4.2 Treatment in the Unsaturated Zone (Aerobic)

Normally, drainfields are designed to discharge the effluent into the unsaturated zone where oxidation of the wastewater can occur. Important oxidation reactions include conversion of organic carbon to CO2 (Equation 5 and 6) and nitrification of NH4+ as shown in Equation 7 (Wilhelm et al., 1994).

CH2O + O2 → CO2 + H2O (5)
CO2 + H2O → H2CO3 → H+ + HCO3 (6)
NH4+ + 2O2 → NO3 + 2H+ + H2O (7)

Table 2 compares dissolved organic carbon (DOC) concentrations in septic tank effluent and the associated groundwater plumes at a number of sites. DOC removal in the drainfields at these sites, ranges from 22 to 97 percent. Greater removal occurs at sites where the wastewater is well oxidized, as indicated by the relatively complete conversion of NH4+ to NO3.

Table 2Comparison of dissolved organic carbon (DOC) concentrations in septic tank effluent and associated groundwater plumes, illustrating organic carbon degradation in the unsaturated zone below the drainfields. Plumes are listed in order of increasing levels of wastewater oxidation as indicated by decreasing NH4+ concentrations in the plumes. Plume values are mean (or representative) values from the proximal plume zones underlying the drainfields, unless indicated otherwise.

Site Effluent ————-Groundwater Plume————- Reference
DOC (mg/L) DOC (mg/L) DOC removal (%) NH4+-N (mg/L) NO3-N (mg/L)
JL, cottage 83 44 47 73 0.1 Robertson et al., 1998
LP2, campground 56 11 80 48 50-80 Robertson et al., 2012
LP2, campground 561 6.3 (5-8) 89 Aukes et al., 2019
SM, campground ~502 13 ~75 37 0.1 Garda, 2018
Otis, municipal wastewater 19 73 63 14 0.1 LeBlanc, 1984
DEL resort, reducing 9 7 22 3 0.1 Robertson et al., 1998
LP1, campground 21 5 ± 0.8 (n=8) 76 1 51 Robertson & Cherry, 1992
PP, campground 32 13 78 0.4 18 Robertson et al., 1998
DEL resort, oxidizing 9 5 45 0.2 13 Robertson et al., 1998
LA, school 28 0.7 97 0.1 131 Robertson et al., 1998
CA, house 40 4 90 0.1 14 Robertson et al., 1998
MU, house 81 3 96 0.1 37 Robertson et al., 1998

1 Effluent DOC value is from Robertson et al., 2012.

2 Effluent DOC value is not available for the SM campground site, but is assumed to be similar to the value at the LP2 campground site, because the two sites have similar effluent Cl and NH4+ concentrations.

3 Plume values are from a monitoring well located 1000 m from the drainfield.

The oxidation reactions generate acidity, but this is usually buffered through a series of rock-water interactions. In calcareous terrain, the dissolution of carbonate minerals (Equation 8), normally restores near- neutral pH conditions and in most cases, also results in increased Ca2+ concentrations in the plume.

CaCO3 + H+ → Ca2+ + HCO3 (8)

In granitic terrain, in the absence of carbonate minerals, buffering can be provided by Al– and Fe-oxyhydroxide minerals (e.g., gibbsite and ferrihydrite) that are usually present as minor constituents in most sediments. However, these minerals are relatively insoluble at neutral pH values. Their dissolution, as shown in Equations 9 and 10, becomes important only under low pH conditions.

Al(OH)3 + 3H+ → Al3+ + 3H2O (9)
Fe(OH)3 + 3H+ → Fe3+ + 3H2O (10)

Septic system plumes in granitic terrain, when well oxidized, are often acidic, with pH values commonly in the range of 4 to 6.

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Septic System Impacts on Groundwater Quality Copyright © 2021 by William Robertson. All Rights Reserved.