| Heavy metal pollution has become an important factor in the soil environment and directly or indirectly endanger human health. Tongling mine area is one of the typical skarn type Cu-Au Mining district in the World, which has been exploited for 3 -thousand-year. With the long-time exploitation of mineral, harmful heavy metals inter into the soil through a variety of ways, such as atmospheric deposition, industrial wastewater and domestic sewage discharge, industrial solid waste , municipal solid waste and pesticide chemical fertilizer, the most of these pollutants focus on the soil surface or topsoil, but with time, the contaminants in the soil will be moved widely and the chemical speciation transform into each other.In this article, we have researched total content and chemical fraction content of heavy metal Cu, Cd in study area. The content of heavy metals in surface soil changed greatly and the variation is also relatively large: the mean amount of Cu is 98.5 (mg kg-1), higher than the background value in study area (32.15 mg kg-1), The chemical fractions is mainly in residual fraction, organic fraction, and followed by Fe-Mn oxidation fraction, the mean amount of Cd is 3.45 (mg kg-1), far higher than the background value in study area (0.09 mg kg-1), maximum up to 24.6 (mg kg-1), organic bound fraction is the minimum (0.51 mg kg-1), followed by exchange fraction (0.77 mg kg-1).According to difference of labile fraction content of heavy metals between clean and contaminated areas, we have estimated anthropogenic input of heavy metals. The results showed that: the content of input of Cu is range from 0μg/g to 458.03μg/g, with average content of 54μg/g; the content of input of Cd is range from 0μg/g to 21.70μg/g, with average of content.63μg/g; and some time, using profile analysis method, we also have analyzed heavy metals Cu, Cd fraction migration characteristics, the results showed that: heavy metals Cu, Cd mainly in exchangeable fraction, carbonate fraction and organic matter fraction migrate.According to delayed geochemical hazards model, we have researched the soil Cu and Cd elements delayed geochemical characteristics in study area The results show that: the relationships between TCASE+C and TRCPE+C+F+O,TCASE+C+F and TRCPE+C+F+O fit mathematical model of delayed geochemical hazard better, there is the possibility of the outbreak of soil delayed geochemical hazard in study area. For the Cu element, in the pH of 5.0,5.0~2.0, burst critical point of delayed geochemical hazards is 222.22,81.68 mg·kg-1 respectively and burst point of delayed geochemical hazards is 438.75,384.99 mg·kg-1 respectively; at the same time, for the Cd element, in the pH of 5.0,5.0~2.0, burst critical point is 11.25,9.7 mg·kg-1 respectively and burst point is 14.61,24 mg·kg-1 respectively. As the acidity increasing, the size of the region with probability of the outbreak of Cu-based delayed geochemical hazards enlarged, burst critical point and burst point are ahead of schedule, while soil quality evaluation process in the lower soil Cu content also need to attract sufficient attention; the sensitivity of Cd on the acidity level less than Cu, with the degree of acidity to enhance its burst critical point is slightly ahead of schedule as well as burst point slight delay and the outbreak of Cd-based delayed geochemical hazards have relations with soil physical and chemical conditions,bio-availability etc. The outbreak of soil delayed geochemical hazard of Cu, Cd have not been yet in study area. The main reasons for possibility of the outbreak of Cu-based and Cd-based delayed geochemical hazards are the accumulation of long-term sustainability entry of soil heavy metals,soil acidification and diminishing environmental capacity in study area.
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