M injun DENG，Yan LUO

1.Guangxi Environmental Protection Monitoring Central Station，Nanning530023，China；2.Guangxi Center for Analysis and Test Research，Nanning530022，China

1 Introduction

With rapid social and economic development，China's urbanization process is constantly accelerating and urbanization level is also rapidly improving.At the same time，however，it also brings serious environ mental pollution and exerts serious influence on quality of agricultural ecological environment［1-5］.Environment quality is a fundamental factor influencing quality of agricultural products.Therefore，analyzing and evaluating heavy metal pollution of major agricultural products and production region environment（soil and water source），and determining the pollution level are of great significance to ensuring our health and providing scientific basis for further development of agricultural products and production of green agricultural products.

Harmlessness is the minimum requirement for safety of agricultural products.Pollution-free agricultural products are products with production region environment，production process，and target product quality conforming to national or industrial standard and production specifications for pollution-free agricultural products，and the quality passing the inspection of production region and market quality supervising authorities，and sold with identification of pollution-free agricultural products.With increasing concern for safety of agricultural products，it becomes particularly important to energetically develop production of pollution-free agricultural products，raise industrial level of agricultural products and improve quality of agricultural products［6-10］.

Pollution and prevention of heavy met als in soil are always key and difficult issues in research of pollution ecology.On the basis of survey in 6 pollution-free agricultural product bases in Guangxi Zhuang Autonomous Region，we analyzed heavy met al pollution of soil，in the hope of providing theoretical and testing basis for understanding situation of ecological environment quality and carrying out environmental pollution control.

2 Experiment and methods

2.1 Experimental methods

2.1.1 Analysis methods of heavy met als in soil.In this study，we mainly measured Pb，Cd，Cr，Hg，and As content in soil of production region of pollution-free agricultural products.Pb and Cd were analyzed with reference to Soil Quality Determination of Lead，Cadmium-Graphite Furnace-Atomic Absorption Spectrophotometry（GB／T17141-1997）；Cr was analyzed in compliance with Soil Quality Determination of Total Chromium-Flame Atomic Absorption Spectrophotometry（GB／T17137-1997）；As was analyzed in accordance with Soil Quality Determination of Total Arsenic-Silver Die thy ldithiocarba mate Spectrophotometry（GB／T17134-1997）；Hgwas analyzed according to Soil Quality Determination of Total Mercury-Cold Atomic Absorption Spectrophotometry（GB／T17136-1997）.

2.1.2 Evaluation methods for heavy met al pollution of soil.We adopted single factor pollution index method and Nemerow synthetic pollution index method to evaluate heavy met al pollution of soil.The current pollution situation is visually reflected through calculation［11-15］.

The calculation formula of pollution index is as follows：

where Iiis the pollution idex of the i-th pollutant in the environment；Ciis the measured content of the i-th pollutant；Siis the standard evaluation value of the i-th pollutant.

The calculation formula of Nemerow synthetic pollution index is as follows：

where Pjmaxis the maximum value of pollution index of all pollutants in the monitoring points and Pjaveis the average value of pollution index of all pollutants in the monitoring points.

Table 1 Grading of pollution index of heavy met als in soil or plant

Table 2 Lim it of content of elements in soil of pollution-free vegetable bases（mg／kg）

2.2 Evaluation methods for heavy met al pollution of soilThe evaluation of upper limit of harmful elements in soil was carried out in accordance with Procedural Regulations regarding the Environment Quality Monitoring of Soil（NY／T395-2000）［11］and Pollution-free Food Environmental Requirements for Producing Areas of Vegetables（NY 5294-2004）.

3 Results and analyses

3.1 Analysis on content of heavy met als in soil of pollution free agricultural product basesFrom Table3，we can know that in all6 pollution-free agricultural product bases，the Cr content in soil was24.3-78.6 mg／kg；Pb content was 22.3-51.2 mg／kg；Cd content was 0.0022-0.28 mg／kg；Hg content was 0.061-0.269mg／kg；As content was 5.7-30.2 mg／kg.All values are lower than standard value for pollution-free soil.

