Study On The Eutrophication And Models Of Hydrodynamic And Water Quality In The Pengxi (Xiaojiang) River Of Three Gorges Reservoir

The eutrophication of the secondary river in the Three Gorges reservoir is one ofthe most severe ecological environmental problems currently in china. After theimpoundment of the Three Gorges reservoir, the nutrient concentrations did not changesignificantly in the Yangtze River and the water quality remained stable in general.However due to water retention time extension, the eutrophication in the secondaryriver shows increased trend, the phenomenon of algae bloom has occurred many times.The change of hydrodynamic condition was the main reason of the water eutrophicationand algae bloom, therefore, it is significant to study the influence of the hydrodynamiccondition on water pollution concentrations and eutrophication in order to control algaebloom.In this paper I analyzed the meteorological condition, water hydrological conditionand pollution load in the Pengxi catchment, the change of water environmental factorssuch as hydrodynamic condition and the water quality in the Pengxi River. Based onthis data, I built the SWAT and MIKE21coupled models to study the feasibility of damregulation to control eutrophication, and to put forward proposal of optimizationoperation schemes. My research provided scientific basis for the water environment andeutrophication control in the backwater area of the Three Gorges reservoir tributary.This research is summarized in the following aspects:â‘ The characteristics of meteorology, hydrology in the Pengxi catchment andbackwater morphology.In the Pengxi catchment the photothermal conditions meet the need of the watereutrophication which occurs frequently at the end of spring and the beginning of thesummer. The rainfall and runoff distribution are uneven. The Pengxi River morphologyis complicated, the cross section areas are from1082m²to75126m²and water surfacewidths vary along the river are from57.60to2026.31m after the impoundment.â‘¡The current situation of water quality and eutrophication in the Pengxi River.I studied relations between environmental conditions and hydrodynamic conditionsbased on environmental monitoring data. In addition, I analyzed the relations betweenthe water pollution and hydrological factors based on the rainfall, runoff and the valuesof temperature, dissolved oxygen （DO）, nitrogen （N）, phosphorus （P）, chlorophyll-a（Chla）, chemical oxygen consumption（COD）, five-day biochemical oxygen demand （BOD₅）, etc. The results showed that the season factors such as temperature and rainfallwere the main influential factors of the water environment. The other factors are fromthe impoundment of the Three Gorges, and the change in water level due toimpoundment ultimately leads to the water environment change. The DO concentrationrose and tended to be even, the peak concentration of total nitrogen（TN） decreasedslightly and the peak concentration of total phosphorus （TP） increased slightly, and N/Pvalue had the tendency to decrease. After the impoundment the hydrological conditionswere helpful to form water eutrophication. The analysis of a year period showed that thehigh concentrations of TN and TP were easy to develop in the high precipitation andhigh discharge period. The high concentrations of TN and TP occurred under theconditions of runoff higher than150m³/s, or daily precipitation higher than30mm,however the concentrations of Chla were relatively low during this period. The highChla concentrations were easy to develop from March to May when the runoff is35-45m³/s and the early precipitation is below the15mm.â‘¢Effect of the Three Gorges reservoir impoundment on the pollutant degradationcoefficients and water environmental capacities of the Pengxi River.Affected by the natural factors and impoundment regulation, the pollutant retentiontime and the degradation rate in the Pengxi water presented dynamic characteristics.The pollutants degradation coefficients after the impoundment were1/20-1/10of thecoefficients before the impoundment, which affected the water environmental capacities.The results simulated by the1D-2D coupled model from seven water function divisionsshowed that after the impoundment of the Three Gorges, the COD_Mn capacity wasreduced by15.6%, the ammonia nitrogen（NH3-N） capacity was reduced by12.2%andthe TP capacity was reduced by28.3%within a year. Water hydrological conditionsafter the impoundment were favorable for the eutrophication in the Pengxi River.â‘£The study on the effect of the impoundment on water dynamic characteristicsin the Pengxi backwater area simulated by the MIKE21model.After the impoundment of Three Gorges project, water depth and the area andwidth of river cross section from the Pengxi backwater water increased. On thecondition of the same runoff, flow rate was reduced as compared to the naturalcondition, and it was inversely proportional to the water level. Before the impoundmentthe minimum flow rate is0.01-0.19m/s and after the impoundment the minimum flowrate is0.001-0.005m/s in the Pengxi backwater. The mean flow rates of five sectionsfrom Quma, Gaoyang, Huangshi, Shuangjiang bridge and Hekou after impoundment were1/2-1/100of those before the impoundment. According to the hydrodynamicconditions and the water quality of the river, the water type was defined as thefollowing: the flow rate of lake type is lower than0.016m/s, the flow rate of thetransition type is0.016-0.050m/s, and the flow rate of the river type is greater than0.050m/s. The dynamic change of water type lead to the change of the applicable waterquality evaluation standards, thus the evaluation result of II-III water quality standardturns to exceeded II-III water quality standard.⑤The study on the non-point pollution load in the Pengxi catchment based on theSWAT distributed hydrological model.Based on the data from digital elevation, soil types, vegetation types, meteorology,hydrology, water quality, administrative divisions, population distribution, etc., thedatabase was set up with these as the SWAT input conditions. The Pengxi catchmentwas divided into twenty five subcatchments and225hydrological response units（HRUs）. The results simulated by SWAT showed that Nitrogen and Phosphorus loadsfrom July to September were high, the mean monthly loads of Nitrogen and Phosphoruswere928.67tons and173.47tons respectively in the flood period; The Nitrogen andPhosphorus loads from December to February is low, the mean monthly loads ofNitrogen and Phosphorus were61.91tons and11.56tons respectively in the dry period.â‘¥The SWAT and MIKE21models were coupled to predict the water quality andwater eutrophication.I simulated the average COD_Mn, TN, TP concentrations and Chla content along thePengxi River on two design conditions:1)175m high water level with low flow and lowpollutant load;2)145m low level with high flow and high pollutant load. COD_Mn, TN,TP concentrations from upstream to downstream tended to increase, and the waterquality at the estuary trended toward the Yangtze water quality. In the period of145mlow water level with large flow and high pollutant load, the Chla content was high. Inthe curve or wide surface sections such as Gaoyang or Shuangjiang Bridge, the flowrate was slow and Chla content was high.The correlation of flow rate （V） and Chla content （S） was fitted under the twodesign conditions, the equations were S=0.1860·V^1.6820（related coefficientr²=0.8380）and S=10.8449·V^-0.2939（related coefficient r²=0.8380） respectively. Thesimulated results showed that the increase of flow rate tended to keep down the Chlacontent. It is feasible to control water eutrophication through the hydrodynamicconditions. ⑦The ecological regulation schemes put forward relied on the Xiaojiangecological regulating dam.Based on the correlation between the flow rate and Chla content, the constraintconditions of hydrodynamic feature and hydraulic project, I established the ecologicalscheduling models. And I put forward the optimization schemes from March to June tocontrol Chla content of lower than20ug/L for the purpose of controlling watereutrophication.