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 Used to Identify a Typical Expected Isotopic Range of Denitrification for NO<sub>3</sub>-Impacted Groundwaters
Lamine Boumaiza, Randy L. Stotler, Bernhard Mayer, Ioannis Matiatos, Elisa Sacchi, Neus Otero, Karen H. Johannesson, Frédéric Huneau, Romain Chesnaux, Mónica Blarasin, Viviana Ré, Kay Knöller
Abstract
High Resolution Image Download MS PowerPoint Slide Stable isotope values of δ 15 N NO 3 and δ 18 O NO 3 of dissolved nitrate (NO 3 ) are commonly used to identify the occurrence of denitrification as there is a progressive increase of δ 15 N NO 3 and δ 18 O NO 3 values accompanied by a decrease in NO 3 concentration. Thus, denitrification results in a positive trendline on the dual plot of δ 18 O NO 3 versus δ 15 N NO 3 . The combination of two trendlines with different slopes provides the “typical expected isotopic range of denitrification (TEIRD)” on the δ 18 O NO 3 versus δ 15 N NO 3 plot. Many studies distinguished denitrified groundwaters by applying the TEIRD concept that is incorrectly introduced because it assumes a single NO 3 source even though multiple NO 3 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 NO 3 sources in aquifer with their measured or reconstructed δ 15 N NO 3 and δ 18 O NO 3 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 δ 18 O NO 3 and δ 15 N NO 3 values. Ultimately, each NO 3 source can have a specific TEIRD, although denitrification trendlines for multiple NO 3 sources can feature the same slope within a single aquifer.