How the δ18ONO3 versus δ15NNO3 Plot Can Be Used to Identify a Typical Expected Isotopic Range of Denitrification for NO3-Impacted Groundwaters

Stable isotope values of δ15NNO3 and δ18ONO3 of dissolved nitrate (NO3) are commonly used to identify the occurrence of denitrification as there is a progressive increase of δ15NNO3 and δ18ONO3 values accompanied by a decrease in NO3 concentration. Thus, denitrification results in a positive trendline on the dual plot of δ18ONO3 versus δ15NNO3. The combination of two trendlines with different slopes provides the "typical expected isotopic range of denitrification (TEIRD)" on the δ18ONO3 versus δ15NNO3 plot. Many studies distinguished denitrified groundwaters by applying the TEIRD concept that is incorrectly introduced because it assumes a single NO3 source even though multiple NO3 sources exist. Also, most TEIRD applications rely on specific denitrification slopes although these are known to vary owing to the aquifer's changing biogeochemical conditions. Alternatively, an accurate delineation of the TEIRD requires the identification of all potential NO3 sources in aquifer with their measured or reconstructed δ15NNO3 and δ18ONO3 values. This allows then for an accurate TEIRD to be traced with two denitrification trendlines having a slope corresponding to that from the correlation of the measured δ18ONO3 and δ15NNO3 values. Ultimately, each NO3 source can have a specific TEIRD, although denitrification trendlines for multiple NO3 sources can feature the same slope within a single aquifer.