Study On The Response Characteristics Of Water Quality To The Seasonal Thermal Stratification And Its In-situ Pollution Control In A Deep Channel Reservoir Of Northern China

Regarding the water pollution characteristics of deepwater reservoirs and the technologies of water mixing oxygenation as the breakthrough point, the method which combines the laboratory monitoring and productive test for water in-situ restoration has been used in this study. It mainly focuses on①the response characteristics of water quality to the seasonal thermal stratification in J-P reservoir;②the performance of water mixing oxygenation of the technology of Water-lifting and Aeration (WLA) under the deepwater environment, as well as its inhibitory effects on internal pollution and algal growth;③optimization of WLA system in J-P reservoir. The main conclusions are as follows:(1) The thermally stratified period of J-P reservoir lasts long (from March to December) and the isothermally mixed period is relatively short (from January to February),which shows a single-cycle mixed pattern. During the thermally stratified period, the existence of temperature gradient effectively restricts the process of heat and mass transfer and exchange among the vertical water layers, which accelerates water to aerobic state at the bottom of reservoir. The average concentration of dissolved oxygen (DO) in the hypolimnion shows a linear decline with time. Seasonal oxygen deficit is considered to be the dominant cause for the decreation of pH. By "bottleneck" effect of thermocline, aerobic conditions at the bottom have limited effect on pH variance of surface water, however, the algal growth and temperature changes closing the air-water interface have great effect on it. Both the water conductivity and ammonium (NH3-N) concentration have significant negative correlation with DO variance, in contrast, this correlation between total phosphrous (TP) and DO is relatively poor, which is due to the water acidity increase. During the isothermally mixed period, the understable failure of stratification structure occurs. The whole reservoir was in aerobic (D0>8mg/L) and slight alkaline (pH:7.1～7.3)state, moreover, the water quality becomes homogeneous under the influence of convective mixing among vertical water layers.(2) The seasonal variance of algae abundance in raw water of J-P reservoir presents a "saddle type" distribution. The spring peak appears in every may and the summer peak more in July and August. Algae abundance has significant positive correlation with water temperature and TP and negative correlation with water level changes, however the correlation with NH3-N is poor, which indicates that the dynamic change of TP content in water is the limiting facter for algae proliferation. During monitoring period,69kinds of typical phytoplankton has been identified, which belongs to5phylums,7classes,12orders,20famils and44genuses based on phycological systematics. The proportion of species from high to low:Chlorophyta> Bacilloriophyta> Cyanophyta> Xanthophyta, Cryptophy, etc. The successional characteristics of phytoplankton are reflected as that the algal community is mainly made of diatoms in spring and winter and blue-green alga becomes dominant in summer and autumn. The concentration of Chlorophyll-a in the surface water has a similar variation tendency with the algal abundance, but not a synchronous proportional relationship. The steady chlorophyll-a content (based on Loiselle’s steady state assumption) has negative expontential correlation with mixing depth and varies unimodally.(3) The evaluation results of water eutrophi cation based on the method of Fuzzy Composite Index (FCI) show that:the current eutrophic state in J-P reservoir has generally achieved medium-high to high nutritional level and becomes more serious year by year. The existence of thermal stratification in J-P reservoir is considered to be an important factor for water increasing pollution and eutrophi cation. In addition, with the running time extension of J-P reservoir, the risk of sudden water pollution caused by understable failure of thermal stratification will increase.(4) The improved floating-type water-lifting aerator (WLA-F) was successfully used for contaminated water restoration in J-P reservoir. Under the mixing and aeration conditions, the thermal stratification structure was disturbed but not destroyed. The vertical water temperature difference was reduced by3～5℃and the surface temperature falled at2.1～3.6℃. The position center of thermocline moved down (drops nearly20～30m in summer) and its temperature difference was reduced by4～6℃. The seasonal aerobic state was withheld at bottom of the reservoir, and the concentration of DO maintained above2mg/L at the sediment surface. The decay rate of DO in the hypolimnion decreased by51.2%and49.6%respectively compared to the same period in2008and2009, which falled to0.0184mgO2/(L·d).The concentrations of TP and NH3-N in the overlying water on sediment surface decreased to0.12mg/L and0.25mg/L respectively, which was reduced more than54%and70%. CH2Cl2was completely removed. The algal abundance in surface water falled below4.5X106cells/L, which was reduced more than75～80%. The relative abundance of cyanophyta decreased from30～31%(monthly average proportion in2008and2009) to6.8%in2010, and bacilloriophyta increased from15%～22%to46.8%. The concentrations of TP, NH3-N, CODMn and chlorophyll-a in the outlet water was respectively reduced by46.7%,69.5%,22.4%and53.6%compared to the year2008and47.2%,67.1%,25.8%,56%compared to2009, which achieved grade Ⅰ-Ⅲ of the Environment Quality Standards for Surface Water (GB3838-2002).However, the present cost of water improvement considering the current water supply capacity (80×104m3) was only0.004Yuan RMB/m3.(5) According to the problems about firmness and stability of WLA-F system existed in the operation process, several measures such as the hole opening on the sidewall, installation of water radiation-flow stabilizer (WRFS) and automatic control system for pressed-water injection have been designed and introduced, which aims at reducing the system operating risks, energy consumption and improving the efficiency of WLA-F.