Abstract: We collected 112 paddy soil and rice samples from five sites in Guizhou Province, including Chengguan Town of Dushan County in southern Guizhou Province, Bagu Town of Duyun City, Longshan Town of Longli County, and Jinzhong Town of Kaiyang County in central Guizhou Province, and Miluo Town of Shuicheng County in western Guizhou Province. We analyzed the concentrations of heavy metals Cd, Cr, Mn, Zn, As and Pb. Monte Carlo simulations were used to estimate the concentrations of these heavy metals in both paddy soil and rice samples. The ecological risk of soil was evaluated by single factor, Nemerow comprehensive index and potential ecological risk index. The health risk evaluation model recommended by the US Environmental Protection Agency was used to assess the potential health risks associated with heavy metals in rice consumption. The results showed that Cd and As were the two heavy metals with the highest average over-standard rate of soil sampling points in the study sites. Specifically, Cd in Duyun had the highest over-standard rate of 81.1%, while As in Kaiyang had the highest over-standard rate of 76.2%. The comprehensive pollution index of heavy metals in soil was highest in southern Guizhou, followed by central and western Guizhou. The ecological risk index (RI) in southern Guizhou was the highest, with Cd being the major risk element. In terms of rice contamination, As had the highest average over-standard rate across the five study sites, with Dushan having the highest over-standard rate of As at 41.2%. The non-carcinogenic risk index (HI) was in an order of Dushan > Longli > Kaiyang > Duyun > Shuicheng, with all values exceeding 1. The non-carcinogenic risk was mainly contributed by As, with an average contribution rate of 45.4%. The cancer risk index (TCR) exceeded the acceptable threshold of 10^-4 in all study sites, with the highest TCR observed in Dushan followed by Longli, Kaiyang, Shuicheng, and Duyun. Cd was identified as the primary contributor to TCR, with an average contribution rate of 72.9%. Our analysis of different population groups showed that both HI and TCR were higher among children than adults. There was a significant positive correlation between Cd, Zn, As and Pb in soil, with Cd and Zn exhibiting a strong correlation coefficient of 0.959, indicating a possible common source for these four elements. The correlation coefficient between Cd and Zn in rice was only 0.414, indicating an antagonistic effect and synergistic absorption mechanism of Zn²⁺ and Cd²⁺. The safety thresholds for total Cd in soil were determined to be 0.33 (pH≤6.5) and 0.42 mg·kg^-1 (pH>6.5). These thresholds are close to the existing risk screening values for soil pollution in agricultural land, which stand at 0.4 and 0.6 mg·kg^-1. Additionally, we identified safety thresholds for soil Cr, As, and Pb based on soil pH levels. For pH≤6.5, the safety thresholds were 25.7, 9.75, and 44.8 mg·kg^-1, respectively, being lower than the corresponding risk screening values of 250, 30, 100 mg·kg^-1; while for pH>6.5, they were 37.6, 17.2, and 54.2 mg·kg^-1, respectively, being lower than the corresponding risk screening values of 300, 25, 140 mg·kg^-1. Our results indicate that paddy soil and rice in the five sites of Guizhou Province were polluted by Cd and As in varying levels, which may pose potential health risks to consumers.

Key words: rice, soil, heavy metal, health risk, high geological background area