[Temporal-spatial Distribution and Health Risk Assessment of Heavy Metals in the Surface Water of Ningbo]

A total of 120 surface water samples were collected from industrial and commercial districts of Ningbo, China in the wet and dry seasons. The concentrations of six heavy metals (Cd, Cr, Ni, Pb, Zn, and Fe) in the samples were measured, the temporal-spatial distribution characteristics of the six heavy metals were analyzed, and Pearson correlation coefficients of the six heavy metals were calculated. Combined with the temporal-spatial distribution characteristics and Pearson correlation coefficients of the six heavy metals, the main pollution sources of the two districts were analyzed, respectively. The risk of heavy metals in surface water to the exposed population was evaluated by calculating the health risk index and carcinogenic risk index. The results showed that the pollution characteristics of heavy metals in the surface water of Ningbo industrial district and commercial district differed greatly in different seasons. In the industrial district, the orders of the average concentration of heavy metals in the wet season and dry season were Fe>Zn>Ni>Pb>Cr>Cd and Fe>Zn>Cr>Ni>Pb>Cd, respectively. The concentrations of Cr, Cd, and Pb in the wet and dry seasons exceeded the class Ⅳ recommended values, following the degrees of Cr>Cd>>Pb and Pb>Cr=Cd, respectively. Sewage containing heavy metals was one of the major pollution sources. In the commercial district, the average concentrations of heavy metals in the wet season and dry season were in the order of Fe>Pb>Ni>Zn>Cd>Cr and Fe>Pb>Ni>Zn>Cr=Cd, respectively. The concentrations of Cd and Pb in the wet season exceeded the corresponding levels (class Ⅳ), and the degree followed Cd>Pb. Only Pb exceeded the standard in the dry season, with the exceeding standard rate of 60%. Road pollution containing heavy metals was the major pollution source, and heavy metals entered surface water mostly with surface runoff and precipitation. The carcinogenic risk posed by heavy metals in the surface water of the Ningbo industrial district and commercial district was very high, and the carcinogenic risk in the commercial district was much higher than that in the industrial district. The main carcinogen was Cr. Compared to the research results of the research group in 2015, the pollution degree of heavy metals has been greatly reduced. In the future, we still need to give adequate attention to the prevention and control of heavy metal pollution in surface water in Ningbo.