Stable isotopes of N ( 15N/ 14N) are broadly used in examining the source of nitrate in groundwater due to the difference of 15N natural abundance of its contamination sources. Nevertheless, nitrogen is not a conservative tracer as its value is a function of its source and isotope fractionation by mineralization, adsorption, nitrification and denitrification that may occur during its generation or groundwater transportation. To identify the source of contamination, it is important to estimate whether the fractionation reaction has happened and what the extent is. In this paper, the author attempt to synthesize the existing techniques using N and O isotopes as indicators of contamination sources. Various methods of ascertaining existence and extent of the fractionation reaction are discussed. Further research is highlighted to improve the data collection and analysis when integrated N cycle and the water cycle are studied. Understanding of the transport and transformation processes of NO3- in the unsaturated zone is also emphasized.