Heavy metals in soil and plants after long-term sewage irrigation at Tianjin China: A case study assessment

•A case study in China was undertaken to understand the long-term effect of sewage irrigation on heavy metal concentrations in soil and plants.•Significant heavy metal (Cd, Zn and Hg) pollution has occurred in soils of areas using untreated waste water irrigation in Tianjin, China.•Based on the soil to plant transfer factor of heavy metals, there is a strong accumulation effect of Cd in vegetables.•Mean Cd, Pb and As concentrations in vegetables were higher than national safety limits, whereas mean concentrations of Cu, Zn and Cr were safety. Due to the constraints of freshwater for agricultural irrigation, sewage water has been used in farmland irrigation as an important supplement and alternative water resources during the past three decades in China, especially in northern areas. However, there is increasing concern about food safety and health risks, hence, a case study was undertaken to understand the long-term effect of sewage irrigation on heavy metal concentrations in soil and plants. Sewage sludge, soil and plant samples were collected and analyzed for heavy metals concentrations. Results showed that sewage irrigated soil contained much higher concentrations of Cd, Cu, Pb, Zn, Ni, Cr, As and Hg compared to clean water irrigated soil. The heavy metal content of the topsoil (0–20cm) was much higher than that of the subsoil. Significant heavy metal (Cd, Zn and Hg) pollution has occurred in soils of areas that had been using untreated waste water irrigation in Tianjin, China. There were significant accumulations of heavy metals in wheat that used sewage irrigation. Compared with the other parts of the wheat plants, the higher contents of Cd, Cr, Pb and As in the roots indicated that roots had a strong absorption ability and maybe cause an obvious barrier effect. Based on the soil to plant transfer factor of heavy metals, there is a strong accumulation effect of Cd in vegetables. Overall, the concentrations of Cd, Pb, Ni, Cr and As were lower than the national permitted safety limits in soil. Mean Cd, Pb and As concentrations in vegetables were higher than the national safety limits, whereas the mean concentrations of Cu, Zn and Cr were below Chinese national safety limits. Therefore, in order to ensure food safety and use of sewage for irrigation, continuous monitoring and pollution control is needed.