Distinct mediating patterns between metal filtering and species coexistence of rare and abundant subcommunities in heavily polluted river sediments

Long-term exposure to fly ash leachate enhances the bioavailability of potentially toxic metals and decreases bacterial community diversity in sediments

The interaction between microorganisms and the physicochemical properties of sediments is the key to maintaining the stability of the ecological environment. However, the effect of fly ash stockpiling on the relationship between sediment bacterial communities and their physicochemical properties remains unclear. In this study, the interactions between geophysical and chemical factors, morphological distribution of potentially toxic metals (PTMs), and bacterial community diversity in sediments affected by long-term ash water seepage were examined. The results showed that (1) Ash water seepage markedly lowered the pH and elevated the electrical conductance; available potassium, available phosphorus, organic carbon contents; small particle size (<0.25 mm), and concentrations of eight PTMs, including nickel (P < 0.05); (2) Ash water seepage considerably raised the relative abundance of Proteobacteria in the sediments, reduced bacterial community α-diversity, and altered the community structure; (3) Bacterial communities in sediments were strongly correlated with the contents of available potassium organic carbon, selenium, arsenic (oxidizable and reducible), antimony (extractable with weak acids), and chromium (extractable with weak acids); and (4) Fly ash perturbation reduced the connectivity and cohesion in the molecular ecological network of sediment bacteria and increased the abundance of pollution-degrading metabolic pathways, such as low-toxicity and organic classes, as well as coupled stimulus-response and chemotaxis-avoidance defense mechanisms. In summary, the results of this study reveal the changes in bacterial communities, major physicochemical factors, and the morphological distribution of PTMs in sediments affected by long-term ash water leakage of fly ash landfills and provides a theoretical basis for ecological environmental management.

Metal contamination, their ecological risk, and relationship with other variables in surface sediments of urban rivers in a big city in Asia: case study of Hanoi, Vietnam

Urban rivers are significantly impacted by anthropogenic pressure. This study presents the updated assessment of the concentrations of 11 metals and other variables (pH, total organic carbon (TOC) and nutrients (total nitrogen, total phosphorus, and total silica)) in the sediments of four urban rivers in inner Hanoi city, Vietnam, during the period 2020-2022. The mean concentrations of Fe, Zn, As, and Cr were higher than the permissible values of the Vietnam National technical regulation on the surface sediment quality. Moreover, Zn and Cr were at the severe effect level of the US EPA guidelines for sediment quality. The calculation of pollution indices (Igeo and EF) demonstrated that Mn, Ni, and Fe were from natural sources whereas other metals were from both anthropogenic and natural sources. The ecological risk index revealed that metals in Hanoi riverine sediments were classified at considerable ecological risk. High values of metals, TOC, and nutrients in the sediments of these urban rivers mostly originate from the accumulation of untreated urban wastewater that is enhanced by low river discharge. Our results may provide scientific base for better management decisions to ensure environmental protection and sustainable development of Hanoi city.