| Sewage irrigation areas are the chief agricultural production areas, which are irrigated by waste water for many years due to the lack of clean water. The pollution of farmland soil is characterized by complex composition of pollutant and the large range of pollution. Based on the improvement of agricultural product quality safety and ensurance of farmer’s incoming, the comprehensive measures were conducted to removal soil pollutants gradually. Finally, the soil of sewage irrigation areas were remediated and utilized safely.(1) This thesis studied the distribution of heavy metals and PAHs in soil, as well as security of local staple crops in Xiaodian sewage irrigation area with the area of137,000hm2. The experiments were conducted based on the area of2×2km2grid points and45sampling sites. The potential ecological risk index was used to evaluate the soil pollution of target areas. The results were shown as follows:The soil of Xiaodian sewage irrigation area has been polluted, with the maximum contents of As. Hg、 Cd、Cr、Pb and PAHs were4.46、8.71、10.72、1.26、1.74and22.87times higher than the soil background values. However, it was found that only one of45sampling sites was measured Cd excess, the corresponding five agricultural products were measured Cd excess and nine agricultural products were measured Pb excess. The exceeding sites were mainly distributed in the intersection of Beizhang waste canal and Taiyu waste canal.The results of potential ecological risk index showed that the main risk factors of Xiaodian sewage irrigation area were Hg and Cd. The single potential ecological risk index ranged between160and320. The ecological risk of Xiaodian sewage irrigation was serious, with the potential ecological risk index of371.48, which was treated as "the stronger ecological risk".The five heavy metal contents in crops were ordered as:leaf vegetables> rhizome vegetables>eggplant vegetables> cauliflower vegetables. The As、Hg、Cd、Cr、Pb contents in seeds of spring maize were higher than in waxy corn. The accumulated factors of heavy metal and PAHs in leaf vegetables were highest. The corresponding values in cauliflower vegetables were lowest, which were0.05%、1.10%、0.08%、0.21%、1.92%for As、Cd、Cr、Pb and PAHs, respectively. Hg was not detected.(2) The accumulations of heavy metals and PAHs by the major crops (maize) and the low cumulative crops (day lily) were studied. The results showed that:The heavy metal and PAHs contents in seeds of maize were relate with their growth period and varieties. The contents of As, Hg, Cr、 Cd、Pb and PAHs in the seeds of milk-ripe stage were much lower than in full ripe stage. Jinxiannuo No.6and Nongda No.84were the favorable varieties in Xiaodian sewage irrigation area.In the soils with different pollution levels, both heavy metal and PAHs contents in the flowers of day lily were very low. The pollutants in economic products (flowers) were lower than the standard values of pollution-free vegetables. The average contents of As, Hg、Cd、Cr、Pb、PAHs were0.023mg/kg、 O.OO1mg/kg、0.07mg/kg、0.007mg/kg、0.180mg/kg、0.023mg/kg, respectively. So they were safest vegetables in the sewage irrigation area.(3) The addition of soil amendments and microorganism agent, as well as the pastures planting influenced the heavy metal and PAHs contents in soil. The results were shown as:The sulphur addition in soils could significantly increase the contents of effective state contents of Hg, Cd, Cr, Pb in soil. The additions of lime, active carbon, diatomite, sodium bentonite and humic acid could significantly decrease the contents of effective state contents of Hg、Cd、Cr、Pb in soil. The addition of humic acid in soil decreased the effective state contents of As, Hg, Cd, Cr, Pb in soil by6.4%、23.6%、41.4%、36.6%、37.32%, respectively. Under the neutral and alkaline soil conditions, the humic acid facilitates the available As transform into other stable form. It reduced the As accumulation of plant and relieve its toxicity.When added the sulphur and enzyme microorganism in soil, the As, Cd, Pb contents in seeds of maize increased. When added the lime, sodium bentonite, humic acid and diatomite in soil, the As, Cd, Pb contents in seeds of maize decreased. The decreasing effect of humic acid on seed accumulation was most obvious; with the values were37.42%,36.96%and39.49%for As, Cd and Pb, respectively.Degradation was the major method for PAHs removal in soils. Plant uptake only accounted for0.33%-3.26%of the total PAHs removal. The PAHs degradation in soil could be increased by1.1-3.5times with the plants existed. The existence of Pennisetum americanum, Gaodan grass and Sudangrass were helpful for increasing the contents of the effective state of heavy metals in soils, so their accumulation was obviously higher than other pastures.The EM microbial agent and OBT oil removal bacteria agent were suitable for the degradation of low molecular weight PAHs; while Nongda microbial agent was suitable for the degradation of medium molecular weight PAHs. After90days of the microorganism culture, the degradation rates of naphthalene, acenaphthylene, acenaphthene, fluorine, luxuriant, anthracene, fluoranthene and pyrene in soil decreased by25.6%、27.0%、27.2%、27.7%、22.3%、23.4%、25.4%、23.4%, respectively. (4) Based on all the research results, we designed the intercropping modes with combination of promotion and control, as well as intercropping and rotation modes with accumulation strengthen. The safe utilization and remediation effects were evaluated in Xiaodian sewage irrigation area by using these two farming modes. The results showed that:After three-year growth of the crops, the As and Cd contents in the seeds of maize planted with two designed modes were lower than the standard values published by "Hygienic standard for grains (GB2719-2005)". When the soil treated with sulphur, enzyme microorganism and Nongda microbial agent, as well as planted with Pennisetum clandestinum, some pollutant contents in soil decreased obviously. The corresponding contents of acenephthene, fluoranthene, BaP, As, Cd and Pb in the treated soil decreased by62.28%、67.95%、69.04%、、23.91%、21.79%and24.67%, respectively. |
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