6 Fate of Nitrogen in Septic System Plumes

At sites where the wastewater has been well oxidized, septic system plumes often have nitrate concentrations that exceed the drinking water limit of 10 mg/L as NO₃^––N. For example, in the Cambridge plume, effluent NH₄⁺–N averaging 30 mg/L, is completely oxidized in the unsaturated zone and as a result, there is an absence of NH₄⁺ in the plume, but NO₃^––N is elevated, and ranges from of 16-38 mg N per liter (Figure 2). Figure 6 shows a histogram of nitrate concentrations measured in 21 septic system plumes in Ontario, and shows that the drinking water limit was exceeded in 16 of the plumes.

Figure 6 – Histogram of mean NO₃^– concentrations in proximal plume zones underlying drainfields at 21 Ontario septic system sites (from Robertson et al., 2020). The nitrate drinking water limit (DWL) of 10 mg N per liter is shown for reference.

Groundwater in densely developed coastal areas of Cape Cod, Massachusetts, USA has NO₃^––N concentrations as high as 24 mg/L, and this is largely attributed to the widespread use of septic systems (Coleman et al., 2018).

Nitrate is potentially reactive however, and can be reduced to di-nitrogen gas (N₂) if reducing conditions are encountered and if potential electron donor compounds are available to act as energy sources to support the transformation reactions. Two commonly occurring NO₃^– transformation reactions are heterotrophic denitrification using labile organic carbon as the energy source (Equation 13) and autotrophic denitrification using reduced sulfur compounds (e.g., pyrite) as the energy source (Equation 14).

These reactions tend to occur abruptly at reaction fronts, rather than uniformly along the plume (Smith et al., 1991). The reaction fronts usually occur at locations where the plumes encounter increased concentrations of electron donor compounds, such as may occur in riparian zone sediments enriched in organic carbon (e.g., Robertson et al., 1991; Böhlke et al., 2002) or in deeper aquifer zones where unweathered pyrite may be present (e.g., Postma et al., 1991; Kölle et al., 1985). The Cambridge plume shown in Figure 2, is suboxic (i.e., dissolved oxygen < 1 mg/L) and could potentially be affected by nitrate transformation reactions. However, NO₃^– concentrations remain consistently elevated and show no evidence of isotopic enrichment, indicating that denitrification is inactive (Aravena et al., 1993). Presumably, this is because there is an absence of suitable electron donor compounds (e.g., labile organic carbon or unweathered pyrite) in the aquifer sediments at this site.

In contrast, at the Long Point site (Figure 3), the proximal plume zone has elevated NO₃^––N of 32 to 79 mg/L (Aravena and Robertson, 1998) and nitrate is completely consumed to < 1 mg/L within 40 m downgradient from the drainfield. Transformation occurs along a reaction front where there are coinciding increases in both alkalinity and SO₄^2–_, indicating that heterotrophic denitrification (Equation 13), as well as autotrophic denitrification (Equation 14), contribute to the observed nitrate loss. At this site, the denitrification is promoted by the presence of trace amounts of labile organic carbon and pyrite in the deeper (unweathered) aquifer sediments (Aravena and Robertson, 1998). The Long Point aquifer is a recent beach deposit that is less than a few thousand years old, whereas the Cambridge aquifer was deposited at the end of the last glacial period, ~10 to 15 thousand years before present. The younger age of the Long Point aquifer, as well as the presence of wetland complexes nearby, may contribute to the richer reserve of labile electron donor compounds at that site. Autotrophic denitrification is known to occur in aquifers and aquitards worldwide because trace amounts of pyrite (or other reduced sulfur compounds) are usually present when the sediments are unweathered (Kölle et al., 1985; Postma et al., 1991; Korom, 1992; Robertson et al., 1996). In most cases this reaction is marked by distinct increases in SO₄^2– concentrations of groundwater in the same zone where NO₃^– values decline.

A third potentially important nitrate transformation reaction that has been recognized recently, is NO₃^–– reduction by NH₄⁺ (anammox, Equations 15 and 16).

These reactions can be active at sites where the wastewater is less well oxidized, such that both NO₃^– and NH₄⁺ occur together in the plume. The reaction is characterized by isotopic enrichment of both the residual NO₃^– and NH₄⁺ (Clarke et al., 2008; Caschetto et al., 2017) and by the presence of anammox-specific bacteria (Moore et al., 2011; Smith et al., 2015). Isotopic and bacteriological characterization has shown that anammox is an important contributor to nitrate transformations observed in a number of septic system plumes, including the Long Point plume (Robertson et al., 2012), the Killarney site campground plume (Caschetto et al., 2017) and the large municipal wastewater plume on Cape Cod (Smith et al., 2015). In the Cape Cod plume, anammox bacteria were found to be present throughout a 3 km-long zone where NH₄⁺ was present, and tracer injection tests established that NO₃^– consumption from anammox and denitrification were of a similar magnitude and occurred at similar rates, when labile DOC was available. However, when DOC supply was limited, NO₃^– consumption became dominated by anammox, accounting for about 90% of the observed NO₃^– loss. Furthermore, in a review of total inorganic nitrogen (TIN) persistence in the 21 Ontario septic system plumes (Robertson et al., 2020), it was discovered that TIN consumption was significantly greater at 10 sites where both NO₃^{– –}and NH₄⁺ were present in the plume zones near the drainfields (62 percent TIN loss), compared to other sites where only NO₃^– was present (3 percent TIN loss). Overall, this evidence suggests that the anammox reaction occurs widely in septic system plumes, and it rivals denitrification in importance for providing consumption of nitrate in these plumes.

The fourth potential nitrate transformation reaction is dissimilatory nitrate reduction to ammonium (Equation 17), is considered less important in groundwater environments (Rivett, et al., 2008) and has not been reported in septic system plumes.