Heavy Metals Fluxes And Their Source And Sink Patterns In A Small Watershed Of Minjiang River

Transformation of heavy metal pollutants in the eco-system is a very complicated process, which is affected by both natural factors and human factors. Heavy metals could concentrate on aerosol particles in the atmosphere, enter surface of soils and rivers through dry or wet sinking,, or into the atmosphere by dust. Plant could absorb heavy metals in the soil, but then return to soils through volatile process, litter decomposition or metabolism of the food chain. These processes act each other. There is a large population in Minjiang river downstream, where has developed social economy, and supply a lot of main food, vegetables, fruit. Wutongqiao district locates in the lower reaches of minjiang river, and also locates in the edge of the Sichuan basin, a typical hilly plain area, where small watershed are widely distributed. Du to the densely population, developed economy, the environment of heavy metal has exceeded the critical value of the environment, and negative effects become increasingly obvious. Thus, the non-point source pollution of heavy metals should be managed urgently. Research on flux of heavy metal in the typical watershed Wutongqiao district is of great significance to understand ecological process of heavy metals in ecological system, and to provide the scientific basis and optimization measures for building area of non-point source pollution control and pollution of agriculture and forestry ecological restoration experiment in Sichuan province.Through sampling in typical watershed in Wutongqiao district section of to know dry and wet deposition or fertilizer, we have obtained the dynamic data on heavy metal content of atmospheric dry and wet deposition, and the input amount of heavy metal originated from fertilizer and pesticide. Secondly, by consecutive sampling of surface runoff, subsoil flow and the main crops in catchment areas, annual output of heavy metals was analyzed, then the differences between land use types in such parameters were compared. Finally, through the above analysis, the key factor affecting heavy metal flux in watershed was found out, regression model of the heavy metal element "source" and "sink" was established based on principal component analysis (PCA). Main results are as follows:(1)The annual precipitation in the catchment was 1202.42 mm, mean pH in precipitation was 5.37, mean contents of NO3--N and NH4+-N were 1.14 mg·L-1 and 1.25 mg·L-1, respectively. Contents of five kinds of heavy metals in winter were significantly higher than those in summer, and those in spring was slightly higher than those in autumn. Heavy metal contents in precipitation increased with the decrease of pH, and there was a linear positive correlation between the nitrogen deposition and heavy metal contents, and as well reached significant or extremely significant level. Total deposition of heavy metals in atmosphere was 32631.83 g·a-1. The As, Cd, Cu, Pb and Zn content was 614.86 g-a"1,358.16 g·a-1,1000.44 g·a-1,3113.42 g·a-1,27544.95 g·a-1, respectively.(2)Total input of heavy metals due to pesticide and fertilizer was 5755.13 g·a-1, annual input of heavy metals in dry land was 5755.13 g·a-1, but in paddy field was 5455.38 g·a-1, the input of dry land was slightly higher than that in paddy field.(3)Total annual output of runoff in the catchment was 8075.16 m3, the annual output of heavy metals was 2167.81 g·a-1, the total output of As, Cd, Cu, Pb and Zn was 24.94 g·a-1, 11.92 g·a-1,36.34 g·a-1,82.75 g·a-1,2011.86 g·a-1, respectively. Compared with the atmospheric deposition, the total output of heavy metal contents was accounted for 6.64% of the input through the atmospheric deposition.(4)Surface flows in the different land use types increased with the atmospheric precipitation in the study area, the seasonal variation were summer>spring>autumn> winter. The order of surface flow was dry land>Eucalyptus grandis plantation> Metasequoia glyptostroboides plantation>Cunninghamia lanceolata plantation> broad-leaved mixed forest, which showed that the effect of interception of atmospheric rainfall in forest ecosystem was better than that in dryland, broad-leaved mixed forest had the best water retention and intercept effect. Heavy metals output of surface flow in Metasequoia glyptostroboides plantation, Cunninghamia lanceolata plantation. Eucalyptus grandis plantation, broad-leaved mixed forest and dry land was 388.26g,394.60 g,867.08g,319.08 g,668.21g, respectively, and the order of output was dry land> Eucalyptus grandis plantation> Metasequoia glyptostroboides plantation>Cunmnghamia lanceolata plantation> broad-leaved mixed forest, indicated that there was a stronger adsorption and absorption of heavy metals in atmospheric precipitation in forestland, and broad-leaved mixed forest had the strongest adsorption and absorption capacity, dryland had showed the weakest effect.(5)The order of heavy metal outputs in main crop in catchment was Oryza sativa> Zea mays> Zingiber officinale> Glycine max> Cucurbita moschata> Saccharum officinarum> Capsicum annuum> Benincasa hispida:the order of output of heavy metals in whole plant was as followed:Oryza sativa> Zea mays> Glycine max> Zingiber officinale> Cucurbita moschata> Capsicum annuum> Saccharum officinarum> Benincasa hispida:the order of output of heavy metals in grain was Zingiber officinale> Zea mays> Glycine max> Oryza sativa> Cucurbita moschata> Saccharum officinarum> Capsicum annuum> Benincasa hispida. Main crops'grain (except soybean) showing enrichment and transfer ability for heavy metals in the study area was not strong. The enrichment coefficient and translation coefficient of Cd, Cu, Pb in Zea mays, Cd in Glycine max and Benincasa hispida was more than 1, respectively. Such results indicated that these plants exhibited important characteristics of hyper-accumulators. It was worth to notice that both the enrichment coefficient and transfer coefficientof Znin Glycine max grain were more than ?, showed it ability of enrichment Zn.(6)The order of soil bulk density in catchment was Eucalyptus grandis plantation> Metasequoia glyptostroboides plantation> dry land> Cunninghamia lanceolata plantation> broad-leaved mixed forest. The order of total porosity was broad-leaved mixed forest> Cunninghamia lanceolata plantation> Eucalyptus grandis plantation> Metasequoia glyptostroboides plantation> dry land. There was a good adsorption effect for As, Cd, Cu and Zn in the soil of 4 different forest types, but Pb was leached out. The order of the total interflow was dry land> broad-leaved mixed forest>Cunninghamia lanceolata plantation> Metasequoia glyptostroboides plantation> Eucalyptus grandis plantation. Seasonal change of interflow in different land use types was summer> spring> autumn> winter. The order of heavy metal outputs in interflow was diy land (896.43 g)> Eucalyptus grandis plantation (500.53 g)> broad-leaved mixed forest (392.12 g)> Cunninghamia lanceolata plantation (357.67 g)> Metasequoia glyptostroboides plantation (250.69 g).(7)The order of heavy metals in total flux in catchment was dry land> Eucalyptus grandis plantation> broad-leaved mixed forest>Cunninghamia lanceolata plantation> Metasequoia glyptostroboides plantation. Based on principal component analysis, the factors of the source and sink can be attributed to two principal components, natural factors and human factors. Combining with the results of regression model, we know that As is mainly affected by runoff of export, fertilization, interflow. Cd is mainly affected by harvesting crops, atmospheric deposition, fertilization. Cu is mainly affected by interflow, atmospheric deposition, runoff of export. Pb is mainly affected by fertilization, atmospheric deposition, surface flow. Zn is mainly affected by interflow, atmospheric deposition, surface flow.