Balances Of Heavy Metals In Small-scale Vegetable Farming System In Typical Peri-urban Areas Of The Yangtze River Delta Region And Associated Ecological Effect

There is growing concern about heavy metals pollution in peri-urban areas with the rapiddevelopment of urbanization in China. Heavy metal balance in an agricultural coo-systemmay be exactly judged by studies on pathways of heavy metal inputs and outputs in a givensystems and quantitative analysis of these ways. These studies and analysis are helpful tothe judgment of heavy metal accumulation tendency and may provide an important base forthe evaluation of ecological pollution risk and agricultural sustainable development.Through the careful investigation and the precise monitoring records, balances andpollution potential of Cu, Pb, Zn and Cd in soils were studied during the years between July,2003 and July, 2005 in small-scale vegetable farming system in peri-urban areas of Nanjingand Wuxi. We also discussed the ecology effect of heavy metal pollution at presentsituation, and made associate ecology effect assessment. Results are described as follows:(1) Pathways of heavy metal inputs to monitoring vegetable field plots in the studyareas included applied chemical and organic fertilizers, irrigation water. The organicmanure inputs were a major heavy metal pathway.Heavy metal inputs to Nanjing monitring plots were as follows: Cu, 1382±1292 g/hm~2;Pb, 248±144 g/hm~2; Zn, 4342±2768 g/hm~2, and Cd, 6.6±4.1 g/hm~2. Heavy metal inputs toWuxi experiment plots were as follows: Cu, 3864±2002g/hm~2; Pb, 245.3±156.5g/hm~2; Zn,6175±2690g/hm~2; Cd, 7.86±4.4 g/hm~2. In comparison with yearly heavy metal inputs inEuropean agricultural area, inputs in the experiment area were quite high.(2) In consideration of various factors (atmospheric deposition; soil leaching andsurface runoff), risk evaluation indicated that there were risks of environment pollutioncaused by heavy metals Cu, Zn and Cd.Balance calculation for heavy metal elements in Nanjing experiment field plotsshowed that heavy metal surplus was found in all plots. This meant that in all cases heavymetal inputs were higher than corresponding outputs. Surpluses of various elements were asfollows: Cu, 1300.1±1254.9g/hm~2; Pb, 222.8±138.1g/hm~2; Zn, 3804.5±2620.4 g/hm~2; and Cd, 4.9±3.9 g/hm~2. Heavy metal accumulation in soil was impacted by farmingmanagement measures with various intensities, and highly intensified managementmeasures accelerated heavy metal accumulation in soil.In Wuxi area, Balance calculation in experiment field plots showed that Cu, Zn and Pbsurplus was found in all plots. Surpluses of various plots were as follows: Cu,3577±2150g/hm~2; Pb, 511.3±234.8g/hm~2; Zn, 4922±3504g/hm~2.The increased Cu, Zn andPb balances in open-field soils were higher than greenhouse soils (p＜0.05), because itaccepted more organic manure and more atmospheric deposition. The soil with short-termplanting showed more obviously the balances. The Cd balances were positive in theopen-field plots, 6.35±3.12 g/hm~2. The Cd outputs were higher than the correspondinginputs in the greenhouse plots. Different management leads to different heavy metalbalance, the risk in open-field plots were higher than greenhouse plots.(3) With reference to Chinese national soil environment safety standard, level 2(GB15618-1995), it might be seen that heavy metal contents in plots concerned were alllower than limits fixed in the standard, and soils in the study areas were still at their safestatus in the experiment initial period. But in the later period, the situation was changed.Some heavy metal contents were approached and even surpass the level 2. Through thecompare between the vegetables soil and the paddy soil analysis in Ding Xiang area, it wasshowed that the highly intensified management not only caused the soil acidification, butalso caused obvious accumulation of the soil heavy metal.(4) The heavy metal content of rhizomes, fruits, melons and legumes vegetables are allin the security rank in research area. The contaminated vegetables all mainly were leafvegetable. The accumulation of heavy metals in vegetables was different between Nanjingand Wuxi area, In Nanjing area, only Pb content of spinach was above the Food HealthStandards, the ratio of exceeding standards was 9.5%. In Wuxi study area, the situation wasmore serious, the ratio of exceeding standards of leaf vegetable was 33.3%. The healthyrisk evaluation result indicated that, the vegetables especially the leaf vegetables in thestudy area had the risk, the risk to human health. Furthermore, the risk was particularly highin Wuxi area.