Delayed geochemical hazard: a tool for risk assessment of heavy metal polluted sites and case study

A concept of delayed geochemical hazard (DGH) was proposed instead of chemical time bomb to represent an ecological and environmental hazard caused by sudden reactivation and release of long-term accumulated pollutants in soil/sediment system due to the change of physicochemical conditions or the decrease of environmental capacity. A DGH model was also established to provide a quantitative tool to assess and predict potential environmental risk caused by heavy metals and especially its dynamic evolutions. A case study of DGH was carried out for a mercury-polluted area in southern China. Results of soil column experiment showed that DGH was directly resulted from the transformation and release of pollutant from the releasable species to the active ones through a mechanism of chain reaction. The most possible chain reaction was summarized as HgE+C+F+O+R→HgE+C+F+O→HgE+C+F→HgE+C→HgE. Although 8.3% of the studied area with the total releasable content of mercury (TRCPHg) exceeded the DGH critical point value of 16.667mg/kg, with the possibility of DGH burst, the area was classified as low-risk of DGH. This confirmed that DGH model could contribute to the risk assessment and early warning of soil/sediment pollution.