Migration And Transformation Of Nutrient Salts In North China Lakes

A lake is a body of relatively still water of considerable size, localized in a basin that is surrounded by land. It's well known as flood resisting, irrigation, navigation, water supply and recreation. In recent years, lakes and reservoirs received an excessive amount of industrial and domestic wastewater with the increase of population and rapid economic development, which greatly accelerates the eutrophication process. So Chinese environmental researchers take a lot of research for the lake eutrophication study, especially at room temperature, which have achieved fruitful results. But the research in low-temperature (<10℃) and ice-bound conditions about lake eutrophication is relatively few. In China, except the Yunnan-Guizhou region which water temperature is above 0℃through years, the other regions have appeared different levels of ice condition. Especially in the northeast plain and mountain lakes, the winter is long, the frozen period even up to 4 to 6 months. In recent years, the lakes in northeast China have developed eutrophication rapidly because of serious pollution. As the plain lakes are shallow and small which flow rate are slow, while serious man-made pollution, eutrophiction even "bloom" phenomenon frequently occured, affecting the growth of aquatic life and people's living. Therefore, it is very important to research the eutrophication with low-temperature and ice-bound condition in north China, especially in northeast region. In this paper, we will carry out a case study on the southren lake, Changchun City.This paper will take migration and transformation of nitrogen and phosphorus nutrient salts as well as eutrophication evaluation and potential risk analysis as the main line, using limnology, environmental engineering, environmental science, environmental water chemistry, chemical kinetics, and statistical theory and methods as guidance, by combining field investigation, testing and simulation, collected 130 water samples, 39 ice samples, 30 sediment samples in 2007 to 2009, carrying out adsorption and release simulative experiment at the same time, for studying migaration and transformation and potential eutrophicaion risk of nitrogen(N) and phosphorus(P) nutrient salts in the condition of room-temperature and ice-bound. The main research results are showing below:(1) The basic chemcial characteristics of lake water is low mineralization and alkaline, and in weak reducing environment. Water chemistry type has a significant seasonal variation characteritics. That is, the water chemistry type changed from HCO₃·Cl-Ca(Ca·Mg) to HCO₃·SO₄^-Ca (Mg / Na)as the season changed from wet to ice-bound. In particular, the raise of SO42- content further proved the lake environmet is weak reduction condition.(2) By test ice and water samples which obtain in field, revealed that the temporal and spatial distribution of different forms of N and P, and chlorophyll-a(Chl-a) and algae. In general, all forms of N contents showed that wet period > ice-bound period > normal water period by temporal; but the contents of all forms of P, Chl-a and algae are showed that wet > normal water > ice-bound period. During the room-temprature period, surface P contents are larger than the underlying in the vertical direction; but during ice-bound period, the contents of nutrient, Chl-a and algae in the aqueous phase are lager than the ice phase. The contents of all forms N are protected area > human activity area > louts pond area on the plane; in lotus pond, the content of P is highest in ice-bound period; in human activity area P is highest in normal water period; and in protected area P is highest in wet period; the contents of Chl-a and algae are protected > lotus pond > human activity area.(3) By researching migration and transformation of nutrients and algea in ice-bound season, and nutrients partition coefficient between the ice and water phase, and conditions of algea recovery, it can dudece that migration and transformation rules of nutrients salts between ice and water during low-temprature and ice-bound condition in north China lakes. In freezing period, ice above the water cut off the access between water and atmosphere, making the water in weak reducing environment, and decreased the biological nitrogen fixation; and nutrients in the water may deposited on the bottom sediment through particulate matter and animal and plant debris, that two reason maked the contents of nutrient salts decreasd slowly; in the thawing period, the upper melt-water mixed with the lower water, playing the role of diluting the nutrient concentration in water; otherwise, it would consume some nutrients when recovery of the growth of phytoplankton and other plants and animals, which make the nutrient contents decreased during