Evaluation of leafy vegetables as bioindicators of gaseous mercury pollution in sewage-irrigated areas

Mercury (Hg) can evaporate and enter the plants through the stomata of plant leaves, which will cause a serious threat to local food safety and human health. For the risk assessment, this study aimed to investigate the concentration and accumulation of total gaseous mercury (TGM) in five typical leafy vegetables (Chinese chives (Allium tuberosum Rottler), amaranth (Amaranthus mangostanus L.), rape (Brassica campestris L.), lettuce (Lactuca sativa L.), and spinach (Spinacia oleracea L.)) grown on sewage-irrigated areas in Tianjin, China. The following three sites were chosen to biomonitor Hg pollution: a paddy field receiving sewage irrigation (industrial and urban sewage effluents) for the last 30 years, a vegetable field receiving sewage irrigation for 15 years, and a grass field which did not receive sewage irrigation in history. Results showed that the total Hg levels in the paddy (0.65 mg kg−1) and vegetation fields (0.42 mg kg−1) were significantly higher than the local background level (0.073 mg kg−1) and the China national soil environment quality standard for Hg in grade I (0.30 mg kg−1). The TGM levels in ambient air were significantly higher in the paddy (71.3 ng m−3) and vegetable fields (39.2 ng m−3) relative to the control (9.4 ng m−3) and previously reported levels (1.45 ng m−3), indicating severe Hg pollution in the atmospheric environment of the sewage-irrigated areas. Furthermore, gaseous mercury was the dominant form of Hg uptake in the leaves or irreversibly bound to leaves. The comparison of Hg uptake levels among the five vegetables showed that the gradient of Hg accumulation followed the order spinach > red amaranth > Chinese chives > rape > lettuce. These results suggest that gaseous Hg exposure in the sewage-irrigated areas is a dominant Hg uptake route in leafy vegetables and may pose a potential threat to agricultural food safety and human health.