[Effects of Bentonite on Chemical Forms and Bioavailability of Heavy Metals in Different Types of Farmland Soils]

Distribution, source identification, and health risk assessment of heavy metals in the soil-rice system of a farmland protection area in Hubei Province, Central China

Heavy metal contamination in farmland soil is of great concern due to the threat to food security arising from the bioaccumulation of heavy metals in crops planted in contaminated soil, such as rice, corn, and vegetables. Cd is the main contaminant in both paddy soils and rice. The purpose of this study was to reveal the spatial distribution of 8 heavy metals (Cd, Cr, Ni, Cu, Pb, Zn, As, and Hg) in the farmland protection areas in northwestern Hubei Province and to evaluate their pollution status, sources, and health risks. The total amounts of these 8 heavy metal elements in the samples were measured, and the health risk posed by their accumulation in rice was evaluated using the health risk evaluation model recommended by the US Environmental Protection Agency (US EPA). The mean contents of Cd, Ni, Cu, Zn, Hg, and As in soil exceeded the background levels (0.17, 37.3, 30.7, 83.6, 0.077, and 12.3 mg kg^-1, respectively) of Hubei Province, and Cd showed the highest enrichment coefficient. The concentration of Cd in 89.1% of samples exceeded the limit values stipulated in the Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land (Trial) (GB15618-2018). The contents of heavy metals showed dissimilar geographical distribution patterns. The principal component analysis (PCA) results indicated that Cd, Zn, Ni, As, and Cu mainly originated from the application of pesticides and fertilizers; Cr mainly originated from soil texture and pedogenesis; exhaust gas generated during transportation was the point pollution source of Pb; livestock wastewater, manure irrigation, and atmospheric deposition were the main pollution source of Hg. The contents of Ni and Cd in 52.2% and 58.7% of the rice samples, respectively, exceeded the limit values stipulated in the Food Safety National Standards for Contaminants in Foods (GB2762-2017), and the average effective Cd content accounted for 81.9% of the total Cd. The average bioconcentration factor of each heavy metal in rice followed the order Cd >Zn >Hg >As >Ni >Cr >Pb. Cd and As were the main noncarcinogenic contributing factors, accounting for 80.8% of the total noncarcinogenic risk. The carcinogenic risk indexes of Cd, As, and Cr exceeded the risk index threshold of 10^-4, indicating a carcinogenic risk to the human body. The highest risks to local residents from heavy metals were found in rice. Cd and As were the main noncarcinogenic and carcinogenic factors and should receive greater attention in risk decision management.