Potentially toxic elements in rainwater during extreme rainfall period in the megacity Beijing: Variations, sources, and reuse potential

Abstract

Air pollution has become a worldwide problem, which resulted in various environmental threats to the human beings and surface ecosystem. It is thus significant to clarify the sink/source process of atmospheric contaminants, such as the potentially toxic elements (PTEs). The rainfall process is a vital sink for atmospheric PTEs, which can effectively reduce their content in atmosphere. However, the understanding blind spot of variations, sources, and reuse potential of rainwater PTEs in megacities (hotspots area of atmospheric PTEs deposition) still existed, particularly in extreme rainfall period. For this, the daily-scale rainwater samples were collected in Beijing City during extreme rainfall period, and the selected PTEs were measured and further analyzed. The results indicated a volume-weighted mean PTEs concentration (in μg/L) order of Zn (10.86) > Mn (3.21) > Fe (0.73) > Cu (0.69) > Ni (0.35) > V (0.22) > Cr (0.09) ≈ Pb (0.09) > Cd (0.04). Zn was the most abundant PTEs, which was 2∼3 magnitude orders greater than the low abundant PTEs, such as Cr, Pb, and Cd. The sources of PTEs and the dissolved processes of atmospheric components (e.g., particulate matter) were the predominant influencing factors of rainwater PTEs variations, and the rainfall amount was also the potential (not the critical) influencing factor, while the influence of rainwater pH is negligible. According to the enrichment factor and multivariate statistical analysis, the Cd, Zn, and Cu were primarily attributed to anthropogenic sources, which were typically derived from fossil fuel burning, vehicle emission, and domestic waste combustion. The crust sources mainly contributed the Fe, Cr, and V, while the other PTEs (Mn, Ni, and Pb) were derived from a combination of human and natural inputs. Reuse potential assessment revealed that the harvested rainwater was suitable for irrigation of urban green space and vegetation (low soil hazard), and may potentially be utilized for drinking purpose under extreme conditions (after addressing the problem of ammonia nitrogen). This work identified the PTEs characteristics in urban wet deposition, which will help knowledge of PTEs biogeochemical cycle and sustainable use of rainwater resources under climate and environmental change.