5.4 Isotopic Tracers–Nitrate-15N

Several constituents in septic system plumes (e.g., Cl ^–, B, NO₃^–) can be isotopically distinct from other sources, therefore isotopic characterization can assist in identifying groundwater impacted by septic systems. Nitrate-¹⁵N (NO₃^––¹⁵N) values have been utilized to distinguish between groundwater nitrate contamination resulting from septic systems and contamination from fertilizers used in agricultural operations. Nitrate derived from chemical fertilizers has lower ¹⁵N values, typically in the range of +3 to +5 per mil (parts per thousand), whereas NO₃^– in septic system plumes generally has higher ¹⁵N values in the range of +7 to +14 per mil (Kreitler et al., 1979; Heaton, 1986; Aravena et al., 1993; Wassenaar, 1995). However, there is some overlap in the isotopic composition of these sources and processes such as NH₄⁺ volatilization, soil zone exchange, and denitrification, can alter the isotopic signature of the source material. In addition, when manure is used in agricultural operations, the isotopic signature of the associated NH₄⁺ can be similar that of domestic wastewater. At some sites it has been noted that groundwater NO₃^––¹⁵N values tend to cluster near the range where fertilizer and wastewater NO₃^–– ¹⁵N values overlap (+5 to +8 per mil) making source identification using isotopic signatures difficult (Komor and Anderson, 1993; Xue et al., 2009). However, the lighter isotopes of NO₃^– (¹⁴N and ¹⁶O) are preferentially consumed during nitrate degradation processes such as denitrification and anammox as discussed in Section 6, and consequently, isotopic analyses can be of considerable value in establishing when such processes are active.