Characterising microplastic pollution in sediments from urban water systems using the diversity index

Abstract

As microplastic pollution (MP) is currently attracting worldwide attention, the characterisation of microplastics (MPs) in terms of pollution level, sources and migration is of great importance. In the present work, both the Simpson diversity index (SDI) and Shannon-Wiener index (SWI) were used to study the MPs present in the sediments from urban water systems including two lakes (Yushan lake and Nan Lake) and two rivers (Yushan River and Yongfeng River) from Ma'anshan City in the lower reaches of the Yangtze River. The abundance of MPs in sediments ranged between 5 and 153 items·kg−1 with high spatial variation depending on the site. MPs detected in the urban water system were dominated by film (69.8%) in shape, transparent (36.4%) in colour, <1000 μm (50.1%) in size, and Polyethylene (PE, 43.3%) and Polypropylene (PP, 35.5%) in composition. Whilst previous studies have used the diversity of MPs to analyse MP in marine environments, this work aims to obtain the diversity information of MPs in urban areas. The diversity indices of polymer, shape, colour and size were 0–0.685, 0–0.578, 0–0.810 and 0–0.816 for SDI, respectively, and 0–1.255, 0–0.935, 0.1.710 and 0–1.748 for SWI, respectively. Pearson analysis indicates a significant correlation between the number of land-use types and the diversity index of MPs (r = 0.507–0.935, p < 0.05 for SDI; r = 0.528–0.857, p < 0.05 for SWI). Additionally, an attractive fitting relationship between SDI and SWI (p < 0.001) was observed in MP analysis. This means that either of the SDI and SWI can be employed for the analysis of MP. Sites where the urban water system flows into the Yangtze River had a higher abundance of MPs (96 ± 66 items·kg−1) than the urban water system (33 ± 30 items·kg−1) (t = −2.727, p = 0.01), implying that MPs tend to accumulate after they enter the Yangtze River from the urban water system.