2 Potential for Contamination from Septic Tank Effluent
A compilation of septic tank effluent composition at sites treating domestic wastewater is provided in Table 1. Nitrogen in domestic wastewater occurs primarily as NH4+–N (nitrogen present in dissolved ammonium) and septic tank effluent values range from 18-108 mg/L. Although nitrate is generally absent in the effluent, when NH4+ is nitrified in the unsaturated zone, nitrate concentrations have the potential to exceed the drinking water limit for NO3––N (nitrogen present in dissolved nitrate) of 10 mg/L. The environmental impacts of groundwater with elevated levels of wastewater-derived nitrate discharging to freshwater lakes and coastal waters are an increasing concern (Persky, 1986; Harris, 1995; Cape Cod Commission, 2015). In addition, effluent phosphorus (P) concentrations (3-15 mg/L, Table 1), are several orders of magnitude higher than guidelines proposed to maintain surface water quality of sensitive lakes and rivers (e.g., 0.01 mg/L P in the USA; USEPA, 2000). Septic system effluent also exceeds drinking water criteria for pathogens and potentially a variety of other trace constituents. Consequently, considering the volume of wastewater generated by septic systems (e.g., 260 L/d/capita in the USA, McCray et al., 2005), septic systems may be considered one of the largest potential sources of groundwater contamination, worldwide. However, on-site treatment such as septic systems, provide a variety of treatment steps in the subsurface that have the potential to diminish the contaminant risk. Treatment is particularly active in the unsaturated zone beneath the drainfield. A number of the more important reactions are depicted in Figure 1 and are discussed in the following sections.
Table 1 – Septic tank effluent composition at sites treating domestic wastewater, including: electrical conductivity (EC), dissolved organic carbon (DOC), alkalinity (Alk), soluble reactive phosphorus or phosphate (SRP), the artificial sweeteners acesulfame and sucralose, and bacteria and virus colony forming units (CFU).
| Parameter | Median, Mean (± std dev), or Range |
Reference (n = number of samples or values) |
| EC (µS/cm) | 1000 | Robertson et al., 1991 (n=2) |
| 2481 | Harman et al., 1996 (n = 8) | |
| 1456 ± 314 | Robertson, 2012 (n ≥ 7) | |
| 1480 ± 131 | Geary and Lucas, 2019 (n = 17) | |
| DOC (mg/L) | 46 ± 27 | Robertson et al., 1998 (n = 8) |
| 11 ± 5.0 | Withers et al., 2011 (n = 37) | |
| 56 ± 26 | Robertson et al., 2012 (n = 3) | |
| Alk (mg/L) as CaCO3 | 316 ± 40 | Wilhelm et al., 1996 (n = 6) |
| 311 ± 102 | Robertson et al. 1998 (n = 8) | |
| 310 ± 105 | Withers et al., 2011 (n = 37) | |
| pH | 7.1 ± 0.4 | Robertson et al. 1998 (n = 8) |
| 7.3 ± 0.2 | Withers et al., 2011 (n = 37) | |
| 7.4 ± 0.2 | Geary and Lucas, 2019 (n = 17) | |
| NH4+–N (mg/L) | 66 ± 41 | Robertson et al. 1998 (n = 9) |
| 4-13 | USEPA, 2002 | |
| 34 ± 10 | Hinkle et al., 2008 (n = 10) | |
| 18 ± 16 | Withers et al., 2011 (n = 37) | |
| 58 | McCray et al., 2005 (n = 37) | |
| 72 ± 37 | Robertson et al.,2019 (n = 111) | |
| 108 ± 16 | Geary and Lucas, 2019 (n = 14) | |
| NO3––N (mg/L) | 0.2 ± 0.3 | Robertson et al. 1998 (n = 9) |
| <1 | USEPA, 2002 | |
| 0.2 | McCray et al., 2005 (n = 33) | |
| 0.03 ± 0.03 | Hinkle et al., 2008 (n = 10) | |
| 4.2 ± 3.2 | Withers et al., 2011 (n = 37) | |
| 0.2 ± 0.2 | Geary and Lucas, 2019 (n = 10) | |
| Total Nitrogen, TKN (mg/L) | 26 – 75 | USEPA, 2002 |
| 53 ± 14 | Hinkle et al., 2008 (n = 10) | |
| Total Phosphorus (mg/L) | 6 – 12 | USEPA, 2002 |
| 4.6 ± 4.2 | Withers et al., 2011 (n = 37) | |
| SRP (mg/L) | 8.4 ± 3.5 | Robertson et al. 1998 (n = 9) |
| 9.0 | McCray et al., 2005 (n = 35) | |
| 3.2 ± 2.6 | Withers et al., 2011 (n = 37) | |
| 8.2 ± 4.9 | Robertson et al.,2019 (n = 123) | |
| 15 ± 1.8 | Geary and Lucas, 2019 (n = 16) | |
| Cl– (mg/L) | 67 ± 64 | Robertson et al. 1998 (n = 9) |
| 32 ± 16 | Hinkle et al., 2008 (n = 10) | |
| 54 ± 16 | Withers et al., 2011 (n = 37) | |
| 64 | Robertson et al.,2019 (n = 106) | |
| Na+ (mg/L) | 54 ± 27 | Robertson et al. 1998 (n = 9) |
| 49 ± 29 | Withers et al., 2011 (n = 37) | |
| K+ (mg/L) | 22 ± 15 | Robertson et al. 1998 (n = 9) |
| 26 ± 8 | Withers et al., 2011 (n = 37) | |
| Ca2+ (mg/L) | 38 ± 36 | Robertson et al. 1998 (n = 9) |
| 96 ± 22 | Withers et al., 2011 (n = 37) | |
| B (mg/L) | 0.51 ± 0.05 | LeBlanc, 1984 (n = 3) |
| 0.11 ± 0.03 | Withers et al., 2011 (n = 37) | |
| 0.28 ± 0.02 | Bassett et al., 1995 (n = 3) | |
| 0.49 | Vengosh et al., 1994 (n = 21) | |
| Fe (mg/L) | 0.45 ± 0.41 | Robertson et al. 1998 (n = 8) |
| 0.12 ± 0.06 | Withers et al., 2011 (n = 37) | |
| Al (mg/L) | 0.1 ± 0.1 | Robertson et al. 1998 (n = 6) |
| Acesulfame (µg/L) | 57 | Snider et al., 2017 (single family, n = 14) |
| 32 | Snider et al., 2017 (communal, n = 36) | |
| 44 ± 32 | Robertson et al., 2019 (n = 56) | |
| Sucralose (µg/L) | 40 ± 25 | Oppenheimer et al., 2011 (n = 8) |
| 51 | Snider et al., 2017 (single family, n = 14) | |
| 28 | Snider et al., 2017 (communal, n = 36) | |
| 40 ± 34 | Robertson et al., 2019 (n = 56) | |
| Fecal Bacteria (CFU/100 mL) | 105 | Viraraghavan, 1978 |
| 106 | Shadford et al., 1997 | |
| 106 – 108 | USEPA, 2002 | |
| 105 | Geary and Lucas, 2019 | |
| Coliphage Virus (CFU/100mL) | 108 | Deborde et al., 1998a |