Quantifying Microplastic Debris Sourcing and Transport for a Karst Aquifer

Microplastic (MP; plastic < 5 mm in size) contamination is ubiquitous, and has been found in environments ranging from deep ocean floors to Artic Sea ice. MPs are a concerning emerging contaminant because they degrade slowly, are highly mobile, and can be ingested by wildlife, causing both physical tissue damage and possible toxicity due to adsorption of other contaminants (e.g., heavy metals) on MP surfaces. MP research has mainly focused on marine settings, and more recently expanded to surface freshwater systems. However, only one previous study has quantified MP pollution in groundwater. Our new study investigates MP sourcing and transport in a limestone-hosted cave system (Cliff Cave, St. Louis, MO). We monitored the cave stream under different flow conditions starting in February 2020. Weekly water samples were analyzed for MP traits (quantity, type, size, color), total suspended solids (TSS), and fluorescence at 436 nm, which is indicative of organic matter (OM) or optical brighteners (OB; tracers of wastewater). We also continuously monitored water level, specific conductance, turbidity, and pH in situ at the cave outlet. MPs were found in all samples, with concentrations ranging from 3.1 to 49.0 counts/L. Most MPs were fibers (95.2%), followed by films (3.4%) and fragments (1.4%). Most of the MPs were clear (64.6%), but some were blue (18.4%), black (12.9%), and red (4.1%). Total MP loads had significant, positive correlations with water level, turbidity, TSS, and OM/OB (R 2 > 0.77; p < 0.0004), and negative correlations with specific conductance (R 2 = 0.45; p = 0.02) and pH (R 2 = 0.14; p = 0.22). However, other MP traits did not correlate with these parameters (p > 0.05). Preliminary results show that MP transport is enhanced during floods, when dilute, sediment- and OM/OB-rich surface runoff enters the karst aquifer through sinkholes and fractures. We observed that high MP and sediment loads occurred well after flood discharge peaks, suggesting an allochthonous MP source, rather than remobilization within the cave. Ongoing analyses of major ions, organic C, and stable isotope ratios (δ 18 O, δ 2 H) will provide additional evidence for MP sourcing and flowpaths. Our study gives new insight into how MP contamination is transported to and through groundwater systems, which will help inform debris mitigation strategies.