| dc.description.abstract | The Danshuei River in northern Taiwan has three major tributaries: Dahan Hsi, Shindien Hsi and Keelung River. The three watersheds have considerable differences in the natural conditions as well as human activities. The two reservoirs located in the drainage basins of the Danhan Hsi and Shindien Hsi are major water supplies to the population in the northern Taiwan. Consequently, the geochemistry of the Danshuei River water is closely related to the water quality of the regional water supply. The source of the alkalinity and major ions in the river water is mainly from weathering of the rock formation in the watershed. The water drains through the surface and groundwater ducts to reach the river channel. Therefore, research on the river water alkalinity can help us understand how the weathering processes may affect the water quality. Besides geochemical processes, the effects of biological activities and precipitation on the water quality were also examined. The relatively long residence time of reservoir water makes the biological effects quite evident. Precipitation in different seasons are from different air masses, which may give waters of different acidities, which in turn may have different effects on water quality. This study comprises three parts dealing with the Feitsui Reservoir, the Taoyuan Irrigation Ditch from the Shimen Reservoir and the whole Danshuei River watershed, respectively. Water samples were collected and filtered for determination of pH, alkalinity, and concentrations of major ions.
The Feitsui Reservoir displays significant biological effects on its water quality. For instance, photosynthesis causes increase of alkalinity, so does denitrification, whereas respiration decreases alkalinity. Different processes occurred in different layers of the stratified water column in different seasons. In the second part, it is shown that the water in the Taoyuan Irrigation Ditch was influenced by acid rains. The alkalinity was high in summer and low in winter. The range was from 1.0 to 1.5 meq/L in summer and decreased significantly in winter because of the neutralizing capacipty of the acid rains. In addition, it was also affected by photosynthesis. The third part explores the relationship between alkalinity and major ions in the Dahan, Hsindien and Keelung rivers. The regression analysis demonstrated a high correlation between calcium ion and alkalinity, while multiple regression using all major ions produced much better correlation. Amonmg the tributaries, the correlation coefficient of Dahan river is the lowest; its R2 is only 0.85; the others are higher than 0.90. The variations of chemical properties of river waters could be related to differences in land-use proportion as well as the elevation of the watershed. The high alkalinity of river waters from the Dahan Stream could be related to the higher elevation and higher percentage of bare soil. The agricultural use of land and bare soil may enhance chemical weathering, which may in turn contribute to the higher alkalinity. On the other hand, the forests appear to produce the lowest alkalinity. | en_US |