Cd accumulation and human health risk assessment of rice in high background areas of heavy metals: A case study of Nagu Town, Huize County, Yunnan Province

A study comparing cadmium (Cd) accumulation in soil and rice in an area of high background heavy metal concentrations in Huize County, Yunnan, showed that, occasionally, the cadmium content in soil exceeds the standard, whereas the cadmium content in crops remains within the standard. However, our understanding of the reasons for this phenomenon is limited. It is of great significance for soil environmental quality evaluation and ecological restoration to study cadmium accumulation in crops and its control factors in these areas. In this study, 41 rice seed samples and 41 corresponding soil samples were collected from the Nagubazi, Nagu Town, Huize County, Yunnan Province, and their cadmium content was determined by ICP-MS. The correlation between rice cadmium and soil elements (indices) was analyzed by partial correlation, R-type clustering, and stepwise linear regression. The results showed that the soil cadmium content ranged from 0.467 to 1.87 mg/kg, with an average of 0.78 mg/kg, which is 5.69 times that of the national soil background value. Moreover, 58.5% of the soil samples exceeded the soil pollution risk threshold for agricultural land. In contrast, the cadmium content of rice (brown rice) ranged from 0.007 to 0.062 mg/kg, with an average of 0.016 mg/kg, which is lower than the national threshold for food safety.The content of cadmium in rice was significantly positively correlated with the content of U and Mo , and significantly negatively correlated with soil organic carbon (TOC) and the content of S in soil, but not significantly correlated with soil cadmium content and pH value. The phenomenon that soil cadmium exceeds the standard but rice cadmium does not exceed the standard may be related to the enrichment of organic matter in the soil. The content of soil TOC is 5.57 times that of the national background value. When TOC is greater than 3%, cadmium content in rice decreases rapidly. Therefore, the organic matter correction method for soil cadmium environmental grade evaluation was proposed, and the 3.42 km² (5129 Mu) of paddy field originally classified as Class Ⅱ in the study area was revised to Class Ⅰ. In addition, some suggestions on soil protection and remediation were put forward.