Enrichment characteristics and ecological risk assessment of heavy metal Cd in soil and sediment along the Yangtze River in Nanjing, Jiangsu Province

The pollution of heavy metal elements such as cadmium (Cd) was a widely concerned environmental issue. In order to explore the enrichment characteristics and pollution situation of heavy metal Cd in different ecosystems along the Yangtze River in Nanjing, and provide a basis for the safe use of soil in this area, a total of 3552 soil / sediment samples were collected from different types of land such as agricultural land, construction land, Qixiashan mining area, Yangtze River sediment and riverside wetland, and the pH value and heavy metal Cd content were determined. The accumulation index (A_Cd), exceedance index method (P_Cd), geo−accumulation index (I_geo), and potential ecological risk index (E_r) were used to evaluate the risk of heavy metal Cd pollution in various regions. The results showed that there had Cd highly abnormal in the Qixiashan mining area, Yangtze River and riverside wetland, and the average contents were 1.25 mg/kg, 0.82 mg/kg, 0.72 mg/kg, respectively. Cd enrichment also occurred in the Yangtze River alluvial plain compared with the hilly area, and the average content was 0.35 mg/kg. The average content of Cd in sequence was Qixia Mountain > Yangtze River sediment> riverside wetland >Yangtze River alluvial plain area>Hilly area>Chuhe River sediment ≈ Nanjing background value. Accumulation index of Cd (A_Cd) was Yangtze River sediment > riverside wetland > Yangtze River alluvial plain > Chuhe River sediment > Hilly area in sequence. The single exceeding standard index of Cd ( P_Cd ) revealed that 95% of the area was in a safe state, and about 5% of the area exceeded the standard, exceed standard level was slight− mild, and the over−standard areas were mainly distributed in the east and south−central Baguazhou. Both the geo−accumulation index ( I_geo ) and the potential ecological risk index ( E_r ) revealed that about 50% of the region was at ecological risk, which was mainly distributed in the Yangtze River alluvial plain and sediment. It was recommended to adopt safe utilization measures such as improving soil pH using soil conditioners, crop rotation, deep tillage or adding heavy metal passivators in the over−standard areas, and meanwhile the monitoring of soil−crop environment should be strengthened to prevent further deterioration of soil environment.