| dc.description.abstract | The Danshuei River has the second largest drainage basin in Taiwan with the river basin spanning across Hsinchu county, Taoyuan County, new Taipei City, Taipei City and Keelung City. The total population in the drainage area exceeds 7 million. Based on previous studies, high levels of water pollutants were found in the Danshuei River. Because of the importantce of the Danshuei River to the inhabitants, we conducted this study on its water quality. This study is to investigate the distribution of nutrients and dissolved oxygen in the Danshuei River and its tributaries and to understand what controls the variation. The results show that, in the upper reach, the concentrations of nitrogen- and phosphorus-bearing nutrients were low and the nitrogen species were dominated by nitrate (5-40 uM), while the dissolved oxygen value was approaching saturation (300uM), during water sampling trips in February and July 2009. As the river water flowing closer to the metropolitant area, the nutrient concentrations increased dramatically (with ammonium up to about 200-300um, and phosphate up to about 5-40uM) and the dissolved oxygen decreased rapidly (down to 50-200uM) because of waste water inflow. The nitrogen species were now dominated by ammonium (5-40 uM). The dissolved oxygen concentration decreased rapidly to 50-200uM due to decomposition of organic matter and nitrification of ammonium. The concentrations of nutrients began to decrease and dissolved oxygen began to increase, when the river water was approaching the sea in the lower reach.
In order to investigate the seasonal variation of water quality and how discharges from sewage treatment plant affected water quality, we conducted weakly water sampling at Chunyang Bridge from May 2010 to August 2011 and monthly sampling from sewage treatment plant. The results show that the contribution of ammonium to the Danshuei River by the Dihua sewage treatment plant was 7.5%, and that of phosphate was 16.5%, indicating discharges from sewage treatment plant was not the dominant source of nutrients in the Danshuei River. Data analysis of the weekly sampling results by the HHT method shows seasonal variation of ammonium over the annual cycle with the lowest value in October, when the discharge was the highest, suggesting dilution effect. By contrast, the phosphate data showed no seasonal trend. The concentration of chlorophyll was the highest in summer and lowest in winter, probably reflecting the variation of the intensity of light.
For better understanding of the trend of water quality in the past few years, we analyzed, using HHT, ammonium data from October 2005 to December 2011 observed at the Fuzhou station of the Dahan River, the Huazhong station of the Shindian River and the Dazhi station of the Keelung River provided by EPA. The results show decreasing trends for concentrations of ammonium in the drainage basin of Danshuei River, Probably attributable to the increasing construction of sanitary sewer systems.
A one dimensional advection-diffusion-reaction model has been applied to simulate the variations of salinity, ammonium, nitrate, chlorophyll and dissolved oxygen along the main channel of the Danshuei River. The purpose is to evaluate the reaction rates and related physical parameters. The results show that the dominant sources of ammonium probably originated from the Shulin and Tucheng areas, which contributed about 40% of the total input, while the tributaries and sewage treatment works contributed another 40% and degradation of organic matter provided the last 20%. 80% of the ammonium was discharged to the sea, while only 20% was consumed by nitrification and phytoplankton growth. Concerning the input of dissolved oxygen, aeration of surface water and primary production contributed 50% and 30% respectively, while only 10% came from upstream. Concerning consumption of oxygen, 70% was by organic degradation, 10% by nitrification, and the last 20% was discharged to the sea.
For December 2009, the model predicted total discharge of DIN was 1.44*10^6 mole/day, which was lower than previously reported discharge of 2.15*10^6 mole/day for December 2001; this is consistent with the trend found in the EPA data. Despite the improved water quality, the pollution in the Danshuei River was still serious, suggesting significant pollutant sources still existed in the drainage basin.
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