地球化学异常区土壤及沉积物中镉的分布、积累特征与释放机制

Cadmium is known to be both extremely toxic and ubiquitous in natural environments. It occurs in almost all soils, surface waters and plants, and it is readily mobilized by human activities such as mining. Ingestion of even trace quantities of cadmium can influence not only the physiology and health of individual organisms, but also the demographics and the distribution of species. Guizhou province in southwest China is covered by large area of carbonate rocks where the average Cd concentrations in soil and sediment is 0.31×10-6 in surface, it's as 2.5-3.5 times as that in other parts of the country, due the high geochemical background Cd concentrations, this natural Cd pollution of the soils and sediments is a matter of great concern. We selected typical carbonate areas of Guizhou province as research area, which distributed in the north, central and south of this province respectively, and selected soils and sediments as the main research objects. The first step of the present study was conducted a serious of field surveys and sample collections, then the speciation of Cd extracted by BCR, and the Cadmium fractionation was known as AS-Redu-OXid-RES according to BCR in soils and sediments. Leaching experiments was used to study the migration characters of Cadmium in soils. The cause and features of Cadmium geochemical anomaly, Cadmium enrichment in the deep layers of soil and its impact factors were studied. The pollution risk of soils and sediments with Cd anomalies was assessed by the Potential Ecological Risk Index ( RI ) and Geoaccumulation Index ( Igeo ). Spatial distribution of Cadmium in sediments was also studied. A simulated experiment exposing Daphnia carinata in the Cd contaminated sediment was conducted in laboratory to study the photoaxis index of Daphnia carinata, and the distribution regularity of Cd in water. The speciation of Cd extracted by Diffusive Gradients in Thin Films Technique (DGT) and BCR and the releasing regularities were determined in sediments to obtain the applicability of predicting sediment quality guidelines.

贵州具有喀斯特碳酸盐岩地质特征，土壤和沉积物中镉(Cadmium)地球化学背景值为0.31×10-6，是中国地球化学丰度值的2.5-3.5倍，这种地球化学异常区域镉在土壤中的运移，在河流沉积物中积累和释放过程，以及镉污染的环境生态风险值得研究。选择典型地球化学异常区土壤和沉积物为研究对象，通过现场调查、淋溶试验以及形态分级等手段，研究土壤镉地球化学特征及成因，解析土壤镉深层富集的过程、迁移速率和影响机制，利用潜在风险指数和地累积指数评估土壤镉的环境生态风险；研究沉积物镉的空间分布规律，利用隆线溞进行生物毒性试验，将隆线溞暴露于河流沉积物镉释放的水体中，研究隆线溞趋光指数与水环境中Cd分布的相关性，用薄膜扩散梯度技术(DGT)和BCR 形态分析法同时测定沉积物中Cd的有效形态和释放规律，探索沉积物中镉的生物有效性及其影响因子，补充和完善沉积物质量基准值测定与环境生态风险评估的研究方法体系。

（1）贵州省碳酸盐岩地区土壤镉（Cd）有较强的活性和迁移特性，地球化学高背景与工矿业、农业等污染叠加，具有一定的环境生态风险。. 研究区域土壤中Cd的总含量变化幅度为0.21~1.81 mg kg-1，Cd平均含量远高于世界土壤背景值、我国土壤背景值和我国主要农业土壤背景值，土壤Cd富集较为严重。土壤Cd以易还原态>酸可交换态>可氧化态>残渣态，易还原态平均为54.10%，活性较强。Cd在土壤剖面中较易迁移，主要受土壤pH、有机质、质地等因素影响。在工矿业污染区，Cd会随有机质在剖面迁移，在未污染区，Cd会附着在土壤粘粒和铁锰氧化物上向下迁移。根据主成分分析等初步推断，研究区域土壤Cd的地球化学异常成因主要是分散的金属矿床点源风化，其次是碳酸盐岩风化产生的，同时受到工矿业开发、农业利用等污染叠加的影响。. （2）喀斯特湖泊沉积物中Cd含量高，主要由有机胶体和细矿物颗粒带入水体。沉积物pH较高，Cd以酸可交换态为主要赋存形态。. 贵州安顺普定夜郎湖表层沉积物中Cd的平均值为2.87 mg kg-1，远高于中国水系沉积物平均值，沉积物Cd在垂直方向随深度增加呈减少趋势，pH的变化幅度在7.22~8.77。沉积物中Cd以酸可交换态为主，占总量的比例为39.21%~76.51%。沉积剖面中Cd含量与pH、有机质含量、粘粒和粉粒含量都呈现显著(r<0.01)正相关，表明镉是随有机胶体和细矿物颗粒进入水体的。. （3）喀斯特湖泊沉积物Cd的沉积速率大于释放速率，沉积物中Cd的生物毒性小，生态环境风险很低。. 薄膜扩散梯度技术（DGT）研究表明，沉积物孔隙水中Cd为0.247~3.995 µg L-1，与沉积物中Cd比值均小于1，表明沉积物中Cd从固相到液相的释放速率小于Cd在孔隙水中的扩散速率。利用蚤状溞进行生物毒性试验，Cd2+对蚤状溞的48 h的半致死浓度LC50为0.50 mg L-1，安全浓度SC为1.2×10-3 mg L-1。模拟沉积物中在固液两相的分配比，推导出沉积物的安全阈值为103 mg kg-1，以此来评价，夜郎湖水环境的生态风险很低。