Risk Analysis And Mitigation Experiment Of Heavy Metal Pollution In Croplands In Vicinity Of Pollution Sources

Soil heavy metals pollution was a seriously problem for safety producing of agricultural products in China. In order to guarantee the sustainable development of the agriculture, it was very important to assess and improve the metal-contaminated cropland. In this paper, we selected two farmlands with different heavy metals pollution sources. One was a wheat field around a scrap dump in the northern of Jiangsu province, another one was a paddy field surrounding a smelter factory located in the southern of Jiangsu province. We analyzed the soil heavy metal contents and spatial variation around the two different pollution sources, and factor analysis was also used to determine sources of heavy metals in soil. For evaluating the safety risk of the metal-contaminated farmland, single factor contaminant index and Nemerow pollution index were used in this study. Finally, a field experiment was performed to study the effect of different modifiers by single and combined application on the activity of soil different heavy metals, and discuss the results of treatments to reduce the absorption of heavy metals in rice grain. The main results were as follows:1. The plough layer soil heavy metals concentration of wheat field around a scrap dump was analyzed in northern Jiangsu. The results showed that Cd, Pb, Cr, Hg and As content of the cropland decreased with the increasing distance from the dump. Soil Pb, Cr, As content were normal distribution and abnormal distribution for the Cd and Hg content. Soil Cd and Hg content also had a high variation for 31.65% and 50.0%, respectively. In contrast with the control field, the main pollution element of the cropland surrounding the dump was Cd. Soil average content of Cd was 1.58 mg kg-1, single and comprehensive pollution index were 5.27 and 3.82, these results indicated that cropland around the dump had been seriously polluted by the heavy metals.2. We studied the characteristics of a paddy soil surrounding a smelter factory in southern Jiangsu, which was contaminated by heavy metals such as Cd, Pb, Hg, As, Zn and Cu. We collected the topsoil (0-15cm) from the paddy field. Results showed that soil total and DTPA-extractable Cd, Pb, Cu, Zn, Hg content decreased with the increasing distance from the pollution source. Cd, Hg had a wide contaminated area and small area for Zn among the investigated heavy metal elements in this studied field. Soil total Cd and Hg content ranged from 3.98 to 44.58 and from 0.36 to 2.01 mg·kg-1 and singe pollution index were 54.49 and 3.28, respectively. Comparing with the seriously pollution elements of Cd and Hg in the tested soil, As was mediate contaminated and light contaminated for Pb, Cu, Zn. The comprehensive pollution index of Nemerow was 39.27 for the studied field, this result expressed that the research area had been seriously contaminated by the heavy metals from the smelting factory. So, there were big risks for food produced and great threats to people consuming in this area.3. Based on the results of the metal-contaminated paddy field around a smelting factory, a field experiment was conducted to discuss the effects of single application or combined application of three different modifiers on rice growing and heavy metals activity in soil. The results showed that different treatments had no significantly effect on rice grain yield. But the Cd contents of rice grain were significantly reduced by 16.55 to 52.91% under different modifier treatments, especially for the combined application of calcium magnesium phosphate 62.5kg hm-2 and calcium hydroxide 150kg hm-2, which could reduce the Cd contents of grain by 52.78% and 37.77% for rice root. Modifiers could also significantly reduce the rice grain content by 27.14 to 47.14%, but no statistical difference could be observed among the different modifiers.4.1) Comparing with the control, soil CaCl2-extractable Cd decreased by 32.85 to 77.37% under different modifier treatments and 5.27 to 35.04% for soil Pb; Modifier treatments could also reduce soil DTPA-extractable Pb from 5.6% to 22.03, but only maximum reduce soil DTPA-extractable Cd by 8.79%. Soil CaCl2-extractable Cd still had a significantly positive co-relationship with rice grain Cd.2) Different modifier treatments could enhance the soil pH, especially for the combined application of Calcium hydroxide and calcium magnesium phosphate fertilizer. Soil pH increased 0.52 pH units under signal calcium magnesium phosphate fertilizer (P) treatments, and enhanced about 1.03,1.39 and 1.57 soil pH units for PCa1, PCa2, and PCa3 treatments. In PSi1, PSi2 and PSi3 experimental plots, soil pH raised by 0.41,0.43 and 0.69 pH units. Correlate analysis result showed that soil pH had a negative relationship with soil available heavy metals, so soil pH raise by the application of modifiers might be an important reason which could explain the inhibition for rice grain absorbing heavy metals under different treatments.