An analysis of microlitter and microplastics from Lake Superior beach sand and surface-water

The abundance of microplastics in natural systems is a concern even in relatively pristine areas such as Lake Superior, on the border between the United States and Canada. In this study, beach sand and surface water were sampled in the Apostle Islands National Lakeshore (APIS) in May and July 2018. Additional sand samples were collected at non-APIS beaches in western Lake Superior in May 2018. Microlitter particles (<4 mm), consisting of microplastics and other low-density particles, were enumerated. Microplastics in sand samples, as identified by melt test, exhibited low abundances (0 to 55 particles/kg dry weight) and were mainly fibers. Microplastics in water samples were also low in abundance (9000 to 40,000 particles/km²) and were mostly fibers. Pyrolysis gas chromatography (pyGCMS) analysis was performed on a subset of microplastics from the sand and water samples. All particles with identifiable mass spectra were polyethylene. When beach sands were processed by density fractionation and filtration, the resulting microlitter samples also contained 260 to 2630 non-plastic particles per kg dry weight. A subset of these non-melting particles was analyzed by Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS). Results indicated that both organic and inorganic particles were included within non-plastic microlitter. Re-analysis of additional aliquots of the same sand samples using oxidation in addition to density fractionation reduced the number of non-plastic particles by roughly half, further highlighting that many of these were organic. Post-oxidation surface-water microlitter (333 μm to 4 mm) also contained non-plastic low-density particles, which comprised 29 to 47% of the total microlitter particle counts. Based upon color distributions, non-plastic microlitter particles in sand and water samples include a small portion of particles identifiable as anthropogenic. The sources for many microlitter particles cannot be distinguished at present and may be natural in origin.