Soil Heavy Metal Process Of Migration-deposition And Ecological Risk Assessment Of Small Watershed Under Ecological Construction

This study takes the controlled basin of typical check dam in the loess hilly-gully region as the research object.It uses radioisotope tracer technology to analyze the change of soil erosion on the slope,reveals the deposition process of the check dam,makes clear the spatial distribution characteristics of the soil heavy metals in the watershed,and discusses relationship about the change of the slope erosion rate and the heavy metals.It reveals the migrated law of heavy metals in the surface soil under the effect of erosion,analyzes the source of heavy metals in the sediment,the law of historical change and the inversion of the environmental changes in the watershed,the risk assessment of the redistribution of soil heavy metals in small watershed is evaluated.The main results of this paper are as follows:(1)By analyzing the spatial distribution characteristics of soil heavy metals under different land use type,the results show that the content of Mn,Zn,Co,Ni and Cu was higher in the shrubland,the content of Cr and As in the sloping farmland was higher;Land use patterns had a greater impact on the contents of Mn?Ni and As in soil 0-20 cm,Zn?Cr and Cu in soil 20-40 cm.The sloping farmland is "strong erosion" on the whole,shrubland and Grassland are "mild erosion".Under erosion,Cr,V,As and Co in surface soil are easy to slope migration and loss,Mn,Cu and Ni are easy to deposit,and heavy metals in the soil are influenced by human activity and erosion process,resulting in the change of their distribution.(2)By analyzing heavy metal content in sediment of check dam.The average content of the heavy metals was above the front of check dam > middle of check dam > behind of check dam,the content of Cr,Co,Cu and As in some samples exceeded the background value.Using 137 Cs dating technique determines the sedimentary age of the swirling layer of sediments.The variation trend of heavy metal content with time decreases first and then increases as a whole.The main sources of V,Mn,Co,Ni,Cu and Zn are natural sources,the sources of Cr,As and Cu are mainly agricultural production activities.(3)The relationship between the heavy metal content and pollution index of sediment and the deposition time of the rotation layer is established,and retrieves the historical change process of the ecological environment.It is divided into three stages,the first stage is non governance phase(from 1956 to 1965),because the less harnessing area of water and soil erosion in the basin,the vegetation coverage is low and the rainfall is more concentrated.The second stage is the effective governance phase(from 1965 to 1980),the soil erosion intensity was reduced by sloping terraced fields and forest restoration in the watershed.The third stage is a period of severe change(from 1980 to 1990),it is subject to the "land contract responsibility system",the impact of heavy vegetation destruction and the aggravation of soil erosion in the watershed resulted in the heavy metal content of the check dam increased again.(4)The pollution degree of the heavy metals in the soil before the check dam is more serious than that in middle the check dam and after the check dam,but the sediment is at a "low risk" level on the whole.The pollution degree of Cr and As is relatively high in the slope farmland,and the potential ecological risk grade of each land use type is "low grade".In general,the small watershed is in a relatively healthy and clean state,but the change of heavy metal content in the check soil before the dam and the sloping farmland should be noticed.