Urban Land Use Sources Microplastic Pollution to Karst Aquifers

Microplastic contamination is ubiquitous in the environment, but the presence of these particles in groundwater reservoirs is not well characterized. Karst systems are particularly vulnerable water resources and habitats because their unique architecture leads to high connectivity between the surface and subsurface, making them susceptible to pollutants from overlying land uses. Thus, our study aims to quantify microplastic contamination in karst groundwater as a function of land use. We collected 4-L water samples from 30 karst springs at baseflow (i.e., > 48 h after precipitation) for microplastic characterization. Sites were distributed across variable land uses in the Salem Plateau and the southeast portion of the Dissected Till Plains in Missouri to establish relationships between contamination levels and land use. A subset (10) of these springs were sampled bimonthly to investigate seasonality of microplastic pollution. Surface streams that featured losing reaches and were near the subset of spring sites were also sampled once. Preliminary results from February 2022 and April 2022 sampling events showed that microplastics were present in our subset of springs, with levels ranging 1.2-15.0 counts/L (median = 3.5 counts/L). Fibers were the most common morphology (90 %), and clear was the most common color (52 %). Recharge area-normalized microplastic concentrations were positively and significantly correlated with urban land cover (ULC; r = 0.52; p = 0.02), showing that urban areas source microplastics to groundwater. Relatively rural springs (< 50 % ULC; n = 4) consistently had higher microplastic levels compared to nearby losing streams, but springs with > 50 % ULC ( n = 3) had lower levels of microplastics compared to proximal surface streams. While the highest microplastic concentrations were observed in our agricultural endmember (62 % agricultural land cover), recharge area-normalized concentrations were negatively, though insignificantly, correlated with agriculture ( r = -0.35; p = 0.33). Ongoing work will entail microplastic characterization for additional water samples across space and time. Developing a holistic understanding of the extent of microplastic contamination in karst environments is essential for mitigating emerging hazards to these fragile water resources and ecosystems.