Research On Ecological Risk Assessment In Heavy Metal Mining Site

In the thesis, the main heavy metals in soil, water and typical plant in a arsenic -lead mine located in Shangyu city, Zhejiang Province were investigated, and the heavy metal distribution, the disciplines of transfer and transformation and their ecological risk were assessed. The main conclusion as following:1 Horizontal distribution of heavy metals in soil of mining siteThe soil in mining site were contaminated in different extent by heavy metals while As and the Pb were the main pollutants with a climax content of 3768.32mg/kg and 16916.55 mg/kg;The content of heavy metals in soil varied much with the distance from mine and gangue offal with distinct decrease grads;The content in lower altitude is higher than upper altitude, some certain heavy metal accumulated in the soil;the content in paddy soil is lower than that in dry land. A majority of heavy metals content in soil were much higher than background value, especially the As and Cd, much exceed the national quality standard, were the main pollutants in fact, and the others were the potential source of pollution.Ammonium nitrate (NH4NO3) extractable heavy metals can well express their availability, and there is good relativity with total content and ammonium nitrate extractable heavy metals (r> 0.74).2 Portrait distribution of heavy metal in soil of mining siteThe results of heavy metals in soil profile showed that, most of heavy metals were in the surface soil and with portrait transfer and accumulation in different extent, the heavy metal content varied with depth and accumulated in the portrait and got its climax content in the depth of 20-40cm;Chemical species analyses showed that hypo-minerals by mining process, deposit and weathering were the main species and the content of heavy metals in larger particle sizes in same to or even higher than that in smaller particle sizes.The As, Cr, Pb and Zn were mainly in species of organic and residual, and Cu was mainly in species of organic and low content in residual;while the Cd was special for its high exchangeable and Fe-Mn oxide and low residual;though the rate of exchangeable species of heavy metals were low in soil, the ecological risk were high.3 The distribution of heavy metals in water body and sediment in mining siteThe pH value in water body of mining site was between 6.80 and 7.73 and most of it was alkalescence except for the underground water in the distance 300m from mine. The As and Cd in part of underground water, poll and river water exceeded the standard of farmland irrigation. Heavy metals complex pollution was caused by acid wastewater from gangue wethering or leach and mass of small size mineral particle in mineral-washing process;The pH value in sediment was between 7.12 and 7.49 and became lower as the distance increased. The content of heavy metals in sediment was lower as the distance increased and especially the As and Pb.4 Tolerant and accumulate capacity of plant in mining siteThere is obvious diversity of tolerant and accumulate capacity among the plants. In the same plant type, the tolerant and accumulate capacity varied with the distance to the mine, and there were positive correlation between soil heavy metal content and plant accumulation while the soil heavy metal content was below to the plant torelance;The accumulation of heavy metal was mainly in the underground part.5 Ecological risk assessment for heavy metal pollution in mining siteEcology risk index by Hakanson was applied for complete assessment of soil and river sediment in mining site. The results showed: soil was light polluted by Cr, Cu and Zn and potential risk was low;The ecological risk in distance 400m was very high and 400-1500 comparatively high with potential risk, while in distance 1500-3000m with middle potential risk. Sediment was light polluted by Cr, Cu and Zn and with low potential risk;The As and Cd with low content but high potential risk;Pb content was high in the distance 500m from mine but with a high decline rate. In summary, there was high potential risk in the district 900m distant from the mine.