7.3 Phosphorus Fate

Phosphorus concentrations in septic tank effluent (e.g., 5 to 15 mg/L total P, Table 1) are several orders of magnitude higher than values capable of stimulating algal growth and eutrophication in aquatic environments (~0.03 mg/L, Dillon and Rigler, 1974). Total phosphorus in septic tank effluent consists of a combination of dissolved phosphate (PO₄^3–), often referred to as soluble reactive phosphorus (SRP), organic phosphorus that occurs as a component of the organic compounds present in the wastewater, and phosphorus adsorbed onto particulate material in the effluent. In septic tank effluent, SRP usually comprises 70-85% of total P (McCray et al., 2005). During infiltration through the drainfield sediments, particulate material is removed by filtration and organic compounds are degraded, and as a result, total P in septic system plumes is mostly SRP (Harman et al., 1996). SRP concentrations in septic system plumes vary, but some sites have P-rich zones where SRP concentrations approach values found in the septic tank effluent, including zones below the drainfields at both the Cambridge and Long Point sites, where SRP values range from 1 to 6 mg/L (Figures 2 and 3). One of the largest reported phosphorus plumes occurs in the Cape Cod municipal wastewater plume, where a P-rich zone with SRP values of 1 to 3 mg/L, extends 600 m downgradient from the wastewater infiltration beds, before discharging to a small lake (LeBlanc et al., 1984; McCobb et al., 2003).

Phosphate is a trivalent anion that can be influenced by a number of reactions that make its fate in the subsurface complex. In most groundwater flow systems, phosphorus is strongly affected by surface-layer sorption reactions because of the presence of minerals with net positive surface charges (e.g., calcite, gibbsite, ferrihydrite at normal pH ranges) in most sediment types. Sorption has the effect of slowing the rate of P migration, but does not permanently immobilize P. The potential for sediment to adsorb phosphorus is typically characterized by a distribution coefficient (K_d), which describes the ratio of the mass of P adsorbed on the sediment solids, to the mass in solution. K_d values for phosphorus can be determined from laboratory testing of sediment samples and are typically in the range of 3 to 40 cm³/g (Walter et al., 1996; McCray et al., 2005). The retardation equation (Equation 26, from Freeze and Cherry, 1979), relates K_d values to a retardation factor for P migration in groundwater.

(26)

where:

The range of K_d values noted above, lead to P retardation factors in the range of 15 to 400 for aquifers with typical sediment properties (e.g., sediment density of 2.65 g/cm³ and porosity of 0.3). In a review of P mobility in 24 Ontario septic system plumes (including the 21 sites noted above), seven sites with well-developed P plumes had P retardation factors that ranged from 11 to 67 (Robertson et. al., 2019), which is generally consistent with the range expected based on K_d values reported in the literature.

In addition to surface-layer sorption, chemical equilibrium models predict that mineral precipitation reactions involving phosphorus should also be active in septic system plumes (Ptacek, 1998; Parkhurst et al., 2003; Spiteri et al., 2007). The above 24 site review (Robertson et al., 2019) observed that distinct zones of sediment P accumulation were present in almost all of the drainfields, and these P-rich zones consistently occurred within 1 to 2 m below the infiltration pipes, even in systems that varied widely in age. Consistent accumulation near the infiltration pipes, suggested that this was the result of mineral precipitation reactions, rather than sorption. Furthermore, sand grains from the P accumulation zones exhibited distinct secondary mineral coatings and these coatings contained phosphorus (Figure 7).

Figure 7 – Secondary P-mineral coating on a drainfield quartz grain from the filter bed at a campground in Ontario, Canada: a) scanning electron microscope image and b) x-ray elemental analysis (Robertson et al., 2019). The coating composition is: Al 19.4, Fe 6.6, P 2.0 and Ca 1.8 percent by weight. (Image courtesy of Surface Science Western, London, Ontario, Canada).

The composition of the coatings was variable, but iron, aluminum and occasionally calcium, were the dominant cations, indicating a relationship to the precipitation reactions noted above (Equations 22 to 25).

The acidity generated by wastewater oxidation, particularly the oxidation of NH₄⁺ (Equation 7), can have an important effect on the fate of phosphorus. Decreases in pH can lead to increased dissolution of gibbsite (Equation 19) and ferrihydrite (Equation 20) which can lead to increased concentrations of Al³⁺ and Fe³⁺. This, in turn, can stimulate the precipitation of variscite (Equation 23) and strengite (Equation 24). This relationship is supported by drainfield porewater analyses at several sites, including the Cambridge site, where zones of sediment P accumulation coincide closely with the zones where wastewater NH₄⁺ is being nitrified (Figure 8).

Figure 8 – Porewater pH and nitrate prevalence in drainfield sediments at three septic system sites in Ontario, Canada where sand aquifers are present: a) Cambridge and b) Muskoka both household systems, and c) Langton public school septic system. Porewater NO₃^––N values indicate percent of total inorganic nitrogen (NH₄⁺ + NO₃^––N) occurring as NO₃^–. Samples with no NO₃^– are the septic tank effluent (100 percent NH₄⁺–N) and indicate the depth position of the drainfield infiltration pipes. Nitrate and pH values from the unsaturated zone are from porewater squeezed from undisturbed sediment cores (from Zanini et al., 1998). Also shown, as hash-marked depth intervals, are the zones where phosphorus has accumulated in the drainfield sediments.

In granitic terrain, acidic conditions can persist in septic system plumes and pH values are commonly between 4 and 6. In the review study noted above (Robertson et al., 2019), P removal was found to be much greater at the sites on granitic terrain where acidic plumes were present (90 percent P removal), and plume SRP values showed a deceasing trend with lower plume pH values (Figure 9).

Figure 9 – Mean or representative soluble reactive phosphorus (SRP) concentrations in the proximal zones of 27 septic system plumes with differing pH values, compared to values predicted considering the solubility of the P-minerals hydroxyapatite, variscite, strengite and vivianite; Open and filled dots (24 Ontario, Canada sites) from Robertson et al. (2019); Cape Cod, USA site from Bussey and Walter (1996); Hollister and Flushing Meadows, USA sites from Robertson (1995). Solubility curves were determined using the chemical equilibrium model, PHREEQC (Parkhurst, 1995) and assume equilibrium with gibbsite and ferrihydrite, Ca concentration of 90 mg/L and for the vivianite solubility curve, ferrous Fe concentration of 1 mg/L (adapted from Robertson et al., 1998). Also shown as the horizontal black line (“Septic tanks, ON” in the legend) is the median septic tank effluent SRP value (8.4 mg/L) and range of effluent pH values at 22 of the Ontario sites (from Robertson et al., 2019).

Phosphorus accumulation in the drainfield sediments has also been observed at other locations, including the Cape Cod municipal wastewater infiltration site (Walter et al., 1996) and at sites in Sweden (Eveborn et al., 2014). The relatively high concentrations of phosphorus that occur in septic tank effluent (i.e., 5 to 15 mg/L, Table 1), compared to most other aqueous environments, increases the likelihood that mineral precipitation reactions will occur.