the thawing period. At the beginning of frozen process, the contents of all forms fo N decreased slowly, and P increased slowly; in general, P partition coefficient between ice and water are higher than N. In both lake water and single substance solution, NO₃^- and PO₄^3- partition coefficent are higher than NH4+'s. During recovery period, algae is growth suitably in temprature between 20℃to 25℃, and 25℃is the optimum temprature for algea growth. During freezing-thawing period, N and P contents were higher than the critial value of eutrophication in aqueous phase, which means the lake is rich in nutrient salts. Although the nutrient in water phase is higher than ice phase, it's only the physical changes, which tatol content is still the same. So it has potential eutrophic risk in the normal water period. (4) By testing the samples taken on the field, it can obtain nutrients' forms in sediment and temporal and spatial distribution characteristics. On the content, N is dominated by organic nitrogen(ON); inorganic nitrogen(IN) is dominated by ammonium; P is dominated by inorganic phosphrous, and IP is dominated by Fe/Al-P. In time series, the content N and P are all wet period > ice-bound period > normal water period, except ammonium. On the plane, the content of N, P are all highest in the region of along coastal area through the louts pond to west to the southwest corner, significantly higher than middle of human activity area and east coastal area in the north of Southern Lake Bridge.(5) By adsorption and release simulation study, it obtained that N and P adsorption kinetics accordance with quasi second-order reaction kinetics; from fitting effects, using Langmuir thermodynamic equation is better than Freundlich equation. According to Langmuir equation fitting results, for site 1-2 and 3-2 sediment, maximum adsorption capacity of ammonium was 1000mg/kg and 1111.11mg/kg respectively, PO43- was 500mg/kg and 476.19mg/kg respectively, which means that sediment has a huge potential adsorption for N and P, that is the theoretical basis which the sedimnet is the eventually sink of N and P. Meanwhile, it can estiamte nutrient salts annual cumulation of the whole lake, with reference to quality balance caculation method, the result showed that the whole lake annual TN cumulation is 4.59t, TP is 1.47t. According to eluted and release simulation experiment, TN release per unit volume of sediment is 37.22 133.26mg/kg, accounted for 3.30% 11.66% of the base value, an average of 5.56%; ammonium release per unit volume of sediment is 15.12 116.41mg/kg, accounted for 5.31% 29.62% of the base value, an average of 10.96%; we can conclude that ammonium is much easier release form sediment. TDP release per unit volume of sediment is 459.4～1149.8μg/kg, PO43- release per unit volume of sediment is 130.7～524.2μg/kg, which is less than TP base value in three order of magnitude, which means it will significantly increase the phosphorus concentrarion even a small amount of phosphorus in the sediment released into the lake. Therefore, a large number of nutrient accumulated in the sediment is the material basis of lake eutrophication. According to exsiting data, Southern Lake water pH value between 7.0 to 8.5, while simulation in room temperature, neutral pH value, so the experiment match practical condition. Temperature and pH value, which has a significant impact on N and P release from sediment. N and P release from sediment increased significantly with increasing temperature. It can release maximum N but minimum P when pH value in neutral. The results provide a theoretical basis for nutrient salts released from sediment in wet and normal water period.(6) Based on the study of field investigation and laboratory simulation experiments, it built a trophic level index(TLI) model to assess the eutrophication and a Bayesian multivariate statistical model to predict the eutrophication risk. The results showed that, The trophic level in wet period is moderate eutrophic, in ice-bound period is mesotrophy, in normal water period is mildly eutrophic; TLI is wet > normal water > ice-bound period, which eutrophication risk decreased in turn. The ice-bound period almost has no risk of eutrophication because of the trophic level is mesotrophy. Different functional areas of the room-temperature period are all mildly eutrophic level. According to principal component analysis, the most relevant factors with Chl-a were TP,SP,TDP and ON, and built the Bayesian multivariate statistical simulation model according to the results. Fitting results show that the TP, TDP and SP effects on the Chl-a is higher than the ON. Chl-a content has a large range in the wet period, which proved wet period are more eutrophication risk in wet period. The results provide a sicentific basis for lake eutrophic management in north China.