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Effects of Human Activities on Spatial Variability of Dissolved Nitrogen Concentration in Danshui River, Taiwan
OECD had published that the nitrogen flux from land to water will increase by 35%-46% in 2000 to 2050. The yields of dissolved inorganic nitrogen for rivers in Taiwan are among the highest in the world, controlled by the population density. However, there are relatively rare studies investigating the sources of nitrogen in the water. In this study, the Keelung area was firstly used as an example aiming to realize spatial variations in dissolved nitrogen concentration. Monthly water samples were taken at 36 stations from 2017/10/24 to 2018/3/27, and electric conductivity (EC) and Kjeldahl nitrogen (TN) was measured. Second, the basin-wide water samples were taken in Danshui Watershed in 2009/7, and d15N-NO3 and d18O-NO3 were measured to identify the source of nitrogen. The results revealed that EC were negatively correlated to forest land use but positively correlated to building and public land use, indicating the contributions of human activities to the ions in river. In terms of the Spearman's rank correlation coefficient, it was found that TN was more positively linked to population density, building, and land use defined as others in the land use map. It was found that the treatment of sewage may be one of the factors reducing the nitrogen concentration in Keelung river. As for the pollution source identified by 15N-NO3 and 18O-NO3, the nitrate in the water mainly came from NH4+ fertilizer, sewage and waste, and soil. When the population density within watershed was more than 50 people/ km2, the signature of sewage and waste can be found in the water. On the other hand, when population density was less than 50 person/ km2, the water quality was more influenced by NH4+ fertilizer and soil.
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