[Cd Pollution and Safe Planting Zoning in Paddy Soils: A Case Study in a District of Chongqing]

Rice has a strong ability to accumulate Cd in soil, and it is of great significance to study Cd pollution and safe planting zoning in paddy soils. In this work, 300 sets of paddy soil-rice samples were simultaneously collected in 22 towns in a District of Chongqing, and soil pH, soil total and available Cd contents, and brown rice Cd contents were determined. Soil Cd pollution was assessed using the geoaccumulation index, bioconcentration factor, and the single-factor pollution index. Based on the Cd pollution indices of soil and brown rice, safe planting zoning for rice was determined. The results showed that the paddy soils were generally acidic, and total Cd contents ranged from 0.09 mg·kg^-1 to 1.60 mg·kg^-1, with 35.0% of sites exceeding the risk screening value. The Cd contents of the brown rice ranged from 0.002 mg·kg^-1 to 0.808 mg·kg^-1 and exceeded the food safety limit in 13.7% of cases. Pearson correlation analysis showed that the Cd content of brown rice was significantly positively correlated with soil total and available Cd (P<0.01). The pollution evaluation indicated that significant Cd accumulation occurred in the paddy soils, with some areas showing light-to-moderate pollution levels. The enrichment coefficients of rice to soil Cd ranged from 0.004 to 1.72. Overall, the paddy soils in the studied area were considered generally safe with respect to Cd pollution, with low-risk areas distributed in the south, west, and east, whereas some medium-high risk areas were detected in eight towns.