Heavy metal transfer in soil-rice systems and health risk assessment in Wanzhou, Chongqing

Objective To investigate the migration characteristics of lead (Pb), cadmium (Cd), chromium (Cr), mercury (Hg), arsenic (As), copper (Cu), zinc (Zn), and nickel (Ni) in soil and rice in Wanzhou district, Chongqing city, and to assess their potential health risks, providing data support for the formulation of scientific prevention and control strategies.

Methods Based on previous soil monitoring data, five representative areas were selected in Wanzhou district, Chongqing city, in August - September 2023. A stratified sampling method was used to establish 25 sampling points. The geo-accumulation index and potential ecological risk index methods were used to evaluate soil pollution. At each sampling point, contiguous paddy fields were selected, and corresponding soil and rice samples were collected, with 25 samples for each. The migration characteristics of heavy metals and their potential impact on human health were analyzed using the bioconcentration factor and health risk assessment model.

Results A total of 25 soil samples were collected, mostly acidic. Except for zinc (Zn), the average values of other heavy metals did not exceed the soil background values. Cadmium (Cd) exceeded the risk screening value at three sampling points but did not exceed the risk control value; other heavy metals did not exceed the risk screening value. Mercury (Hg) had the largest coefficient of variation, > 1 (1.03), indicating its highly uneven distribution. The geo-accumulation index assessment showed no overall soil pollution. Ecological risk assessment indicated that Hg and Cd contributed the most to ecological hazards, at 44.65% and 36.76%, respectively. Among the 25 rice samples, Cr had the highest over-standard rate at 80.00%, followed by Hg, As, and Cd, all at 12.00%. Enrichment factor analysis showed that Cd and Hg had higher enrichment factors in rice, at 0.341 and 0.329, respectively. Health risk assessment showed that, except for As, other heavy metals posed low chronic health risks, but the combined health risk of heavy metals in rice may have chronic effects on human health (mean = 3.53).

Conclusions There is a certain degree of Cd and Hg migration pollution in the soil and rice of Wanzhou district, Chongqing city. Arsenic (As) may have chronic health effects on humans, and there may be synergistic effects of multiple metals.