Researches On Chemical Behavior And Influencing Factors Of Heavy Metals In Soil And Corps Of Xiang'an, Xiamen

Xiang'an is the key farmland base of Xiamen. Soil samples collected from Xiang'an were digested and the contents of heavy metals were detected by ICP-MS. Ranges and average values of concentration of heavy metals we have got are: Zn(45.520～229.080 mg/kg,153.800 mg/kg), Pb(24.372～111.504 mg/kg,73.258 mg/kg),Cr(6.249～134.400 mg/kg,48.504 mg/kg), As(5.783～56.05230 mg/kg,27.092 mg/kg), Cu (4.573～46.032 mg/kg,18.399 mg/kg), Ni(1.870～5.696 mg/kg,9.606 mg/kg), Cd(0.024～0.313 mg/kg,0.174 mg/kg), Hg(0.002～1.261 mg/kg,0.108 mg/kg). The content sequence of these elements is: Zn>Pb>Cr>As>Cu>Ni>Cd>Hg. The concentrations of the investigation area are accord with the first environmental soil quality criterion of China on the whole. So the soil can be considered as non-polluted soil. The column sample was from paddy field in this research. It can be seen from the research results that, heavy metals enrich mainly in the depth 10-15 m, not in 0-5 cm or 5-10 cm. Deep-ploughing of the soil and the inundation of surface layer(0-10 cm) can make the elements run off and eluviate, which may cause the decreasing of the contents of elements. Below the depth 25 cm, the contents of heavy metals are stable.For the main crops in Xiang'an farmland, the concentration of Ni is in the range 0.002～0.147 mg/kg, the average concentration is 0.081 mg/kg, For Pb, the concentration range is 0.004～0.556 mg/kg, the average concentration is 0.208 mg/kg. For Cr, the range is 0.169～1.428 mg/kg, average value is 0.742 mg/kg. For Zn, the range and the average value are 0.709～12.425 mg/kg and 6.161 mg/kg respectively, and for Cd is 0.001～0.138 mg/kg and 0.037 mg/kg. Other than some exceptional kinds of vegetables, As and Hg can not be detected. Content of heavy metals in crops is as following: Zn>Cd>Cr>Cu>Pb>Ni>Hg>As. Enrichment index was calculated by comparing with contents of heavy metals in soils, the index ranges and the average indexes for the metals are listed as below: Ni: 0.009～0.056, 0.032, Pb: 0.002～0.512, 0.056, Cr: 0.006～2.300, 0.386, Cu: 0.037～0.752, 0.326, Zn: 0.321～1.444, 0.759, Cd: 0.008～10.570, 2.856. Concentrations of heavy metals in rainfall and irrigating water of Xiang'an are almost under theⅠnation water environmental quality criterion of surface water. Contents of Cr and Ni are lower than in rainfall, which is diluted by precipitation. Total fluxes per year by atmosphere subsidence are: Cr: 23.2 g/(hm2·a), Ni: 0.44 g/(hm2·a), Cu: 12.1 g/(hm2·a), Zn: 1076 g/(hm2·a), Cd: 1.88 g/(hm2·a), Pb: 34.2 g/(hm2·a). For Zn,Cd,Cu and Ni, wet subsidence is the dominant, and for Cr and Pb is dry subsidence.Sequential extraction was adopted to analyze the fractionation (exchangeable forms, carbonates forms, Fe-Mn oxide forms, organic forms, residual forms) of Cd,Cr,N,Pb and Zn in the soil. Residual forms of all the metals are all above 30% except for Pb and Cd. Organic forms of Pb and Cu is the highest in all the metals, and the lowest is Cd (12.1%). Fe-Mn oxide forms is not the main form, only for Cd the proportion is 36.7%, for other metals the proportions are all below 15%. Carbonates forms and exchangeable forms of Cd is 9.7% and 41.5% respectively, while the proportion of these two forms are relatively low for other metals (all below 4%). Potential ecological risk research was done by Hakanson potential risk index method. The risk for the eight heavy metals is: Cd>Hg>As>Pb>Cu>Ni>Cr>Zn. From the results we can come to the conclusion that, the soil of Xiang'an is exposed to the strong pollution of Cd and the moderate pollution of Hg and As. The potential risk index for other metals is relatively small. The general potential risk index is 316.8, so the ecological risk can be defined as moderate risk.Soil fertility and physicochemical environment of Xiang'an are analytic determination in the research. Content range of organic matter of farmland soil in Xiang'an is 1.3459%～4.1749%, and the average content is 2.3994±0.59%. Total N content is 0.9420～1.1120 g/kg, and the average content is 0.9797 g/kg. Total P content is 0.4073～1.7879 g/kg, and the average content is 0.9842 g/kg. Fertile soils make up a majority of farmland of Xiang'an. When pH is lower than 6.5 and during 6.5～7.5, heavy metal total amount are compared. It indicates that heavy metal total amount in neutral soils are higher than that in acidic soils. In these soils from Xiang'an district, sand class are composed of non-clay mineral with most quartz. Coarse components have most feldspar and white mica, when the fine components have most heavy mineral.The aim of this research is to provide scientific bases for the prevention and management of the heavy metals pollution in the soil of Xiang'an. By the analysis and assessment, we can evaluate the security of crops, forecast the transformation trend in the future and make constructive advices for the government to put forward steps to protect the limited soil resource and to improve economical worth of agriculture products. It will be of great practical importance for effectively exploiting and utilizing farmland.