Subsurface dynamics of microplastics in the land of headwaters: a subsurface study of the spatial distribution and accumulation of microplastics near the headwaters of major U.S. rivers

Despite growing attention, most microplastic (MP) research has not examined subsurface soil, limiting understanding of terrestrial MP fate and transport (Fig. 1A). Few studies have extracted soil cores or soil pits, leaving vertical MP distribution largely unexplored (Fig. 1B). Here we attempt to bridge the gap by using soil coring with fine sampling increments, and radioactive dating of soil pits to align MP characteristics with deposition and accumulation rates. We established a transect from a dual-use road/ trail downslope to La Poudre Pass Creek (a headwater of the Cache La Poudre River main stem) in northern Colorado and collected 25 soil cores across five sites. We also excavated three soil pits and deployed a passive atmospheric dust collector (Fig. 1C, D). MPs are being isolated by density separation, grains will be characterized with a Dynamic Image Particle Analyzer, and soil properties have been quantified (e.g., moisture, organic matter, and texture). We hypothesized MPs would move more easily through coarse, angular, well-draining soil, while a fine-grained, organic-rich, wetter soil will retain more MPs (Fig. 1E). Preliminary results of soil property analyses show trends that reflect the samples’ proximity to the floodplain, as well as to-be-explained vertical variation in properties like pH, electrical conductivity, texture, C/N, and moisture. This will provide context for why some MPs are more likely to mobilize in surface runoff versus penetrating deeply into the soil. Next, we will use the soil cores and soil pit samples to quantitatively classify the MPs via microscopic FTIR, as well as date the soil using 210Pb and7Be. Multi-variate regression will be used to evaluate predictors of MP distribution, and spatial statistical analysis will be performed to characterize spatial continuity of MP distribution. Radiometric dating will be used to construct a model for MP accumulation over time.