| Along with the city construction, contradictions between the water source protection and economic development are becoming more and more aculeate. As the main source water supply of Xi'an city, Heihe Reservoir is facing some problems that the water quality is deteriorating and the reservoir is gradually becoming eutrophic. Therefore research on the water quality and algae growth is important to the source water protection and pollutant control.Based on the field monitoring and laboratory assay, the characteristics of the water quality and seasonal thermal stratification in Heihe Reservoir were investigated and the main conclusions are as follows:(1) The concentration of TN in the source water, with an average of 1.29 mg/L, has reached far beyond the regulated limit. Besides TN, the content of TP has also reached 0.031 mg/L and has been increasing year by year. Eutrophication is apparent in Heihe reservoir. Runoff water quality deteriorates in the rainy season becasue rain erosion makes a large number of surface pollutants go into the river and lead to a dramatic rise in total nitrogen and organic matter contents. The average content of TN can rise up to 1.60 mg/L, CODMn up to 4.0mg/L or so, both of which are 1-2 times higher than those in the sunny season. Using with the data of reservoir runoff upstream, the upstream input pollutant loads are approximately estimated to be:TN 605.76t/a,TP 15.41t/a.(2) The algal growth showed a double peak law in Heihe Reservoir in 2009. Algae blooms occurred from April to May and from August to September, and the highest cell densities reached 6.1 million/L and 7.7 million/L respectively. A single dominant species, mainly Cyclotella, grew in spring; the diversity of algal species in the summer and autumn increased, and green algae and diatoms are dominated species. The algal growth was mainly restricted by phosphorus content. In addition, the dilution effect of flood flows is also important to the algal growth. According to the 2008 and 2009 monitoring results, algae cell density and the reservoir water level has significant negative correlation.(3) Heihe Reservoir is stratified. March～May is the formation period of thermal stratification, June～early October is the stable stratification period, October～December is the weak thermal stratification period, and from late December to February in the next year is the winter mixing period of the reservoir. The cold runoff can form undercurrent layer in summer, breaking the natural thermal stratification with a depth range of 20m～40m. The vertical temperature change and increase in mixing depth in the stratified reservoir in winter is a gradual process, which starts from gradual partial mixing and ends with complete mixing after a large number of cold-water sink finally. Heihe Reservoir went into completely mixed state in the middle of January 2010:bottom water was rapidly reoxygenated, pH and conductivity distributions over the water depth were also consistent. |
