沘江流域农田土壤重金属污染特征及原因剖析

以沘江流域农田土壤为研究对象,通过土壤采样测试分析,运用单因子评价法、内梅罗综合污染指数法和相对污染指数法,分析了沘江流域农田土壤重金属As、Cd、Pb和Zn的污染特征及原因。结果表明:按照国家土壤环境质量标准(Ⅱ),沘江流域农田土壤中Cd大范围严重超标,其次是Pb和Zn,而As没有超标;土壤单因子污染指数CdZnPbAs,农田土壤广泛受到重金属Cd的极重污染,大部分农田土壤受Zn重污染,Pb中度污染且集中在矿区2号段,而As仅对2号段少部分农田土壤有中度污染;内梅罗综合污染指数显示,农田土壤内梅罗综合污染指数达到重污染等级;相对污染指数显示,农田土壤As、Cd、Pb和Zn相对污染指数均超标,超标率PbZnCdAs,说明兰坪铅锌矿的开发对土壤污染影响最大的是Pb和Zn。沘江流域农田土壤中重金属污染不仅由金顶铅锌矿开发所造成,而且与其高背景值也有关系。沘江上游到下游沿岸农田土壤重金属污染空间变化显著。矿区上游土壤的重金属单因子污染和综合污染均最低,离矿区最近的土壤污染最高,离矿区稍有一定距离的土壤污染迅速下降,中游的4号段污染有一定升高,到5号段云龙县城附近污染又逐渐上升,但上升幅度较小。沘江流域农田As、Cd、Pb和Zn污染之间的相关性显著,同源的概率非常大,存在4种重金属复合污染的危险。

Characteristics and Reasons of Heavy Metal Pollution in Farmland Soils in Bijiang Watershed

With farmland soil along Bijiang watershed as study object,the pollution characteristics and reasons of As,Cd,Pb and Zn were analyzed through soil sampling and testing analysis and by applying single-factor pollution index(SPI) evaluation,Nemerow composite pollution index(NCPI) evaluation and relative pollution index(RPI) evaluation.The results indicated that in accordance with Environmental Quality Standard for Soils(II),the content of Cd contained in the farmland soils has severely exceeded the standard in a large scale,followed by Pb and Zn.However,the content of As is maintained within the specified standard;The SPI values of soils are in the following sequence: Cd>Zn>Pb>As.The pollution level caused by the heavy metal "Cd" to the farmland soils is extremely heavy in a wide range,and a majority of the farmlands are heavily polluted by Zn.The farmlands with moderate pollution by Pb are centered at Plot 2 in the deposit,and only a few farmland soils are moderately polluted by As at Plot 2 in the deposit;Based on the NCPI,the results indicate that the NCPI of the farmland soils has reached to the degree of heavy pollution;It is indicated based on the RPI evaluation that the RPI values of As,Cd,Pb and Zn contained in the farmland soils have exceeded the standard in the following sequence: Pb>Zn>Cd>As,which illustrates that during the development of Jinding Lead-Zinc Deposit in Lanping County,the heavy metals imposing the most profound influence on the soil pollution are Pb and Zn.The heavy metal pollution in the farmland soils from the upper reaches to the lower reaches of the Bijiang watershed is not only caused by the development of Jinding Lead-Zinc Deposit in Lanping County,but is also associated with its high soil background value.There is a remarkable spatial variation of heavy metal pollution in farmland soils from the upper reaches to the lower reaches of the Bijiang watershed.Both the SPI and the NCPI values of heavy metals in the soils within the deposit at the upper reaches of the Bijiang watershed are the lowest;the pollution index of the soils closest to the deposit are the highest,and the pollution index of the soils with a certain distance from the deposit drops swiftly;the pollution index of Plot 4 rises to a certain degree at the middle reaches,and gradually ascends near the Yunlong County seat at Plot 5,however,with a comparatively small growth rates.The correlation among the pollution caused by AS,Cd,Pb and Zn to the farmlands along the Bijiang River is outstanding with an extremely high possibility of homology,and there exists the risk of combined pollution caused by four heavy metals.