Research On The Transportation And Change Mechanism Of Fluorine In Water And Soils In Yangshapao Reservoir, Western Jilin Province

The subject is chosen and specific studies are carried out according to the problem of water quality improvement of Yangshapao Reservoir for Water Supply Project from Nenjiang River to Baicheng City, which is one of the most important water resources projects of"the Eleventh Five-year Program"in Jilin Province. The Yangshapao (also name as Yangsha lake) reservoir is located in the arid and semiarid low plain area in western Jilin Province, where the fluorine content in environment is high. The reservoir is a natural lake and its water comes mainly from the seasonal recharge of upstream Erlongtao River. So the lake has been dried for several times, and the total dissolved solids, concentration of fluorine and iron are quite high, and fluorine has great effects on the water quality. Therefore, it is of great practical value to find out not only the origin, distribution characteristics and influencing factors of the fluorine, but also the long-term release and transfer principle of fluorine in the bottom mud, which also has great academic and practical value for the management and protection of water resources in arid and semiarid areas.Since there has been few the domestic and foreign references about the research on spacial & temporal changes of fluorine in high concentration lakes, the principle of fluorine transfer through the interface of water and soil, the release characteristics of the fluorine in the bottom mud of lake, moreover, the research on the absorption and desorption of fluorine by soil is very limited in the practical projects because of the short period and little scale. It is necessary to carry out experiments, to lengthen the research period, and to enlarge the scale for the research on long-term release characteristics of the fluorine in bottom mud of lake, for the study of the effects on fluorine in the water environment by the release of the bottom mud, and for the analysis of transfer principle of fluorine in water & soil environment. According to the experiment data and analysis results from the actual samplings in situ, the spacial & temporal changes of fluorine in Yangshapao reservoir are forecast after the diversion project put into use. Sampling was carried out during six different periods and 84 samples of water and ice from Yangsha Lake, 15 samples from groundwater around the lake, 14 samples from Nenjiang River, 2 samples from the Erlongtao River, 88 samples from the bottom mud, and 15 samples from the lake banks were analyzed. The sampling periods was over a year, which covers 4 different seasons (spring, summer, autumn and winter), low and high water periods, frozen and thaw periods, which made the samplings typical. The experiments were designed as follows: beaker experiment, fluorine release experiment from the bottom mud in flume (or trough) and in column, water quality mixture simulation experiment, surface runoff simulation experiment from the bank soil, chemical dynamic fluorine release experiment from the bottom mud or soil, and so on. Each experiment was designed for several schemes. In practice, the beaker experiment period was one day; fluorine release experiment from bottom mud in flume took 176 days and in four schemes with 38×4 samplings;fluorine release experiment from bottom mud in column took 171 days and in six schemes, with 18×6 samplings;water quality mixture simulation experiment took 149 days and in one scheme, with 7 samplings each time and with 231 total samplings; surface runoff simulation experiment took one day, with 34 samplings; chemical dynamic release experiment from bottom mud tookr four days and twice, with two schemes and 40 samplings. And 13 samplings were analyzed for different forms of fluorine.The fluorine concentration was high at the beginning of 2007, which is about 5～10 times of the drinking water standard level. The distribution characteristic in the lake is that it is the highest near the southeast bank among Chagan, Dongping and Xiyangsha, and the lowest in Donyangsha. However, the concentration gradient in water is not big.The main influencing factors are the fluorine concentration in bottom mud, the water body entering the lake, the hydrodynamic conditions, the lithology, wind dynamic conditions and so on. The fluorine in spring, or thaw period, is the lowest, and increases gradually. The change of fluorine in bottom mud is big. The total fluorine content in the bootom mud is 588～1157 mg/kg and that in the western part of the lake is higher than that in the eastern part. The fluorine in soluble salts is 13.13～82.53 mg/kg in bottom mud, and the fluorine is high among the Xinyangsha and Donyangsha, while low in southeast, which decreases from west to east. The fluorine in unconfined pore water is 2.05～4.20 mg/L, and that is more than 1.0mg/L and less than 1.5mg/L in confined water.The experiment indicated that the max fluorine release quantity from the bottom mud is 81.58～132.11mg/kg, which was big at the beginning of release progress, and the increasing amplitude decreases gradually, and so is the release