Heavy Metal Pollution Character In Soil-Plant System And Study On Lead Isotopic Tracing

As an important component of the ecological environment, soil is the foundationfor human existence. And soil-plant system is the energy circulation centre forecosystem as well as the bridge connecting organic and inorganic world. Because ofwastewater discharge, irrigation, atmospheric dustfall, mining smelting, pesticidesand fertilizers using, etc., this system is gradually polluted, especially by heavymetals enrichment. Migration of heavy metals in soil-plant systems directlyinfluences plant growth, further more, influences the plant quality and output.It has become a pressing issue to find out the source of pollution and to controlpollution. At present, there are three main methods to identify the source of pollution:statistical method, computer graphics method, and isotopic tracer method. The fronttwo methods mainly make a statistical analysis and quality evaluation for the totalcontent and chemical speciation of heavy metal elements in the soil and plants, butcan not make an effective discrimination and analysis on the soil-plant heavy metalmulti-source system. Pb isotope has an absolute advantage in tracing multi-sourcepollution. Different Pb sources have different Pb isotopic composition, which has norelation with the changes of physical and chemical conditions in the transfer processof heavy metals, and Pb isotope has a high abundance ratio and stable isotope ratio,so it is easily measured. Moreover, it suffered little impact from the late geologicaland geochemical process. Therefore, Pb isotope tracer is gradually becoming apowerful technique means, especially for studying the pollution sources of heavymetal including Pb or other sulphophile elements (Hg, Ag, Tl, Sb, Zn and Cu etc.).Using the "fingerprint" feature of Pb isotope, as long as the Pb isotopic compositionof the subject investigated and possible sources is determined, we can accuratelyidentify the pollution sources and achieve effective pollution abatement. However, up to now, the study still stays in the heavy metal pollution of soil, air and water, andthere is relatively less study on the tracing of plant's heavy metal sources, which isalso rarely reported at home and abroad. The difficulty lies in the fact that the Pbcontent in plants is very low, and we must do more exploration and research in theexperimental technique and laboratory condition, so it is restricted in the application.Based on the above purpose of study, this paper takes three types of cities inChengdu economic area (Chengdu, Deyang, Pengshan and Pujiang) as an example.The main content of the research is as follow:1. It has used the theory of element geochemistry to study various types of cities'heavy metal pollution profile in Chengdu economic area, and made an evaluation onthe heavy metal pollution of soil and plants.2. It has used the "fingerprint" feature of Pb isotope to trace the soil, plants (root,stem, leaf) and the near-surface atmosphere dust of the adjacent areas, and revealedthe pollutant sources and the relative contribution rate of the objects of the study.The main results are as follow:1. The content of Hg, Zn, Cd in the agricultural soil of Chengdu, the content ofHg, Cr, Cd in the soil of Deyang's, and the content of Cr, Cd in the soil of Pujiang andPengshan, have exceeded the second national level soil standard, and have causedharm to plants and the environment. In Chengdu city, the soil of the south and eastarea suffers the most serious pollution, and in Deyang city, the soil of the north andwest area suffers a relatively serious pollution.2. By using the geological accumulation index method, the potential ecologicalharm method, and the Nemerow comprehensive index method respectively, we havetaken a comprehensive evaluation of soil quality in various areas. The results of thegeological accumulation index and potential ecological risk index show that thecontent of As, Pb, Zn in the soil of Chengdu area, the content of Hg, Cu, Cd in thesoil of Deyang area, and the content of Cr in the soil of Pujiang and Pengshan areahave caused serious pollution. The variation sequence of the potential ecological riskindex (RI) in multi heavy metal ecological system is: Deyang＞Chengdu＞Pujiangand Pengshan. The comprehensive evaluation of agricultural soil based on the soil'sbackground value in Chengdu shows that the comprehensive pollution index is largerthan 3 in Chengdu, which suffers a serious soil contamination. The comprehensivepollution index is between 2 and 3 in Deyang, which suffers a medium soilcontamination. The comprehensive pollution index is between 1 and 2 in Pujiang and Pengshan, which suffer a light soil contamination.3. The statistical analysis of heavy metals in Chengdu's vegetables according tofood hygiene standard shows that in Chengdu's vegetables the exceeding standardrate of Pb is 32.7%, the exceeding standard rate of Cr is 14.5%, and the exceedingstandard rate of Hg is 12.7%, but As, Cd, Cu, Zn have not exceeded the standard. Thestudy of concentration factor shows that the concentration factors of Cd, Zn, Cu invegetables are relatively high, which can be seen as the high concentration elements.Hg's concentration factor in lettuce leaves is high, but its concentration factor in othervegetables is low. The concentration factors of Pb, Cr, As in various vegetables arerelatively low, which can be seen as the low concentration elements. From therelationship between heavy metal concentration in vegetable and total content in soil,only Pb showed highly significant correlation in lettuce and soil, while most of themhad no evident correlation. Absolute value of regression coefficient was quite loweven for heavy metals which appeared to be significantly correlated. Therefore, therewere some import influence factors such as soil properties, etc.4. We have used the "fingerprint" feature of Pb isotope to trace theenvironmental lead pollution in Chengdu. The results show that the environmentallead pollution in Chengdu comes mainly from coal and automobile exhaust, and thelead from firing coal is more than that from automobile exhaust (firing oil). Thedifference of the end-member component's lead isotopic composition can completelytrace and identify the source of the environmental lead pollution in Chengdu.5. The soil lead pollution in Chengdu area has two pollution sources, firing coaland firing oil. The soil lead isotopic composition falls within the scope of the leadfrom firing oil and the lead from firing coal. The soil in the area of heavy trafficsuffers a serious automobile exhaust pollution, and the pollution source of lead comesmainly from firing oil. In the coal dust source area near thermal power plant or therural areas far from highways, the dust from firing coal is the main soil pollutionsource. In addition, in the areas having many factories, the lead in the soil is mainlythe mixed multi-source lead.6. The isotopic ratios vary greatly between underground parts of plants(rootstock) and the aboveground parts of plants (leaves). The lead isotopic ratio isrelatively high in the rootstock, and relatively low in the leaves. The lead isotopiccomposition both in the rootstock and the leaves falls within the scopeof the mixing process of the end-member of the lead from firing oil and the lead from firing coal. The isotope in the rootstock has the feature of the soil lead (a mixture ofoil exhaust and firing coal, and leans to the lead from firing coal), and the isotope inthe leaves leans to the lead from firing oil. This distribution rule of lead isotope inplants has been verified in several plants.The main innovation of this paper lies in using lead isotope tracer system toeffectively reveal that the lean in some plants' rootstock and leaves has different leadsources. The leaves are mainly influenced by the lead from firing oil, and are closelyrelated to the local near-surface atmosphere dust. The lead isotopic composition in therootstock mainly reflects the characteristics of the lead source in the soil.