湘西岩溶区土壤剖面Cd的超常富集行为及其环境影响评价

Supernormal Enrichment Behavior of Cadmium and its Environmental Impact Assessment in Soil Profile of Karet Area in Western Hunan

选择湘西永顺县王村镇一条白云岩发育的土壤剖面(WC),通过对剖面中Cd的地球化学特征研究,结合质量平衡计算、基岩动态淋溶实验以及土壤层活性态Cd的提取,获得了以下主要认识。1)贫Cd碳酸盐岩(0.095 mg/kg)的酸不溶相中,Cd显著富集(2.12 mg/kg),质量分数是其在基岩中的22倍;另一方面,基岩中80%以上的Cd赋存在碳酸盐相。因此,碳酸盐岩风化成土过程中,Cd呈现出低背景、强亏损、高富集的特征。2)Cd在WC剖面紧挨岩-土界面土层的超常富集(13.24 mg/kg),可能主要源于岩-土界面作用中残余的富Cd酸不溶物基础上,叠加了从土壤层长期演化中进一步淋滤下渗的Cd。3)土壤层中Cd的质量分数不仅高于或显著高于农用地土壤Cd污染风险筛选值,而且在土壤层底部还明显超过Cd污染风险管制值,甚至活性态Cd的质量分数都高于Cd污染风险管制值。

A soil profile developed on dolostone,in Wangcun town,Yongshun county,Western Hunan,namely Wangcun profile(WC)was taken as the research target.Through the study on geochemical characteristics of cadmium(Cd)in the profile,combining with mass balance calculation,bedrock dynamic leaching experiment and the extraction of mobile Cd in the soil layer,the main understandings were obtained as follows.(1)Cd is significantly enriched in acid-insoluble phase(2.12 mg/kg)in carbonate bedrock poor in Cd(0.095 mg/kg),and its concentration in the former is 22 times higher than that in the latter.On the other hand,more than 80%of Cd in bulk rock occurs in carbonate phase.Therefore,during weathering and pedogenesis of carbonate rock,Cd presents the characteristics of low background value,strong loss and high relative enrichment.(2)Supernormal enrichment of Cd(13.24 mg/kg)in the soil layer close to the rock-soil interface in the profile WC may be based on the residual Cd enriched in acid-insoluble matter of bedrock in the interaction of rock-soil interface,superimposed with Cd further leached from the profile in the long-term evolution process.(3)The concentration of Cd is not only higher or even significantly higher than the Cd risk screening value for soil contamination of agricultural land in the soil layer,but also obvious higher than the Cd risk intervention value for soil contamination of agricultural land at the bottom of the soil layer,and even the concentration of mobile Cd is higher than the Cd risk intervention value.