Spatial autocorrelation analysis of monitoring data of heavy metals in rice in China

The aim of this study was to analyze the spatial distribution of heavy metal monitoring data in paddy rice to provide a scientific basis for food safety risk assessment and suggestions for possible risk management. In this study, the spatial distribution data of heavy metals (cadmium, lead, total arsenic, total chromium, and total mercury) in rice collected in the main production provinces of China were analyzed by multidimensional visualization and aggregation analysis. The spatial correlation of elemental contamination was also identified. Monitoring data of cadmium, lead, and arsenic content in different varieties of rice was compared with their mean confidence intervals. Results showed that cadmium content in rice was higher than the limit value in some areas of Hunan, Sichuan, Guangxi and Anhui Provinces in China. With respect to other heavy metals, a small area of Sichuan Province experienced lead levels in rice higher than the limit value. Also, the arsenic level in rice was higher than the limit value in Jiangxi Province, a northern area of Liaoning Province and most parts of Guangzhou and its surrounding areas. In contrast, chromium was only detected at excessive levels in southern Sichuan Province. In addition, a small part of the eastern Sichuan Province was found to have excessive levels of arsenic. Moran's I index of cadmium, arsenic, chromium, lead, and mercury in rice was 0.50, 0.55, 0.21, 0.09, and 0.05, respectively, which revealed a spatial autocorrelation. Overall, there was moderate aggregation of cadmium and arsenic in the monitoring areas, while lead, chromium and mercury showed low aggregation. Geographically for the provinces, the high aggregation of cadmium in rice was evident in Hunan and Jiangxi Provinces and Guangdong border areas. The arsenic in Jiangxi Province and border areas of Jiangxi and Guangdong Provinces also showed high level of aggregation. Meanwhile, the parameter testing of the samples showed that the concentration of cadmium and lead were significantly higher in late Indica rice compared to early Indica rice, while the arsenic and chromium showed the opposite effect. In view of the high levels of certain heavy metals in rice in some provinces, more refined dietary intake assessments of rice as consumed are necessary to determine if populations are exposed to levels that exceed the health-based guidance levels.