5.1 Plume Tracers–Major Ions; (EC, Cl-, NO3-, Na+)

Septic system plumes usually have elevated concentrations of major ions such as Cl^–, Na⁺, NO₃^– and sometimes NH₄⁺, derived from the wastewater, as well as increased alkalinity (Equation 6) and other constituents resulting from rock-water interactions in the drainfield (e.g., Ca²⁺, Equation 8). All of these constituents are potentially useful as plume tracers. Figure 2 shows that sodium values in the plume core at the Cambridge site (>50 mg/L) are about five times higher than background values (< 10 mg/L). Furthermore, elevated concentrations of dissolved constituents impart increased specific conductance (EC) to the plume water and as a result, septic system plumes can usually be identified in the field using a specific conductance meter. Table 1 provides a summary of typical major ion concentrations occurring in septic tank effluent at sites where domestic wastewater is being treated. The main limitation for using major ions as plume tracers is that all of these constituents are not unique to domestic wastewater. Fertilizers used in modern agricultural operations contain potash and nitrogen formulations that can lead to elevated K⁺, Cl^– and NO₃^– levels in impacted groundwater, of the same magnitude as those found in septic system plumes (Spalding and Exner, 1993; Goss et al., 1998). In northern climates, de-icing salts applied to roadways and parking lots in winter, can lead to Cl^– and Na⁺ concentrations similar to those found in septic system plumes (Baer et al., 2019). In the Cambridge plume (Figure 2) for example, neither Cl^– nor NO₃^– could be used directly as plume tracers, because similar concentrations occurred in the background groundwater, as a result of fertilizer use on the adjacent farm fields. In the Long Point plume (Figure 3), Cl^– use as a tracer was limited because background values were elevated as a result of the use of CaCl₂ on the campground roadways to suppress dust. In both the Cambridge and Long Point plumes, however, Na⁺ and EC remained effective as plume tracers.

Whereas major ions may serve as useful plume tracers in the proximal plume zones, in more distal zones, their usefulness is often diminished because of the presence of similar contaminants from other sources. This circumstance can limit the usefulness of major ions as indicators of septic system impact in situations where impacts from multiple contaminant sources may overlap, such as may occur in water withdrawn by wells from broad contributing areas (e.g., Figure 5).

Figure 5 – Conceptual view of the capture zone of a water supply well encountering a septic system plume and other potential sources of similar contamination, including a shopping mall parking lot that contributes sodium and chloride from de-icing salts, and a golf course that contributes nitrate from fertilizer. Adapted from Einerson and Mackay (2001).