Nitrate contamination in stream water and groundwater is a serious environmental problem that arises in areas of high agricultural activities or high population density. It is therefore important to identify the source and flowpath of nitrate in water bodies. In recent decades, the dual isotope analysis (δ¹⁵N and δ¹⁸O) of nitrate has been widely applied to track contamination sources by taking advantage of the difference in nitrogen and oxygen isotope ratios for different sources. However, transformation processes of nitrogen compounds can change the isotopic composition of nitrate due to the various redox processes in the environment, which often makes it difficult to identify contaminant sources. To compensate for this, the stable water isotope of the H₂O itself can be used to interpret the complex hydrological and hydrochemical processes for the movement of nitrate contaminants. Therefore, the present study aims at understanding the fundamental background of stable water and nitrate isotope analysis, including isotope fractionation, analytical methods such as nitrate concentration from samples, instrumentation, and the typical ranges of δ¹⁵N and δ¹⁸O from various nitrate sources. In addition, we discuss hydrograph separation using the oxygen and hydrogen isotopes of water in combination with the nitrogen and oxygen isotopes of nitrate to understand the relative contributions of precipitation and groundwater to stream water. This study will assist in understanding the groundwater flowpaths as well as tracking the sources of nitrate contamination using the stable isotope analysis in combination with nitrate and water.

In this study, according to determination of nitrogen (δ 15 N) and oxygen (δ 18 O) from nitrate (NO 3) in water for 2 years, 2016-2017, Gunwi, Korea, the origin of pollutants in supply water could be found. The sampling sites are including 6 points, Donggok, Yonga, Janggok1 and 2, reservoir of dam and Ingak temple around Gunwi Dam, Korea. The water samples were determined NO 3-N using Ion Chromatography and Nitrogen (δ 15 N) and Oxygen (δ 18 O) stable isotope ratio in nitrate and isotope ratio mass spectrometry, respectively. The standard deviation of determined standard materials is less than 0.2‰ for δ 15 N AIR and 0.2‰ for δ 18 O VSMOW in water. The origin of the supply water pollution was analyzed through the correlation between the values of δ 15 N and δ 18 O in nitrate. The values indicated that these sources were originated sewage and livestock manure in the Dam. The dominant contribution of water pollutants was calculated about 50% from Donggok location in this study. In addition, the location of Yonga where has the small inflow rate also constantly contributed to the Dam.

To identify the origin of major pollutants, we determined the distribution of stable isotopes of δ15N and δ18O in nitrate extracted from water samples and sediments collected at two-month intervals during 2019-2020 from the North Hangang River, Korea. Stable isotope ratios were measured to investigate nitrogen contamination sources in the water system. This study determined that the North Hangang River was affected by soil organic matter, livestock manure, and domestic sewage, and that the streams and rivers flowing into the main stream of the North Hangang River were affected by domestic sewage or livestock manure, resulting in relatively heavy nitrogen isotope ratios. Generally, the concentration of nitrate nitrogen was higher in the inflowing streams (Geumsan, Hongcheon, Mukhyeon, and Gyeongan district) than at the main stream. Nitrogen pollution due to chemical fertilizers and soil organic matter were different in 2019 and 2020, and the ratio of stable nitrogen isotopes decreased owing to the influence of rainfall. Sediment in the North Hangang River has been affected by agricultural soil organic matter and livestock manure, showing the necessity to nitrogen pollutant management, particularly with pollutants such as soil organic matter and livestock manure.