Isotope signatures to trace the origin and fate of nitrate from an agricultural sub catchment within Soyang Lake Watershed, South Korea

Abstract

Abstract : Monsoon season in South Korea has a great influence on the biogeochemical and hydrological processes in the entire country, but is specifically of concern in the Soyang Lake Watershed, the main drinking water reservoir for the 20 million metropolis Seoul. Stable isotopes are a useful tool to quantify and determinate the origin of nitrate inputs into the Soyang Lake. The I´ 15 N values of nitrate from different sources often show overlapping ranges but the additional measurement of the I´ 18 O values allows a more precise classification (Durka et al. 1994; Mayer et al. 2001; Deutsch et al. 2005). According with this principle the nitrate derived from sewage or manure or fertilizes is isotopically distinct between each other and from the other sources. Preliminary results suggest the nitrogen fertilization and its influence in the denitrification process in the agriculture-dominated Haean basin as the major contributor to the nitrate output into the ground water systems, posterior discharge into the Mandae River. Soil microbial nitrification only partly can explain the isotopic composition of the 15 N and 18 O abundance in the nitrate discharge. Also the isotope values from mineral fertilizers applied in the area cannot explain the comparatively high 15 N abundance found in the rivers, suggesting that nutrient addition plays an important role in the biogeochemical process instead of being only washed out into the river systems by the heavy monsoon rain. Furthermore the impact of extreme weather monsoon conditions drives not only the biogeochemical processes but also hydrological. The monsoon is the most important driver for the nitrate release between the different types of crops being responsible for more than the 80% of input from the total nitrate transport within summer season.