rate, which keeps steady about 0.01～0.06 mg/kg.d, until the release was over. The fluorine released from the bottom mud can be quickly mixed with water in the lake. The wuantity of water in upper part has great influence on the fluorine release from the bottom mud, the greater the quantity of water, the greater the fluorine release from bottom mud. The accumulative fluorine release matches the logarithm curve distribution law. The difference of fluorine concentration in extraction liquid from the bottom mud can influence the fluorine release, so the fluorine extracted by the distilled water is higher than that by tap water (fluorine is about 0.32 mg/L). The absorption dynamic release equation is in accordance with the distribution character of logarithm curve.The formation of the high fluorine Yansha lake is the result of long-term evolvement of the surface envioronment. The volcanic rocks of Daxing'an Mountain and unconsolidated deposits of Songnen low plain provides matter basis bearing abundant fluorine for Yangshapao reservoir, rivers and groundwater runoff provides dynamic conditions for fluorine movement, typical low lying topography provides geographical conditions for the formation of high fluorine, arid and semiarid climate promotes the formation of high fluorine environment. The main sources of fluorine in Yangsha Lake include water influx from upstream, rainfall taking, influx of runoff around and the release from the bottmom mud in the lake. The average annual fluorine quantity from upstream Erlongtao River makes up 94.68-98.85% of annual fluorine source, rainfall taking makes up 1.05-5.17%, and runoffs around makes up 0.15%. The strong evaporation and concentration action without sources outside entering increases the concentration of fluorine in water by 0.38-1.47mg/L.According to the theory of chemical dynamics, long-term experimental study on the law of fluorine release in high fluorine bottom mud is carried out importantly; meanwhile, the model to describe the long-term release characteristics and law of fluorine in high fluorine bottom mud is presented. The expanding equation of the model is logarithm curves. The fluorine release quantity from the bottomb mud increases with time, but the increase extent decreases until the end of the total release. The chemical reaction rate model has been presented by using the results of fluorine dynamic release experiment, and the model basically matches Elovich pattern.The release of fluorine from the high fluorine bottom mud is related closely with pH, total dissolved solids and the ratio of Na/Ca. Alkalinous environment is proportional to the release of fluorine; the content of fluorine increases with the increase of total dissolved solids, which indicates that with the increase of salinity or the evaporation and concentration of water, the fluorine increases or be concentrated as the same. The content of fluorine increases with the increase of the ratio of Na/Ca, which indicates that the increase of the ratio of Na/Ca is proportional to the release of fluorine. Freezing in winter has an effect of rapid concentration on the fluorine in water body, which makes the uniform distribution of fluorine in water body.The total fluorine content in bottom mud in Yangsha Lake is is 2.94-5.79 times more than the global one with an average of 200 mg/kg. The water-soluble fluorine is 26.77-112.39 mg/kg. The content of different types of fluorine in the bottom mud in Yangsha Lake shows that: residual fluorine >> water-soluble fluorine > organic fluorine > iron-manganese combined fluorine e >exchangeable fluorine. The reasons of the high concentration of water soluble fluorine in bottom mud are:①Yangsha Lake belongs to the high fluorine environment, and the content of fluorine in bottom mud is higher than that in the soil around;②Some parts of the fluorine in bottom mud in Yangsha Lake comes from the mother material of soils, while others comes from the influx of river runoff;③Yangsha Lake has become dried several times because of the strong evaporation and concentration, and the fluorine in water is deposited into the bottom mud, which makes the content of water-soluble fluorine is very high;④The bottom mud in Yangsha lake belongs to alkaline environment, and the pH is between 8.0-9.5, which is favourable to the release of fluorine.In soil-water environment of Yangsha Lake, fluorine is migrated and transformed through the processes of precipitation - solution, adsorption– desorption, iron exchange and replacement. The content of total fluorine in the bottom mud is relatively high, and this provides an enogh matter basisi for the migration and transform of fluorine in the water and soil environment. The content of soluble fluorine in the bottom mud is relatively high, it is easy for fluorine to be dissolved or desorbed under long-time hydro dynamic conditions; Biological fluorine and invertible abiological fluorine in the bottom mud of Yangsha Lake can be totally desorbed into water ultimately. The residual minerals in the bottom mud include biotite, phosphorite or apatite, cryolite and fluorite, in which the fluorine can gradually and continuously migrate into water in long-time