TOWARD A METHOD OF ASSESSING MICROPLASTIC ACCUMULATION IN BEACH SAND DEPOSITS

Since the 1920s plastic waste management issues have escalated and the impact of microplastics in natural environments has been increasingly recognized. Research pertaining to microplastic delivery, accumulation, and inclusion in marine and terrestrial depositional environments has only effectively begun in the past two decades. There is now an urgent need for more insight into all aspects of microplastics occurrence in the world’s sedimentary environments. Here we explore a low-cost, simple method to measure microplastic concentration in beach depositional environments.

In this study, we compared density separation techniques using four saline solutions to float-separate microplastic debris of six major plastic types and three size fractions from sand samples. We contaminated clean siliciclastic sands with known quantities of polyethylene terephthalate (PET), high density polyethylene (HDPE), polyvinyl chloride (PVC), low density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS) at three different grain sizes; fine (4 phi), medium (2 phi), and coarse (1 phi). We then density separated these microplastics back out of the sand using four solutions of 10% NaCl, 30% NaCl, 30% CaCl₂, and 74% ZnCl_2.

There were no trials in which any test solution failed to recover a mass of microplastic particles. Coarser grained microplastic particles yielded a higher recovery percentage than finer grained particles with 61-78% recovery of the coarse size fraction and 12- 52% recovery of the fine size fraction. HDPE and PP plastics had the highest average recovery rates among the four solutions. It is notable that 61% and 76% of the plastics with the highest published densities (PET and PVC) were recovered despite being floated by solutions of similar or lower densities. The 10% NaCl and 74% ZnCl₂ solutions exhibited the highest recovery, however all fell within the error of each other.

We then employed the 10% NaCl solution method on collected siliciclastic beach sand samples from the east coast of North America and report the results by plastic concentration and grain size. This low-cost, simple method shows promise for widespread application in the development of a better understanding of the degree of beach sand contamination by microplastics on the modern shorelines and in the recent sedimentary record.