when dilute and sediment-rich surface runoff enters aquifers through sinkholes and fractures. Antecedent moisture conditions may also play a role in microplastic transport because floods occurring after dry periods tend to have higher microplastic loads compared to floods following wetter conditions, even when peak stage values are comparable. Our study gives new insight into how microplastic contamination is transported to and through carbonate CZs, which will help inform debris mitigation strategies.