Water Quality Simulation And Management Strategies Of Two Reservoirs For Drinking Water Supply In Zhuhai City, South China

Fenghuangshan reservoir and Nanping reservoir play an important role in water supply in Zhuhai city. Since most water was pumped from polluted down steram of Pear River, the water quality of the two reservoirs has greatly deteriorated in recent years, and cyanobacteria bloomed ocasionally The blooming of cyanobacteria not only depends on nitrogen and phosphorus loadings, but also on hydrodynamic factors associating with hydrology and meteorology.We used ELCOM-CAEDYM 3D hydrodynamic coupled with aquatic ecological model to simulate temperature, nutrient concentration and cyanobacterial concentration of the two reservoirs in Zhuhai. The model simulation showed that the temperature ranged from 14℃to 28℃in Fenghuangshan reservoir. The concentration of total nitrogen significantly varied spatially from southern part to northern part, while nitrogen concentration was the highest in the inlet zone near the dam, up to 2.0 mg/L. The total nitrogen concentration is the lowest in April, and highest in end of October. The monthly average total phosphorus concentration over the whole reservoir ranged from 0.006 mg/L to 0.030 mg/L. The total phosphorus concentration had a decreasing trend from April to October. The concentration of total phosphorus gradually rose up in October. The cyanobacterial concentration in the reservoir peaked in the inlet zone, from which it decreased towards from the southern part to the northern part. A relatively better water quality appeared in the central zone with the lowest cyanobacterial concentration.The annual water temperature ranged from 14℃to 30℃(October) in Nanping reservoir. The temperature showed a decreasing gradient from the west to the east zone, and the inlet water temperature was higher than that of the outlet. The concentration of total nitrogen was higher in February and December than in October, up to 2.0 mg/L in the inlet zone. The monthly average total phosphorus concentration ranged from 0.016 mg/L (October) to 0.200 mg/L (February). However, the cyanobacterial concentration increased apparently in May, showing an outbreak of bloom with the biomass reached 120μg/L in the inlet area. In October, the cyanobacterial concentration began to decrease rapidly towards the lowest in a year. The cyanobacterial concentration varied from the west to the east zone.The highest biomass occured in the inlet zone.The model simulation showed that inflow water quality had a bad effect on water quality of Fenghuangshan reservoir, and the input flow to the Nanping reservoir can hold back the accumulation of cyanobacteria. Cyanobacteria usually peak in the beginning of April in Fenghuangshan reservoir and the middle of May in Nanping reservoir. It is strongly suggested that hydrodynamic regulation could be a good measure to improve the water quality.