Accumulation characteristics and risk of heavy metals and microbial community composition in bioretention systems: A case study of a university campus

Abstract

Bioretention systems have proven to be an effective stormwater treatment technology for reducing runoff pollution. However, the accumulation risk of heavy metals and characteristics of the microbial communities in the filter media is still unclear under long-term runoff infiltration. This paper reports on a field study that investigated the ecological impact of common heavy metals and the structures of microbial communities in nine bioretention systems during two seasons. Samples were collected from the filer media to determine the concentrations of heavy metals and the microbial information was obtained by high-throughput sequencing technology. The results indicated that Cr, Ni, Cu, and Zn accumulated significantly in the bioretention system, with mean concentrations 2–3 times higher than the corresponding background values. Heavy metals were mainly trapped in the surface media (0-10 cm) and concentrations varied with season. The comprehensive pollution status of the bioretention systems was at a light to medium level. In particular, Ni accounted for the majority of the potential ecological risk index values, with an ecological hazard of above moderate levels. The results of high-throughput sequencing showed that Actinobacteria (22.51–42.16%) and Proteobacteria (19.67–34.46%) are the dominant phyla in the bioretention media under the long-term pollutants accumulation. Microbial diversity was negatively correlated with the heavy metals content of the media (p < 0.05). Heavy metals inhibited the growth and reduced the diversity of media microorganisms to varying degrees, with Ni contributing the most to microbial community changes. Our findings are expected to help in controlling the heavy metals pollution in the bioretention systems and provide practical guidance regarding the systems operation and media micro-ecosystem maintenance.