Comparative Analysis Of Internal Pollution Control Technique In Dianchi Lake

The essence of internal pollution is sediment contamination. In the appropriate environmental conditions, nitrogen and phosphorus would be released to water resulting from some extent concentration difference in sediment-water interface, which would cause the eutrophication. Similarly, heavy metals and refractory organics in sediments would be released under certain conditions also. They could be enriched through the biological enrichment and cause strong poison effect, while may threat the human health through food chain. Currently, the research of internal pollution management has become one of the hot issues in the international environmental protection. Large numbers of internal pollution management researches have been developed in major lakes in China. Some projects, such as sediments dredging, ecological restoration, water diversion and situ-control have been taken into measures in different lakes. However, the treatment technology of internal pollution has long been focused on the related analysis and outcome prediction for one specific technology, while the comprehensive comparison and study for different treatment techniques has not been carried out. So it is difficult for decision makers of environmental protection management to choose the appropriate technology, which can gain the maximum environmental benefits with limit management funds.As a difficulty of management among the Three Lakes, the pollution problem of Dianchi Lake had attracted the government and scholars'widely attentions. At present, external pollution of Dianchi Lake has well controlled under the strict environmental monitoring and managing system. However, the internal pollution has become the severe challenge to water pollution control. In view of the urgency and importance of the Dianchi Lake management, as well as the lack of comprehensive comparable research within the different internal pollution management technologies, Dianchi Lake was chosen as the study area in this paper that the effects of Dianchi Lake polluted sediment dredging and disposal project, captive breeding of Water Hyacinth and resource utilization project, and Niulan River-Dianchi Lake water diversion project in managing internal pollution were studied systematicly. It aims at approving scientific basis to internal pollution management in Dianchi Lake and other lakes. (1) Through comparing dredging and un-dredging areas with with and without comparison method, the influences of dredging project on physical and chemical properties, nitrogen and phosphorus pollutants, heavy metals and As loading, and biodiversity in water and sediment were analyzed in this paper, also the social benefits the project brought.The results showed that there were some improvements on the physical and chemical properties of the water and sediment. Compared with un-dredging areas, the average turbidity was5FTU lower and average transparency was11.5cm higher in dredging areas of southern Caohai, while there was little difference in Panlong River and Daqing River. Among the three dredging areas, the water conductivity decreased5-13s/m, and DO, pH, ORP didn't change obviously. The organic matter contents and ignition loss decreased sharply, while the moisture contents didn't change too much in dredging areas of Panlong River and Daqing River. However, these indexes were opposite in Caohai dredging areas. The silt contents increased and the clay contents decreased to different degrees in the three dredging areas.The contents of various forms of nitrogen and phosphorus in dredging areas were basicly lower than the un-dredging areas, while heavy metals and As were not found in water. The pollutants contents in dredging areas sediment were sharply lower than the un-dredging areas. It was estimated that11,500tons TN,5,432tons TP,458.7tons Pb,439.8tons Cu,1,030.5tons Zn,5.3tons Cd,365.7tons Cr, and85.9tons As were wiped off through the dredging project. The different forms nitrogen and phosphorus in core sediments of the two dredging areas in Waihai have been fractionized, the results showed that the bio-available nitrogen decreased sharply, and only parts of the surface sediments appeared potential releasing trend. Ca-P and Fe-P took a large portion of TP, and the variation trend with depth was gradually decreased, similar to TP. Dredging didn't cause the Ca-P and Fe-P releasing largely, which had little affected on TP contents in water. By using the geological cumulative index method and potential ecological risk assessment method, the potential ecological risk of heavy metals and As have been assessed, the results showed that the pollution levels of five heavy metals and As in dredging areas were lower than the un-dredging areas. The ecological risk in dredging areas was less than the un-dredging areas resulting from the assessment of potential ecological risk factors of single heavy metal. While the ecological risk of heavy metals and As in surface sediment was in a slight degree, using the assessment of potential ecological risk factors of multiple heavy metals. The research of plankton diversity showed that, the alga biomass and diversity indexes in dredging areas were more than that in un-dredging areas, and the biodiversity was in a low level overall, while dredging areas in Panlong River and Daqing River were still polluted seriously.In this paper, the social benefits of the project were discussed through questionnaire. The results showed that the project was endorsed by the local people and the enterprises, the majority of the respondents were willing to donate for the project, believing that the dredging project had good social benefits by providing some employment opportunities, improving the ecological landscape and city investment environment and promoting tourism development. Meanwhile, the project was also a vivid propaganda about environmental protection which enhanced the citizens' environmental protection consciousness.(2) Through8months monitoring of water quality in Caohai during the captive breeding of water hyacinth experiment, the removal efficiency of nitrogen and phosphorus by captive water hyacinth was investigated preliminary, and the influences to the pore water by long-term planting water hyacinth was analyzed, as well as the effects of improving internal pollution were estimated. The results showed that, the physics and chemistry index was better in different degrees before Oct15th in Caohai, transparency, dissolved oxygen, oxidation-reduction potential increased, while pH and conductivity decreased. However, the physics and chemistry index became worse since Oct15th. In the whole experimental periods, the various forms of phosphorus were purified notably, the contents of TP, DTP, and PO43-were decreased from0.44±0.09mg/L to0.07±0.01mg/L, from0.17±0.01mg/L to0.01±0.001mg/L and from0.15±0.01mg/L to0.003±0.001mg/L respectively, removal rates reached84.09%,94.12%and98.00%; While the various forms of nitrogen were purified poorly, the highest removal rates of TN, NO3--N and NH4+-N were17.24%,50.07%and75.76%before Oct15th. However, the contents of TN and NO3--N increased obviously since Jan15th in the following year, were5.09±0.76mg/L and3.34±0.08 mg/L respectively, while the contents of NH4+-N was comparatively lower,0.26±0.08mg/L. NH4+-N and PO43-were the essential components nutriment in the pore water, and the contents of various forms of nitrogen and phosphorus were increased at first and then decreased. It is indicated that the captive water hyacinth has affected the releasing of nutrient from sediment. The correlation analysis of the water quality indexes showed that temperature and pH were the key factors which affected the purifying capacity of water hyacinth, rising temperature or reducing pH could promote the purification effect; On the contrary, the nutrient in the sediment would release with the rising of pH.(3) The allowed maximum of pollutants emissions, supply water quantity of eco-environment and water supplement effect were forecasted scientifically according to the informations of the water supplement project in Niulan River. The pollutant levels in the year2020were calculated by two-dimensional water quality model that the average concentration of TP and TN were0.10mg/L and1.45mg/L respectively, the concentration of permanganate index reached the requirement of grade Ⅲ which meet the water quality of grade IV in Waihai of Dianchi Lake in2020. By using Zero-dimensional model, the attenuation coefficients of TN and TP were11.27/a and10.88/a in Waihai,2.41/a and2.73/a in Caohai, respectively. The maximum permissible quantity of TN and TP pollutants were increased by about20%. In order to meet the project objective, the environment diversion quantity was about5.5*109-6.5*109m3/a in Dianchi Lake.In this paper, the results of comparative study of the three internal pollution control technologies showed that the sediment dredging project should be carried out preferentially in the areas where sediment and water pollution was quite serious such as Caohai, the estuary and eastern, northern part of Waihai, then taken the water hyacinth control measure. Considering the high water pollutant loading and long lake residence period in Waihai of Dianchi, in order to improve the water quality quickly, water diversion project could be adopted if the water and funds are sufficient.