3.1 Bacteria
Examples of well-developed groundwater plumes below drainfields at a household site (Cambridge site) and at a campground (Long Point site) are shown in Figure 2 and Figure 3 respectively.
Figure 2 – A plan view (a) and cross-sectional view (b) of a groundwater plume from a household septic system at the Cambridge site in Ontario, Canada, showing: Na+ concentration (mg/L), the extent of E. coli (> 1 cfu/100 mL) presence, and phosphate (P) zones with concentration greater than 1 mg/L, dashed lines, and range of NO3––N concentrations (mg/L) in the plume core. Septic tank effluent has mean Na+ of 98 mg/L, NH4+–N of 30 mg/L and SRP of 8 mg/L (adapted from Robertson et al.,1991; Shadford et al., 1997; and Robertson, 2003).
Figure 3 – Septic system plume from a large campground at the Long Point site in Ontario, Canada, showing concentrations of the artificial sweetener, acesulfame (µg/L); and the extents of the zones (dashed lines) of NO3––N (>1 mg/L), phosphate-P (>1 mg/L) and E coli (>10 cfu/100 mL), along the plume centerline. Dots indicate multilevel monitoring points. The septic tank effluent has mean acesulfame concentration of 50 µg/L, NH4+–N of 100 mg/L and SRP of 6.3 mg/L (adapted from Robertson et al., 2013).
Fecal bacteria are largely removed within the unsaturated zone at both sites. In a test of bacteria mobility at the Cambridge site, tracer bacteria (E. coli NAR) were dosed into the septic tank over a three-week period and then monitored in the unsaturated zone below the drainfield and in the underlying groundwater plume, over an 84-day period (Shadford et al., 1997). The tracer bacteria were detected in unsaturated zone lysimeters within 30 days of the start of the injection, but only at concentrations that were much lower than in the septic tank (~1000-fold decrease). In the groundwater plume, the tracer bacteria were almost entirely absent, with detection occurring in only one of 255 samples (Shadford et al., 1997). The Cambridge unsaturated zone is 1.5 m thick, consists of fine to medium sand, and has a wastewater retention time of about 15 days (Robertson, 1994). In similar testing at two other sites, but with shallower water tables (<1 m depth), Shadford and others detected the tracer bacteria in groundwater directly below the drainfields, but not in other monitoring wells located only a few meters farther downgradient, with the exception of a few very low concentrations. Reneau and Pettry (1975), Viraraghavan (1978), and Chen (1988) report fecal coliform concentrations in a number of household and cottage septic system plumes (at three, one and seventeen sites, respectively). However, most of these sites had shallow water tables and shallow lower permeability sediments or bedrock. Consequently, monitoring was focused in the shallow soil zone where groundwater flow likely occurred through macropores. At all of these sites, fecal coliforms were either not detected in the groundwater plumes, or were depleted by at least 2 to 3 orders of magnitude, within a few meters downgradient from the drainfields.
In contrast, evidence for rapid transport of bacteria has been shown at sites with more favorable hydrologic conditions. In a tracer test conducted in the Cape Cod municipal wastewater plume, where relatively rapid groundwater velocities of 0.2-0.7 m/day occur in the medium to coarse sand aquifer, only a 10-fold reduction of a bacterial tracer occurred over a travel distance of 6 m and the arrival of the bacterial tracer coincided with the arrival of the Br co-tracer (Bales et al., 1995).