Shallow groundwater was observed in the upper part of impermeable clay layer in the study area, with water level of 11.8 ~ 18.7 m. The high concentration of NO 3 -N in shallow groundwater induced nitrate contamination in deep groundwater by inflows of shallow groundwater directly into the deep groundwater well. NO 3 -N concentration in shallow and deep GW were affected by rainfall, usage of fertilizers, hydrogeology. There were differences between both groundwater system in the pattern of fluctuation of the NO 3 -N concentration By cluster analysis, the shallow groundwater was grouped into 3 groups (Group A, Group Band Group C) and deep groundwater samples were divided into 4 groups (Group Ⅰ, Gro- up Ⅱ, Group Ⅲ, Group Ⅳ ), clustering groups of shallow and deep groundwater were reflec ted their hydrogeochemical properties. The sources of NO 3 - is a complex origins of chemical fertilizers, organic soil and denitrifi- caiton in shallow and chemical fertilizers, organic soil in deep groundwater. :Three oxygen molecules of nitrate produced by nitrification, two are coming from the water and one comes from dissolved atmospheric oxygen (Hollocher,1984) :δ 18 O values of nitrate derived from nitrification in the study area: 3.03~3.30‰ :Enrichment factor(ε) is useful to compare denitrification rates in ground water (Mariotti et al., 1998) Nitrate Contamination and Source Identification of Groundwater in the Agricultural Fields, Gosan, Jeju 1 Eun-Hee Koh ･ 1 Dugin Kaown ･ 2 Bong-Rae Kang ･ 1 Hee Sun Moon ･ 1 Kang-Kun Lee 1 School of Earth and Environmental Sciences, Seoul National University, Korea 2 Institute of Environment research, Jeju Special Self-Governing Province, Korea Introduction - Agricultural field is distributed broadly in Gosan area where located in the western part of Jeju Island (Fig.1) and nitrate contamination in deep groundwater has been observed. - Shallow groundwater in the top soil layer was observed in the upper part of imper- meable clay bed through this study (Fig.2). - For fully understanding nitrate contamination in groundwater in Gosan, Jeju Island, characterization nitrate contamination considering shallow groundwater as well as deep groundwater is needed. -Objectives of this study are i) to define hydrogeochemistry properties of shallow an d deep groundwater, ii) to figure out the hydrogeochemical characteristics and diffe- rences between shallow and deep groundwater using statistical approaches, iii) to identify sources of NO 3 - contamination in shallow and deep groundwater using stabl- e isotope analysis. Fig.1. Location map of the study site, Gosan. Fig.2. Schematic of hydrogeology in study area. Methods - Total 22 wells were installed to identify hydrogeological properties of shallow groundwater through 2009 ~ 2010 ( Fig.3) - Hydrogeochemical species in samples were analyzed by using IC (ion chromatography) with AS-19 column for anions, and with CS-12 for cations (Dionex ICS-3000, USA). - To analyzed stable isotopes, δ 15 N and δ 18 O of NO 3 -, Shallow GW, Deep GW were sampled in 05, The water samples were analyzed in th e Isotope Science Laboratory at the University of Calgary, Canada - To monitor hydrogeochemical charicteristics, Groundwater Sampling was conducted 04, 2009 ~ 09, 2010 and shallow GW was sampled using a hand pump, deep GW was collected directly from the taps at the agricultural wells. - Cluster analysis is a method for grouping the water quality data according to “similarity” of the each other. - In this study, Hierarchical clustering (cluster group can be included in the other group but allowed not to overlap with each groups) and Squ are Euclidean distance was used for measuring distances between two observations. Fig.3. Well location map. Group Ⅰ Group Ⅱ Group Ⅲ Group Ⅳ Group A Group B Group C Shallow GW Deep GW Timal variation of nitrate-nitrogen in shallow and deep GW Source identification by stable isotopes Cluster analysis Piper diagram Major ions spatial distributions of shallow and deep GW ReferencesAcknowledgement This research was financially supported by Basic Science Research Program through the Na tional Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology( ) and the Ministry of Knowledge Economy (MKE), Korea Institute for Advancement of Technology (KIAT) and Jeju Leading Industry Office through the Leading Industry Development for Economic Region. - Hollocher, T.C., 1984, Source of oxygen atoms in nitrate in the oxidation of nitrate by nitrobactor agilis and evidence against a P-O-N an hydride mechanism in oxidative phosphorylation. Archives of Biochemistry and Biophysics 233, pp Mariotti, A., A. Landreau, B. Simon., N isotope biogeochemistry and natural denitrification process in groundwater: Application to the chalk aquifer in northern France. Geochimica et Cosmochimica Acta 52, Results and Conclusions Deep GWShallow GW