Accumulation of Heavy Metals in Agricultural Soils and Crops from an Area with a High Geochemical Background of Cadmium, Southwestern China

Abstract

In order to investigate the accumulation and transfer of heavy metals in agricultural soils and crops in an area with a high geochemical background of cadmium, soil and crop samples from a black shale outcropped area in Chongqing were collected and analyzed, and the results were then compared with those from other representative black shale outcropped areas. The results showed that some soil samples had a very low pH, and the metals Cd, Cr, Ni, and Zn were enriched. Cadmium concentrations in soil samples exceeded the safety limit, followed by Cr and Ni. Overall, 91.3% of soil samples were heavily to extremely polluted by Cd. The residual fractions accounted for more than 80% of the total metals, except for Cd and Pb. The weak acid soluble fraction of Cd accounted for 27.0%±6.4% of the total Cd, followed by Zn and Ni. The results demonstrate that weathering of black shales can result in elevated heavy metals in soils, and Cd is the primary contaminant in local soils. The high bioavailability of Cd and the high acidity of soils induced the enrichment of Cd in local crops. Cadmium has a higher transfer factor than other metals, and the crops were seriously polluted by Cd, particularly the leaf vegetables, which presented a high concentration of 11.5 mg·kg-1 based on dry weight, and thus, these vegetables are not suitable for cultivating as food stuff. In addition, the risks from Cr should be of concern as well. Therefore, it is recommended that countermeasures be carried out to address the pollution situation, for example, by classifying the pollution levels of agricultural soils and adjusting the planting structures accordingly to reduce the health risks to local inhabitants.