water environment by iron exchange and replacement, and some residual fluorine in the bottom mud migrates into water. Besides, the alkaline water and soil environment of Yangsha Lake reduces the appetency and adsorption capacity of irons by solid phase surface, which accelerates the desorption of anions, and improves the exchange ability of fluorine, making F- in crystal lattice of clay minerals exchange out. The alkaline environment is favorable to separating and release of fluorine in the bottom mud of Yangsha Lake. Therefore, the fluorine migrating from soil (bottom mud) to water becomes the main direction in the water and soil environment of Yangsha Lake.On the basis of previous study results, the water quality of the lake influenced by the bottom mud and ice layer is analyzed and predicted by multiple technical methods. Using the coupling model of water budget equation and solute conservation law, and a month as a time step, considering following 8 programs for prediction: (1) hold raw water, and do not consider urban water supply; (2) consider urban water supply in some time-intervals; (3) discharge intensively, only hold dead storage, and do not consider urban water supply; (4) discharge intensively, only hold dead storage, consider urban water supply in some time-intervals; (5) empty out the storage, do not consider urban water supply; (6) empty out the storage, consider urban water supply in some time-intervals; (7) empty out the storage continuously in spring of 2 years, consider urban water supply; (8) empty out the storage continuously in 3 years, consider urban water supply in some time-intervals. The results show that emptying out all storage before diversion in spring, the water quality is relatively good during water diversion, and the conerntration of fluorine in water is relatively low and urban water supply can be considered, and the conerntration of fluorine in water can meet the drinking water standard in the fifith year. Therefore, it is suggested to use the program that empty out all storage before diversion in spring, then transmit water, and supply the city with water in different time-intervals.Under the conditions of not emptying out the current high-fluorine water and continuously water divertion, the conerntration of fluorine in water is relatively high in three years and can not fully meet the drinking water standard forecasted by using simulation experiment.In order to predict and forecast the spatio-temporal distribution characteristics of fluorine in the reservoir further more, the law of distribution and migration of fluorine in water is numerically simulated by using the software Delft 3D. Based on actual results of measurement and engineering design, hydro-dynamic numeric simulation model is established firstly, finite difference element approach is adopted to get solution; then the numeric simulation model of water quality trsportation is established, the calculating area is meshed with orthogonal system of curves, while the grid step is controlled between 60～65m and 8725 grid units in total. The prediction of 3 programs is carried out according to the design characteristics of fluxes at inlet and outlet, and the result is similar to the results from previous analytical and experimental simulation methods and shows that the water quality will be improved to different degrees under different water changing conditions. Based on above results of water quality prediction, available program of water quality improvement in Yangsha Lake is suggested.Against the situations that the study scale is small and the time is short during former experiments, the time is prolonged and study scale is amplified during this research. It is the first time to research the long-term fluorine release law in the bottom mud in high fluorine lake and reservoir experimentally by using combined methods of indoor experiments and filed sampling analysis, and the long-term slow fluorine release law and transportation mechanism in the high-fluorine lake and reservoir has benn revealed. The long-term slow fluorine release rate and the fluorine release quantity from the bottom mud are analyzed and calculated and this provides a basis for the reseaches on the long-term slow fluorine release law from the bottom mud in high-fluorine lakes and reservoirs in China. It is the fist time to cary out qualitative analysis and quatitative calculation about the effects of ice layer on the fluorine transportation in the water and soil environment, and the results expands the research on the influencing factors of fluorine transportation in water and soil environment. According to the theory of chemical dynamics, the experimental research of long-time fluorine release law in high fluorine bottom mud is importantly carried out, and the dynamic fluorine release model for describing the long-term slow fluorine release in the bottom mud of high fluorine lake or reservoir is established for the first time, and accumulated fluorine release equations under multiple conditions are fitted, the study results fill the blank of the study on the long-term fluorine release law of the bottom mud in high fluorine lakes and reservoir in China.