Hydrochemical and stable isotopic assessment of nitrate contamination in an alluvial aquifer underneath a riverside agricultural field Gi-Tak Chae a,b , Seong-Taek Yun a,c, *, Bernhard Mayer c , Byoung-Young Choi a , Kyoung-Ho Kim a , Jang-Soon Kwon a , Soon-Young Yu d a Department of Earth and Environmental Sciences, Korea University, Seoul 136-701, Republic of Korea b CO 2 Sequestration Research Department, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea c Department of Geoscience, University of Calgary, Calgary, AB, Canada T2N 1N4 d Department of Earth Sciences, University of Waterloo, Waterloo, ON, Canada N2L 3G1 1. Introduction Nitrate contamination of alluvial groundwater underneath agricultural field causes serious environmental and health problems in many countries. If used for drinking water purposes, the high concentration of nitrate in groundwater may cause various diseases including methemoglobinemia (Spalding and Exner, 1993; Starr and Gillham, 1993; Bouman et al., 2002; Costa et al., 2002) and it can ultimately contaminate surface waters as it discharges to streams resulting in eutrophication and acidification (Maı ˆtre et al., 2003; Thorburn et al., 2003; Monaghan et al., 2007). Therefore, an accurate estimate of nitrate leaching from agricul- tural fields is crucial to environmental impact studies (Kraft and Stites, 2003). Nitrate contamination in alluvial groundwater is partially controlled by geology including soil characteristics and land use of the area (Burt et al., 1999; Min et al., 2003; Chae et al., 2004). Dilution and/or denitrification can result in the decrease of nitrate concentrations in groundwater and the latter process often occurs in parts of the alluvium covered by thick silt layers suitable for the establishment of reducing conditions (Cey et al., 1999; Pauwels et al., 2000; Panno et al., 2001; Peterson et al., 2002; Thayalakumaran et al., 2008). In contrast, nitrate is biogeochemi- cally stable and usually poorly adsorbed in oxygenated sandy soils. Therefore, it is easily leached during groundwater recharge, Agricultural Water Management 96 (2009) 1819–1827 ARTICLE INFO Article history: Received 24 July 2008 Accepted 5 August 2009 Available online 31 August 2009 Keywords: Nitrate Groundwater pollution Riverside alluvium aquifer Nitrogen and oxygen isotopes Mass balance modeling ABSTRACT Major ions and stable isotopic (dD water , d 18 O water , d 15 N nitrate , d 18 O nitrate ) measurements in concert with hydrochemical modeling were used in order to elucidate the sources and geochemical processes controlling nitrate contamination of shallow alluvial groundwater underneath a riverside agricultural field in the Buyeo area, Korea. Beneath vegetable fields in the sandy soil, the mean nitrate concentration of groundwater was 148.6 mg/L, which is significantly higher than in groundwater (mean 28.8 mg/L) beneath silty soils underneath rice paddy fields. Nitrogen isotope data indicate that synthetic fertilizers are the predominant source of groundwater nitrate in the study area. Denitrification during recharge through rice paddy soils appears to be responsible for the lower nitrate concentrations in groundwater beneath the silty soil zone. The relationship between nitrogen and oxygen isotope data of nitrate also suggests mixing of two different groundwater bodies with nitrates from the silt zone and the sand zone. Geochemical mass balance modeling on hydrochemical data indicates that various agricultural chemicals such as urea, lime, magnesium sulfate and potassium chloride dissolve in vegetable fields of the sandy zone, resulting in significant enrichment of various solutes such as K + , Ca 2+ , Mg 2+ , NO 3  , SO 4 2 and Cl  . As a consequence of over-utilization of synthetic nitrogen fertilizers, the sand zone is characterized by very high nitrate concentrations in the groundwater. This study suggests that a reduction of over-fertilization especially on vegetable fields in the riverside sand zone is required to minimize the nitrate contamination of groundwater. This study also shows that combination of geochemical and isotopic techniques with simple mass balance modeling provides information about the causes and processes of nitrate contamination of groundwater underneath a riverside agricultural field. The study also provides sustainable measures to optimize fertilization rate as an important basis of eco-friendly agriculture. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author at: Department of Earth and Environmental Sciences, Korea University, Anam-Dong, 5-Ga, Seongbuk-Ku, Seoul 136-701, Republic of Korea. Tel.: +82 2 3290 3176; fax: +82 2 3290 3189. E-mail address: styun@korea.ac.kr (S.-T. Yun). Contents lists available at ScienceDirect Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat 0378-3774/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2009.08.001