Nitrate pollution in surface and groundwater has become a worldwide environmental problem. For better water quality management, it’s important to know total NO3- loads and identify the proportional contributions from multiple NO3- sources. Stable isotopes of NO3-, the δ15N value and the δ18O value, have been frequently used to identify NO3- sources in water. However, due to complexity in the N cycle, mixing multiple sources and fractionation during biological processes often caused biased and uncertain results. One possible solution is to use SIAR, a linear mixing model running in R, to estimate the proportional contribution from multiple NO3- sources. Unfortunately, SIAR cannot distinguish manure and septic sources, which requires the analysis of other chemical components, such as sodium, caffeine. A research approach is proposed to pilot a unified method to identify NO3- sources, determine their isotopic signature, and estimate each source’s contribution by using the Fall Creek Watershed as a proof- of-concept study area.