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Retention And Removal Effect Of Ditch Wetlands On Non-point Source Pollutants From Agricultural Drainage

Posted on:2005-01-15Degree:DoctorType:Dissertation
Country:ChinaCandidate:C L JiangFull Text:PDF
GTID:1101360122985652Subject:Hydrology and water resources
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Non-point pollutants from agricultural sources are major cause of surfacewater eutrophication, which can be traped and removed by adsorption, sedimentation, assimilation, and microbe decomposition of wetlands. The ditches are the first pool of diffuse nutrients from rain runoff and field drainage, and are shaped to a net work in the area of the lower reaches of the Changjiang River where the cultivation manner of fields are rotation of rice and wheat. Now we have learnt little about the mechanisms of pollutants trasport and transformation in ditch wetlands. The following questions have been studied in this paper.1. The retention and accumulation of organic matters, N and P in sediment of wetlandsThe contents of organic matters and TN (Total Nitrogen) are higher in the layers below 40cm in reed (Phragmites communis) and wild rice (Zizania latifolia) wetland sediments, compared with that in the same layers of field soil. TP (Total Phosphorus) has accumulated in the whole layers of sediments from surface to 100cm.2. The variation of non-point source pollutants after input to wetalnds with rain runoff and fields drainageThe concentrations of COD, TN and NH/-N are decreased after input to wetalnds with rain runoff and fields drainage. It means that wetlands have the ability of rentention and purification of non-point source pollutants. The concentration of NO3-N is never exceeded 1.5 mg/L, there is no pollution potential of it in reed and wild rice wetlands. TP is rise with high frequency of rainfall and field drainage from June to September. The decomposition of hydrophyte residua led to high concentration of COD in winter and the start of spring.3. The transformation rate of pollutants in sedimentThe main source of organic matters of wetlands is the residua of reed and wild rice. From December of 2002 to March of 2003 (just the decompositin period of water plants), the mineralization rates of organic matters are 51.3g/m3/d in reed sediments and 142.0g/ m3/d in wild rice sediment. The denitrification rates in reed sediments of the nonharvested and harvested plots are 23.5mg/m3/d and 22.7mg/m3/d respectively, and in wild rice sediment is 21.4mg/ m3/d.4. The correlation of organic matters, TN and pH valueThe correlation of organic matters and TN in sediments is high significant, with 0.9653 (n=27) coefficient in reed wetland and 0.9102 (n=19) in wild rice. This testified that the main composition of TN is organic N. In the process of ammonification, nitrification of organic N, H+ will be released, which causes the drop of pH value. There is significant negative relationship between TN and pH value. The coefficients of TN and pH in reed and wild rice sediments are -0.8181 (n=27) and -0.6932 (n=19) respectively.5. The variation characteristic of pollutants in harvested and nonharveated sediments of reed wetlandsTwenty days after the harvest of reed, the concentration of organic matters (21.33 mg/L) and TN (1.04 mg/L) is lower than that in nonharvested plot. The contents of organic matters, TN and TP in sediment layer from 0-20cm also decreased after 2 months of harvested, but NH/-N and NO3--N increased due to the amelioration of transperancy and aeration, which accelerated ammonification and nitrification.6. The assimilation ability of N and P by hydrophyte and prevention of the secondary pollutionThe reed, wild rice and Zizania caduciflora Turez Hand-mazt can remove 818 kg/hm2,131 kg/hm2 and 200kg/hm2 of N and 103.6 kg/hm2, 28.9 kg/hm2 and 21.1kg/hm2 of P respectively from wetland after harvested each year. Even though reed and wild rice have good assimilating ability of N and P, they have low value in use. However, Z. caduci flora is a kind of vegetable, the planting of it to replace reed and wild rice in ditches can not only absorb N and P, but also bring ecomomic benefit for farmers and promote the active reap of them.The reasonable utilization of ditch wetlands for their capacity of retention, transformation and absorbtion of nutrients can be an effective tool in reducing...
Keywords/Search Tags:ditch, wetland, non-point source pollutant, organic matters,nitrogen,phosphorus,hydrophyte, retention and removal effect
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