Characterizing heavy metals in combined sewer overflows and its influence on microbial diversity

This study characterized the pollution levels and potential ecological risk of heavy metals in combined sewer overflows (CSOs) and their effects on microbial diversity in nearby riparian sediments. The chemical fractionations of Zn, Cd, Cr, and Cu in dry-weather flows, wet-weather flows (CSO discharges), sewer sediments, and surface runoffs were determined. Geo-accumulation (I_geo) and ecological risk (RI) indexes were employed for metal risk assessment. DNA extraction and polymerase chain reaction (PCR) amplification on the Illumina MiSeq platform were conducted. The results show that heavy metals contents in fine-sized fractions have higher values than those in coarse-sized fractions. Chemical fractionation analysis suggests that Zn and Cd are two of the most bioavailable metals impacted by anthropogenic activities. Cr and Cu contents in CSOs are relatively stable and could exist for extended periods. According to the RI analysis, CSOs pose a considerable risk (RI-G2) to receiving waters due to the higher bioavailability of Cd, which was consistent with the I_geo index. Furthermore, under the stress of the highly-bioavailable Cd and Cu, Gram +ves in the riparian benthic sediment gradually became dominant with metal-tolerance property. Therefore, long-term exposure to highly bioavailable metals could exhibit great impacts on microbial diversity.