| With the speeding up of urbanization process and the rapid development of industry and agriculture, socio-economic development of our country has been greatly promoted in recent years. However, the discharge and exudation of huge amount of "three industrial wastes", municipal solid wastes and sludge resulted in sever pollution of heavy metals in soil and water systems. Irrational application of pesticides and fertilizers also aggravated the accumulation of heavy metals in soils. The safety problem of agricultural products and enormous economic loss caused by pollution of heavy metals have resulted in great concerns both at home and abroad. The plain in northern Zhejiang Province around Taihu Lake (south of Taihu Lake) is one of the most famous area for agricultural production in China, especially for rice grain and fish production. However, great economic loss because of agricultural product safety problem caused by heavy metal pollution has happened from time to time in recent years. Therefore, for the sustainable agricultural development and the human health, it is necessary to study on the spatial variability of soil heavy metals and their pollution risks, and further to put forward the preventing and controlling countermeasures.Based on above reasons, this research mainly focused on the cultivated land of the plain in northern Zhejiang Province around Taihu Lake. Spatial variability of six heavy metals (As, Cd, Cr, Cu, Hg and Pb) under three different scales of 1:250000, 1:50000 and 1:10000 was investigated based on field investigation and lab analysis, and with combined geostatistics and GIS techniques. The pollution risks of these heavy metals were then assessed. Possible causes and the corresponding controlling countermeasures were also analyzed. Sampling strategies were further studied at the small scale. And the management information system of soil environmental quality on the basis of WebGIS was finely developed. It is expected that the study could provide scientific basis for conserving the farmland resources, insuring food safety and human health, and formulating farmland soil pollution management policies and countermeasures on regional (large) , county (medium) , and township (small) scales. The main results and conclusions are as follows:(1) The soil heavy metals in the study areas at three scales all presented great spatial variability and differentiated obviously from one another, which had been influenced by human activities. Specially, on the large scale (the plain of northern Zhejiang Province around TaiHu Lake) , the spatial variability of soil Hg, Cd, As and Cuwas large, with the coefficient of variation (CVs) between 25.19%~58.81%, among which the CVs of Hg and Cd reached 58.81% and 33.34%, respectively. The CVs of Pb and Cr were relatively small, being 17.33% and 17.43%. On the medium scale (Changxing County) , Hg, Cd and As exhibited the greatest spatial variability, the CVs being 54.85%, 36.32% and 29.60%, respectively. Spatial variability of Cu, Pb and Cr was relatively small, with a range from 21.24% to 22.85%. On the small scale (Zhicheng township) , CVs of Hg, As and Cd were high, reached 54.02%, 31.21% and 26.08%, respectively. And the CVs of Cr, Pb and Cu were low, ranging from 18.07% to 22.31%.(2) Based on the second level standards of "Environmental Quality Standard for Soils" (GB15618-1995 ) , the single factor method was used to assess the pollution risks of the studied heavy metals. The results showed that on the large scale, soil contents of Pb and Cr were lower than the standards, indicating no pollution risks. Soil As also maintained in a clean condition. However, both soil Cu and Cd exhibited slight pollution risks with overproof rates of 1.23% and 2.72% respectively. But soil Hg pollution was most serious with overproof rates as high as 10.87%. On the medium scale, soil contents of Cu, Pb and Cr were lower than the standards and had no pollution risks. Soil As was also clean. Soil Cd and Hg, however, showed different levels of risks with overproof rate of 4.06% for Cd and of 6.17% for Hg. On the small scale, soil was seriously polluted by Hg, with overproof rate of 9.80%. The other five heavy metals showed no pollution risks. These results confirmed that the cultivated land in plain of north Zhejiang Province around TaiHu Lake had been polluted by heavy metals to various extent. Specially, the pollution of Hg was most serious and could become the main limiting factor to the regional socio-economic development.(3) The risk assessment of soil heavy metal pollution and the analysis of pollution causes indicated that, on the large scale, the soil environment was polluted by Hg, Cd and Cu to various extent, and the distribution of high risk areas was found to have a good correlation with the distribution of cities and industrial and mining enterprises. Therefore, it is considered that the pollution risks of Hg, Cd and Cu in the study areas could be caused by increasing discharge and exudation of three industrial wastes and municipal solid wastes as the development of industrial and mining enterprises and urbanization. Also, excessive application of pesticides, fertilizers and the poultry manure with high Cu content could be related to soil pollution by heavy metals. On the medium scale, the distribution of Hg and Cd risk areas in Changxing County was also well correlated with the distribution of cities, industrial and mining enterprises, and the railways. And becausethe distribution was also closely related to the air pollution of Changxing by coal smoke, it was extrapolated that the deposition of pollutants in atmosphere was the main cause for the pollution of Hg and Cd. On the small scale, soil was polluted only by Hg, and Hg was thus the limiting factor of the soil quality. This was most possibly resulted from the secondary pollution of atmospheric deposition of pollutants emitted from thermal power plant, cement and fire-proof material producing enterprises.Based on above discussions, measures of the source control and integrated remediation were proposed for counteracting soil heavy metal pollution for each scale. On the large scale, emphasis should be given to macro level establishment and/or improvement of regulations and policies related to the prevention and control of soil heavy metal pollution. On the medium scale, both macro level management and micro level countermeasures should to emphasized to enhance the decision making ability of managing industrial and agricultural production and preventing and controlling pollution. Effective remediation measures should also be investigated and demonstrated. On the small scale, great emphasis should be given to application of practical countermeasures to prevent and remediate soil pollution, such as adjustment of cultivating systems and integration of physical, chemical and biological methods.(4) Sampling strategies were studied on the small scale of 1:10000. The results indicated that the predicting precision was nearly the same as that using 100% of initial samples (153 samples) even the samples were gradually reduced to 60% of the initial ones. Thus, it is considered that 60% of the initial sampling numbers was reasonable sampling density for similar studies. Using this sampling density, not only could the predicting precision of soil heavy metal be assured, but also could the cost be reduced.(5 ) The spatial variability of soil heavy metals in cultivated land of the study areas presented no scale regularity, which was mainly related to the influence of human activities. On the other hand, all six heavy metals in the study areas showed a medium spatial correlation in general. Therefore, it seemed appropriate to use geostatistics methods to predict the spatial distribution of soil heavy metals at the scales. The spatial distribution maps of soil heavy metals disclosed obviously their spatial distribution tendency. And the results were coincident well on all three scales. Therefore, based on the results on the large scale, the pollution situation of heavy metals can be qualitatively acquired, it is helpful for the decision making of macro level management. Based on the results on the medium scale, the distribution of pollutants can be understood in more detail and the possible causes can be analyzed, it is significant on county level for management of industrial andagricultural production and decision making for pollution control. Based on the results on the small scale, specific pollution status and its distribution can be understood in great detail. Also, the pollution causes can be identified, it is helpful for the selection of concrete countermeasures to control and remediate the pollution.(6) The attribute and spatial databases of soil heavy metals in cultivated land of the study areas were established for all three scales. And the management information system of soil environmental quality on the basis of WebGIS was developed. The system can be used to analyze heavy metal pollution status in the the study areas, predict the potential development tendency of pollution, and realize on screen demonstration of pollution status, online inquiry of pollution development tendency and announcement of real-time news. It can not only provide scientific support for decision making of controlling heavy metal pollution in the plain of northern Zhejiang province around Taihu Lake, but also promote control of soil heavy metal pollution all over Zhejiang Province or even China as a whole.
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