How the δ¹⁸O_NO3 versus δ¹⁵N_NO3 Plot Can Be Used to Identify a Typical Expected Isotopic Range of Denitrification for NO₃-Impacted Groundwaters

Stable isotope values of δ¹⁵N_NO3 and δ¹⁸O_NO3 of dissolved nitrate (NO₃) are commonly used to identify the occurrence of denitrification as there is a progressive increase of δ¹⁵N_NO3 and δ¹⁸O_NO3 values accompanied by a decrease in NO₃ concentration. Thus, denitrification results in a positive trendline on the dual plot of δ¹⁸O_NO3 versus δ¹⁵N_NO3. The combination of two trendlines with different slopes provides the “typical expected isotopic range of denitrification (TEIRD)” on the δ¹⁸O_NO3 versus δ¹⁵N_NO3 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 δ¹⁵N_NO3 and δ¹⁸O_NO3 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 δ¹⁸O_NO3 and δ¹⁵N_NO3 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.