| Land quality in farmland area is closely related to chemical composition and metal element enrichment in topsoil,especially in wet areas with abundant precipitation in China.How the water cycle conditions of surface-underground runoff affect the Migration-Accumulation of metal elements in soil,and then form the regional difference characteristics of soil element content distribution in different units on watershed scale,and what are the dominant influencing factors and mechanisms.It is of great significance to clarify these scientific issues for the delimitation and protection of permanent basic farmland in humid areas of China.This study focused on the farmland distribution area of the national important commodity grain base in Nanling County,Wuhu City,Anhui Province.Based on 2379 groups of surface water,soil water and groundwater samples,5803 groups of surface soil samples(0-20 cm depth),1962 groups of aerated zone soil samples(0-60 cm depth,10 cm spacing)and field investigation and monitoring data.Through the study of the influence characteristics and mechanism of water circulation conditions such as formation lithology,aeration zone permeability,runoff hydraulic gradient,water potential elevation,phreatic water level burial depth and upstream inflow flux on the distribution differences of elements such as Zn,Cu and Cd in soil,the following main innovative understandings were obtained.1.A zoning method was established to study the effects of watershed-scale water cycle conditions on the distribution of elements in soils.The divisional research unit based on water circulation system was divided.The water circulation conditions of surface-subsurface runoff in different units were identified,including the lithology and permeability of aeration zone,groundwater burial status,groundwater dynamic field system composition and units,upstream inflow flux,water potential elevation and hydraulic gradient.The distribution of Zn,Cu,Fe,Cd,Cr,Pb,Hg and As and available elements such as Zn,Cu,Cd in soils of different units were studied.The above laid a foundation for the study of the characteristics and mechanism of the influence of water cycle conditions on the difference of soil element distribution.2.It was clarified that the contents,abundance and potential ecological effects of Zn,Cu,Cd,Cr,Pb,As,Hg elements in surface soils of different zones from upstream to downstream of the same river basin were obviously different,with obvious selective aggregation characteristics.The order of variation degree of these metal elements in soil was as > Cd > Hg > Zn > Cu > Pb > Cr.The spatial variability of Zn,Cu,Cd,Pb,Hg and As in the upper hilly-hilly region(unit IV,V)was greater than that in the lower plain region(unit I,II).In hilly-valley region(unit III)with large surface-underground runoff flux,the distribution of these heavy metals in soil also shows obvious zonal differences in the upper,middle and lower reaches.3.It was found that the water circulation conditions,such as lithology,stratigraphic structure and permeability,hydraulic gradient of runoff,water potential elevation,buried depth of phreatic water level and inflow flux of upstream water,had significant effects on the enrichment of elements such as Zn,Cu and Cd in soils of humid areas.The contents of Zn,Cu and Cd in soil increased with the increase of clay mineral proportion(permeability decreased)and decreased with the increase of runoff hydraulic gradient.In the depth of groundwater level < 2.0 m area,the lower the groundwater depth,the higher the contents of Zn,Cu and CD in the surface soil.The lower the plain area,the higher the content of Zn,Cu and CD in the soil corresponding to the unit volume water flux.4.The mechanism of the effects of the water cycle conditions on the enrichment of Zn,Cu and Cd in soil was revealed.There are many factors "coupling enhancement"(1+1>1)in the area far from the slope-flood source influence area,and there are many factors "coupling weakening"(1+1<1)in the area of slope-flood source influence area.In the downstream plain area,the "1+1>1" effect of many factors coupling is significant,and the correlation is strong.The coupling effect of the lithology of aeration zone,the proportion of clay minerals affecting soil permeability and the upstream inflow flux showed the "coupling enhancement"(1+1>1)effect.In the upstream low-mountain-hilly area,the coupling effect of many factors is significant and the correlation is poor.The coupling effect of water potential elevation and groundwater level burial depth shows a "coupling weakening"(1+1<1)effect.5.Using different evaluation methods,the result shows the comprehensive potential ecological risk effect of metal element content in surface soil is generally "moderate" in the study area.The percentage of potential ecological risk distribution in each zone is the lower part of the plain(unit I)> the upper part of the plain(unit II)> low mountain-hilly area(unit IV?V).The downstream plain area should be a key area of prevention and control,mainly based on Cd and Hg ecological risk prevention and control.In particular,there are more potential ecological risk areas such as Cd,As and Hg in the soil of the lower reaches Qing Yi jiang River Basin.In the middle and upper reaches,we should strictly control the entry of Cd,Hg and As pollutants from human activities into surface and underground water.It is not suitable to solve the problem of lack of K and P content in the soil of this area by increasing the application amount of K and P fertilizer,which will affect the centralized supply of drinking water supply conservation function in this area. |
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