3.2 Evaluation of heavy met al pollution of soilFrom Table4，it can be seen that taking limit of pollution-free soil quality，Hg，Pb and Cr are clean in all 6 pollution-free agricultural product bases，As and Cd reach the alarming level in some bases，but the soil is moderately clean.Therefore，the soil quality in those pollution-free agricultural product bases is suitable for producing pollution-free agricultural products.

Table 3 Heavy met al content of soil in different regions（mg／kg）

Table 4 Evaluation of heavymet al pollution of soil in different regions

Continued Table4

4 Conclusions

Through determination of Pb，Cd，Cr，Hg and As content in soil of pollution-free agricultural product bases in Guangxi Zhuang Autonomous Region，it evaluated the pollution of soil using single factor pollution index method and Nemerow synthetic pollution index method in combination with evaluation standard of heavy metals in soil and grading standard for soil pollution.The results show that major pollution-free agricultural products bases in Guangxi Zhuang Autonomous Region have excellent soil quality which is suitable for producing pollution-free agricultural products.

［1］CHEN HM，ZHENGCR.The current situation of heavy met al pollution of soil in China and the countermeasures［J］.AMBIO：A Journal of the Human Environment，1999，8（2）：130-134.（in Chinese）.

［2］WEN F，TAOHY，LILQ.Advances on heavy metals pollution［J］.Gansu Science and Technology，2008，24（24）：118-121.（in Chinese）.

［3］LU S.Analysis of heavy met als concentrations in crops by influencing of soils factors［J］.Journal of Dandong Textile College，2004，11（3）：34-35.（in Chinese）.

［4］LIQJ，HUANGYP，GUOZ.Analysis of scientific views on agro-food quality and its safety［J］.Food and Nutrition in China，2008（8）：11-13.（in Chinese）.

［5］ZHANG SG.Investigation and evaluation on heavy met als pollution［J］.Journal of Anhui Agrotechnical Teachers College，2001，15（4）：23-24.（in Chinese）.

［6］ZHANGBW，HAO ZH.Safety of China's farm products［J］.Food and Nutrition in China，2000（6）：5-7.（in Chinese）.

［7］ZHANGSY，ZHAOXJ，QIAOWD，et al.Environment-friendly vegetables and the ordinary vegetables quality analysis［J］.Modern Vegetable，2006（9）：37-38.（in Chinese）.

［8］CHEN YY.Current situation and prospect of vegetable consumption demand of inhabitants in our country［J］.Food and Nutrition in China，2005（7）：38-39.（in Chinese）.

［9］ZHANG JX.An overview of standard production techniques for pollutionfree agricultural products［M ］.Yangling：Northwest A＆F University Press，2002.74-242.（in Chinese）.

［10］Focus on Promoting 50 Techniques by the Ministry of Agriculture＂Fifteen＂.Production techniques of no-polluted vegetable［J］.World Agriculture，2001，267（7）：41.（in Chinese）.

［11］Ministry of Agriculture of the People's Republic of China.Procedural regulations regarding the enviroment quality monitoring of soil（NY／T395-2000）［M］.Beijing：Standards Press of China，2001.（in Chinese）.

［12］LUO XM，ZHANG YR，YANG DQ.Pollution analysis and assessment of heavy met als in vegetables from Chengdu［J］.Sichuan Environment，2003，22（2）：49-51.（in Chinese）.

［13］DENGQJ，SONGCR，HE JL.Distribution and evaluation of heavy met als in cultivated soil of Guiyang［J］.Soils，2006，38（1）：53-60.（in Chinese）.

［14］LIQL，HUANG Y.Change of the content of heavy met al and contamination in soil of the vegetables bases in suburbs of Chongqing［J］.Chinese Journal of Soil Science，2002，33（2）：158-160.（in Chinese）.

［15］YANG YG，HU AT.Evaluation on harmful met al contamination in harmless vegetable bases［J］.Journal of Environment and Health，1999，15（5）：213-214.（in Chinese）.

［16］XIN LY，JIA BH.Analysis on status of Tianjin coastal urban agriculture［J］.Food and Nutrition in China，2008（8）：8-10.（in Chinese）.

［17］Ministry of Agriculture of the People's Republic of China.Non-polluted food：The environment of growing vegetables in greenhouse（NY 5294-2004）［M］.Beijing：Standards Press of China，2004.（in Chinese）.