How the δ<sup>18</sup>O<sub>NO<sub>3</sub></sub> versus δ<sup>15</sup>N<sub>NO<sub>3</sub></sub> Plot Can Be\nUsed to Identify a Typical Expected Isotopic Range of Denitrificatio

Stable isotope values of δ<sup>15</sup>N<sub>NO<sub>3</sub></sub> and δ<sup>18</sup>O<sub>NO<sub>3</sub></sub> of\ndissolved\nnitrate (NO<sub>3</sub>) are commonly used to identify the occurrence\nof denitrification as there is a progressive increase of δ<sup>15</sup>N<sub>NO<sub>3</sub></sub> and δ<sup>18</sup>O<sub>NO<sub>3</sub></sub> values accompanied by a decrease in NO<sub>3</sub> concentration. Thus, denitrification results in a positive trendline\non the dual plot of δ<sup>18</sup>O<sub>NO<sub>3</sub></sub> versus δ<sup>15</sup>N<sub>NO<sub>3</sub></sub>. The combination\nof two trendlines with different slopes provides the “typical\nexpected isotopic range of denitrification (TEIRD)” on the\nδ<sup>18</sup>O<sub>NO<sub>3</sub></sub> versus δ<sup>15</sup>N<sub>NO<sub>3</sub></sub> plot. Many studies distinguished\ndenitrified groundwaters by applying the TEIRD concept that is incorrectly\nintroduced because it assumes a single NO<sub>3</sub> source even\nthough multiple NO<sub>3</sub> sources exist. Also, most TEIRD applications\nrely on specific denitrification slopes although these are known to\nvary owing to the aquifer’s changing biogeochemical conditions.\nAlternatively, an accurate delineation of the TEIRD requires the identification\nof all potential NO<sub>3</sub> sources in aquifer with their measured\nor reconstructed δ<sup>15</sup>N<sub>NO<sub>3</sub></sub> and\nδ<sup>18</sup>O<sub>NO<sub>3</sub></sub> values. This allows\nthen for an accurate TEIRD to be traced with two denitrification trendlines\nhaving a slope corresponding to that from the correlation of the measured\nδ<sup>18</sup>O<sub>NO<sub>3</sub></sub> and δ<sup>15</sup>N<sub>NO<sub>3</sub></sub> values. Ultimately, each NO<sub>3</sub> source can have a specific TEIRD, although denitrification trendlines\nfor multiple NO<sub>3</sub> sources can feature the same slope within\na single aquifer.