| Lake is one of the most important freshwater resources, which plays a very important role in national economic development. Ulansuhai Lake is one of the eight great freshwater lakes in China, is also a multi-function lake which is extremely rare in the semi-desert area in the whole world. Ulansuhai Lake is polluted more and more seriously recently because of the climate change and interference of human activities, which has attracted widespread attention of government and scholars. A large number of studies on Ulansuhai Lake have been carried out, most of which focused on ice-free period. But Ulansuhai Lake is located in high-latitude area, in which the ice season is longer and the characteristics of pollution must be different from the ice-free period. Focusing on longer ice season, Ulansuhai Lake is selected for study object to dicuss the migration mechanism and effect of pollutants in freezing process and its application in pastoral area for drinking water safety. The results could be concluded as follows:(1) A complete set of instruments for collecting ice samples, water samples, sediment samples, and simulating in laboratory is developed. Compared to the existing sampling method, the adaptability and feasibility of these instruments are described:the ice drill is easy to use, which solves the crucial high-price problem of imported instrument; water and sediment samplers well combined with ice drill can collect samples at every part consist of water nearby ice-water interface and water-sediment interface, water in sediment pores, and core sediments without disturbance.(2) The field observation data shows:The effect of temperature on the under-ice water temperature is not significant, while ice temperature significantly increases as temperature rises and as ice depth deepened. The empirical formula of Zubov model, that is, FDD=0.002T2-0.2251T, can well describe the relationship between cumulative freezing-degree-day and corresponding ice thickness. The ice density changes not significantly in freezing process, it increases as ice depth deepened. The bedding structure of ice changes gradually as ice thickness grows. There was a good negative correlation between water depth and ice thickness in Ulansuhai Lake, and its correlation coefficient is0.53. The under-ice water is almost still during the icebound season.(3) Ice samples and water samples at different depths were collected as ice thickness grows. The pollutant content of samples were analyzed to dicuss the migration mechanism in freezing process by means of crystallography and thermodynamic theory.TDS profiles are drawn to show the distribution of TDS and to describe TDS migration qualitatively and qualitatively. The results showed that between ice and water, about80%, that is360158400kg, TDS migrated from ice to water during-the whole growth period. Within ice layer, TDS migration only occurred during initial ice period, then stopped almost. The ice TDS decreased with increasing ice thickness, following a negative exponential-like trend. Within under-ice water, TDS migrated from ice-water interface to the entire water column under motive power of concentration difference until the water TDS was uniform. Between water and sediment,60440950kg (16.78%of360158400kg) TDS migrated from water to sediment. The fitting curves for transfer flux of both ice-water and water-sediment to growth rate displayed good linear positive correlation relation.pH profiles are also drawn to describe the change of pH value in freezing process. The results showed that it was not significantly that pH value as ice thickness grows because of buffer effect of carbonate system, which made the change of pH value different from TDS.The space-time curve of N, P, Chlorophyll a, COD and BOD5were drawn to elaborate the migration mechanism and effect in freezing process based on analyzing the content of samples from upper ice, middle ice, lower ice, water at ice-water interface and water-sediment interface and middle water at different ice thickness. The results showed that exclusion effect of freezing process on N, P, Chlorophyll a, COD and BOD5was different, and the concentration of them in under-ice water increased as ice thickness grows. The concentration of TN, NO3-, NO2-, NH4+in under-ice water was respectively2.06,1.77,1.26,2.35times of that in ice. The concentration of TN, NO3-, NO2-, NH4+in water at ice-water interface was higer than that in middle water and at water-sediment interface, while the concentration of the latter two position was similar. The concentration of TN, NO3-, NO2-, NH4+in upper ice was higer than that in middle ice and lower ice. The difference between the two decreases with the increase of ice thickness, which revealed various forms of nitrogen migrated within the ice. The concentration of TP, DTP, PO43-and Chlorophyll a in under-ice water was respectively1.50,1.57,1.82and1.64times of that in ice. The concentration of Chlorophyll a in water at ice-water interface was higer than that in middle water and at water-sediment interface. There was no obvious rule among the concentration of Chlorophyll a, TN and TP in ice, but they were consistent in water.The exclusion effect of freezing process on organics was more obvious than that on inorganics. COD in under-ice water was3.13times of that in ice. COD in middle ice was higer than that in upper ice and lower ice while COD in water increased from top to bottom due to the sedimentary action. BOD5in under-ice water was3.24times of that in ice. BOD5in lower ice was higer than that in middle ice and upper ice. BOD5in water at water-sediment interface was higer than that in middle water and at ice-water interface. The PBOD5/PCOD in under-ice water was higher than that in ice, which revealed the biodegradability of under-ice water was better than that of ice. There was positive correlation between COD and BOD5in every sample.(4) The eutrophic state of Ulansuhai Lake before and after icing was assessed by mean of fractal theory to discuss the environmental effect of freeaing process. The results showed that the eutrophic state of water before icing, ice and under-ice water was respectively eutrophication, eutrophication and heavy eutrophication, that is, freeaing process made the eutrophic state of Ulansuhai Lake more heave because part of cleaner water was freezed in ice. The result can provide theoretical basis and data support for sediment dredging that is an effective nutritious repair countermeasure.The concentration of TDS in under-ice water in freezing process was forecasted by mean of weighted markov chain, which was accurate and able to meet the forecast requirements.(5) The idea that freeze purification process can be used easily and widely in that was rich in cold energy, wind energy and solar energy but existed drinking water safety hazard was proposed. Furthermore, the purification process was designed. The operating parameters in the purification process, that was ice thickness, freezing temperature, rate of area and depth of container, ice seed and freezing series, were optimizated based on that the feasibility of laboratory simulation was verified. The purification process can provide reference for drinking water safety in pastoral area. |
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