广西环江铅锌矿尾砂坝坍塌对农田土壤的污染及其特征

广西环江县因铅锌金属矿区尾砂坝坍塌导致大面积农田污染甚至绝收.为此,对矿区下游污染区和非污染区的农田土壤、尾砂和河流沉积物进行了系统调查和研究.调查结果表明,农田遭受As、Pb、Zn和cd污染,土壤酸化严重,pH值最低至2.5,全硫含量高达2.29%.X-衍射鉴定结果表明,受污染土壤中存在大量硫铁矿,这是导致土壤酸化的主要物质.由于强酸性淋溶作用的影响,污染农田中La、Ce和Nd等稀土元素发生明显的向下淋溶现象,导致表层土壤La、Ce和Nd元素含量明显低于未污染农田.从土壤剖面分布来看,污染点的土壤中As、Pb和Zn仍主要集中分布在表层0～30 cm范围,发生土壤酸化现象的土层深度仍局限于0～70 cm范围.

Pollution of agricultural soils resulting from a tailing spill at a Pb-Zn mine:A case study in Huanjiang,Guangxi Province

The collapse of the tailing dam in the Beishan Pb/Zn Mine of Huanjiang, Guangxi Province led to the spread of mining waste on the farmland along the Huanjiang River. Samples of polluted soil, unpolluted soil, mine tailing and river sediments were analyzed for pH, S content, heavy metals and rare earth elements in surface soil and soil depth profiles to assess the level of soil pollution. The results show that the polluted soils had increased concentrations of As, Pb, Zn and S and decreased pH, La, Ce and Nd. The maximum concentration of total soil S increased up to 2.29 % and the initial oxidation of metal sulphides from pyrite residues reduced the soil pH to extremely acidic (pH=2.5). Five years after the spill, there were still high concentrations of FeS2 in the soils, as identified by X-ray diffraction. The lower concentrations of La, Ce and Nd in the polluted soils than in the unpolluted soils resulted from leaching to deeper layers of the soil profile under the acidic conditions. The elevated concentrations of As, Pb, Zn were only found in the upper soil layer, up to 30 cm deep, indicating that the leaching of heavy metals was minimal under the local subtropical conditions. The maximum depth of decreased soil pH, resulting from spills of tailings containing pyrite, was in the upper 70 cm of the polluted soils.