(6) Denitrification and nitrification

Due to denitrification, nitrate and nitrite are reduced to gaseous nitrides and nitrogen in an anaerobic environment, which may change the composition of δ <sup>15</sup>N(NO3) and <sup>18</sup>O(NO3) of different nitrogen sources. Therefore, an important prerequisite for using δ <sup>15</sup>N(NO3) and δ <sup>18</sup>O(NO3) isotopic values to identify the nitrogen source is that no significant denitrification occurs [43]. If denitrification occurs, the residual NO<sup>3</sup> − would enrich <sup>15</sup>N(NO3), and the content of NO<sup>3</sup> − decreases [44]. The occurrence of denitrification can also be judged by the enrichment coefficient (εN/εO). According to the research results of Bottcher et al. [45] and Fukada et al. [46], the enrichment coefficient (εN/εO) should range between 1.3–2.1.

### (7) δ <sup>15</sup>N(NO3) – δ <sup>18</sup>O(NO3) dual isotope technique

When using the <sup>15</sup>N(NO3) alone, the δ <sup>15</sup>N value ranges between different sources overlap, which leads to multiple solutions. The <sup>15</sup>N(NO3) – <sup>18</sup>O(NO3) dual isotope technique provides a useful and powerful tool to identify nitrate contaminations by using the stable isotopes <sup>15</sup>N and <sup>18</sup>O of nitrate together [8]. Combined with hydrochemical methods, the source of nitrate can be accurately identified, which is of great significance to the prevention and control of nitrogen contamination.

Nitrate in groundwater may come from atmospheric deposition, inorganic fertilizer, soil, manure and sewage, and different nitrate sources have specific δ <sup>15</sup>N(NO3) and δ <sup>18</sup>O(NO3) value ranges [18,47]. Based on previous research [47–49], the ranges of δ <sup>15</sup>N(NO3) and δ <sup>18</sup>O(NO3) originating from different sources were classified and are shown in Table 2. Then, the sources of nitrate can be identified by using the δ <sup>15</sup>N(NO3) – δ <sup>18</sup>O(NO3) dual isotope technique.

**Table 2.** Ranges of δ <sup>15</sup>N(NO<sup>3</sup> ) and δ <sup>18</sup>O(NO<sup>3</sup> ) originating from different sources (unit: ‰).

