Study On The Ecological Risk Assessment And Precaution Of Heavy Metal In The Farming Soil Of Manganese Mining Area Based On GIS Technology

Liancheng manganese mining area in Fujian Province was selected in this study in order to provide a scientific basis for safe use and protection of the farming soil in the area.By sample surveys in the study region,spatial variation and pollution degree evaluation of heavy metal content in the farmland of the mining area were conducted.On this basis,potential ecological risk assessment and precaution of the heavy metal were studied at last.GIS and geostatistics were used in the whole process.(1) The over standard rate of six heavy metals in the study region were 8.64%,16.67%,25.93%,38.27%,73.46% and 87.65%, respectively,and the maximum of them were 109.6 mg/kg,695.1 mg/kg,753.7 mg/kg,1.02 mg/kg,9.04 mg/kg and 12312 mg/kg, respectively.The result suggested that the maximums of all heavy metals were much higher than the standard.The permissible limits of China standard was used for Cu,Pb,Zn,Hg and Cd,and Fujian soil element background value was used for Mn.By correlation analysis,Cu,Pb,Zn,Cd and Mn showed highly significant positive correlation.By principal component analysis of single pollution index of Cu,Pb,Zn,Hg and Cd,the result showed that Cu,Pb,Zn and Cd belonged to the same type of pollution factor,caused by the influences of both background value and mining activities;Hg belonged to a separate type,which caused by the influence of background value.(2) The spatial variation of Cu, Pb, Zn and Cd in the study region were basically the same:Most of them distributed in the middle and east of the study region, and the content of them were generally higher especially in low-lying areas near the two main streams, the reservoir and mining sites.The distribution of Mn in the north of the study region was similar to that of Cu, Pb, Zn and Cd,but the high-cotent area of Mn was much smaller,which mainly distributed in the middle and north of the study region.The distribution of Hg was large,and its content was higher in central and southwest regions.The uncontaminated zone of Cu and Pb individually accounted for 94.91% and 88.20% of the study region,the rest was low degree of contaminated area.The contaminated zone of Zn and Hg individually accounted for 24.43% and 37.14%,most of which was low degree of pollution area.The pollution area of Cd accounted for 94.94%,most of which were moderate and considerable degree of pollution areas.The pollution state was Cd> Hg> Zn> Pb> Cu.According to the integrated pollution status,most of the study region was heavy pollution area,accounting for 45.09%;Second was light pollution area,accounting for 29.82%;Third was medium pollution area,accounting for 13.47%.It illustrated that the heavy metal pollution in the study region was already quite serious.(3) The potential ecological risk assessment result illustrated that Cu,Pb and Zn only showed low degree of ecological risk.The low,moderate and strong degree of ecological risk area of Hg accounted for 62.87%,35.23% and 1.91%,respectively,which showed that most of the study region was at low degree of ecological risk,distributing in the southwest and northeast far away from the entrance to the mine and streams;A small part showed moderate and stronger degree of ecological risk,distributing in settlements and surrounding area. The low,moderate,strong,stronger and strongest degree of ecological risk area of Hg accounted for 12.97%,29.90%,29.04%,24.28% and 3.81%,respectively,which showed that Cd had a high contamination risk,and the moderate degree and above area was up to 87.03% of the study region, with the risk value gradually reducing from the center near mining area and streams to surrounding area.The potential ecological risk of the 5 heavy metals was Cd> Hg> Pb> Cu> Zn.According to the multi-factor potential ecological risk,more than half of the study region was at low ecological risk level and only a very small part was at very high ecological risk.The risk trended to reduce from the central to the surrounding area.(4) 85.68% of the study region was already contaminated,more than half of which was heavy pollution area.In the situation of isokinetic accumulation,the over standard ratio of Cu,Pb,Zn,Hg and Cd would respectively be 58.05%,73.30%,85.10%,98.91% and 99.89%.In the situation of accelerated accumulation,the over standard ratio of Cu,Pb,Zn,Hg and Cd would respectively be 93.42%%,91.86%%,98.56%%,99.87% and 99.92%.In both cases, in addition to the area over standard of Cu,Pb and Zn,the study area that would exceed standard in 100 years most distributed in the central area and near the streams,and the time it would take tended to be longer from center to surrounding.However,the rest that would exceed standard after 100 years most distributed in the southwest and northeast.The area that would exceed standard of Hg in 100 years would be small,and it’s the smallest of Cd.Compared to the situation of isokinetic accumulation,over standard area of each heavy metal would be more in the situation of accelerated accumulation in 100 years.According to this, it would have a direct impact on the heavy metal pollution in the study region in the future,such as the strength of the environmental protection efforts,improvements of mining production process and so on.