Source Analysis Of Soil Heavy Metals In Small Watershed Of Northwestern Guizhou Based On Chemical Mass Balance Method

The northwestern part of Guizhou is a typical superimposed area of heavy metal geological background and exogenous pollution,at the same time,its soil pollution is serious.Source analysis of soil heavy metals can provide theoretical basis and data support for regional soil heavy metal pollution control.The typical polluted watershed Jinzhong(JZ)and the control small watershed Lushan(LS)were selected for 3 consecutive years of research.Collected soil samples from agricultural land,wasteland,forest land as well as rock and fertilizer samples.Atmospheric deposition,TSP,crop,and surface runoff samples were collected in four seasons.By detecting their Cd,Hg,As,Pb,Cr,Cu,Ni,Zn contents,the distribution characteristics,input and output fluxes of heavy metals were studied.The chemical mass balance method(CMB)combined with principal component(PCA)and positive definite matrix factor(PMF)were used for soil heavy metal source analysis.(1)The contents of Cd,Pb,Zn in the soil of JZ were 6.93,305.91,612.21 mg/kg,which were 1.35,5.75,2.58 times that of LS.While the contents of other heavy metals were relatively low,and most of them exceeded the regional soil background values.Generally,the heavy metals were in the order of agricultural land>wasteland>forest land.Affected by the pollution source,the distribution law of soil heavy metals in JZ decreased with the increase of the distance from the pollution source,while the LS showed a trend of high in adjacent residential areas and low-lying areas.(2)The total atmospheric deposition and heavy metal content were high in winter and near pollution sources,while low in summer and far away from pollution sources.The annual total dry deposition of JZ and LS were 102.96 and 35.88(t/km~2)·yr.The atmospheric dry and wet deposition of Cd,Pb,Zn in JZ were 50.36,4763.92,6764.29 mg/kg and 7.89,71.75,309.39μg/L,respectively,which were 2.64,3.50,8.84 times and 5.52,2.44,2.23 times that of LS.For the human health risk assessment of atmospheric TSP by respiratory exposure route,As in JZ and LS atmospheric TSP had a certain risk.(3)Based on inputs such as atmospheric deposition and agricultural inputs,as well as outputs such as crop harvesting and surface runoff,almost all soil heavy metals in the two small watersheds were net inputs.The net input of heavy metal in the soil of JZ was much larger than that of other regions.The net input of Cd,Pb,Zn were 91.03,4987.01,8769.38 g/ha/yr,which were 11.34,7.87,9.01 times that of LS.Its environmental risk was high.(4)Comparing the results of PCA and PMF models,the analytical results of CMB model were relatively accurate.The soil heavy metals in JZ mainly from agriculture,historical industry,geology,in-production industry,dust,traffic,as well as the contribution rates were 4.9%,45.2%,26.0%,16.0%,4.8%,and 3.1%.About LS,they mainly from agriculture,geology,dust,traffic,domestic combustion,and the contribution rates were 15.5%,61.1%,9.7%,2.4%,and11.3%,respectively.The background values of soil heavy metals such as Cd,Pb,Zn in the karst area of northwestern Guizhou are high.Due to the influence of metal smelting and historical zinc smelting,the soil pollution is seriously superimposed.At this stage,atmospheric deposition of heavy metals caused by metal smelting and dust is serious.Therefore,it is necessary to control the pollution source,at the same time,carry out ecological restoration to reduce soil erosion and dust,thereby reducing environmental impact.