Investigation and evaluation of the risk of Cd contamination of agricultural land in typical areas

Abstract

The issue of food safety being compromised by heavy-metal contamination in soil has become the focus of increasing attention. Sampling points with a low, middle, and high Cd content in the soil were selected in Hunan Province. By sampling the soil and rice point by point, the total and the available Cd content and the content in rice were monitored. The 85 samples were subdivided into three groups according to the total Cd concentration: for the 25 low-risk samples, the content was 0.19–0.80 mg/kg; in the 51 moderate-risk samples, the content was 0.31–3.62 mg/kg; and in the nine high-risk samples, the content was 1.52–24.25 mg/kg. The results showed that: (1) the mean Cd concentration of the soil in the three groups was 0.45, 0.92 and 5.94 mg/kg, respectively; the available concentration was 0.30, 0.57 and 3.71 mg/kg; and the concentrations in rice were 0.09, 0.19, and 0.64 mg/kg. The total Cd content in the soil, the available content and the content in rice increase from low risk group to high risk group. (2) The Cd contents in rice show a significant positive correlation with the total amount in both the soil (the correlation coefficient (r) = 0.343) and the available contents (r = 0.377).(3) The total Cd content in the low-risk group is small, with three of the 25 rice samples exceeding the standard; the effective rate (available content/total content) of Cd was 50.7–97.5%. This showed that rice could still exceed the standard even when the total Cd content of the soil was low but the available content of Cd was relatively high. (4)Among the nine samples in the high-risk group, three did not exceed the standard; the effective rate of Cd was 32.7–62.5%, obviously lower than that in the rice samples that exceeded the standard (51.2–99.4%). This showed that even the total content was high and the available content was low, the Cd content in rice might not exceed the standard. (5) According to the total Cd content in the soil and the amount of Cd exceeding the standard in rice, the risk level was adjusted. After adjustment, the low-risk group contained 61 samples, the rate at which the standard was exceeded was 0%; the moderate-risk group had 12 samples, the excess rate was 75%; and 12 samples formed the high-risk group, where the excess rate was 100%. The adjusted classification is more consistent with what is expected from the standard grouping. The modification of the standard proposed here should improve the accuracy and thereby the usefulness of the standard, and may be useful for the improvement of similar laws and regulations in other countries.