喀斯特地区典型风化剖面重金属超标程度及元素迁移特征研究

我国西南喀斯特地区是著名的重金属元素地球化学异常区。除了人类活动所导致的土壤污染问题以外,地质背景很可能是造成土壤重金属超标的重要因素。为了弄清岩石风化和成土过程中重金属的释放规律及其环境效应,本次研究以贵州省罗甸县典型的碳酸盐岩和碎屑岩风化剖面为载体,采用实地调查、地球化学测试分析等手段,结合质量平衡系数的计算方法对研究剖面的重金属元素超标程度及元素迁移规律展开调查。研究结果表明:碳酸盐岩比泥质岩具有更高的重金属元素释放潜能,其风化和成土过程向环境中释放重金属的量与风化壳的发育程度有关。研究区石灰土剖面重金属元素超标程度严重,且以Cd和As的超标程度最大。地带性黄壤中Cr、Ni元素的富集很可能与剖面发育过程中铁氧化物的沉淀富集有关。综合分析认为岩性不同所导致的风化速率的差异很可能是造成碳酸盐岩和泥质岩剖面重金属淋失程度差异的主要原因。而且,重金属元素在风化剖面中的迁移规律受元素自身地球化学性质、母岩类型和风化壳发育程度等多重因素控制。

Excessive Degrees and Migration Characteristics of Heavy Metals in Typical Weathering Profiles in Karst Areas

The karst area in southwest China is a famous geochemical anomaly region of heavy metals. Besides human activities, the regional geological background could be another significant factor for which results in excessive heavy metals in soils. In order to figure out releasing rules and environmental effects of heavy metals during weathering and pedogenesis processes of rocks, investigations of excessive degrees and migration rules of heavy metals in representative weathering profiles of carbonate and clastic rocks in the Luodian County, Guizhou, were carried out using field surveys, geochemical analyses and mass balance calculations. The results show that, compared to clastic rocks, carbonate rocks have higher potential to release heavy metals, and that the amount of heavy metals released from carbonate rocks by either weathering or pedogenesis process is closely related to the weathered crust development degree. In the study area, concentrations of heavy metals in calcareous soil profiles exceed the national standards remarkably, especially of Cd and As. The enrichments of Cr and Ni in yellow soils are probably associated to the precipitation and enrichment of iron oxides during the development processes of weathering profiles. The difference of weathering rates caused by different lithology is probably the main reason for the difference of heavy metal leaching degrees between carbonate rocks and mudstone profiles. Moreover, the migrations of heavy metals in weathering profiles are controlled by their geochemical properties, parent rock types and development degrees of weathered crusts.