The anomaly features of As and Cd in the karst area in Yunan province

The content of As and Cd, reaches 6.7~14.9ppm and Cd 0.14~0.21ppm in the stream sediments in Yunnan karst areas, is higher than other areas in Yunnan. The high anomalies in As and Cd content differs a lot in different karst areas in Yunnan. The largest concentration in As and Cd content appears in southeast Yunnan and the anomalous intensity is also biggest in this area, followed by east and northwest Yunnan as well as the Baoshan to Cangyuan area of west Yunnan. Moreover, in northeast Yunnan, the anomalous intensity of Cd is prominent while that of As is not obvious. As and Cd is of high enrichment from rock to soil and the content enrichment ranges reach4 to5 times. Through the geochemical survey and geological environmental analysis on the high anomalous areas in Xiaojiang River basin of Luxi County, it is found that the absorbing amount of As in vegetables is low. Only 14.8% of vegetables exceed the national standard of China. But the absorbing amount of Cd in vegetables is much higher and 74.1% of vegetables exceed the national standard. Through geo-environmental comparative research, the authors summarized that the high anomalies of As and Cd come from the bedrock where is of high background value (As 12.89~28.86 ppm, Cd 0.031~0.314 ppm) in the karst areas. Among all the bedrocks, the background value of carbonate rock is the highest, which is the main cause of the anomalies. The main mechanism of the enrichment is the adsorption and accumulation of clay mineral and organic material in soil, which causes the high enrichment of As and Cd in soils in Yunnan karst areas.